Download Atmospheric Pressure Plasmas for Surface Modifications

UNIVERSITÀ DEGLI STUDI DI MILANO-BICOCCA
SCUOLA DI DOTTORATO DI SCIENZE
CORSO DI DOTTORATO DI RICERCA IN FISICA E ASTRONOMIA
Riccardo A. Siliprandi
Atmospheric Pressure
Plasmas for
Surface Modifications
Relatore:
Prof. Claudia Riccardi
Coordinatore: Prof. Claudio Destri
Ciclo XX 2004-2007
to my family
Avrò piacere d’esser illuminato e tratto d’errore
Simplicio in Dialogo sopra i due massimi sistemi del mondo
Galileo Galilei, 1632
Contents
1
2
Introduction
1
1.1
Cold Atmospheric pressure plasmas . . . . . . . . . . . . . . .
1
1.2
Surface modifications with atmospheric plasmas . . . . . . . .
2
1.3
Motivations and thesis outline . . . . . . . . . . . . . . . . . .
3
Atmospheric pressure discharges and surface processes
5
2.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5
2.2
Electrical Breakdown of Gases
. . . . . . . . . . . . . . . . .
6
2.2.1
Townsend breakdown mechanism . . . . . . . . . . . .
7
2.2.2
Streamer breakdown mechanism . . . . . . . . . . . .
10
2.3
2.4
Dielectric Barrier Discharges
. . . . . . . . . . . . . . . . . .
15
2.3.1
Overview and properties of dielectric barrier discharges 16
2.3.2
Dielectric barrier discharge regimes . . . . . . . . . . .
16
2.3.3
Streamer Discharge Regimes . . . . . . . . . . . . . .
18
2.3.4
Micro-discharge interaction and pattern formation . .
20
Plasma-surface interactions . . . . . . . . . . . . . . . . . . .
21
2.4.1
Gas-phase chemistry and processes . . . . . . . . . . .
21
2.4.2
Surface kinetics and processes . . . . . . . . . . . . . .
24
VII
VIII
3
Dielectric barrier discharge devices
29
3.1
DBD device for surface modifications . . . . . . . . . . . . . .
29
3.1.1
Plasma reactor . . . . . . . . . . . . . . . . . . . . . .
30
3.1.2
Pumping system and gas distribution . . . . . . . . .
30
3.1.3
Electric power supply and configuration . . . . . . . .
33
3.1.4
Diagnostics . . . . . . . . . . . . . . . . . . . . . . . .
33
DBD device for streamer regime characterization . . . . . . .
34
3.2.1
Plasma reactor . . . . . . . . . . . . . . . . . . . . . .
34
3.2.2
Diagnostics . . . . . . . . . . . . . . . . . . . . . . . .
35
3.2
4
CONTENTS
Plasma and material diagnostics
37
4.1
Optical emission spectroscopy . . . . . . . . . . . . . . . . . .
37
4.1.1
Determination of molecular vibrational temperature .
38
Voltage Current measurements . . . . . . . . . . . . . . . . .
38
4.2
4.2.1
4.3
Implementation of Rogowski coils for measurements
nanoseconds current pulses . . . . . . . . . . . . . . .
38
Characterization of the materials surfaces . . . . . . . . . . .
43
4.3.1
Infrared spectroscopy (FTIR/ATR-FTIR) . . . . . . .
44
4.3.2
Atomic force microscopy (AFM) . . . . . . . . . . . .
45
4.3.3
Contact angle measurements and surface energy determination . . . . . . . . . . . . . . . . . . . . . . . .
5
46
Statistical characterization of a streamer discharge regime
51
5.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . .
51
5.2
Statistical characterization of current signal . . . . . . . . . .
53
5.2.1
streamers. . . . . . . . . . . . . . . . . . . . . . . . . .
54
Discharge current regimes . . . . . . . . . . . . . . . .
56
Statistical analysis of temporal behavior . . . . . . . . . . . .
62
5.2.2
5.3
5.3.1
Inter- and intra-bump correlations: surrogate model
and Hurst exponents . . . . . . . . . . . . . . . . . . .
62
Temporal correlations between streamers . . . . . . .
68
Concluding remarks . . . . . . . . . . . . . . . . . . . . . . .
75
5.3.2
5.4
Structure of the discharge current: bumps, bursts and
CONTENTS
IX
6
Characterization of the DBD device in nitrogen atmosphere
79
6.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . .
79
6.2
Experimental setup and methods . . . . . . . . . . . . . . . .
80
6.3
Discharge regimes in Nitrogen Atmosphere . . . . . . . . . . .
80
6.3.1
Characterization of the discharge as a function of injected power . . . . . . . . . . . . . . . . . . . . . . .
6.3.2
6.4
7
sure and gas fluxes . . . . . . . . . . . . . . . . . . . .
85
Concluding remarks . . . . . . . . . . . . . . . . . . . . . . .
89
91
7.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . .
91
7.2
Materials and methodology . . . . . . . . . . . . . . . . . . .
92
7.3
Characterization of the deposition process . . . . . . . . . . .
94
7.3.1
Plasma characterization . . . . . . . . . . . . . . . . .
95
7.3.2
Thin film characterization . . . . . . . . . . . . . . . .
97
Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . 105
Fluorination of polymer surfaces
107
8.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
8.2
Experimental, diagnostics and methods . . . . . . . . . . . . 109
8.3
Characterization of the fluorine grafting process . . . . . . . . 110
8.4
9
Characterization of the discharge as a function of pres-
Deposition process of organosilicon thin films
7.4
8
83
8.3.1
Plasma-phase characterization
. . . . . . . . . . . . . 110
8.3.2
Material surface characterization . . . . . . . . . . . . 113
Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . 119
Plasma Application for modification of paper surfaces
9.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
9.1.1
9.2
9.3
121
Cellulose and paper . . . . . . . . . . . . . . . . . . . 121
Deposition of organic silicon compounds for hydrophobicity . 123
9.2.1
Experimental setup and diagnostics . . . . . . . . . . 124
9.2.2
Hydrophobicity of treated paper surface . . . . . . . . 125
Fluorination process for oil repellency . . . . . . . . . . . . . 128
9.3.1
Experimental setup and diagnostics . . . . . . . . . . 129
X
CONTENTS
9.3.2
Oil repellency of paper surfaces . . . . . . . . . . . . . 129
10 Conclusions
135
Bibliography
139
CHAPTER
1
Introduction
1.1 Cold Atmospheric pressure plasmas
Atmospheric pressure plasmas are growing as an interesting alternative to
low pressure plasmas for several applications. The possibility to develop continuous processing without the costs of vacuum technologies has attracted in
the last decade the efforts of several industries and research groups all over
the world. Within the different types of atmospheric pressure non-thermal
plasmas, dielectric barrier discharges (DBDs) are the most interesting solution. DBDs are a well known type of gas discharge. They have been
widely used in industrial applications like ozone generators, plasma display
panels, excimer lamps, volatile organic compounds destruction and surface
modifications [1, 2, 3, 4, 5, 6].
DBDs are low temperature, non-equilibrium, transient gas discharges
operating in a quasi-continuous discharge regime. They usually consist of
two electrodes (at least one of them covered with a dielectric material) to
1
2
INTRODUCTION
which an AC high voltage is applied for frequencies generally varying in the
range of 102 Hz to 102 kHz. At low pressures DBDs operate in a Townsend
breakdown regime [5] generating a diffuse glow discharge. At atmospheric
pressure, the realization of a diffuse discharge is restricted to limited conditions of geometry, electrical parameters and gas composition [7, 8, 9, 10, 11],
and DBDs operate usually in a streamer discharge in which several narrow
discharge filaments are typically formed. The streamer regime constitutes
a strongly interacting system of discharges exhibiting cooperative behavior.
This leads, under specific conditions, to the formation of coherent spatial
configurations that have been observed in different types of experimental
setups [12, 13, 14, 6]. However, micro-discharges seem, to some extent, to
occur at random within the discharge gap for most applications of DBDs.
Despite of the existence of several industrial applications and intense study
during decades, DBDs still lack a clear physical interpretation of the discharge regimes and of the complex chemistry involved in the processes.
1.2 Surface modifications with atmospheric plasmas
Plasma-surface interaction is a rather complicate process which involves
several complex chemical and physical mechanisms [15, 16]. For this reason plasma processing is the subject of study in many research areas like
plasma physics, surface science, gas-phase chemistry and atomic and molecular physics. The common theme is the creation and use of plasmas to activate a chain of chemical reactions at a substrate surface. At low pressure
the behavior of many chemical processes in presence of a plasma have been
the subject of intense research in the recent years and is now a well established industrial process[17, 15, 18, 19, 2]. An example is the semiconductor
industry which successfully employs plasma processes for the production of
integrated circuits.
DBDs are already employed in industry for modification of material surfaces [1]. This application has regarded mainly the processes in atmospheric
Air for treatments of polymer surfaces to attain wettability, printability and
adhesion properties [1, 20, 21]. Its use in different reactive atmospheres
has proved more difficult because of the strong dependence of the discharge
regime on the atmosphere composition and the absence of an environment
1.3 MOTIVATIONS AND THESIS OUTLINE
3
sealed from contaminations [1, 22]. Moreover, the behavior of several reactive atmospheres, well known at low pressure, is different to some degree
when used at atmospheric pressure. This is due to the great difference between the discharge regimes that can be realized at high pressure and the
substantial change in chemical and physical processes both in gas-phase and
on the surface [23, 24, 25, 26].
1.3 Motivations and thesis outline
This thesis concerns the study of atmospheric pressure plasmas realized in
dielectric barrier discharges and their applications to surface modifications
of materials.
The first part of the work is dedicated to the study and the characterization of the streamer regime. Streamer development is still subject of
intense study and several theoretical models and few experiment describe
the development of the single phenomena. However, due to the complexity of the interaction between micro-discharges, a clear physical picture of
their behavior as a whole system is not presently available. In this study a
novel approach to the problem is used, and the tools of statistical analysis
are used to investigate the temporal behavior of micro-discharges through
the measurement of current signal. Several interesting feature regarding
the discharge dynamics and the temporal propagation of correlations are
discussed.
The second part of this work is dedicated to the study of plasma processing of material surfaces. These studies are performed using a newly
built plasma reactor which gives the possibility to study both the discharge
physics and the plasma-surface interaction during continuous processing in
a wide range of pressures and compositions of the atmosphere. Two specific
processes are studied: a deposition process of thin organosilicon films for the
creation of hydro-repellent coatings and a grafting of fluorine atoms process
to produce hydro- and oil-repellent properties on the surfaces of organic soft
matter. The applications of these processes to the modification of paper
surfaces is then studied.
The thesis is organized as follows. In Chapter 2 a brief review of the
physical foundations of plasma discharges and plasma-surface interactions
4
INTRODUCTION
is given. Chapter 3 describes the plasma devices that have been realized
and used in the experiments. Chapter 4 describes the diagnostics both
for the plasma and materials. The description of study and realization of
high bandwidth Rogowski coils for the measurement of fast current pulses is
given. Chapter 5 involves the study of the streamer regime and its characterization by means of statistical analysis of current signals. Chapter 6 gives
a characterization of plasma discharges in nitrogen atmosphere. Chapters 7
and 8 describe two atmospheric pressure processes of surface modifications.
A deposition process of thin organosilicon films and a grafting process of
fluorine radicals. Chapter 9 is finally devoted to the presentation of some
results concerning the application of studied processes for modification of
paper surfaces.
CHAPTER
2
Atmospheric pressure discharges
and surface processes
In this chapter the general concepts of and features of electrical discharges
in gases the the plasma-surface processes are briefly reviewed. Attention is
focused on physical phenomena interesting for the arguments of the present
research. For a more complete insight of the problems reader is advised to
refer to literature [5, 2, 15, 1].
2.1 Introduction
One of the simplest way to produce a plasma is applying an electric field to
a neutral gas. These artificially generated plasmas can be classified into two
main categories: thermal and non-thermal ones.
In a plasma, as in any gas, the temperature is determined by the average
kinetic energy of its components. However, a plasma can exhibit multiple
5
6
ATMOSPHERIC PRESSURE DISCHARGES AND SURFACE PROCESSES
temperatures, usually one for the heavy particles and one for the electrons
(Ti and Te respectively) unless sufficient collision occur between them. Because of the large difference in mass between electrons and other particles
the temperatures of these two species remains different in many conditions.
When Te ≃Ti the plasma is considered in local thermodynamic equilibrium ,
LTE, (which requires also the absence of chemical gradients) and termed as
thermal plasma. These discharges are characterized by high temperature of
the gas. Examples of thermal plasma are the Plasma Torches or the fusion
plasma devices. Otherwise, when large deviations from LTE are present (i.e.
Te > Ti ) the plasma is not thermalized an is called non-equilibrium or nonthermal plasma. The main feature of non-thermal plasmas is that the most
part of the electrical energy injected in the system is used for the production
of energetic electrons rather than heating the gas, while the neutral species
and ions remain relatively cold because of the low energy exchange with light
particles. The electrons have enough energy to ionize other molecules and
atoms generating excited species, other electrons and free radicals. They
can achieve sufficient energy to initiate chemical reactions usually forbidden
to standard chemistry in the same condition. Plasma can initiate several
physical and chemical processes on material surfaces which can provide an
efficiency increase in processing methods and very often can reduce environmental impact in comparison to more conventional processes.
2.2 Electrical Breakdown of Gases
Electric breakdown is referred to the process that transforms a non-conducting
material to a conducting one when a sufficient strong electric field is applied.
Although the breakdown is a rather complicate process that strongly depends on the system conditions, it begins always with an electron avalanche,
i.e. the multiplication of some primary seed electrons in cascade ionization
when accelerated by the electric field. After this initial stage the following
development of the discharge depends on several parameters as gas composition, pressure, distance between electrodes, frequency of the applied field
and geometry of the system. For sufficient low pressure the mean free path
of the electrons is high and the initial avalanche proceeds until the plasma is
generated in the whole discharge gap. For relative High pressure the mean
2.2 ELECTRICAL BREAKDOWN OF GASES
7
Figure 2.1: Townsend electrical breakdown in a gap d with constant electric field
E = V /d. Secondary electrons emitted by the cathode generate the multiplication
of avalanches [2].
free path of the electrons is drastically reduced and the avalanche ionization
can generate a great number of electrons giving rise to a localized space
charge which propagates in the discharge gap creating a thin conductive
channel named streamer. If no means are taken to limit the current in
the system, the the streamer is only the initial stage of the so called arc
discharge.
2.2.1 Townsend breakdown mechanism
The discharge process at low pressure or for low values of pd products,
where p is the pressure and d is the inter-electrode gap distance, is called
Townsend1 . For the sake of simplicity consider a system of parallel plate
electrodes at a distance d to which is applied a DC Voltage V that provides
a constant field E = V /d. The seed electrons generated from an external
source (for example cosmic rays or natural radioactivity) are accelerated by
the electric field in the gap and reach the anode unless they are lost in the
way by ion recombination or interaction with the chamber wall. The greater
the external electric field (i.e. faster electrons), the smaller the the fraction
1
From the name of John Sealy Townsend who first introduced this model to explain
electrical breakdown in gases.
8
ATMOSPHERIC PRESSURE DISCHARGES AND SURFACE PROCESSES
of the electrons lost before they reach the anode. As a result, the electric current i in the circuit, which is proportional to the number of charged
species which reach the electrodes, initially increases with increasing voltage
V . Beginning at a certain voltage, quite all the charged particles (electron
and ions) reach the electrodes and the current reaches a saturation i0 and
ceases to depend on V . At this point the discharge is non self-sustaining,
i.e. the discharge depends on the presence of an external sources (point A
in Figure 2.2). At still higher voltage, the electron impact ionization on
neutral gas molecules starts initiating the avalanche process (Figure 2.1)
and amplifying the initial current i0 due to the external source. It is convenient to describe the ionization in an avalanche by the Townsend ionization
coefficient α that express the electron production per unit length:
dne
= αne −→ ne (x) = n0e eαx
dx
(2.1)
where x is the distance from the cathode, ne the electron density and n0e is
the initial electron density created by the external sources. For simplicity
here the electron losses due to recombination and attachment to electronegative molecules are neglected. The current at the anode (and so the current
in the closed circuit) is equal to: i = i0 eαd where i0 = en0e and e is the
charge of the electron. The primary process of electron impact ionization
generates n0e [eαd − 1] ions during the avalanche propagation which become
important when the voltage is furthermore raised and hitting the cathode
they can generate γn0e [eαd − 1] electrons in the process of secondary electron
emission. γ is the secondary electron emission coefficient and it depends on
cathode material, state of the surface and electric field (which define ion energy). Taking into account this secondary process the current in the circuit
is:
i=
i0 eαd
1 − γ[eαd − 1]
(2.2)
which is called Townsend formula and was first derived in 1902 to describe
the breakdown process in electric discharges. The transition from non selfsustaining to self-sustaining discharge is controlled by the denominator in
Equation (2.2). If µ = γ[eαd −1] < 1 the discharge is still non self-sustaining,
but when µ approaches to unity the current grows to infinity and the discharge becomes self-sustaining, i.e. the breakdown occurs. The simplest
9
2.2 ELECTRICAL BREAKDOWN OF GASES
Figure 2.2: Voltage-current characteristic of low temperature discharge between
electrodes for a wide range of currents. A: region of non-self-sustaining discharge,
(BC) Townsend discharge, (CD) subnormal glow discharge, (DE) normal glow discharge, (EF) abnormal glow discharge, (GH) arc discharge [5].
breakdown condition can be expressed as:
µ = γ[eαd − 1] −→ αd = ln(
1
+ 1)
γ
(2.3)
which means that each primary electron generated in the initial avalanche
and lost at the anode is replaced by another electron generated by secondary
emission at the cathode. This represents a steady self-sustained current in
the homogeneous field Et = Vt /d (point B in Figure 2.2), where Vt is the
breakdown voltage and is determined from Equation (2.3) as a function of
d an in terms of γ and the known function α(E).
In the presented ideal framework the current i for a voltage V = Vt
would increase to infinity. Any real circuit of the type described above has
an ohmic resistance Ω which sets a limit to the current achievable for a
given electromotive force E. In the case Ω is so great that only a really
small current can flow through the gap and the electrode gap is small in
comparison to electrode dimensions, the field is constant and equal to the
field in absence of discharge. The potential will be equal to the breakdown
voltage Vt . This stable self-sustained discharge with extremely low current is
called Townsend dark discharge (segment BC in Figure 2.2). Let us gradually
increase the current. This can be realized by reducing the load resistance Ω
10
ATMOSPHERIC PRESSURE DISCHARGES AND SURFACE PROCESSES
or by increasing the electromotive force E. The voltage across the electrodes
begins to decrease after a certain current is reached. The fall then stops
and the current remains almost constant over a fairly wide range of values
(sometimes of several orders of magnitude). This is the so-called normal glow
discharge (segment DE in Figure 2.2). The lower part of the transition region
(segment CD in Figure 2.2) corresponds to a sub-normal glow discharge. The
normal discharge has one remarkable property. As the discharge current is
varied, its density at the cathode remains unchanged while changes the area
through which the current flows. When Ω or E is varied, the luminous
current spot on the cathode surface expands or contracts. When no more
free surface is left on the cathode, the current is increased by increasing the
voltage, hence extracting more electrons from unit surface area. Indeed, the
cathode current density must grow. This discharge is said to be abnormal
(segment CD in Figure 2.2). The glow first covers the entire cathode surface
facing the anode, then reaches every spot unprotected by dielectric on the
lateral and inner surfaces and on the support pin, and only having exhausted
these possibilities does it become more extended and intense to a degree
typical of the abnormal discharge. When i ∼ 1A, the glow discharge cascades
down to an arc discharge which is characterized by high current and low
voltage. The segment FG in Figure 2.2 describes the transition, and GH
represents the arc discharge.
2.2.2 Streamer breakdown mechanism
When the pressure is high an the pd > 100 Torr·cm, the Townsend breakdown cannot describe the discharge development. This mechanism is based
on the emission of secondary electrons from the cathode and develops in
time of the order 10−5 − 10−3 . For high values of pd the breakdown develops
much faster and the independence of the breakdown voltage on the material of the cathode, established by very accurate measurements, is evidence
against the participation of cathode processes in the breakdown mechanism.
This different breakdown is called streamer breakdown for the thin localized
plasma channels that are generated in the process. The concept of streamer
was originally developed by Raether [27], Loeb [28] and Meek [29].
Also at high pressure an individual avalanche is a primary and compulsory element of the breakdown mechanism. Consider an avalanche in a
11
2.2 ELECTRICAL BREAKDOWN OF GASES
uniform external field E0 between plane electrodes. Let it be initiated by
a single electron that leaves the cathode at the time t = 0. The x axis is
directed from a point on the cathode to the anode. The radial distance from
the x axis is denoted by r. Taking into account the possible formation of
negative ions, we find the total numbers of electrons and ions increasing as
the avalanche moves forward:
dN+
dN−
dNe
= (α − η)Ne ,
= αNe ,
= αNe ,
dx
dx
dx
Ne = e(α−η)x , N+ =
α
α
(Ne − 1), N− =
(Ne − 1),
α−η
α−η
(2.4)
(2.5)
where α and η are the ionization and attachment coefficients. All the new
electrons fly to the anode in a group at the drift velocity vd = µe E0 where
mue E0 is the electron mobility. However, free diffusion (De )makes the electron cloud spread around the central point x0 = vd t, r = 0. Taking into
account both the effects the electron density can be expressed as:
(x − vd t)2 + r 2
ne = (4πDe t)−3/2 exp −
+ (α − η)vd t
4De t
(2.6)
The density ne decreases with distance from the moving center following a
Gaussian law. The radius of the sphere on which the density is exactly e
times less than that at the centre, ne (x0 , 0, t), increases with time (during
the progress of the avalanche) by the characteristic diffusion law:
rD =
p
4De t =
s
De x0
4
=
µe E0
r
4Te x0
.
eE0
(2.7)
The ions remain practically fixed during the time of flight of the avalanche
to the anode. (see Figure 2.3). Thus, they accumulate at each point. The
positive ion density is
n+ (x, r, t) =
Z
t
αvd ne (x, r, t′ )dt′ ,
(2.8)
0
In the absence of attachment in the limit t → ∞ and for regions not too far
from the axis, an approximate calculation of the integral using Equations
12
ATMOSPHERIC PRESSURE DISCHARGES AND SURFACE PROCESSES
Figure 2.3: Formation of a streamer[5].
(2.6) and (2.8) gives
n+ (x, r) =
α
r2 exp
αx
−
πra2 (x)
ra2 (x)
(2.9)
where ra2 (x) is the avalanche radius defined by Equation (2.7). The ion
concentration in the trail of the avalanche is growing up along the axis
in accordance with exponential (2.4) increase of number of electrons. The
qualitative change in avalanche behavior takes place when the charge amplification exp(αx) is high. In this case the production of a space charge
with its own significant electric field E ′ takes place. This local electric field
E ′ should be added to the external field E0 . Because the electrons are
much faster than ions the electrons always run at the head of avalanche
leaving the ions behind and thus creating a dipole with the characteristic
length 1/α (mean distance for an electron before creating an ion) and charge
Ne ∼ exp (αx). The fields E ′ and E0 in front of the avalanche head add up
to give a field stronger than E0 . The fields E ′ and E0 in the zone between
the centers of the space charges of opposite signs point in opposite directions
and the resultant field is weaker than E0 . When the avalanche reaches the
anode, the electrons sink into the metal and only the positive space charge
of the ionic trail remains in the gap (Figure 2.3). The field is formed by the
ionic charge and by its ”image” in the anode. The image in the relatively
13
2.2 ELECTRICAL BREAKDOWN OF GASES
distant cathode plays a rather insignificant role. The field close to the anode
is less than E0 , but exceeds it farther off. The field reaches its maximum at
the axial distance from the anode of the order of one ionization length α.
When the number of charges Ne is high, the diffusional spreading of
the electron cloud is replaced by their electrostatic repulsion. The law of
expansion R(x), is given by:
R=
3e 1/3
αx 3E ′
exp
.
=
αE0
3
αE0
(2.10)
The fast growth of the transverse avalanche size restricts the electron density
in the avalanche by the value: ne = (3Ne )/(4πR3 ) = (αE0 )/(4πe). When
the transverse avalanche size reaches the characteristic ionization length
1/α (about 0.1 cm at atmospheric pressure in Air), the broadening of the
avalanche head slows down dramatically. Obviously, the avalanche electric
field is about the external one in this case (see eq. 2.16). The typical values
of maximum electron density in an avalanche are about 1012 − 1013 cm3 .
When the avalanche head reaches the anode, the electrons sink into the
electrode leaving the ions occupy the discharge gap. At the electron absence,
the total electric field is due to the external field, the ionic trail and also
the ionic charge image in the anode. The resulting electric field in the ionic
trail near the anode is less than the external electric field, but farther off
the electrode it exceeds E0 . The total electric field reaches the maximum
value on the characteristic ionization distance (about 1 mm from the anode).
A strong primary avalanche amplifies the external electric field leading to
formation of thin weakly ionized plasma channel, so-called streamer. The
avalanche-to-streamer transformation takes place, when the internal field of
an avalanche becomes comparable with the external one, that is when the
amplification parameter αd is big enough. At a relatively small discharge
gaps, the transformation occurs only when the avalanche reaches the anode.
Such a streamer is known as the cathode-directed or positive streamer. If the
discharge gap and over-voltage are big enough, the avalanche-to-streamer
transformation can take place quite far from anode. In this case the socalled anode-directed or negative streamer is able to grow toward the both
electrodes.
The cathode-directed streamer starts near the anode. It looks like and
14
ATMOSPHERIC PRESSURE DISCHARGES AND SURFACE PROCESSES
operates as a thin conductive needle growing from the anode. The electric
field at the tip of the anode needle is very high, which stimulates the fast
streamer propagation in direction of cathode. Usually the streamer propagation is limited by neutralization of the ionic trail near the tip of the needle.
The electric field there is so high, that provides electron drift with velocity
about 108 cm/s. One hypothesis states that the decisive role is played by
energetic photons that are emitted by atoms excited in the avalanche and
produce photoionization in the vicinity of the primary avalanche. (Events
of production of electrons at the cathode or far from the trail are unimportant in this context because they result in avalanches similar to the primary
one.) Electrons produced by photons initiate secondary avalanches that are
pulled into the trail due to the direction of the resulting field. Secondaryavalanche electrons intermix with primary-avalanche ions and form a quasineutral plasma. They also excite atoms, so that new photons are emitted.
Secondary-avalanche ions en- enhance the positive charge at the cathode
end of the evolved plasma channel. This charge attracts the electrons of
the next generation of secondary avalanches, etc. This is how the streamer
grows. The process of ionization along the ion trail of the primary avalanche
begins at the spot where the positive charge and the field are the highest,
that is, at the anode, provided the degeneration condition E ′ ∼ E0 has been
reached there. This is the situation shown in Figure 2.3.
The anode-directed streamer occurs between electrodes, if the primary
avalanche becomes strong enough even before reaching the anode. The
streamer propagates in direction of cathode in the same way as cathodedirected streamer. Mechanism of the streamer growth in direction of anode
is also similar, but in this case the electrons from primary avalanche head
neutralize the ionic trail of secondary avalanches. However, the secondary
avalanches could be initiated here not only by photons, but also by some
electrons moving in front of the primary avalanche
When the streamer channel connects the electrodes, the current may be
significantly increased to form the spark or arc discharge which are characterized by high current and low voltage. This would lead to Joule heating
of the gas and the generation of a thermal plasma.
2.3 DIELECTRIC BARRIER DISCHARGES
15
Figure 2.4: Examples of dielectric barrier discharge systems[2].
2.3 Dielectric Barrier Discharges
As it has been shown in Section 2.2.2 once the conducting streamer channel
is established electrons can flow trough it and sink at the anode until current
rises and the streamer converts to a spark. If no means are used to limit the
current, the temperature of the gas will rise rapidly due to Joule heating
(thermal plasma). The simplest solution to the problem is to place a dielectric barrier between the electrodes which prevents the electrons to reach
the electrodes and sink. At this point after the streamer channel is created
only a limited current for a short time can flow and the temperature of the
gas remains quite low while the electrons posses temperatures of the order
of electronvolts. This solution establishes a transient discharge which must
be reactivate by the external circuit using an alternating or pulsed current
power supply. With such system it is possible to obtain a quasi-continuous
regime. These are called dielectric barrier discharges (DBDs). Example of
DBD system are shown in Figure 2.4
16
ATMOSPHERIC PRESSURE DISCHARGES AND SURFACE PROCESSES
2.3.1 Overview and properties of dielectric barrier discharges
Dielectric barrier discharges have a high number of industrial applications
[19, 18, 1, 2] because they operate at strongly non-equilibrium conditions
at atmospheric pressure and at reasonably high power levels, without using
sophisticated pulse power supplies. This discharge is industrially applied
in ozone generation [30], CO2 lasers, and as a UV-source in excimer lamps
[31, 32]. In addition, the DBD in air is commonly used to treat polymer
surfaces in order to promote wettability, printability, and adhesion [1, 22].
DBD application for pollution control is quite promising, but the largest
expected DBD application is related to plasma display panels for large-area
flat television screens. Strong thermodynamic non-equilibrium and simple
design these distinctive properties of DBD allow hoping on expansion of
its applications in low temperature atmospheric pressure plasma chemistry.
DBD has a big potential to be a prospective technology of exhaust cleaning
from CO, NOx and volatile organic compounds [33, 34]. Successful use of
DBD reported in recent research on plasma-assisted combustion may result
in new applications [35].
2.3.2 Dielectric barrier discharge regimes
Usually at atmospheric pressure for values of product pd > 100Torr·cm the
breakdown is the streamer breakdown (see Section 2.2.2) which leads to the
formation of several narrow micro-discharges. The origin of the streamer
is a large electronic avalanche creating enough ions to localize the electrical
field. It is observed when the gas gap becomes large compared with the electron mean free path. However, yet in 1968, Bartnikas found that helium ac
discharges between closely spaced plane-parallel electrodes, metallic or covered with a dielectric layer, can exhibit diffuse glow discharge characteristics
[36]. After this first observation several research groups studied this particular regime finding that stable diffuse discharges could be obtained in gases
including helium, neon, argon, nitrogen, oxygen, and air [7, 8, 37, 9, 10, 11].
However, this diffuse regimes remains extremely unstable and tends to convert to the more stable streamer regime.
A detailed explanation of the operation of diffuse discharges is not known.
It is clear, however, that streamers can be avoided by using an applied elec-
2.3 DIELECTRIC BARRIER DISCHARGES
17
tric field below the Meek criterion. The requirement for establishing a stable
diffuse discharge is that the number of seed electrons is large enough to cause
appreciable overlap and merging of the primary avalanches. This results in
a smoothing of the field gradients due to space-charge at the stage when
streamer formation would otherwise occur. The governing parameters of
this transition are the effective first ionization coefficient ᾱ (which is defined as ᾱ = α − η where η is the electron attachment coefficient) and the
secondary electron emission from the cathode γ. The coefficient ᾱ, or bet-
ter the quantity ∂(ᾱ/n)/∂(E/n) (evaluated at the breakdown) is bound to
the radius of the propagating streamer channel [4, 6, 38]. A low value of
this quantity results in a wider streamer channels that overlap more easily to form a diffuse discharge. The increase of streamer radius has also
been observed experimentally [39]. However, the fundamental mechanism
that ensures the presence of enough seed electrons is the so called Penning
ionization [40, 41]. A Penning mixture usually consists of a gas with small
admixture of impurities. If the components of the impurity B have a ionization potential lower than the metastable potential of the gas A, then the
metastable atoms of the main gas can ionize the molecules of the admixture
according to
A ∗ +B−→A + B + + e− .
(2.11)
Usually, the probability of this process is so high that very small admixtures
may have considerable influence on the discharge development. For example, in Helium which possess highly energetic metastable levels (e.g. He[23 S]
and [21 S]), for the creation of a Penning mixture the background impurities may be enough. The presence of seed electrons lowers the breakdown
voltage allowing the discharge to develop without the creation of intense
field gradients due to space-charge. However this condition requires that
the slope of the voltage versus time is limited. Thus, in presence of a sinusoidal excitation voltage, its amplitude and frequency that allows to obtain
a diffuse discharge are limited. Very high values of these parameters induce
instabilities which lead to a pure filamentary discharge and limits the discharge power in this regime. Another problem of the diffuse discharge is
that the presence of electronegative gases in the mixture can rapidly quench
the seed electrons reducing their number. This leads again to the streamer
18
ATMOSPHERIC PRESSURE DISCHARGES AND SURFACE PROCESSES
regime, limiting the reactive atmosphere that can be eventually employed
in diffuse plasma processes.
2.3.3 Streamer Discharge Regimes
The dielectric barrier discharge gap usually includes one or more dielectric
layers, which are located in the current path between metal electrodes. Two
specific DBD configurations, planar and cylindrical are illustrated in Figure 2.4 . Typical discharge gaps varies from 0.1 mm to several centimeters.
Breakdown voltages of these gaps with dielectric barriers are practically the
same as those between metal electrodes. If the dielectric-barrier discharge
gap is a few millimeters, the required AC driving voltage with frequency 500
Hz to 500 kHz is typically about 10 kV at atmospheric pressure. The dielectric barrier can be made from glass, quartz, ceramics or other materials
of low dielectric loss and high breakdown strength. Then a metal electrode
coating can be applied to the dielectric barrier. The barrier-electrode combination also can be arranged in the opposite manner, e.g. metal electrodes
can be coated by a dielectric. As an example, steel tubes coated by an
enamel layer can be effectively used in the dielectric-barrier discharge. In
most cases, dielectric barrier discharges are not uniform and consist of numerous micro-discharges distributed in the discharge gap as can be seen from
figure 2.5. The physics of micro-discharges is based on an understanding of
the formation and propagation of streamers, and consequent plasma channel
degradation. The electrons in the conducting plasma channel established by
the streamers dissipate from the gap in about 40 ns, while the heavy and
slowly drifting ions remain in the discharge gap for several microseconds.
Deposition of electrons from the conducting channel onto the anode dielectric barrier results in charge accumulation and prevents new avalanches and
streamers nearby until the cathode and anode are reversed (if applied voltage
is not much higher than voltage necessary for breakdown). The usual operation frequency used in the dielectric barrier discharges is around 20 kHz,
therefore the voltage polarity reversal occurs within 25 µs. After the voltage polarity reverses, the deposited negative charge facilitates the formation
of new avalanches and streamers in the same spot. As a result, a manygeneration family of streamers is formed that is macroscopically observed
as a bright filament that appears to be spatially localized. It is important
2.3 DIELECTRIC BARRIER DISCHARGES
19
to clarify and to distinguish terms streamer and micro-discharge. An initial
electron starting from some point in the discharge gap (or from cathode
or dielectric that cover the cathode in the case of well developed DBD)
produces secondary electrons by direct ionization and develops an electron
avalanche. If avalanche is big enough the cathode directed streamer is initiated (usually from the anode region). Streamer bridges the gap in few
nanoseconds and forms a conducting channel of weakly ionized plasma. Intensive electron current will flow through this plasma channel until local
electric field is collapsed. Collapse of the local electric field is caused by
the charges accumulated on dielectric surface and ionic space charge (ions
are too slow to leave the gap for the duration of this current peak). Group
of local processes in the discharge gap initiated by avalanche and developed
until electron current termination usually called micro-discharge. After electron current termination there is no more electron-ion plasma in the main
part of micro-discharge channel, but high level of vibrational and electronic
excitation in channel volume along with charges deposited on the surface
and ionic charges in the volume allow us to separate this region from the
rest of the volume and call it micro-discharge remnant. Positive ions (or
positive and negative ions in the case of electronegative gas) of the remnant
slowly move to electrodes resulting in low and very long ( 10 µs for 1 mm
gap) falling ion current. Micro-discharge remnant will facilitate formation of
new micro-discharge in the same spot as the polarity of the applied voltage
changes. That is why it is possible to see single filaments in DBD. If microdischarges would form at a new spot each time the polarity changes, the
discharge would appear uniform. Thus filament in DBD is a group of microdischarges that form on the same spot each time polarity is changed. The
fact that micro-discharge remnant is not fully dissipated before formation of
next micro-discharge is called memory effect. The principal micro-discharge
properties for most of the frequencies do not depend on the characteristics
of the external circuit, but only on the gas composition, pressure and the
electrode configuration. An increase of power just leads to generation of
a larger number of micro-discharges per unit time, which simplifies scaling of the dielectric barrier discharges. Modeling of the micro-discharges is
closely related to the analysis of the avalanche-to-streamer transition and
streamer propagation. Detailed 2D-modeling of formation and propagation
20
ATMOSPHERIC PRESSURE DISCHARGES AND SURFACE PROCESSES
Figure 2.5: Example of stable pattern formation in a one dimensional DBD in air
at a pressure of 500 mbar.
of streamers can be found literature [42, 43] where also the mutual influence
of micro-discharges is considered [43].
2.3.4 Micro-discharge interaction and pattern formation
Although the DBDs in a streamer regimes have been studied and utilized
in industries for several decades, the interaction between micro-discharges is
still subject of intense studies and a clear physical picture is yet to be found.
In the past decades several experiments have been performed showing that
under specific conditions regular pattern can be obtained [44, 14, 13, 12].
These patterns have been modeled using methods that apply generally to
pattern formation in nonlinear dynamical systems [45, 46]. Thus, the dynamical interactions between filaments, as well as the chemical and electronic interactions within filaments, needs yet a clear explanation. The
development and propagation of a single streamer have been studied both
from a theoretical point of view [42, 47, 48] and in few experiments [49, 50].
Also some efforts have been performed to describe the interaction between
streamers during their initial stage and propagation [43] but up until now,
the only possibility to investigate 3D patterns on the time scale of many
breakdowns was Monte Carlo simulation of the micro-discharges distribution [51, 52]. An example of pattern formation in a dielectric barrier discharge is shown in Figure 2.5. What is still to be explained is the role of
the interaction between developing discharges and the interaction between
the micro-discharge remnants on the dielectric surface and discharge volume
(a sort of interaction-through memory effect). In Chapter 5 a different approach based on the temporal analysis is proposed to explain some features
and limitations of the memory effect.
2.4 PLASMA-SURFACE INTERACTIONS
21
2.4 Plasma-surface interactions
Plasmas are largely employed for the modification of surface properties of
materials. Plasma technologies have a great importance in several industrial
fields for the optical, physical, and chemical modifications of materials surface. For example about one-third of the processes needed to make a modern
semiconductor chip involve a plasma-based process. Indeed, Materials and
surface structures can be fabricated that are not attainable by any other
method, and the surface properties modifications are unique.
In a typical reactive plasma the gas phase chemistry is extremely complex
because the highly energetic electrons can activate a great number of reactions. In a plasma the species include neutral atoms and molecules, positive
and negative ions, radicals, electrons and photons. These species interact
with the surface of materials activating a number of processes which can
be reassumed as: reaction of atom or chemical groups insertion (grafting),
generation of free radicals on the surface (activation), deposition of a thin
layers adherent to the surface (film deposition), chemical or physical ablation
of the material surface (etching). Often in reactive plasmas all of the cited
processes are present, thus the knowledge both of the gas-phase and surface
chemistry is fundamental for the development of plasma applications.
2.4.1 Gas-phase chemistry and processes
Describing the complexity of the processes and reaction occurring in a
plasma is not a simple task and is far beyond the scope of this introduction
(see Ref. [15, 2]). Here are briefly introduced the fundamental processes of
a reactive plasma.
Ionization processes
The fundamental process in a plasma is ionization because it is responsible
of its generation and sustainment. There are different kind of such processes.
Direct ionization by electron impact is the basic plasma reaction and
include the ionizations of non-excited atoms, molecules and radicals. It
involves the interaction of an enough energetic electron hitting the other
neutral species when its energy is high enough to create an ion-electron
pair.
22
ATMOSPHERIC PRESSURE DISCHARGES AND SURFACE PROCESSES
Preliminary exited neutral species can undergo further ionization in a
stepwise ionization by electron impact. This kind of process is important in
thermal or highly energetic discharge when the degree of ionization (ratio of
electron and ion density) is high.
Ionization by collision with heavy particles can generate electrons during
ion-molecular or ion-atomic collisions involving also vibrationally or electronically excited species. Chemical reactions are involved too.
Photoionization processes generate electron in the collision process between an heavy particle and a photon. Photoionization is important in thermal plasma and in the propagation process of a streamer channel (Section
2.2.2).
Surface ionization with electron emission can be provided by ion, electron or photon collisions or just by surface heating (thermoionic electron
emission). One of the most important processes is the secondary electron
emission (or Auger emission) involving the neutralization of ions at the surface.
Electron and charged particles losses
Many processes bring to the loss of a free electron and charged particles. The
balance between theses processes and the ionization processes determines the
degree of ionization and plasma density.
Electron-ion recombination processes involve the neutralization of a positive ion with an electron. It is a highly exothermic reaction which need a
channel for accumulation of the energy released during the process. This can
lead to molecular dissociation, creation of excited species, photon emission,
etc.
Especially in presence of an electronegative gas (O2 , CO2 , SF6 , CF4 ,
etc.) the electron attachment processes are extremely important and are
often responsible for the balance of charged particles. An attachment process typically take place in electronegative gases when a molecular fragment
(dissociation products) has a positive electron affinity.
When the electron attachment processes are involved in the balance of
electrons and ions (electronegative gases), the actual losses of charged particles are mostly due to ion-ion recombination processes which are the mutual
neutralization of positive and negative ions in binary or three-body colli-
2.4 PLASMA-SURFACE INTERACTIONS
23
sions.These processes can proceed by many different mechanisms and have
very high rate coefficients.
Finally, as for ionization, must be considered in the balance of charged
particle losses the surface recombination processes. These processes are the
most important in low pressure discharges because they are usually kinetically limited by the diffusion of charged particles to the walls.
Gas-phase chemical reactions
Along with the processes described above, in a typical reactive gas a wide
variety of chemical reactions are to be considered in the gas-phase chemical equilibrium. The radical production processes are responsible for the
creation of extremely reactive species that can interact with other elements
of the atmosphere or on the surface. These species are usually extremely
important for plasma processing. A wide variety of gas phase chemical reactions involving all the active species in the plasma are also to be considered.
Usually the number of these reactions is very high and the complete description of the chemical equilibrium of a reactive plasma can become an
overwhelming task.
Excited atoms and molecules in plasma
Excited species are extremely important in plasma chemical kinetics. High
electron temperatures and thus highly energetic electrons, can provide a
high excitation rates of different electronically excited state of atoms and
molecules by electron impact. If the radiative transition to the ground state
is not forbidden by quantum selection rules, such a state is called resonant
excited state. It has typically a short lifetime (10−8 ÷ 10−6 sec.) and does
not interfere with chemical kinetics. Otherwise, if the radiative transition
is forbidden, this state is called metastable excited state and because its
lifetime can be very long (10−2 ÷ 102 sec.), it can significantly contribute to
the chemical kinetics.
In presence of molecules in the plasma an extremely important process is the vibrational excitation of molecules by electron impact. Indeed,
in a molecular gas, most of the electron energy can be transferred to the
vibrational excitation. For this reason, the vibrational excitation, relax-
24
ATMOSPHERIC PRESSURE DISCHARGES AND SURFACE PROCESSES
ation, and reaction of vibrationally excited molecules strongly influence the
chemical kinetics of the plasma. Several relaxation processes are important: vibrational-translational (VT) processes convert vibrational energy in
kinetic energy of the whole particle and it is the loss mechanism of vibrational energies. Vibrational-vibrational (VV) processes rearrange the energy
between vibrational levels and are responsible for the creation of highly vibrationally excited molecules which are extremely influent in the chemical
equilibrium. In fact, these molecules can posses enough energy for dissociation and/or other endothermic chemical reactions. The vibrational levels are
usually thermalized and a vibrational temperature Tv can be defined. However, because the VT processes are often weakly efficient in non-thermal
discharges, molecular vibrations ”trap” the electron energy, and Tv > T0
where T0 is the ion and gas temperature.
Also the rotational levels of molecules can be excited by rotational excitation of molecules by electron impact processes. Similarly to vibrational
levels, the relaxation of rotational levels can happen through rotationalrotational (RR) or rotational-translational (RT) relaxation processes. However, the probability of RT (and RR) processes in very high because of the
smallness of rotational quanta, and, in many non-thermal plasmas, the rates
of rotational relaxation processes are comparable with the rate of translational thermalization (TT processes). In this cases the defined rotational
temperature Tr ∼ T0 .
2.4.2 Surface kinetics and processes
Physical and chemical surface processes are central to plasma processing.
Some of these processes, which are important for sustainment of the discharge and its chemical equilibrium, have been described in Section 2.4.1.
Indeed, the surface and gas-phase reactions sets are strongly coupled and
cannot be considered separately. Here attention is concentrated on those
surface processes which are fundamental in plasma processing of materials
and on the kinetics of surfaces.
2.4 PLASMA-SURFACE INTERACTIONS
25
Figure 2.6: Typical reaction set for a surface process.
Surface kinetics
Surface reaction mechanism for most plasma processes are still not well understood and characterized. However, adsorption and desorption of reactive
species on the surface are usually part of the complex surface processes.
Adsorption is the mechanism that brings an atom (or molecule) to form
a stable bond with the surface. There are two kind of adsorption processes:
physiosorption, which is the creation of a bound due to the weak attractive
Van der Waals forces between the atom and the surface, and chemiosorption
which is due to the formation of a chemical bond between the atom (or
molecule) and the surface. These two kind of adsorption mechanism are
often found in the same system with different regimes favored depending on
surface temperature and chemical environment. Desorption is the reverse
reaction to adsorption and, in thermal equilibrium, the two reaction must
be balanced. In Figure 2.6 is illustrated a typical reaction set for a surface
process. Reactive species diffuse or flow to the surface with rate constant
K1 , where they are adsorbed (K2 ) and react (K3 ). If the process generates
by-products (for example in chemical etching) they can desorb (K4 ) and
diffuse or flow into the gas phase (K5 ). In addition, must be considered the
desorption without reaction of the reactive species (K6 ) and the backward
adsorption of the eventual by-products. This is the most simple reaction
scheme that must be considered for the description of a surface process.
26
ATMOSPHERIC PRESSURE DISCHARGES AND SURFACE PROCESSES
Etching processes
Plasma Etching is a fundamental process for the removal of material from a
surfaces. The process can be chemically selective (i.e. can remove a specific
type of material leaving others unaltered) and is the only commercially viable technology for anisotropic removal of material (i.e. can remove material
at the bottom of a trench while leaving the same material on the side walls
unaffected). Most application of plasma etching are in the field of integrated
circuit fabrications, but other applications exist (in association with other
processes) for polishing, cleaning or sterilization of surfaces.
Etch process are typical at lower pressure and comprise three main processes. Sputtering is the ejection of atoms from surface due to energetic ion
bombardment. It is an unselective (i.e. sputtering yields do not change too
much with material), highly anisotropic process, and is the only one which
can remove in-volatile products from a surface. In pure chemical etching
the discharge supplies gas-phase etchant atoms or molecules that chemically
react with the surface to form gas-phase product following a the scheme
shown in Figure 2.6. This process is highly chemical selective. Ion-enhanced
etching is a process in which the discharge supplies both etchants and energetic ions to the surface. The energetic ions increase the etching rate and
anisotropy but reduce the selectiveness of pure chemical etching.
Deposition processes
Plasma assisted deposition, implantation and surface modification processes
are extremely important for the creation of thin films on surfaces and for
the modification of surface properties. Plasma-generated thin films can
have unique chemical composition and morphology that are not attainable
with conventional chemical vapour deposition (CVD) and other processes.
Plasma enhanced chemical vapour deposition (PECVD) consists of a plasma
activated set of gas-phase and surface reactions that produce a solid product at the surface. Reaction scheme is rather complicate and can involve
also polymerization processes both in the gas-phase and on the surface. A
deposition process of this kind is studied in Chapter 7.
2.4 PLASMA-SURFACE INTERACTIONS
27
Plasma grafting
Plasma grafting is the insertion through chemical bonding of a specific functional group on the surface. It can be considered a deposition processes
which follows the reaction scheme illustrated in Figure 2.6. It starts like a
deposition with the generations in the gas-phase of a reactive species which
is adsorbed on the surface where form a stable chemical bond. However, are
not present reactive species that can start a polymerization process neither
in gas-phase nor on surface. This results is the creation of a single molecular (or atomic) layer on the original surface. A grafting process of fluorine
atoms is studied in Chapter 8.
Plasma Activation
What is called plasma activation is usually a combination of an etching and a
grafting process that in intended to modify or improve surface properties in
order to attain, for example, better adhesion of polymeric webs to coatings,
painting, gluing, wetting, etc. Plasma activation can also promote crosslinking and is always present also during deposition processes and increases
the bonding of reactive species to surfaces. Grafting of specific functional
groups can be promoted (for example polar groups in air plasma treatment of
polymer surfaces to attain wettability), and the removal of weakly bounded
layers through etching can be attained.
28
ATMOSPHERIC PRESSURE DISCHARGES AND SURFACE PROCESSES
CHAPTER
3
Dielectric barrier discharge devices
This chapter is devoted to the description of the experimental setups utilized
in the research. The plasma device developed for the study of surface process
in a controlled atmosphere is described in details as long as the motivations
of the choices. The diagnostics are mentioned here only when needed in the
description and are discussed more deeply in Chapter 4
3.1 DBD device for surface modifications
The main advantage of the atmospheric pressure DBD is its easy adaptability to continuos material processing [1, 2, 3, 20]. The idea behind the
realization of this experimental setup is the possibility to study both the
discharge physics and the plasma-surface interaction during continuos processing in a wide range of pressures and compositions of the atmosphere.
Continuous treatment of web material is a central feature of industrial applications and the roll-to-roll configuration is a compulsory characteristic of
29
30
DIELECTRIC BARRIER DISCHARGE DEVICES
the plasma discharge system.
3.1.1 Plasma reactor
The dielectric barrier discharge (DBD) device used for the experiments is
mostly similar to the typical corona treater already used at industrial level
for adhesion improvement. The choice of this configuration is motivated by
the ease of development and scaling of this type of configuration.
A schematic representation of the experimental setup is given in Figure
3.1. The electrode system consists of two parallel high voltage electrodes
and a rotating cylindrical grounded electrode. The high voltage electrodes
are two cylindrical rods 230 mm long with a 8 mm diameter, coated with
pure (> 99.7%) Al2 O3 sintered ceramic dielectric of 2 mm thickness. The
grounded rotating electrode is a void steel cylinder coated with ceramic
dielectric of 5 mm thickness. Distance between electrodes can be varied
between 0.5 and 5 mm. An electric motor with a controller and a motion
vacuum feed-through can rotate the grounded cylinder with tangent speeds
between 0.1 and 100 m/min.
The electrodes are enclosed in a vacuum chamber (Copra Cube by CCR
Technologies Gmbh) 40x40x40 cm where particular gaskets have been employed to avoid leakage both in vacuum and in over-pressure. These particular solution allows to perform experiments without contamination not
only in under-pressure, but also in slightly over-pressure. The affordable
working pressure range is between 10−1 and 1300 mbar, limited below by
the evacuation of impurities and above by the leakage of gaskets.
3.1.2 Pumping system and gas distribution
The control of the reactive atmosphere is a key feature of this plasma device.
To ensure a minimal concentration of uncontrolled contaminations during
the experiments, a double-stage rotary pump (SD-301 by Varian) is used to
evacuate the atmosphere to a limiting pressure of 5 · 10−3 mbar. Because
this pump cannot work efficiently at high pressure without a considerable
overheating, a second dry pump (ZA60 by Rial) is used at higher pressure
experiments. The desired pressure is maintained constant balancing the
inlet fluxes through the regulation of dosing valve V2 (see Figure 3.1).
3.1 DBD DEVICE FOR SURFACE MODIFICATIONS
31
Figure 3.1: Schematic representation of the DBD reactor. The electrodes are
enclosed in a vacuum chamber. A motor with a rotation control (RC) rotates the
grounded electrode. A rotary pump (P1) is used to evacuate the chamber and a
piston pump (P2) with a dosing valve (V2) are used to stabilize the desired working
pressure balancing the inlet fluxes. A Pirani pressure gauge (PG1) and a capacitive
gauge (PG2) measure the pressure in the chamber. Three mass flow controllers
(MFC1,2,3) with different capacity can mix gases and a controlled evaporator and
mixer (CEM) can also mix liquid (MFCL) as vapours in a carrier gas (MFC). An
amplified signal generator and a current transformer provide the high voltage to the
electrodes. Current and voltage are acquired in a oscilloscope with a specifically
designed Rogowski coil (ROG) and a high voltage probe (HVP). Optical emission
is acquired through an optical fibre with an UV-VIS spectrometer.
32
DIELECTRIC BARRIER DISCHARGE DEVICES
Figure 3.2: Layout of a section of the discharge region. The two high voltage
electrodes are in front of a rotating grounded electrode. A polycarbonate injection
system guarantee the uniformity of gas and vapour flow over the width of the
electrodes.
Two pressure gauge are used to measure low and high pressure ranges: a
Pirani pressure gauge (Ttr 91 by Leybold) measures the pressure respectively
in the range 10−3 ÷ 5 mbar and a capacitive gauge (DI2000 by Leybold)
measures the pressure 1 ÷ 2000 mbar.
The inlet fluxes are controlled by a gas and vapour mixing system (Figure 3.1). Process gases with high purity level are regulated and mixed using
a system of 3 mass f low controllers (El-flow by Bronkhorst) with different
capacity. To use liquid precursors at atmospheric pressure a controlled evaporator and mixer (Bronkhorst CEM System) is also attached to the inlet allowing the mixing of vapours with concentration up to the saturation at the
given conditions of temperature and pressure in the vacuum chamber. The
inlet fluxes generated by the gas and vapour mixing system are injected into
the vacuum chamber directly between the high voltage electrodes through an
injection nozzle. The nozzle is a polycarbonate shower specifically designed
and realized to ensure uniform fluxes on the whole width of the electrodes
up to 50 ln /min.. Polycarbonate have an upper limit working temperature
around 80 ◦ C and good chemical resistance. A sectional view of the injection
nozzle arrangement is shown in Figure 3.2.
3.1 DBD DEVICE FOR SURFACE MODIFICATIONS
33
3.1.3 Electric power supply and configuration
The AC is applied by an amplified signal generator with frequencies between
10 KHz and 50 KHz and through a high voltage transformer. The power
supply is composed by a current rectifier which brings the line current from
230V AC to a 310 V DC. A transistor switching system create the AC current
which is connected to the primary winding of the high voltage transformer.
The secondary of the transformer is then directly connected to the electrodes
and the whole system consist of a resonant circuit. The voltage applied to
the primary winding of the transformer is constant and the voltage applied
to the electrodes is controlled by the resonance between the proper frequency
of the system and the applied frequency. Usually the complete voltage (and
power) range of the device is within a span of few kilohertz.
3.1.4 Diagnostics
We characterize the plasma discharges principally by means of optical and
electrical diagnostics. The emission spectra of the discharges have been measured with a wide band spectrometer A complete description of instrument
and measuring techniques is given in Section 4.1.
Both the current and the voltage are acquired in a Nicolet 450 oscilloscope respectively with a specific designed Rogowski coil (see Section 4.2.1)
and a high voltage probe (Tektronix P6015A). In Figure 3.3 are plotted the
amplitude-frequency and phase shift-frequency response of the Rogowski coil
used for current measurements in the experiments with the present setup.
Current measurement are performed with two different Rogowski probes.
The first probe has a lower bandwidth (5 kHz-25 MHz) and is used to measure the displacement current and longer current pulses without introducing
dephasing. The second probe is used to record the shape of the fast current
pulses due to micro-discharge formation and has an higher bandwidth in
the range 400 kHz-120 MHz. Probe type have been selected and adjusted
to necessity of different experiment. A complete description of development
and calibration of Rogowski coils is given in Section 4.2.1.
34
DIELECTRIC BARRIER DISCHARGE DEVICES
Phase [π]
Attenuation [dB]
10
-3
-2
10
10
-1
0
10
1
10
2
10
3
0
-3
-6
-9
-12
0.4
0.2
0
-0.2
-0.4
10
-3
-2
10
-1
0
10
10
Frequency [MHz]
1
10
2
10
Figure 3.3: Amplitude vs. frequency (upper panel) and phase shift vs. frequency
(lower panel) response of the two Rogowski coils. Circles represent the values for
the ferrite core coil used for displacement current measurements with a 5 kHz-25
MHz bandwidth. Diamonds represent the values for the NiZn core coil used as
a reference for the fast current pulses, with a bandwidth in the range 400 kHz120 MHz. The dotted lines in the upper panel represent the usual 3 dB limit to
determine the probe bandwidth.
3.2 DBD device for streamer regime characterization
In order to use the simplest configuration for the characterization of the
streamer regime it has been used a different configuration with respect to
the one described in Section 3.1. The parallel rod electrodes allow a better
understanding of the discharge properties from the analysis of the currentvoltage signals, however the same amplified signal generator as in Section
3.1 have been used in nearly the same frequency range.
3.2.1 Plasma reactor
The DBD device used here consists of two high voltage (center grounded)
electrodes working under atmospheric pressure conditions in air. They are
constituted by two rod electrodes 290 mm long with a 9 mm square section,
coated with pure (> 99.7%) Al2 O3 sintered ceramic dielectric, with an external 15 mm square section and 3 mm thickness. A schematic diagram of
the experimental setup is shown in Fig. 3.1.
The steel electrodes are cave and a cooling gas flow pass through them
35
3.2 DBD DEVICE FOR STREAMER REGIME CHARACTERIZATION
ROG
T
OSCILLOSCOPE
HV
HV
Figure 3.4: The DBD device is made up of two rod electrodes of square section coated with pure (> 99.7%) Al2 O3 ceramic dielectric. The distance between
electrodes is 4 mm. An amplified signal generator and a current transformer (T)
provide the high voltage to the electrodes. Current and voltage are acquired with
a specifically designed Rogowski coil (ROG) and a high voltage probe (HV).
to keep the temperature low. The length of the discharge gap is fixed at
4 mm. The AC voltage is applied using the same amplified signal generator
used for the device described in Section 3.1. The applied voltage to the
primary winding of the high voltage transformer is constant and the one
applied to the electrodes is controlled by the resonance between the proper
frequency of the system and the applied frequency. The frequency range
spans between 31 kHz and 36 kHz in the affordable voltage (and power)
range of the device.
3.2.2 Diagnostics
As for the other device both the current and the voltage are acquired with
a Nicolet-Multipro oscilloscope. Using a high voltage (Tektronix P6015A)
probe, voltage is acquired ( on one of the hot wire and doubled to consider
the symmetry of the electric circuit. The current signal is acquired using
a specific designed Rogowski coil with a ferrite magnetic core coil and a
bandwidth of 50 kHz-70 MHz) and a , respectively. A second NiZn core
coil with a bandwidth of 400 kHz-120 MHz have been used (see Fig. 3.5)
in order to control the response of the ferrite coil to short current pulses
due to streamers. The response of the two coils to the shortest current
pulses measured in the experiments were almost undistinguishable from each
other. Because of the low frequency range of the generator, lying below the
36
DIELECTRIC BARRIER DISCHARGE DEVICES
probe bandwidth, the displacement current response is underestimated and
dephased. However, this does not affect our analysis of fast current pulses
associated to the discharge current. Probe type have been selected and
adjusted to necessity of different experiment. A complete description of
development and calibration of Rogowski coils is given in Section 4.2.1.
-2
Phase [π]
Attenuation [dB]
10
-1
10
10
0
1
10
2
10
3
0
-3
-6
-9
-12
0.4
0.2
0
-0.2
-0.4
-2
10
-1
10
0
1
10
10
Frequency [MHz]
2
10
Figure 3.5: Amplitude vs. frequency (upper panel) and phase shift vs. frequency
(lower panel) response of the two Rogowski coils. Circles represent the values for
the ferrite core coil used for measurements with a 50 kHz-70 MHz bandwidth.
Diamonds represent the values for the NiZn core coil used as a reference for the
fast current pulses, with a bandwidth in the range 400 kHz-120 MHz. The dotted
lines in the upper panel represent the usual 3 dB limit to determine the probe
bandwidth.
CHAPTER
4
Plasma and material diagnostics
In this Chapter are described the diagnostics used to characterize the plasma
discharges and the modifications induced on material surfaces. Both the
instruments and the analysis methods are described.
4.1 Optical emission spectroscopy
The emission spectra of the plasma discharges have been measured by means
of a wide band, low resolution spectrometer (PS2000 by Ocean Optics).
The spectrometer, equipped with a 10 µm slit, a holographic grating (600
lines/mm, blazed at 400 nm) and a 1024 pixels CCD, has a resolution of
1.02 nm and a spectral band extending from 200 to 850 nm. Integration
time is changed depending on the discharge brightness. Emission spectra of
the discharges have been recorded through an UV enhanced optical fiber,
connected to the device by a vacuum feed-through. Depending on the gas
composition, intensities of the emission lines can allow the calculation of
37
38
PLASMA AND MATERIAL DIAGNOSTICS
several properties of plasma discharge and gas-phase chemistry like concentrations, vibrational and electron temperature.
4.1.1 Determination of molecular vibrational temperature
When a molecular gas is present in the discharge atmosphere the excitation
of its vibrational and rotational levels becomes a dominant process and in
some cases the most part of the electron energy is spent in these processes
(see Section 2.4.1). Because the vibrational levels are in thermal equilibrium, supposing a Boltzmann distribution it is possible to determine the
vibrational temperature Tv . The method depends on the molecule and gasphase chemistry and will be explained for mixtures containing nitrogen in
Chapter 6
4.2 Voltage Current measurements
Voltage and current measurement are fundamental to plasma discharge understanding. Both signals are digitally acquired in a Nicolet MultiPro (3
channels, 200 MHz, 8 bit, 1 GS/s) or a Nicolet 450 Oscilloscope (4 channels,
200 MHz, 2 GS/s) or a Tektronix TDS 4020 (2 channels, 60 MHz, 1 GS/s)
and analyzed with the aid of the computer.
Voltage is usually acquired with a wide bandwidth, high voltage probe
(Tektronix P6015A, 40 kV, 75 MHz) which allows to recognize eventual
fluctuations of the applied sinusoidal voltage.
Because of the presence in the current signal of very fast processes due
to micro-discharges, particular attention must be paid to the current measurement. The development and calibration of home-made Rogowski coil
sensor will be described in the subsequent Paragraphs.
4.2.1 Implementation of Rogowski coils for measurements nanoseconds current pulses
Detailed measurements of the current response of a dielectric barrier discharge require particular attention to the bandwidth of the probe. The
displacement current response will be at the same frequency of the applied
voltage, but the discharge current is generally in pulses with duration from
39
4.2 VOLTAGE CURRENT MEASUREMENTS
microseconds down to tenth of nanoseconds depending on the discharge
regime [4, 6, 10, 9]. An easy way to measure the current is to introduce
a shunt in series with the electrical circuit. The current shunt has a good
response bandwidth but the protective circuit needed to avoid damage to
measuring instruments can cause distortion of the read waveform. The Rogowski coil is galvanically separated from the main circuit and can be designed for a precise measurement of the nanosecond current pulses typical
of the streamer discharge regime[53, 54].
Theory and principle of Rogowski coils
A Rogowski coil is a conducting wire that is wound in a spiral around a
magnetic or non magnetic core and then returns to the original point. The
coil is placed around the conductor to couple the pulse signals. The operating principle was formulated by Rogowski and Steinhaus in 1912 [55]. In the
original design Rogowski coils were air cored to avoid saturation of magnetic
core when measuring high currents. In the present measurements, currents
are constantly far below saturation and it will be shown how the choice
of the magnetic core influences the bandwidth of the coils. A schematic
representation of a Rogowski coil is shown in Figure 4.1. The current I
flowing in a cable generate an electromotive force (emf) E at the output of
the coil which is proportional (following Faraday law) to the rate of change
of the current: ∂I/∂t. The signal E must be integrated with a passive or
active circuit. The high frequency behaviour of the coil, in particular its
bandwidth and susceptibility to high frequency oscillations, is significantly
influenced by the integration circuit impedance. With the right choice of the
configuration of the coil the integrating circuit can be reduced in a simple
resistance [56, 57, 58, 53, 54]. A lumped parameter model can be introduced
to describe the circuit behavior (Figure 4.2). The variable current i1 (t) produce a magnetic field and the rate of change in current produce a voltage in
the coil equal to
Ui (t) = M
di1 (t)
,
dt
(4.1)
40
PLASMA AND MATERIAL DIAGNOSTICS
Figure 4.1: Schematic representation of a Rogowski coil. The current I flowing
in a cable generate a emf E at the output of the coil which is proportional to ∂I/∂t
and must be integrated with a passive or active circuit.
Figure 4.2: The equivalent circuit diagram (lumped parameter) (M , mutual inductance; Ls , self-inductance; Cs , stray capacitance; Cp , turn-to-turn capacitance,
which can be ignored in the spaced winding in the design because it is very small;
Rs , equivalent resistance of coil; R, integral resistance larger than Rs in the design;
U0 (t), voltage of the integral resistance; Ui (t), the induced voltage) [54].
41
4.2 VOLTAGE CURRENT MEASUREMENTS
where M is the mutual inductance between the measured circuit and the
coil. The transfer function for the lumped parameter model of Figure 4.2 is:
U0 (t) =
Ls Cs
Rs2
R
Ui (t),
+ (Ls + Rs Cs R)s + Rs + R
(4.2)
where the generic impedance of the integrating resistance have been considered a pure resistive load. If the integrating resistance R is chosen in order to
p
have R ≪ Ls /Cs the pole of the transfer function 4.2 move along the real
axis so the system does not oscillate and the Rogowski coil is self-integrating
between the two poles [53].
For toroidal coils with rectangular square section the lumped parameters
can be calculated as:
Ls =
A
d2
µN 2 h
log
= µN 2
2π
d1
l
2
4π ǫK1
Cs =
log K1 /K2
(4.3)
(4.4)
where l is the length of the effective magnetic path, µ is the magnetic permittivity of the core, A is the cross-sectional area of the core, d2 and d1 are
the outer and inner diameters of the coil respectively, h is the height of the
coil, N is the number of turns in the coil, ǫ is the dielectric constant of the
core, K1 = (d2 + d1 )/2 and K2 = (d2 − d1 )/2. From the lumped parameter
model represented in Figure 4.2 can be deduced the following equations:
fl =
fh =
1
R + Rs
1 R
≈
2π Ls + RRs Cs
2π Ls
(4.5)
1 1
1 Ls + RRs Cs
≈
2π RLs Cs
2π RCs
(4.6)
where fl and fh are the lower and upper frequency limits respectively. From
the above equations is possible to determine which construction parameters
must be changed in order to obtain the desired bandwidth. Thou, In order
to increase the frequency band, fl should be as low as possible, while fh
should be as high as possible. According to equations (4.4), (4.5) and (4.6),
it can be seen that to obtain the wider bandwidth Ls should be as large
as possible while R should be as small as possible. A bigger inductance Ls
can be obtained increasing µ or, more efficiently, increasing N which gives a
42
PLASMA AND MATERIAL DIAGNOSTICS
quadratic dependency. With increasing N and decreasing R, the bandwidth
will become wider, but the sensitivity will become lower. A balance in these
parameters must be attained in order to have the wider bandwidth and
ensure a good sensitiveness.
me ? m0
N—–2
Although bandwidth of the current transducer can reach the desired
range by controlling R and Ls in theoretical analysis, the frequency range
and configuration parameters of the magnetic core play important roles in
determining bandwidth of the current transducer. First, the frequency range
of the magnetic core should include the desired frequency range. Two kind
of magnetic core have been used to build the Rogowski coils used in the experiments. For the lower bandwidth (and in order to measure displacement
currents of the order of kilohertz) a ferrite material with initial permeability µ = 4300 NA−2 , coercive field strength Hc = 0.19 Oe and saturation
flux density Bs = 3900 Gauss, have been used which gives an higher sensitiveness but cannot resolve higher frequencies. For the detection of fastest
current pulses due to micro-discharges a Nickel-Zinc core have been used
which guarantees an higher bandwidth at the cost of sensitiveness. Its parameters are: initial permeability µ = 1500 NA−2 , saturation flux density
Bs = 2800 Gauss, coercive field strength Hc = 15 Oe. Also the geometrical
parameters of the core can have some influence the final bandwidth [54],
but here they are not considered because in the experiments they are constrained by dimensions of the cables and cannot be varied. From the value
of Bs the maximum measured current Imax can be calculated according t
[54]:
Imax =
0.8Bs l
.
µ
(4.7)
In all the experiments and for all the build current sensor the measured
current is always under the maximum value (I < Imax ).
Calibration of the Rogowski coils
The calibration circuit (Figure 4.3) consists of a wide bandwidth signal
generator which is connected to a 50Ω non inductive resistor through an
RG 58 BNC cable. The cable is split on the resistor side with the hot wire
4.3 CHARACTERIZATION OF THE MATERIALS SURFACES
43
Figure 4.3: The calibration system used to evaluate the frequency and amplitude
response of the Rogowski coils.
passing through the Rogowski probe to be tested. Acquiring the voltage
drop across the resistor and the probe signal it is possible to determine the
amplitude response, the phase shift and the sensitiveness of the Rogowki
Coil.
Several combination of magnetic core and number of turns N have been
tested while the integrating impedance have been kept constant to a 50 Ω
non-inductive resistance. In Figure 4.4 are showed the Bode plots for some
built Rogowski coils. The parameters are the type of magnetic core are
indicated in the figure legend.
The Bode plots for the specific coils used in the single experiments are
showed in Sections 3.1 and 3.2 in the description of the experimental setups.
We utilized two different Rogowski probes. The first one with a ferrite
magnetic core and a bandwidth of 5KHz-25MHz to detect the displacement
current and longer current pulses. The second one with an high frequency
NiZn magnetic core and a bandwidth of 250KHz-120MHz to detect the current pulses of single streamers.
4.3 Characterization of the materials surfaces
In this section are briefly described the diagnostics used for the characterization of the material surfaces after the plasma treatments. At a microscopic level both the chemistry and the morphology are characterized
44
PLASMA AND MATERIAL DIAGNOSTICS
Figure 4.4: Amplitude vs. frequency (upper panel) and phase shift vs. frequency
(lower panel) response of some Rogowski coils with different construction parameters and magnetic core.
with infrared spectroscopy and AFM/SEM/FIB measurements respectively.
Modified macroscopic properties are evaluated with contact angle measurements.
4.3.1 Infrared spectroscopy (FTIR/ATR-FTIR)
Fourier transform infrared spectroscopy (FTIR) can be used to identify
chemical composition of the realized coatings. FTIR is perhaps the most
powerful tool for identifying types of chemical bonds (functional groups).
Molecular bonds vibrate at various frequencies depending on the elements
and the type of bonds. For any given bond, there are several specific frequencies at which it can vibrate. The wavelength of light absorbed is characteristic of the chemical bond as can be seen and identified in the spectrum.
Sometimes transmission measurements cannot be performed on several
specimens and a surface analysis must be used. An attenuated total reflection (ATR-FTIR) technique operates by measuring the changes that occur
in a totally internally reflected infrared beam when the beam comes into
contact with a sample. An infrared beam is directed onto an optically dense
crystal with a high refractive index at a certain angle. This internal reflectance creates an evanescent wave that extends beyond the surface of
4.3 CHARACTERIZATION OF THE MATERIALS SURFACES
45
the crystal into the sample held in contact with the crystal. This evanescent
wave protrudes only a few microns (0.5 µm- 5 µm) beyond the crystal surface
and into the sample. Consequently, there must be good contact between the
sample and the crystal surface. In regions of the infrared spectrum where the
sample absorbs energy, the evanescent wave will be attenuated or altered.
The attenuated energy from each evanescent wave is passed back to the IR
beam, which then exits the opposite end of the crystal and is passed to the
detector in the IR spectrometer. The system then generates an infrared
spectrum. The measurement have been performed with a Nicolet Avatar
360 with a resolution of 4 cm−1 in the range 400÷4000 cm−1 , and equipped
with an ATR accessory (PIKE-Technology).
4.3.2 Atomic force microscopy (AFM)
The atomic force microscopy (AFM)is a rather recent technique to measure the morphology of surfaces down to nanometer scale resolution. The
functional scheme of an AFM is represented in Figure 4.5. The AFM head
uses a beam deflection scheme to monitor the cantilever displacement. This
scheme is quite simple and permits registration of both normal deflection of
the cantilever with sub-angstrom resolution and its twisting angle, so normal
and lateral force can be measured simultaneously. A laser beam is focused
onto the back surface of cantilever close to tip position, and reflected beam
falls onto the quadrant photodiode. Cantilever deflection causes displacement of the reflected beam over sections of the photodiode. An amplified
differential signal from the quadrant photodiode permits measurement of
angular deviation with the accuracy of less than 0.1 degrees, that corresponds to normal cantilever deflection of the order of 0.05 nm. Among the
several techniques used to measure the morphology of surfaces, have been
used contact mode and semi-contact (tapping) mode. In contact mode cantilevers touches the surface while scanning in repulsive mode (like a needle
of gramophone), but can scratches softer surfaces. The semi-contact mode
is a special modulation technique for non-destructive imaging of soft samples as well as of hard. It measures topography by tapping the surface with
an oscillating probe tip. The measurements in the present work have been
performed with a NT-MDT Solver P47H-pro.
46
PLASMA AND MATERIAL DIAGNOSTICS
Figure 4.5: Functional scheme of an AFM.
4.3.3 Contact angle measurements and surface energy determination
Contact angle measurements have been performed to evaluate macroscopic
properties of material surface such as surface energy and wettability. Static
and dynamic measurements have been performed using a video-supported
contact angle measuring instrument Dataphysics OCA 20.
Wettability and surface energy
Wetting [59]describes the ability of a liquid deposited on a solid substrate to
spread out or remain confined. When the surface energy of a dry substrate
is higher than the energy of the wetted one (by some liquid), the liquid
spreads completely on the surface in order to lower its energy (for example,
the behaviour of water on a clean glass surface). On the contrary, when the
surface energy of a dry substrate is lower, the liquid partially wets the surface
forming drops (for example, water on a plastic surface). At the contact line
between the three phases (liquid, solid, gas or vapour) the contact angle
between the liquid drop and the surface is determined by the equilibrium of
the surface tensionsσ (or surface energies) of the interfaces according to the
4.3 CHARACTERIZATION OF THE MATERIALS SURFACES
47
Figure 4.6: Young equilibrium between surface tension determining contact angle.
Young’s equilibrium [60] (see Figure 4.6):
σlv cos θe = σsv − σsl ,
(4.8)
where the subscripts indicate the inter-phase between liquid (l), solid (s)
and vapour (v). The equilibrium contact angle θe is a physical constant
depending only on the materials, and in no other way on the particular
configuration considered.
According to the Owens-Wendt two-parameter model the surface tensions of the solid-vapor and liquid-vapor inter-phases consist of two components: a dispersive one accounting for van der Waals and other non-sitespecific interactions and a polar one accounting for dipole-dipole, dipoleinduced dipole, hydrogen bonding and other site-specific interactions[61].
The surface tensions of the liquid and the solid (in contact with vapour) can
be expressed as:
d
p
σsv = σsv
+ σsv
,
(4.9)
p
d
+ σlv
.
σlv = σlv
(4.10)
The solid-liquid interfacial tension can be expressed as [62]:
q
q
p p
d
d
σsl = σsv + σlv − 2 σsv σlv + σsv
σlv
(4.11)
48
PLASMA AND MATERIAL DIAGNOSTICS
Figure 4.7: Contact Angles as a function of drop volume when it is increased
(right-hand arrow) and decreased (left-hand arrow) whit a syringe. Advancing and
receding contact angle are determined as indicated.
Combining equations (4.8) and (4.11) yields:
σd + σp
(1 + cos θe ) lvq lv =
d
2 σlv
q
d +
σsv
p
p
σsv
s
p
σlv
.
d
σlv
(4.12)
d and σ p in equation
The two unknown components of the surface tension σsv
sv
(4.12) can be determined from the measured contact angles against at least
d and
two test fluids with known values of surface tension components
σlv
r
p
σlv
p
for different
σlv
. A plot of left hand side of equation (4.12) versus
σd
lv
liquids yields the dispersive component (square of the y-intercept), the polar
component (square of the slope) and consequently the surface tension of the
solid-vapor interface σsv from equation (4.10).
4.3 CHARACTERIZATION OF THE MATERIALS SURFACES
49
Contact angle hysteresis
According to Young’s equation (4.8) the static equilibrium contact angle θe
is related to the surface tension of the solid-vapor and solid-liquid interfaces,
and it is ideally a unique property of the material system being considered,
but practically a hysteresis often arises depending on how the interfaces
form. If a liquid droplet is quietly settled on a solid surface (or if its volume is
slightly increased after it settling) thus measured contact angle is larger, even
up to several tens of degree, than the angle measured for the same droplet
after reducing its volume. The advancing angle θa is the largest contact angle
achievable before the wetting line begins to move in the direction of the gas
phase and the receding angle θr is the smallest contact angle achievable
before the wetting line begins to move in the direction of the liquid phase.
Many theories of the contact angle hysteresis have been proposed [63, 64,
65, 66] even if a clear interpretation of this effect still lacks. Hysteresis
is usually connected with changes of roughness and chemical heterogeneity
of the surface [59]. The measurements of advancing and receding contact
angles can give interesting informations on the solid-liquid interactions.
With the OCA20 instrument the advancing and receding angles have
been measured by modifying the volume of a drop by inflating and deflating
liquid with a computer controlled syringe. Recording a movie of the dynamic
contact angle it is possible to obtain the estimate of advancing (receding)
angle (see Figure 4.7) as the maximum (minimum) value before the drop
base diameter increases (decrease).
50
PLASMA AND MATERIAL DIAGNOSTICS
CHAPTER
5
Statistical characterization of a
streamer discharge regime
5.1 Introduction
In this chapter, we investigate the temporal behavior of current pulses for a
streamer regime in a DBD at atmospheric pressure. As explained in Chapter
2, at low pressures DBDs operate in a Townsend breakdown regime [5] generating a diffuse glow discharge. At atmospheric pressure, the realization of
a diffuse discharge is restricted to limited conditions of geometry, electrical
parameters and gas composition, and DBDs operate usually in a streamer
discharge in which several narrow discharge filaments are typically formed
(see Section 2.3.2). The streamer regime constitutes a strongly interacting system of discharges exhibiting cooperative behavior. This leads, under
specific conditions, to the formation of coherent spatial configurations that
have been observed in different types of experimental setups [12, 13, 14, 6].
51
52
STATISTICAL CHARACTERIZATION OF A STREAMER DISCHARGE REGIME
However, micro-discharges seem, to some extent, to occur at random within
the discharge gap for most applications of DBDs. To our knowledge, the
statistical properties of such discharges in air have not been discussed with
sufficient insight in literature so far, even if similar studies have been performed in dealing with the so-called partial discharges [67, 68]. In particular, we find the existence of two different streamer regimes as a function
s (here V s ≃ 23.6 kV). The two
of applied voltage, separated by a value Vpp
pp
different regimes can be characterized by the first moments of the discharge
s.
distributions, suggesting a way for determining the separatrix voltage Vpp
The peculiar feature of DBDs (see Chapter 2) is that the charge transported by the micro-discharges to the dielectric cannot reach the conducting
electrode and accumulates near the surface until the change in the local electric field extinguishes the filament. Because of the slow diffusion of charges
on the surface, in the subsequent half-cycle of the driving voltage the locally
modified field promotes the formation of a streamer in the same spot. This
so called ‘memory effect’ is a dominant feature in DBDs (see Section 2.3.3).
A typical streamer has a lateral spatial extension of about 0.1 mm and a
duration of the order of nanoseconds depending on the device configuration
and type of gas. The presence of the memory effect suggests temporal correlations may be found in the discharge current signal. In particular it is found
that in the studied streamer discharge regime the existing residual correlations propagating between the discharge processes (half-cycles) are only an
effect of the non-stationariety of the discharge current response, thus, correlations vanish outside the single discharge process. On the contrary, by
analyzing the current signal inside the half-cycle, it is found that on time
scales of the order of hundreds of nanoseconds (i.e., within a single current
burst, in which the streamers develop sufficiently close in time), strong correlations exist which also reveal a peculiar ordered temporal structure of the
discharge current signal. The experimental setup and the diagnostics used
here are described in Section 3.2 and Section 4.2, respectively. In Figure
5.1(a) is shown a schematic representation. The current signal is recorded
at constant intervals of τ0 = 5 ns for a total of 3 × 105 steps, for different
applied peak-to-peak external voltages in the range (22÷26) kV.
53
5.2 STATISTICAL CHARACTERIZATION OF CURRENT SIGNAL
ROG
T
OSCILLOSCOPE
HV
HV
(a) Experimental setup.
(b) Simplified electrical equivalent circuit.
Current [mA] Current [mA]
Figure 5.1: The DBD device is made up of two rod electrodes of square section coated with pure (> 99.7%) Al2 O3 ceramic dielectric. The distance between
electrodes is 4 mm. An amplified signal generator and a current transformer (T)
provide the high voltage to the electrodes. Current and voltage are acquired with
a specifically designed Rogowski coil (ROG) and a high voltage probe (HV).
200
100
0
-100
-200
0
10
20
30
40
50
60
70
80
10
20
30
40
50
60
70
80
400
200
0
-200
-400
0
time [µs]
Figure 5.2: Typical current signal of a DBD device. The upper and lower panels
refer respectively to a low voltage (23 kV) and high voltage (25 kV) current signal, representing the two typical discharge patterns observed in the device. The
continuous sinusoidal line is the displacement current of the system.
5.2 Statistical characterization of current signal
Because the interest is in a detailed analysis of fast current pulses due to
micro-discharges, the displacement current of the system must be determined and subtracted. Because the ionization of the gas is very low, it is
suitable to assume that the capacitance of the gas does not change during
the discharge process and use a simplified electrical equivalent circuit of the
discharge system (Figure 5.1(b)). Thus, the current measured in the system
54
STATISTICAL CHARACTERIZATION OF A STREAMER DISCHARGE REGIME
can be considered to be made of two components: a sinusoidal displacement
current which does not depend on the presence of the plasma and the response current due to the discharge process [9, 11, 5]. The displacement and
discharge currents can be then calculated as
dV (t)
dt
Idisch (t) = Itot (t) − Idisplace (t),
Idisplace (t) = Cx
(5.1)
(5.2)
where Cx is a capacitance including both dielectric layers and gas gap, V
is the applied voltage, Itot the total measured current, Idisplace the displacement current and Idisch the discharge current. An example of the separation
of the two components is given in Figure 5.2.
5.2.1 Structure of the discharge current: bumps, bursts and streamers.
Once the displacement current has been subtracted from the signal we are
left with a series of discharge patterns of alternating sign. Since we are interested in the discharge amplitudes, we have first checked that both positive
and negative discharge values occur with a similar distribution, indicating
that we can treat them on the same foot. Then, we change the sign to
those negative discharge patterns. However, small negative current values
still occur in the series which are due to errors introduced by the sinusoidal
fit and the intrinsic errors of the current probe. To this end, we introduce a
cut-off threshold for the current, below which it is set to zero. The cut-off
value Icut is taken as the minimum value of the current measured within the
full time steps, Icut = |min I0 (t)|. Then, the discharge current is taken as
I(t) =
(
I0 (t), if I0 (t) ≥ Icut ,
0
, if I0 (t) < Icut .
(5.3)
For the present results, we find Icut ≃ 10 mA.
An example of the resulting signal within a single half-cycle, which we
denote as a discharge bump is shown in Figure 5.3. It can be noted that
a bump is composed of several well separated discharge bunches, which we
call bursts. The bursts are made of a sequence of single streamers (their
structure and temporal behavior will be discussed in detail in Section 5.3.2)
55
5.2 STATISTICAL CHARACTERIZATION OF CURRENT SIGNAL
500
λ=50 mA
-1
PDF [mA ]
-1
400
IB [mA]
10
300
10
10
200
-3
-5
0 100 200 300 400
IB [mA]
100
0
0
2
4
6
8
10
time [µs]
Figure 5.3: Discharge current signal IB (t) within a half-cycle as a function of
time t [µs], for a high voltage situation (25.3 kV, see also
lower panel of Figure
5.2). The continuous line is the mean current response IB , Eq. (5.4). The inset
represents the probability distribution function (PDF) where the straight line is an
exponential fit using Eq. (5.5) with λ = 50 mA.
which are clustered together. This clustering is a result of the presence of
strong short-time correlations in the discharge patterns. We are going to
analyse these correlations below in Sect. 5.3.1. In what follows, we perform
a statistical analysis of bumps.
In our analysis, we consider from Eq. (5.3) values of I(t) only within
an effective time interval tmin < t < tmax , for a fixed applied voltage Vpp .
To stress this fact, the discharge current within a bump is indicated as
IB (t). The lower bound tmin is defined as the lowest time, within a halfcycle, at which I(t) > 0 for the first time, calculated among all bumps.
The upper bound tmax is defined as the largest time, within a half-cycle,
at which I(t) > 0 for the last time. The total number of bumps, NB , is
typically NB ≃ 100 in the recorded interval, while ∆t = tmax − tmin varies
in the range 10 [µs] < ∆t < 20 [µs], depending on Vpp . We denote as T the
half-cycle period.
The continuous line in Figure 5.3 represents the mean discharge current
56
STATISTICAL CHARACTERIZATION OF A STREAMER DISCHARGE REGIME
response of the DBD device within a half-cycle. It is calculated as
NB
1 X
(i)
IB (t),
IB (t) =
NB
(5.4)
i=1
for tmin < t < tmax , and IB = 0 otherwise, where i denotes the bump
index. Note that max{ IB (t) } ≪ max{IB (t)}, where max{IB (t)} is the
maximum value of IB (t) within a bump. In the case of the bump shown in
Figure 5.3 we find max{IB (t)} ≃ 400 mA, while max{ IB (t) } ≃ 70 mA.
The inset to Figure 5.3 displays the probability distribution function of
IB . It can be approximately fitted by the exponential form (see also Figure
5.6 and related explanation) valid for IB ≥ 0,
P (IB ) = P0 δ(IB ) +
PA
exp(−IB /λ),
λ
(5.5)
where P0 represents the fraction of zero current values inside the bump,
indicating the existence of a characteristic current intensity. Exponentially
decaying functions are typical of random systems displaying uncorrelated
R∞
fluctuations. Using the normalization condition, 0 dIB P (IB ) = 1, with
R∞
the convention that 0 dIB δ(IB ) = 1, one has PA = 1 − P0 , representing
the fraction of positive current events, denoted also as activity ratio, inside
a bump. We have verified that the PDF is only weakly dependent on the
current cut-off.
5.2.2 Discharge current regimes
The evolution of the mean discharge current is plotted in Figure 5.4 (lower
panel) for different applied voltages. The maximum of IB (t) tends to
occur at early times, t ≃(2-3) µs, while only at large voltages the mean
response spans the whole bump width (see e.g. curve (e) in Figure 5.4). The
upper panel in Figure 5.4 displays the corresponding PDFs, which seem to
attain a limiting shape, independent of Vpp , for Vpp > 23.5 kV. This is a
first indication that discharge currents may be organized into two different
discharge regimes. We explore this possibility further in the following.
An important quantity assessing the efficiency of the discharge device is
the total charge, Qtot , transferred during each half-cycle of the system.
57
5.2 STATISTICAL CHARACTERIZATION OF CURRENT SIGNAL
23
-1
-1
P(IB) [mA ]
10
10
10
20
-3
0
-5
-7
0
200
400
IB [mA]
100
(a) 22.9 kV
(b) 23.0 kV
(c) 23.5 kV
(d) 23.7 kV
(e) 24.7 kV
80
<IB> [mA]
40
Vpp [kV]
10
80
60
λ [mA]
22.9 kV
23.0 kV
23.5 kV
23.7 kV
24.7 kV
λ=55 mA
25
24
60
40
20
e
dc
0
0
b
a
2
4
6
time [µs]
8
10
Figure 5.4: (Upper panel ) PDF’s P (IB ) [mA−1 ] vs IB [mA], for different applied
potentials Vpp indicated in the plot. The straight line displays an exponential
function with decay constant λ = 55 mA
(see Eq. (5.5)) and is shown as a guide.
(Lower panel ) Mean discharge currents IB (t) versus time [µs], for the same values
of Vpp considered in the upper panel.
The total charge can be calculated from the mean discharge current IB (t)
as,
Qtot =
Z
tmax
tmin
dt IB (t) ,
(5.6)
where tmin and tmax are the temporal bounds for bumps. The total charge
Qtot is plotted in Figure 5.5, where one can see the emergence of two
distinct regimes separated by a threshold value Vpp ≃ 23.55 kV. The latter
is consistent with a similar behavior obtained from the shape of the PDF’s
58
STATISTICAL CHARACTERIZATION OF A STREAMER DISCHARGE REGIME
10
3
0.6
I
II
2
10
0.4
0.3
1
0.2
10
Activity Ratio
<Qtot> [nC]
0.5
0.1
10
0
23
24
Vpp [kV]
25
0
Figure 5.5: Mean total charge Qtot [nC] vs applied voltage Vpp [kV], transferred
by the discharge currents within a half-cycle (open circles, left-scale). The dashed
and continuous lines are quadratic fits to the numerical data and are shown as a
guide to stress the presence of a two-regime discharge pattern in the DBD, separated by a value Vpp ≃ 23.55 kV. Also shown, the activity ratio PA (open squares,
right-scale) plotted vs applied voltage. The dashed (continuous) line is a linear
(quadratic) fit plotted as a guide to the eye.
displayed in Figure 5.4.
In support to these findings, we can add that visual inspection of actual
discharge patterns show also two qualitatively different behaviors at low
and high applied voltages. In the low voltage regime, few moving discharges
occur at random along the electrodes. At higher voltages, many discharges
covering essentially the full electrode length occur in a fully random fashion.
The activity ratio of the DBD, that is the fraction of time within a
half-cycle in which a positive current is measured, is also plotted in Figure
5.5. It does not show such a clear change of behavior as Qtot does. Yet,
we have found that to obtain accurate fits the data need to be separated
into two parts, one below Vpp ≃ 23.55 kV, where just a linear dependence
occurs, and one above it, in which a quadratic function is required. In this
sense, also the activity ratio reflects the presence of two different regimes.
5.2 STATISTICAL CHARACTERIZATION OF CURRENT SIGNAL
59
We can also mention that an accurate quadratic fit to the data for Qtot
can not be obtained for the whole interval of voltages considered. This
difficulty reflects to some extent also the presence of an underlying tworegime discharge process.
Using the approximate form for the discharge PDF, Eq. (5.5), we comment on the two observed discharge regimes. For low voltage values, Vpp <
23.55 kV, the decay parameter λ increases rapidly with applied voltage (see
upper panel in Figure 5.4), i.e. the larger the value of λ the more likely the
higher current values are. Similarly, the activity parameter PA (Figure 5.5)
increases also, and it does it linearly in regime I. Based on evidence that
the charge transported by a single streamer does not seem to depend on
the applied voltage [6, 5], we suggest that the increase in height and duration of bursts we observe is due to the increasing number of simultaneous or
close-in-time streamers as a function of the applied voltage. In this plausible
scenario, new streamers can occur spanning the largely available space on
the dielectric without strongly experiencing the repulsive interaction with
residual charges deposited from previous micro-discharges within the same
half-cycle (bump).
For higher voltages, Vpp > 23.55 kV, λ stops growing, indicating that
a limiting shape of the discharge PDF has been reached. Thus, an upper
number of simultaneous streamers seems to occur, as suggested by the shape
of the mean discharge current shown in the lower panel of Figure 5.4, shape
which becomes broader in time but reaching a limiting upper value as the
voltage is increased. In other words, for low values of Vpp bursts are made
up of few streamers and IB remains low, spreading in time. By rising the
applied voltage, IB first increases in height up to a limiting value, then its
temporal duration starts to grow.
The two regimes identified previously can be further characterized by
looking at selected moments of order n of the discharge current IB (t), which
can be calculated according to,
1
In =
∆t
Z
tmax
tmin
dt IBn (t) ,
(5.7)
where here 1 ≤ n ≤ 4 and ∆t = tmax − tmin . The symbol
denotes an
average over different bumps. In addition to the mean, I1 , and standard
STATISTICAL CHARACTERIZATION OF A STREAMER DISCHARGE REGIME
100
100
80
80
σI [mA]
<I1> [mA]
60
60
40
20
40
20
23
3
24
25
Vpp [kV]
23
24
25
Vpp [kV]
23
24
25
Vpp [kV]
10
8
FI
SI
2
1
0
60
6
4
23
24
25
Vpp [kV]
2
Figure 5.6: First moments of the current
signal IB (t) versus applied voltage
Vpp [kV]. Shown are: the mean value I1 , standard deviation σI , skewness SI
and flatness FI . The horizontal line represents the values of SI and FI for an
exponential PDF, Eq. (5.9). The vertical dashed line indicates the separatrix value
Vpp ≃ 23.55 kV.
deviation, σI , of IB (t), we consider, in order to characterize the discharge
current, also the skewness, SI = I3 /σI3 and the flatness, FI = I4 /σI4 .
It is worth noticing that the total charge and mean current are related
to each other according to Qtot = I1 ∆t. The results are shown in
Figure 5.6. As one can see from the figure, the mean and standard deviation
display two different regimes clearly separated by the value Vpp ≃ 23.55 kV,
consistent with our previous results (see Sect. 5.2.1). The first two moments
strongly increase with applied voltage, and tend to stabilize above the value
Vpp ≃ 23.55 kV.
Higher moments, such as skewness and flatness of the distributions can
be compared with the values expected from an exponential PDF,
P (I) =
1
exp(−I/λ),
λ
I ≥ 0,
(5.8)
yielding the moment of order n,
In = λn
Z
∞
0
dy y n exp(−y) = λn Γ(n + 1),
(5.9)
160
160
120
120
σδ [ns]
<τδ> [ns]
5.2 STATISTICAL CHARACTERIZATION OF CURRENT SIGNAL
80
40
23
24
25
Vpp [kV]
23
24
25
Vpp [kV]
23
24
25
Vpp [kV]
15
3
10
Fδ
Sδ
80
40
4
2
5
1
0
61
23
24
25
Vpp [kV]
0
Figure 5.7: First moments of the burst lengths τδ versus applied voltage Vpp [kV].
Shown are: the mean τδ , standard deviation σδ , skewness Sδ and flatness Fδ .
The horizontal line represents the values of Sδ and Fδ for an exponential PDF,
Eq. (5.9). The vertical dashed line indicates the separatrix value Vpp ≃ 23.55 kV.
where Γ(n) is the Gamma function. According to Eq. (5.9), the skewness
√
and flatness take the values SI = 3/ 2 and FI = 6, respectively. The latter
are displayed in Figure 5.6 by the horizontal lines. The good agreement of
the higher moments confirms that the choice of the exponential distribution
as an approximation was adequate. The slightly deviation of flatness from
the predicted value is due to the poor statistic for high current values.
A similar analysis can be performed for the burst duration, denoted here
as τδ . The result for the corresponding moments are displayed in Figure 5.7.
Again in this case, the change of discharge regime becomes apparent around
Vpp ≃ 23.55 kV, consistent with our previous findings. The mean value
τδ first increases with applied voltage, while above 23.55 kV the burst
length stops growing, suggesting that a saturation number of simultaneous
streamers has been reached. A similar behavior is displayed by the standard deviation σδ , telling us that also fluctuations around mean values are
bound when Vpp > 23.55 kV. The further increase of the first two moments
for higher voltages may be due to the apparent overlap of nearby bursts not
resolved with the present diagnostic resolution. Higher moments of distributions are finally compared with the values expected for an exponential PDF,
62
STATISTICAL CHARACTERIZATION OF A STREAMER DISCHARGE REGIME
Eq. (5.8), suggesting that the actual PDF deviates a bit from an exponential
shape.
5.3 Statistical analysis of temporal behavior
The peculiar presence of memory effect in DBDs (see Introduction 5.1 and
Section 2.3.2) suggests that temporal correlations may exist also in the apparently random behavior of micro-discharges in the streamer regimes analyzed here and patterns (not visible with the eye) are formed. In this
section it will be studied the existence and propagation of temporal correlations both between discharge processes (bumps) and inside the single
discharge process.
5.3.1 Inter- and intra-bump correlations: surrogate model and Hurst exponents
Inter-bump correlations
In the following we deal with the question of correlations between discharges.
We consider first correlations between bumps, or inter-bump correlations.
To this end, we study the quantity CBi Bj (τ ), representing the correlations
between bump Bi and Bj , separated by a time lag τ = kT , where k = |i − j|
and T is the half-cycle period,
CBi Bj (τ ) =
where the symbol
∆t
IBi (t) − IBi · IBj (t) − IBj ∆t
σ Bi σ Bj
(5.10)
indicates the average over the N∆ = ∆t/τ0 values
of IB (t) present inside each bump. We find that CBi Bj (τ ) ≃ const (i.e.
independent of τ ) for k ≥ 1, indicating that a residual correlation is present
between any pair of bumps. By averaging over the total number of bump
pairs in the signal we obtain the mean residual correlation between bumps
as,
CBB =
X
1
CBi Bj |k≥1 .
NB (NB − 1)
(5.11)
i6=j
The mean correlation CBB is shown in Figure 5.10 as a function of
applied voltage Vpp , where CBB ≃ (0.2 − 0.3) for Vpp > 23.5 kV. Note
63
5.3 STATISTICAL ANALYSIS OF TEMPORAL BEHAVIOR
2.2
400
IB [mA]
2.4
2.6
2.8
3
3.2
400
300
IB [mA]
500
Surrogate
Real
<IB>
200
100
300
time [µs]
0
200
100
0
0
2
4
6
8
10
time [µs]
Figure 5.8: Surrogate uncorrelated time series generated with the model compared
with the original source data.
that there occurs a maximum of CBB around Vpp = 23.55 kV, suggesting
another way of determining the separatrix voltage between regimes I and II.
In order to understand the origin of such correlations, we implement a
surrogate model in which fully uncorrelated discharges (streamers), IS (t),
occur inside a bump. Several methods exist for the generation of an uncorrelated time series with a specific PDF [69, 70]. A simple rejection method
can be used to obtain a stationary time series with a PDF like those represented in Figure 5.4.
To make the model more realistic, we take into
account the intrinsic non-stationarity of the process, that is represented by
the time dependence of the mean discharge response. To do this, we calculate N∆ local PDFs, PL (t), one for each time step inside a bump, obtained
from the total number of bumps in the signal, NB . Then, for each point
inside a bump, we generate an uncorrelated surrogate current signal IS (t)
according to the local PDF, PL (t). In Figure 5.8 the original signal is compared with the surrogate one and it can be observed that the original shape
of the bump is well reproduced. In Figure 5.9 are compared the P (IB ) of the
surrogate generated time series and it can be observed that the two PDFs
overlap very well although a little underestimation of the zero component
of the original signal is present. This is probably due to the limited number
of data available for the calculation of local PDFs PL (t). A consequence
64
STATISTICAL CHARACTERIZATION OF A STREAMER DISCHARGE REGIME
0
10
10
-2
-1
P(IB) [mA ]
Surrogate
Real
10
10
-4
-6
0
100
200
300
IB [mA]
400
500
Figure 5.9: Comparison between the PDF of the original source data with the
PDF of the surrogate uncorrelated time series generated with the model.
is a slightly overestimation of lower current values, which brings, for exam
ple, to an increase of about 15% of the calculated Qtot values. However,
this little differences do not affect the correlation analysis performed in the
following. As is apparent from Figure 5.8, the surrogate signal displays a
similar shape as the real current, but it looks more uniformly distributed as
the discharge clustering typical of bursts is not implemented in the model.
Yet, the clustering is not important for determining the residual correlations
between bumps as shown by the correlation values CSS (open diamonds in
the Figure 5.10). We conclude that the residual correlation between bumps
we observe in the discharge patterns is due to the non-stationarity of the
signal. This conclusion is further supported by the calculation of CBB for
the detrended signal, that is the discharge current normalized by its mean
value, i.e. ID (t) = IB (t)/ IB (t) . A similar definition is applied to the surrogate signal. As clearly seen from Figure 5.10, the cross-correlations between
bumps vanish for the detrended signals, indicating that residual correlations
are a result of the non-stationarity of the discharge process. Similar behavior
65
5.3 STATISTICAL ANALYSIS OF TEMPORAL BEHAVIOR
2
6
4
0.4
<CBB>
8
I [mA] 100 200 300
0.5
time [µs]
Real Data
Detrended Real Data
Surrogate
Detrended Surrogate
0.3
0.2
0.1
0
23
24
Vpp[kV]
25
Figure 5.10: Mean value of the residual cross-correlation CBB between discharge
processes as a function of the applied voltage Vpp [kV]. Both real and surrogate
signals are shown in comparison with their respective detrended signals. The inset
shows an example of the original signal, a surrogate and the mean response function
for Vpp = 25 kV.
is displayed by the surrogate current.
Intra-bump correlations
Verified that the memory effect between half-cycle has no influence on the
temporal behaviour of the discharge it is interesting to see if correlations
survive inside the single discharge process. The role of clustering (bursts)
becomes apparent when studying intra-bumps correlations, as we do next.
To study intra-bump correlations, or autocorrelations in the discharge signal
IB , we apply the method known in literature as the fluctuation analysis (FA)
based on Haar wavelets (HW) [71, 72]. We briefly summarize the FAHW
method in the following.
The FA approach is based on random walk concepts. One regards the
fluctuation of the signal,
∆IB (t) = IB (t) − IB (t) ,
(5.12)
as a jump performed by a random walker at time step t (in units of τ0 ),
66
STATISTICAL CHARACTERIZATION OF A STREAMER DISCHARGE REGIME
where tmin ≤ t ≤ tmax . Then, one calculates the position W (ti ) of the RW
at time ti = iτ0 , with 1 ≤ i ≤ N∆ , as the sum over all previous jumps
∆IB (tj ), tj ≤ ti ,
i
X
(IB (tj ) − IB (tj ) ),
W (ti ) =
(5.13)
j=1
which is also denoted as the ‘profile’ of the random walk. Once the profile
has been obtained, we study the scaling behavior of W (t) on the time scale
τ . To do this, we divide the total number of points inside the bump, N∆ ,
into consecutive non-overlapping segments of length ℓ ≥ 1, corresponding to
the time scale τ = ℓτ0 . Inside each segment m, 1 ≤ m ≤ N∆ /ℓ, we evaluate
the average of W according to,
ℓ
1X
W (t(m−1)ℓ+j ).
ℓ
Bm (ℓ) =
(5.14)
j=1
The FAHW approach consists in studying the fluctuations of the profile on
the ‘time scale’ ℓ, defined as
F12 (ℓ) =
2 Bm+1 (ℓ) − Bm (ℓ) ,
(5.15)
where the subindex 1 in F1 (ℓ) refers to the first-order Haar wavelet, and the
average is performed over all consecutive boxes m and m + 1. Higher-order
wavelets can be introduced [71], allowing for eliminating possible higherorder trends in the profile. The dependence of F1 (ℓ) on ℓ is expected to
obey a scaling behavior of the form,
F1 (ℓ) ∼ ℓH ,
(5.16)
which defines the Hurst exponent H. The value H = 1/2 indicates uncorrelated fluctuations, or standard random walk behavior. Cases in which
H 6= 1/2 correspond to signals in which autocorrelations are present. If
this occurs for ℓ → ∞, one says that the signal features long-time correla-
tions. Cases in which H > 1/2 denote persistence, and cases with H < 1/2
anti-persistence. More common situations are those in which a power-law
exponent H 6= 1/2 occurs only on finite time scales, typically at short time
scales. These methods have been also employed for the analysis of turbulent
67
5.3 STATISTICAL ANALYSIS OF TEMPORAL BEHAVIOR
2
<F1(l)>B
10
H=0.50
H=0.85
1
10
0
10
2
<F1(l)>B
10
H=0.50
H=0.88
1
10
0
10
0
10
1
10
2
10
Time [ns]
3
10
4
10
Figure 5.11: (color online)
Fluctuation analysis of intra-bump correlations.
Shown is the quantity F1 (ℓ) versus time scale τ = ℓτ0 [ns]. The average of
F1 has been performed over all bumps in the signal. The straight lines have slopes
H, yielding the Hurst exponents. The time series analysed corresponds to a high
voltage case, Vpp = 25.5 kV. The open circles represent the original signals, while
the open squares the detrended ones. Upper panel : Real data. The diamonds were
obtained by excluding the zero current values, yielding H ≃ 0.5. The vertical line
indicates the time scale τ = 165 ns. Lower panel : Surrogate model. The detrended
surrogate signal displays uncorrelated fluctuations H ≃ 0.5.
behaviors in magneto-plasma devices [73, 74].
Results of the FAHW analysis performed for real discharge currents and
for the surrogate model are displayed in Figure 5.11. We observe that within
a time scale of the order of 160 ns, the real data display strong autocorrelations with H ≃ 0.85. These correlations reflect the discharge clustering
within bursts. The detrended signal behaves similarly as the original one for
time scales within bursts, suggesting that bursts clustering is a robust type
of correlation, even when non-stationarity of the signal is eliminated. For
time scales larger than mean bursts width, i.e. 160 ns, we observe different
68
STATISTICAL CHARACTERIZATION OF A STREAMER DISCHARGE REGIME
behaviors between the original and the detrended signal: The original signal
seems to display persistent correlations at such long time scales, while for
the detrended one fluctuations become flat. The latter behavior may suggest
the presence of long-time anti-correlations. These two unexpected behaviors
can be shown to be an artefact of zero-current events in the calculation of
F1 (ℓ). To show this, we have studied the scaling behavior of F1 (ℓ) for the
case of positive current values, by excluding zero-current events from the
analysis. The corresponding points are displayed by the open diamonds in
the upper panel of Figure 5.11. As expected, current fluctuations between
bursts are uncorrelated yielding the standard behavior H = 1/2 (dashed
line). The surrogate signal displays an effective Hurst exponent H ≃ 0.88,
suggesting strong autocorrelations inside a bump. These correlations are
shown to be an artefact of the non-stationarity of the model and vanish for
the detrended signal. This correlation analysis suggests that inside the single discharge process (bump) exists a cooperative behaviour of the streamers
occurring sufficiently close in time (i.e. inside a burst), however the eventual
pattern formed by discharge remnants on the dielectric is destroyed by the
subsequent burst which does not retain memory of the previous one.
5.3.2 Temporal correlations between streamers
The presence of strong correlations inside the single burst found with the
Hurst analysis (Section 5.3.1) suggests that a deeper insight into the discharge process is required in order to understand the nature of these correlations. To this end, a specific, short time scale analysis of the burst
structure has been performed. The lower panel of Fig. 5.12 displays the
short-time scale of a single typical burst. One can anticipate the existence
of an internal structure of the burst by the presence of several emerging
peaks, aside from few significant ones, that represent the micro-discharges
or the temporal superposition of more micro-discharges. From now on they
will be all referred to as streamers. The aim is to extract information about
the temporal streamer distribution by performing an accurate fit to the full
burst shape using Gaussian functions as the basis set. Thus, the single burst
5.3 STATISTICAL ANALYSIS OF TEMPORAL BEHAVIOR
69
Figure 5.12: Upper panel : A typical discharge pattern [mA] within a half-cycle vs
time [µs] for Vpp =25 kV. The continuous line represents the mean (absolute value)
discharge response of the system, averaged over all half-cycles in the time series.
Lower panel : The internal structure of a single burst taken from the upper panel
(zoomed around 4 µs). The Gaussian fits are physically identified as streamers.
The fit virtually coincides with the discharge pattern. Here, τa represents the time
interval between two adjacent streamers, and τb the time separation between two
adjacent bursts.
shape IB (t) is written according to,
NB
X
1
(t − ti )2
,
IB (t) =
ρi √
exp −
2σi2
2πσi
i=1
(5.17)
where ρi is the streamer charge, σi the standard deviation and ti the temporal location of the streamer. First it is required that the number of Gaussians
to be used be reduced to a minimum. This is done by searching for their
possible locations using information from the (numerically evaluated) first
70
STATISTICAL CHARACTERIZATION OF A STREAMER DISCHARGE REGIME
10
0
intra-B (52 ns)
inter-B (293ns)
3.7 -6
F=τ ,τ
F=log-normal
-1
σ P(τ)
10
10
4.8
F=τ Gauss
-2
-3 3.7
-6
10
-4
10
10
1
10
2
τ [ns]
10
3
4
10
Figure 5.13: The scaled PDF, σP (τ ), of intra-burst (open circles) and inter-burst
(open squares) times vs τ [ns]. The lines are different types of fits to the numerical
data. Intra-burst times: power-law fits ∼ τ 3.7 for τ < 50 ns and ∼ τ −6 for τ > 50
ns (continuous line); ∼ τ 4.8 ×Gaussian (dashed line). Inter-burst times: log-normal
fit (continuous line). The mean intra-burst and inter-burst times are indicated in
parenthesis. The corresponding standard deviations are: σa = 17 ns and σb = 250
ns, respectively. Averages over different applied Vpp ranging from 24.5 kV to 25.5
kV have been performed.
(0)
derivative of IB (t). The obtained initial locations ti
are used to initiate the
search. The fit parameters for all Gaussians inside a burst are then determined using a recursive least-square method. The latter is implemented by a
random search of the parameter values using a simulated annealing [75] type
of strategy. The final fit yields global absolute error of the order of 10−4 mA
in most cases, and the fit is generally indistinguishable from the experimental data, as illustrated in the lower panel of Fig. 5.12. The reconstruction
algorithm described above allows to perform a detailed study of the statistic
of time intervals between discharge processes, which are represented by the
quantities τa which is the time interval between two adjacent streamers, and
τb which represents the time separation between two adjacent bursts. The
aim here is to identify the nature of the correlations individuated in Section
5.3 STATISTICAL ANALYSIS OF TEMPORAL BEHAVIOR
71
5.3.1.
A first insight can be observed in the behavior of PDFs of waiting times.
In Figure 5.13 are represented the separated distributions of waiting times
within bursts τa and inter-burst times τb . Data have been accumulated
for applied Vpp voltages ranging from 24.5 to 25.5 kV after having verified
the statistical properties do not depend significantly on voltage. It can be
observed that the distributions of waiting times inside a single burst and
waiting times between bursts follow different distribution laws. While the
inter-burst times τb show a fast-decaying typically uncorrelated behavior,
which in Figure 5.13 is fitted with a log-normal distribution, the intra-burst
waiting times τa show a non-trivial power law decaying character. Although
the PDF analysis in not enough to recognize a correlated behavior, the
presence of power laws in distributions have already been connected with
anomalous behaviors and presence of correlations (for example see literature
on fluctuation analysis in magnetized plasmas [76, 77, 78, 79, 80, 81, 73, 74]).
This anomalous behavior of τa distribution suggests the presence of an intraburst structure. To show this, two correlation functions are constructed to
analyze the time series of the occurrence of streamers (or superposition of
contemporary streamers) and the charge transported by them. The first is
the temporal pair distribution function g(τ ) and is defined by:
ρ̄τ =
Z
0
∆τ
dτ g(τ ) = Ns − 1
(5.18)
where Ns is the total number of streamers pairs within ∆τ , ∆τ is the time
lag limit and ρ̄τ = Ns /∆τ . g(τ ) can be calculated as a discrete quantity:
g(τ ) =
Ns
Ns X
∆τ /dτ X
δdτ ((ti − tj ) − τ )
Ns (Ns − 1)
(5.19)
j=1 i=1,i6=j
where dτ is the discrete time interval and δdτ is a Dirac function. The function g(τ ) is constructed identically to the pair distribution function which is
used to recognize spatial correlations in liquid or solid state matter [82, 83].
g(τ ) is its temporal equivalent and expresses the probability to find two
streamers occurring at a distance τ in time. Note that g(τ ) tends to unity
for large values of τ .
72
STATISTICAL CHARACTERIZATION OF A STREAMER DISCHARGE REGIME
3.00
1.0
Cqq(τ)
0.5
2.00
0.0
g(τ)
0
100
50
τ [ns]
150
200
1.00
0.00
0
100
200
300
τ [ns]
400
500
Figure 5.14: The streamers temporal pair correlation function, g(τ ) (open circles),
vs time lag τ [ns], from
in Fig. 5.13. The dashed line is the fit
shown
the data
y = 1 + 4.6 exp(−τ / τa ), with τa = 52 ns. The inset shows the autocorrelation
function, Cqq (τ ) (open circles), of streamer charge transfer Q vs time lag τ [ns].
The horizontal line is a guide.
In Figure 5.14 it is represented g(τ ) for the time series of occurrence of
Gaussian functions (5.17) obtained with the reconstruction algorithm described above. It is evident a persistent characteristic time interval for the
occurrence of streamers. This suggests the presence of a characteristic ”frequency” of occurrence which disappears outside the single discharge burst.
The presence of this strong correlation between streamers and its vanishing for time intervals longer than typical burst duration, confirms what was
found with the analysis of Section 5.3.1. Furthermore, the presence of a
characteristic time interval can be connected to the discharge development
in which the occurrence of a streamer is somehow produced by a previous
one. A possible interpretation can be given with the following mechanism.
When a streamer occurs in some point on the dielectric surface, in its surroundings exist several other ”seed” micro-discharge remnants that may
not have yet reached the breakdown conditions. At this point the photons emitted by the excited atoms in the first streamers may ”induce” the
5.3 STATISTICAL ANALYSIS OF TEMPORAL BEHAVIOR
73
other remnants to attain the breakdown condition. This photo-ionization
induced mechanism is influenced by many parameters such as the time for
the streamer to reach an adequate emission intensity, remnants on the dielectric surface and others, but finally shows a characteristic time which is
evident in the temporal structure shown in Figure 5.14. The influence of
radiation on micro-discharges has been reported in the past [84, 85]. If one
assumes that this excitation mechanism exists, then the τa of Figure 5.14
is its characteristic time. The nature of this interaction, however, cannot
be determined by the present analysis as it requires a different diagnostic
approach.
Another interesting quantity is the charge associated to every Gaussian
streamer which is proportional to the number of real micro-dischargers occurring at the same time. To evaluate the presence of temporal correlations
in this quantity it is introduced the function:
1 X (qj − q ) · (qj−τ − q )
Cqq (τ ) =
,
Nτ
σq2
(5.20)
j
where the sum over j indicates the sum on the total number of time steps
Nτ and q and σq2 are the mean value and variance of the distribution of
charges ρ in the time series. The function (5.20) is normalized to unity
by definition and indicates whether or not correlations between transfered
charge are present at a certain distance τ in the time series of Gaussian
streamers. In the inset of Figure 5.14 is plotted the function Cqq for the
data shown in Figure 5.13. It can be seen that a correlation persists on
shorter time scales than the g(τ ). This suggests that the number of microdischarges activated by the hypothetical excitation mechanism stated above
has wider fluctuations and de-correlates faster within the time length of
the single current burst. It is possible to interpret the two functions (5.19)
and (5.20) as the description of two aspects of the discharge development.
The former describes the temporal connections in micro-discharges formation which is independent on their number and thus it is more independent
on geometrical constraints like the electrode dimensions (but still depends
on the gap distance and atmosphere composition and pressure). That is, it
describes a more general property of the discharge process: i.e. the temporal
aspect of a possible reaction mechanism in which the occurrence of a micro-
74
STATISTICAL CHARACTERIZATION OF A STREAMER DISCHARGE REGIME
discharge in a certain position in the gap promotes the formation of other
micro-discharges within a well defined time interval. Obviously, this can be
verified only with a spatiotemporal analysis which is beyond the capabilities of the present diagnostics. The function (5.20), on the contrary, takes
into account the number of micro-discharges occurring in the gap and thus
can be considered more dependent on electrode dimensions which limit the
intensity of current pulses (see Section 5.2.1). With this interpretation g(τ )
should be found independent of the streamer regimes described in Section
5.2.2 while Cqq should depend on them. Unfortunately, the non-interacting
(low-voltages) regime has a poor statistical basis to perform the analysis
described here and a diagnostic improvement is needed. It is interesting to
note a similar behavior of the current signal (called multi-peaks which have
been observed in diffuse DBDs [86, 87, 88]). Even if discharge conditions,
gas compositions and time scales are completely different, similarly, a propagation mechanism (in this case of the ionization front) has been proposed
to explain the effect [89, 87, 90].
The temporal analysis performed above requires to prove that the peculiar structure is due to correlations between streamers. To this end, an
uncorrelated model time series is generated and analyzed with the function
(5.19). To make the comparison possible the surrogate time series are generated with a rejection method [69, 70] starting from the P (τ ) fit function
which is defined as:
P (τ )R = 0.061
(τ /52)−3.75
.
1 + (τ /52)9.7
(5.21)
Also a Gaussian distribution with same mean value and standard deviation
is considered. In Figure 5.15 are compared the generating functions with
the PDFs of the obtained series. The time series of streamer occurrence are
then obtained by simple integration of the waiting time series.
In Figure 5.16 the function g(τ ) is calculated for the surrogate time series and compared with the experimental data. It is evident that in absence
of temporal correlations the oscillation of g(τ ) vanishes almost immediately,
revealing that the temporal structure of the experimental data is effectively
due to the presence of temporal correlations between streamers in the discharge process. This is also confirmed by the behavior of the power spectra
75
5.4 CONCLUDING REMARKS
-1
10
Uncorrelated Process
<τ>= 51.9 στ = 17.0
Gaussian Function
Uncorrelated process
<τ>=53.0 στ=18.3
τa Fit Function
-1
P(τ) [ns ]
-2
10
-3
10
-4
10
0
10
1
10
2
10
τ[ns]
3
10
4
10
Figure 5.15: The PDF, P (τ ), of surrogate waiting time series compared with the
generating functions. Circles refers to time series generated with equation (5.21).
Diamonds refer to time series generated with a Gaussian function with same mean
value and standard deviation. (Top-right legend) Because the time series refers to
waiting times between streamers, the negative values are ignored.
of g(τ ) (inset of Figure 5.16) where the presence of a characteristic time for
the real data is well evidenced.
5.4 Concluding remarks
The streamer regime of a DBD in air has been characterized by means
of the statistical analysis of the discharge current. The presence of two
different discharge regimes has been observed in several quantities both regarding the statistical properties of the current intensity and its temporal
behavior. These regimes have been found to be dependent on the applied
voltage. It has been shown that below a threshold value of the applied voltage, the streamers generated in the discharge process can span the largely
available space on the dielectric without being affected by the repulsive interaction with residual charges deposited from previous micro-discharges.
This brings a rapid growth of the charge transferred by the system within a
76
STATISTICAL CHARACTERIZATION OF A STREAMER DISCHARGE REGIME
F[g(τ)]
3
2
-2
10
0
g(τ)
19 MHz
-1
10
20 40 60 80
f [MHz]
1
Exp. <τ>=52.0 ns στ=17.0 ns
Gauss. <τ>=52.0 ns στ=17.0 ns
P(τ)R <τ>=53.0 ns στ=18.3 ns
0
0
100
200
300
τ[ns]
400
500
Figure 5.16: The streamers temporal pair correlation function, g(τ ) (open circles),
vs time lag τ [ns], from the data shown in Figure 5.13. The squares and diamonds
represent g(τ ) for the streamer occurrence time series calculated from the waiting
time time series represented in Figure 5.15. (Inset) The power spectrum of the g(τ )
functions
single discharge process. For higher voltages, a limited number of simultaneous streamers seem to occur, as suggested by the behavior of the discharge
current shape and temporal properties. In this discharge regime, where the
streamers strongly interact, the rate at which energy is transferred by the
system to the plasma discharge gets slower with increasing voltages.
The presence of correlations between discharge processes and within the
single discharge process have been studied. With the help of a surrogate
model it has been shown that the observed residual cross-correlations between half-cycles are only an effect of the intrinsic non-stationarity of the
signal, indicating that no memory persistence is present in the temporal
structure of the discharge. Also it has been shown that, within the discharge process, strong correlations are present in the current signal within a
short time scale of the order of the mean value of the burst duration. This
suggests that the interaction between streamers can act only when they oc-
5.4 CONCLUDING REMARKS
77
cur close in time and the eventual memory left as discharge patterns on
the dielectric is destroyed by the subsequent burst. Decorrelation between
bursts and bumps promotes uniformity of energy pattern deposition over
time.
Using newly defined correlation functions, the temporal structure of
bursts have been revealed to be extremely correlated and the existence of
a characteristic frequency in the occurrence of streamers have been found.
This frequency is possibly related to the propagation of discharge in the gap.
78
STATISTICAL CHARACTERIZATION OF A STREAMER DISCHARGE REGIME
CHAPTER
6
Characterization of the DBD device
in nitrogen atmosphere
6.1 Introduction
In this Chapter the newly developed plasma device described in Section 3.1
is characterized in an atmosphere of pure nitrogen. The aim is to find the
device capabilities by exploring the control parameters and give a description
of the plasma discharge device in nitrogen atmosphere, which is often chosen
as carrier gas for the development of plasma processes for applications. The
controlled parameter for such device are: the power injected into the system,
flux of the nitrogen gas through the injection nozzle and pressure. Plasma
discharges in the DBD will be characterized as a function of these three
parameters using voltage and current measurements and optical emission
spectroscopy.
Nitrogen discharges have been the subject of studies already for many
79
80
CHARACTERIZATION OF THE DBD DEVICE IN NITROGEN ATMOSPHERE
decades [91, 92, 42]. It can form stable atmospheric pressure discharges and,
being an electropositive gas, it does not have the tendency to quench electron
activity in the plasma. Moreover, nitrogen is mostly chemically neutral and,
for example, does not alter chemical composition of thin film deposited with
plasma processes (see Chapter 7), even if active species are formed during the
discharge. These species, principally molecules in metastable states, due to
their long lifetimes and elevated potential energy can transfer energy to other
species, for example a reactive compound added to the mixture. Last, but
not least, nitrogen is extremely cheap with respect to rare gases like helium
or argon. For all this reasons nitrogen, as a basis of plasma discharges, is
often chosen to develop plasma processes for applications [93, 94, 95, 96].
6.2 Experimental setup and methods
The experimental setup and the diagnostic utilized are described in Section
3.1 and Section 4.2, respectively. The experiments have been performed as
follows. The discharge chamber has been evacuated with the rotary pump
P1 (Figure 3.1) down to 5 · 10−3 mbar to avoid contaminations, then a
calibrated flux from the injection system fill the chamber up to the desired
working pressure. After the working pressure is reached the dry pump P2
is used to balance the inlet flux and keep the pressure stable. The current
and voltage signals are acquired with a time step of 5 ns for a total length
of 0.5 ms. The inter-electrode gap has been kept fixed at 2.5 mm.
6.3 Discharge regimes in Nitrogen Atmosphere
Usually, in atmospheric pressure DBDs in nitrogen, the discharge regime is
a filamentary one even if, under specific conditions, a homogeneous diffuse
discharge may be obtained [86, 9, 97]. However, it is generally difficult
to obtain and reliably control such homogeneous discharges at atmospheric
pressure. For example, minor changes in the electrode configuration or small
variations of the amplitude or repetition frequency of the applied voltage
can cause a transition from the relatively unstable diffuse mode to that of
a much more stable filamentary discharge. For many potential industrial
applications, the diffuse behaviour is a severe disadvantage compared to the
6.3 DISCHARGE REGIMES IN NITROGEN ATMOSPHERE
(a) Discharge in Nitrogen: flux of 2 ln /min., (b) Discharge in ambient Air:
injected power 220W.
2 ln /min., injected power 200W.
81
flux of
Figure 6.1: Typical current voltage characteristics for discharges in nitrogen atmosphere and ambient air at high injected power. Lower panel: applied voltage
and total current. Upper panel: applied voltage and discharge current according to
equation (5.2).
easier implementation of filamentary DBDs. Moreover, ways can be found
for ensuring that the intrinsic instantaneous inhomogeneity of this random
filamentary DBD does not lead to global inhomogeneity.
Important informations can be obtained by the analysis of the currentvoltage characteristics. For all the discharges in the present setup, because
the ionization of the gas is very low, it is possible to consider that the
capacitance of the gas does not change during the discharge process [9, 11].
The discharge current is thus calculated with equations (5.2) following the
procedure described in Section 5.2. In Figure 6.1 are compared the typical
voltage and current waveforms for discharges in nitrogen and ambient Air. It
can be observed that both show the typical current pulses due to streamers
(which are also visible by the eye) but substantial differences are present.
In air, current bursts are typically short and more intense (see Chapter 5
for a complete description of the streamer regime in air) while in nitrogen
they seem to be lower in height and longer in time with a peculiar slowly
decaying current tail. This behavior can be connected to the dimension
and duration of single micro-discharges. As already discussed in Chapter
5, current bursts are the temporal superposition of more micro-discharges.
It has been shown that oxygen admixtures to nitrogen can lead to plasma
channel reduction [98, 4, 38]. Thus, the lower intensity of the current bursts
observed in nitrogen can be possibly explained by the presence of wider
82
CHARACTERIZATION OF THE DBD DEVICE IN NITROGEN ATMOSPHERE
Figure 6.2: Typical emission spectra for discharges in nitrogen atmosphere. Flux
of 2 ln /min., injected power 100W, pressure 500 mbar.
streamer and, thus, less contemporary streamers developing close in time.
Also the different charge transported by the single discharge process must
be considered. Moreover, the longer duration of current bursts in nitrogen,
characterized by the slowly decaying current tail, suggests the presence of
active species with longer lifetime like N2 (A3 Σ+
u ) metastable molecules [99]
that maintain active the discharge. The presence of electronegative oxygen
gas quenches more rapidly this activity.
Useful information on the plasma phase can be achieved from the analysis of emission spectra. Nitrogen is a very active species which has a complex reaction scheme involving electronic, vibrational and rotational excited
states along with ionized species. Particularly important is the role of vibrational excitations. The creation of excited vibrational state by electron
impact is highly favorable while the relaxation processes (see Section 2.4.1)
are less effective. Thus, vibrational states adsorb a large part of the energy
and act as a sort of reservoir. These energies are typically high enough to
activate chemical reactions with other species [2]. In Figure 6.2 is shown the
typical emission spectra of a discharge in nitrogen. The spectrum is shown
between 300 nm and 500 nm because outside this region the emission lines
are absent or too weak . The spectrum is dominated by the second positive system (SPS) of N2 (C 3 Πu →B 3 Πg ) [100, 101]. Vibrational levels are
usually thermalized because vibrational-vibrational transition processes are
6.3 DISCHARGE REGIMES IN NITROGEN ATMOSPHERE
83
very effective. Thus, a vibrational temperature Tv can be calculated from
the SPS structure by determining the populations of the vibrational levels
of N2 [102, 103, 104, 105, 106] and according to the formula:
nN2 (C,ν) =
EN (C,ν)
X IN2 (C,ν)→N2 (B,ν ′ )
2
− kT
v
∝
e
4
νN2 (C,ν)→N2 (B,ν ′ )
′
(6.1)
ν
where ν and ν ′ are the vibrational level index, IN2 (C,ν)→N2 (B,ν ′ ) is the intensity and ν the frequency of the electronic transition between N2 (C 3 Πu ) and
N2 (B 3 Πg ) levels of nitrogen. It is interesting to recall that also the electron
temperature could be determined by the vibrational population levels [107]
and, generally, it increases in the same way as Tv . An emission line from the
2 +
2 +
first negative system (FNS) of N+
2 (B Σu →X Σg ∆ν = 0) is also visible at
391.3 nm. This emission line is usually connected to the electron energy and
electron energy distribution function because the ionization threshold, from
2 +
3
a neutral N2 molecule, of the N+
2 (B Σu ) is higher than N2 (C Πu ). Thus, to
a first approximation, the ratio between the intensities of two characteristic
lines of FSN and SPS is a monotone function of the electron temperature
[108, 109]. The ratio between 391 nm line of FSN and 357 nm line of SPS will
be used in the following to estimate the variation of electron temperature
as a function of discharge parameters.
6.3.1 Characterization of the discharge as a function of injected power
The DBD discharge device is powered by a simple transistor switching system which does not allow to control separately frequency and voltage (see
Section 3.1.3). The voltage applied to the electrode is varied by varying
the difference of the frequency with respect to the resonance frequency of
the system. The generator also provides a measure of the power injected.
The generator can support powers up to 600W but in the present experiments the power levels have been kept under 250W to avoid overheating and
damage to the polycarbonate injection nozzle. As a first characterization, a
current-voltage plot is obtained (Figure 6.3(a)) spanning the available power
range of the system. It can be observed that the behavior is not unique in
the above range and two regions are evident. By comparing the root mean
square (rms) value of displacement and total current as a function of the rms
84
CHARACTERIZATION OF THE DBD DEVICE IN NITROGEN ATMOSPHERE
(a) Current-voltage characteristic of the (b) Mean value
transferred charge in
¸
˙ of total
DBD device.
a half period Qtot .
Figure 6.3: Discharge regimes: Voltage-Current characteristic and total transferred charge. Flux of 2 ln /min., pressure 900 mbar.
rms
has a rough linear
applied voltage V rms , it is evident that, while Idisplace
rms shows a two-stage behavior. This is the same
dependence on V rms , Itot
effect that has been characterized for discharges in Air in Chapter 5. In the
lower voltage regime new streamers can occur spanning the largely available
space on the dielectric without strongly experiencing the repulsive interaction with residual charges deposited from previous micro-discharges within
the same half-cycle. In the high voltage regime the micro-discharges are
forced to a strong repulsive interaction which limits the possibility to add
more streamers to the electrodes. In the same way and similarly to equation
(5.6) the two discharge region can be well recognized by measuring the mean
value of the total charge transferred by the discharge process which can be
calculated as:
Qtot =
Z
0
T /2 Idisch (t)dt ,
where the integration is over a half period and the mean
(6.2)
is calculated
over all the half periods recorded in the time series. It is worth to mention
that, even if the electrode system is not symmetric, no asymmetries have
been found in the measure of Qtot . Qtot has been calculated according to
equation (6.2) and the results are shown in Figure 6.3(b). A separatrix rms
voltage Vsrms ≃ 5.1kV is found.
In Figure 6.4(a) the vibrational temperature Tv is shown, calculated
according to equation (6.1). It can be noted that the vibrational temperature
6.3 DISCHARGE REGIMES IN NITROGEN ATMOSPHERE
85
(a) Vibrational temperature as a function of (b) 391nm/357nm line intensity ratio as a
power. The solid red line is a linear relation function of injected power.
plotted as guideline.
Figure 6.4: Variation of vibrational temperature and 391nm/357nm line intensity
ratio as a function of the injected power.
is of the order of 2500 K, which means that in such discharges also the
neutral molecules have thermal energy sufficient to influence directly the
chemical kinetics evolution of the gas-phase. A slow decrease of Tv with
increasing injected power is evident which means that at higher power levels
it is allowed a more pronounced quenching of the excited vibrational levels
respect to its ground state. In Figure 6.4(b) the 391nm/357nm line intensity
ratio is plotted as a function of the injected power. It is evident that no
clear trends can be recognized in the plot because all variations seem to be
within the error of the measure. This means that no substantial variations
are present for the electron temperature.
6.3.2 Characterization of the discharge as a function of pressure and gas
fluxes
One of the most interesting capabilities of this DBD device is the possibility
to work in a completely controlled atmosphere being the electrodes inside a
vacuum chamber. This characteristic allows to work also at lower pressure.
In the following the electrical and optical behaviour of the discharge is observed as a function of pressure in the range 50÷900 mbar. It has not been
possible to perform experiments below 50 mbar, because the system tends
to realize a diffuse discharge that is not confined in the electrode gap.
In Figure 6.5 are shown the current and voltage waveforms for exper-
86
CHARACTERIZATION OF THE DBD DEVICE IN NITROGEN ATMOSPHERE
(a) Discharge at 50 mbar, 75W
(b) Discharge at 200 mbar, 100W
(c) Discharge at 500 mbar, 100W
(d) Discharge at 900 mbar, 100W
Figure 6.5: Current-voltage characteristic for discharges at various pressure.
Lower panel: applied voltage and total current. Upper panel: applied voltage and
discharge current according to equation (5.2).
iments at various pressures. At first glance, it is evident that for lower
pressure the discharge seems to occur in a sort of continuous mode while
at higher pressure the identification of current bursts is possible. This can
be explained by the fact the duration of current pulses due to streamers
have been measured to be proportional to inverse square of the pressure [4].
As it is known, when the pressure (number density) is low the ionization
by direct electron impact α is lower and a localized space charge (which is
the origin of the streamer channel) is not created. Moreover, a fundamental
role is played by metastable species which (by Penning ionization, equation
2.11) keep the number of seed electrons high and lower the breakdown voltage. The requirement for establishing a stable diffuse discharge (dominated
by Townsend breakdown mechanism) is that the number of seed electrons
is large enough to cause appreciable overlap and merging of the primary
avalanches. With a better analysis of the discharge current for the 50 mbar
6.3 DISCHARGE REGIMES IN NITROGEN ATMOSPHERE
87
(a) Rms voltage and total charge according (b) Mean value
transferred charge in
¸
˙ of total
to equation (6.2).
a half period Qtot .
Figure 6.6: Rms voltage, total charge and absolute intensity of emission as a
function of pressure. Flux of 2 ln /min., power 100 W (75W for 50 mbar).
discharge (Figure 6.5(a), top panel) it is possible to see that, even if the
discharge seems a single process, several well separated current peaks can
be observed. This kind of behaviour is different from the current characteristic of a well developed diffuse discharge [110, 108]. This can be possibly
explained by the fact that, even if the lower pressure would allow the development of a diffuse discharge, does not exist an effective process to maintain
the number of secondary electrons high enough. Thus, the discharge processes last on smaller time scales giving origin to the observed current peaks.
The transition between these two regimes is not clear even if they have been
studied as a function of gas composition [111], electrical and geometrical
parameters [10, 112] and pressure [108]. A simple visual observation suggests that a diffuse discharge exists up to 300 mbar, but this is due to the
superposition of thousands discharge processes. It has been shown that the
use of fast cameras can reveal the presence of the streamers [113, 110, 114].
An interesting observation is that a substantial reduction of the duration of
the discharge process occurs with increasing pressure. Moreover, the same
occurs for the slowly decaying current tail.
In Figure 6.6 are shown the behaviours of the rms voltage and total
charge transferred according to equation (6.2) as a function of pressure and
for a constant injected power of 100 W. As it can be expected [5, 1], the rms
voltage increases with pressure because the breakdown voltage increases.
The interesting aspect is the behaviour of Qtot which decreases with pres-
88
CHARACTERIZATION OF THE DBD DEVICE IN NITROGEN ATMOSPHERE
Figure 6.7: Left scale: Vibrational temperature as a function of pressure. Right
scale: 391nm/357nm line intensity ratio as a function of pressure.
sure. This means that for the same power level the system at lower pressure
transfers more efficiently energy to the plasma. This is also confirmed by the
behaviour of the absolute intensity of the discharge (Figure 6.6(b)) which
is higher for lower pressure. In Figure 6.7 are shown the behavior of the
optical emission spectra by measuring vibrational temperature and mean
electron energy variations. The grow of the mean electron energy with the
decrease of pressure has been observed elsewhere [108] and it is also evidence
that the discharge regime moves toward a diffuse one [114, 99]. The similar
behaviour of the vibrational temperature is consistent with the general relation between these two quantities [107]. What is more surprising is that
Tv shows a minimum at 300 mbar and increases for increasing pressures
while the 391nm/357nm ratio seems to reach a plateau with only a slightly
increase with pressure. The behaviour of the latter quantity has already
been observed elsewhere [108] for air and has been connected to the transition to filamentary discharge. The observed increase of Tv above 300 mbar
has no simple explanation. Assuming that electron temperature does not
change too much (as indicated by the 391nm/357nm line intensity ratio),
some change in the kinetic equilibrium favours the excitation of vibrational
states, moreover, a role of the quenching processes of the vibrational state
should be considered. However, a deeper study of the phenomena also with
6.4 CONCLUDING REMARKS
89
Figure 6.8: Left scale: Vibrational temperature as a function of nitrogen flux.
Right scale: 391nm/337nm line intensity ratio as a function of nitrogen flux.
other diagnostics should be performed in order to give a complete explanation.
Finally, it has been observed also the behaviour of the discharge when the
inlet fluxes are changed. In Figure 6.8 are shown the behavior of the optical
emission spectra by measuring vibrational temperature and mean electron
energy variations. It is evident that the adjoint of gas convection does
not influence the discharge in a measurable way. Also electrical discharge
behavior remains unchanged.
6.4 Concluding remarks
The capabilities of the developed DBD device have been verified by exploration of parameter space. A nitrogen atmosphere has been chosen because
of its capability to transfer energy to other species and generate reactive
environments without influencing too much the chemistry of the processes
and being often the best basis for the study and development of plasma
processes for applications. It has been observed that the system shows the
presence of two discharge regimes as a function of the applied voltage as
already observed for the system described in Chapter 5. For higher voltages, because of the limitation in the number of simultaneous streamers, the
90
CHARACTERIZATION OF THE DBD DEVICE IN NITROGEN ATMOSPHERE
rate at which energy is transferred by the system to the plasma discharge
gets slower with increasing voltages. It has been observed also that a possible change in the quenching mechanism of the vibrational state generates
a slightly decrease in the vibrational temperature with increasing injected
power. An interesting behaviour of electrical and optical measurements has
been observed when the pressure is varied even if a complete transition to a
diffuse discharge regime cannot be reached.
CHAPTER
7
Deposition process of organosilicon
thin films
7.1 Introduction
In this chapter a deposition process of thin organosilicon films at atmospheric
pressure is investigated as a method to obtain and control hydrophobicity
of materials surface.
Recently, plasma deposition at atmospheric pressure has become a promising alternative to low pressure plasma enhanced chemical vapour deposition
(PECVD) [25, 24, 115, 116, 117, 23, 118]. The main advantages are the
possibility to avoid the expensive vacuum systems, to decrease the time of
treatment, and to simplify the technological transfer where the processes of
production are making in continuous mode.
The use of organosilicon compounds as precursors for deposition processes of thin films of silicon compound has been studied for several pur91
92
DEPOSITION PROCESS OF ORGANOSILICON THIN FILMS
poses like vapour and gas barrier creation [119, 120, 121], wear and friction
reduction [122, 123], anti-corrosion protection [124, 125], biocompatibility
[126, 127], hydrophobicity of surfaces [128].
Modification of hydrophobicity properties of surfaces with plasma treatment can be obtained with fluorination processes, coating processes with
fluorocarbon of hydrocarbon films. However, these processes may become
unstable and show aging by oxidation [129] or other more complex aging
processes depending on substrate (see for example Chapter 9). Low surface
energies, which mean also hydrophobicity, can be attained with high retention in the coating of methyl groups (CH3 ) which have a non-polar character
and tend to repel highly polar water molecules. Starting from an organosilicon precursor like hexamethyl-disiloxane (HMDSO, see Figure 7.1) it it
possible to obtain a highly organic deposit taking advantage of the elevated
intrinsic stability due to their partly cross-linked Si-O chains (backbone)
which results in an enhanced long-term durability (See Chapter 9).
Here are presented results regarding the deposition process of thin organosilicon films generated at atmospheric pressure in nitrogen with small
admixtures of HMDSO vapours. The plasma source is the DBD described
in Section 3.1 which works in a roller configuration and is able to simulate
continuos treatments of material surfaces and operates in controlled atmosphere.
7.2 Materials and methodology
The liquid HMDSO is introduced as a vapour in small quantities using the
evaporator system described in Section 3.1.2. The carrier gas which dilute
the vapor is nitrogen and concentrations of HMDSO are varied up to 1.2%.
According to data from [130], at a pressure of 1 bar and at room temperature
(25 ◦ C), the maximum concentration of HMDSO before condensation occurs
is 5.5%.
The vacuum chamber is initially evacuated with the rotary pump P1
(Figure 3.1) down to 5 · 10−3 mbar to avoid contaminations, then calibrated
fluxes from the injection system fill the chamber up to a working pressure of 900 mbar. Although the chamber is provided with gaskets both
for under- or over-pressure, a slightly lower pressure ensure a better insu-
93
7.2 MATERIALS AND METHODOLOGY
(a) Chemical representation.
(b) Graphic representation.
Figure 7.1: Representation of the hexamethyl-disiloxane (HMDSO). Formula:
C6 H18 OSi2 . Molecular weight: 168.38 amu. Boiling point: 373 ± 2 K.
lation from possible contaminations. Experiments have been performed to
verify that the hundred millibar difference does not affects the deposition
process. After the working pressure is reached the dry pump P2 is used to
balance the inlet fluxes and keep the pressure stable. The carrier nitrogen
gas is maintained at 2 ln /min, while the liquid flux is regulated as needed
to obtain the desired concentration of HMDSO. In the present study the
experiments are implemented at constant power of 170 W injected in the
system. The specimens undergoing the treatment are exposed to the plasma
at the tangent speed of 1 m/min for 15 times. For this kind of geometric
configuration is convenient to use the so called corona dose which is defined
Power
as D =
which has the dimensions of energy
electrode width×tangent speed
on surface. Thus the experiments are performed with D = 728.6 kJ/m2 .
The rather high energy dose has been chosen to reduce errors on experimental measurements (weighting, FTIR, thickness). To estimate the residence
time it can be considered the diameter of the rod electrodes (12 mm) as the
discharge length1 as a rough estimate of its dimension. Thus, for these experiments, the total residence time is around 21 seconds which is extremely
low with respect to low pressure plasma processes to obtain the same results.
The plasma discharge is characterized by current-voltage measurements
and the acquisition of optical emission spectra (see Sections 3.1, 4.2 and
1
This is actually an over estimate because the typical length of the discharge is less.
Thus, the actual value cannot be measured independently on power and other parameters
in such DBD devices
94
DEPOSITION PROCESS OF ORGANOSILICON THIN FILMS
4.1).
The deposits are characterized with several methods. KBr salt pellets
of 8 mm diameter are prepared by compression starting from the powder
(Fluka) and exposed to the plasma. The transmission infrared spectra of
the pellets are measured with the FTIR spectrometer (see Section 4.3.1) before and after the treatment and their difference is considered. Small sheets
(10x4 cm2 ) of low density polyethylene (PE) 0.2 mm thick, is washed in acetone and attached to the grounded rotating electrode. It is used to evaluate
the morphology of the deposits using the atomic force microscope (AFM)
(see Section 4.3.2). Mass deposition rates were evaluated by weighting larger
sheets with an analytical balance before and after the exposition to the
plasma with an appropriate mask (15x15 cm2 ). Small pieces (∼10x5 mm2 )
cut from (100) silicon wafer where also exposed to the treatment. Where
needed, the specimens were attached to the grounded electrode with tape.
7.3 Characterization of the deposition process
As already mentioned in Section 7.1 HMDSO possesses some features that
make it extremely effective for the realization of hydrophobic coatings. The
main polymerization process is through the creation of Si-O bonds with
the creation of a highly cross-linked inorganic backbone. This kind of reactions are chemically favorable once in the plasma are generated radicals
by fragmentation of the original compound. For this reason organosilicon
compounds can produce better deposition rate than simply organic precursors. From Figure 7.1 it is evident that the monomer is initially highly
organic. The retention of initial methyl groups is controlled by discharge
conditions. Higher levels of power injected into the system usually promote
a higher grade of fragmentation which induce a loss in organic character
of the deposit. When a completely inorganic coating is needed (silica-like),
for example for the realization of barrier effects, usually oxygen is added
to the gas mixture in order to promote the oxidation of the organic compounds. Under specific conditions a nearly complete elimination of organic
character can be achieved [122, 119, 120]. In the present experiments, on
the opposite, it is searched the highest retention of methyl group in order to
obtain the best achievable hydrophobicity. In the following, the best degree
7.3 CHARACTERIZATION OF THE DEPOSITION PROCESS
95
Figure 7.2: Current-voltage characteristic of a discharge in nitrogen with 0.45%
of HMDSO vapour.
of organic retention is searched by analyzing both the plasma and deposit
characteristics.
7.3.1 Plasma characterization
Important informations can be obtained by the analysis of the current voltage characteristic. The typical current and voltage waveform of the discharge
process is plotted in Figure (7.2). From the presence of fast current pulses
it is possible to recognize the typical behaviour of the streamer regime already described in the preceding Chapters. In comparison with nitrogen
atmosphere (Figure 6.1(a)), it is evident the streamer have a shorter duration. Moreover, a visual observation reveals thinner plasma channels.
Possibly this effect can be similar to that observed in air (see Chapter 6)
where the presence of oxygen leads to plasma channel reduction [98, 4, 38].
The uniformity of the treatment is then guaranteed by the mean effect due
to the large difference between the time scales of the treatment (seconds),
of the discharge process (microseconds) and of the typical duration of the
streamers (nanoseconds). It is well proved that uniformity is achieved down
to the microscopic scale as it is evident from the roughness analysis (see
Figure 7.5). Thus, the presence of a streamer regime dose not undermine
uniformity issues usually fundamental in plasma applications.
Useful information on the plasma phase can be achieved from the analysis
of emission spectra. In Figure (7.3) is depicted the typical emission spectra of
96
DEPOSITION PROCESS OF ORGANOSILICON THIN FILMS
the discharge. The spectrum is shown between 300 nm and 500 nm because
outside this region the emission lines are absent or too weak . The spectrum
is dominated by the second positive system (SPS) of N2 (C 3 Πu →B 3 Πg )
[100, 101].
From the SPS structure it is possible to determine the populations of
the vibrational levels of N2 molecules and calculate the vibrational temperature Tv [102, 103, 104, 105, 106] which is an interesting plasma parameter
because processes such as vibrational relaxation and excitation can strongly
influence plasma chemistry [2]. This is because the vibrational levels are
mostly excited by direct electron impact and vibrational-translational relaxation processes are not efficient in converting vibrational energy into kinetic energy (heating of the gas). Thus, energy remains ”trapped” in the
vibrational levels which give to molecules a reservoir of energy to activate
several chemical reactions (see also Section 2.4.1). The determined values
are 2000± 100 K for all concentrations of HMDSO explored. These temperatures are lower than temperature achieved in pure nitrogen atmosphere
(∼2700±40 K). This finding suggests that, being Tv a monotone function
of the electron temperature [131, 107], also the latter is lower in these discharges.
In Figure (7.3) an emission line from the first negative system (FNS) of
N+
2
2 +
(B 2 Σ+
u →X Σg ∆ν = 0) is also visible at 391.3 nm. This emission line
is usually connected to the electron energy [108, 109], but here is not easily
observed because of the presence of the CN bands.
The most interesting feature of the spectra is the presence of the CN
2 +
violet system at 388 nm and 422 nm (B 2 Σ+
u →X Σg ) which is a consequence
of the chemistry of N2 +HMDSO vapour mixtures in the plasma state. A
complete evaluation of the concentration of active chemical species, ions or
radicals from OES diagnostics requires a detailed modeling of the excitation
and quenching processes for each light emitting energy level observed in the
spectra. However relative information could be inferred by normalizing the
emission intensities of different emitting molecules. The CN line intensity
at 387.1 nm has been normalized to that of one of the brightest band of the
SPS of nitrogen at 357 nm. In this way the dependency of absolute intensity
from high energy electrons density in the discharge region is factored out.
Moreover, assuming that electron temperature does not change too much
7.3 CHARACTERIZATION OF THE DEPOSITION PROCESS
97
Figure 7.3: Emission spectrum from the plasma of a mixture of nitrogen and
0.15% HMDSO vapour.Inset: CN(387.1)/N2 (357.7) line intensity ratio as a function
of HMDSO concentration.
and, since dissociation level is usually very low, that the absolute density
of nitrogen is constant, the the intensity ratio should be proportional to
the relative concentration of CN during the discharge[132]). We can see
in the inset of Figure 7.3 that this quantity shows a stepwise behaviour
evidencing a threshold value of concentration around 0.3% after which the
CN line intensity abruptly decreases. The presence of cyano radical CN is
connected to the fragmentation of initial HMDSO monomer in plasma. The
formation of CN requires carbon atoms which can come only by the monomer
and are created by consecutive fragmentation of the organic components of
HMDSO. The vanishing of the CN emission band possibly means others
reaction channels are preferred at higher concentrations of HMDSO and the
monomer retains more of the initial organic character. From the analysis of
the emission spectra it is possible to suggest the presence of two different
discharge regimes in which the chemistry of the plasma changes in some way.
This behaviour will be observed in other quantities further in the following.
7.3.2 Thin film characterization
Although the plasma phase analysis can give useful information on the development of the deposition process, it cannot give too much hints on the
98
DEPOSITION PROCESS OF ORGANOSILICON THIN FILMS
plasma-surface interactions and surface reactions complexity. Usually at atmospheric pressure the role of energetic ions less important and the typical
reaction scheme is more like the one described in Section 2.4.2, where the
radicals, compounds and ions created in the plasma phase are adsorbed to
the surface where chemical reactions take place.
The change in the morphology of the depositions have been measured
on PE substrates with an atomic force microscope as a function of HMDSO
concentration. In Figure (7.4) are compared images of the deposits at different concentration of HMDSO in comparison with the untreated PE surface
(a). At a concentration of 0.05% (b) we can see that the deposition process is generate a ”dust” like film with evident nanoscale structures. At a
concentration of 0.15% (c) and 0.3% (d) it is possible to see that still some
structure is present which is embedded in a structureless deposit. At higher
concentrations (e and f) the formation of nanoscale structures is no longer
visible and the deposition is extremely smooth. The change in the morphology is evident also in the roughness of the surface which is evaluated
from theqroot mean square (RMS) of the heights of the surfaces defined as
2
RMS =
h2 − h , which is the standard deviation of probability distri-
bution function of heights.
The change in the morphology can be explained with the presence of
two mechanism of deposition. When the fragmentation of monomer is high,
a plasma-phase polymerization with subsequent adsorption and reaction on
the surface is predominant with the result of a higher grade of cross-linking
and the creation of nanoscale structures. These structures are often observed
in other experiments both at atmospheric pressure [24] and low pressure (in
this case usually with smaller characteristic dimensions [133]). On the other
end, when the concentration of monomer is high a different deposition process takes place. As mentioned before, the residence time of specimens in the
discharge region is of 21 seconds while the treatment times lasts for around
10 minutes. Thus, the specimens are exposed, for most of the time, to the
neutral atmosphere containing the monomer vapour. Possibly, if the concentration is high enough, the monomer is absorbed on the substrate surface
and reacts with the radicals created before by the plasma or is activated
when exposed to the plasma in a sort of mechanism of adsorption/reaction
polymerization. This creates a smoother, softer, structureless deposit which
7.3 CHARACTERIZATION OF THE DEPOSITION PROCESS
99
Figure 7.4: AFM images of thin films deposited on PE substrate at different
concentrations of HMDSO. a=untreated, b=0.05%, c=0.15%, d=0.3%, e=0.45%,
f=1.2%.
100
DEPOSITION PROCESS OF ORGANOSILICON THIN FILMS
50
Untreated PE
RMS [nm]
40
30
20
10
0
0
0.2
0.4
0.6
0.8
HMDSO [%]
1
1.2
Figure 7.5: Roughness of treated PE substrates estimate with RMS as a function
of HMDSO concentration.
embeds the morphological structure of the plasma phase polymerization.
This second kind of deposit is usually really soft and flexible.
Large differences can be found also in the chemical composition of the
deposited thin film which have been characterized measuring the infrared
absorption spectra. Figure (7.6) shows the spectra of the deposits at different HMDSO concentrations. The spectra show the typical bands already
recognized in the literature [134, 135, 124] and indicated in the Figure (7.6).
According to the literature the stronger absorption band in the range 10001150 cm−1 can be assigned to the Si-O-Si asymmetric stretching mode.
Other typical absorption band can be assigned: the CH3 symmetric bending
in Si-CH3 at 1260 cm−1 , the CHx symmetric and asymmetric stretching at
2900-2960 cm−1 , the CH3 rocking in Si-(CH3 )2 at 800 cm−1 and the Si-CH3
rocking vibration in Si-(CH3 )3 at 840 cm−1 . Bands at 800 cm−1 , 840 cm−1 ,
1260 cm−1 and 2900-2960 cm−1 indicate retention of methyl group in the
plasma deposit, which brings the condition for the creation of hydrophobic
surfaces with HMDSO plasma. It is interesting to observe the two peaks at
800 cm−1 and 840 cm−1 . It can be seen a rapid growth of the Si-(CH3 )3 peak
against the Si-(CH3 )2 as the concentration of HMDSO grows. Si-(CH3 )3
groups are termination sites in the network structure of the deposited films.
The abundance of such groups at higher monomer concentration indicates
the films are composed of shorter chains having a less cross-linked structure.
Observing the spectra it is evident an increase of all the bands relative to
7.3 CHARACTERIZATION OF THE DEPOSITION PROCESS
101
Figure 7.6: FTIR spectra o the thin films at different HMDSO concentrations.
a=0.05%, b=0.15%, c=0.3%, d=0.45%, e=1.2%. The spectra are normalized on
the Si-O-Si peak intensity.
organic compounds which are responsible for the hydrophobic character of
the resulting surface.
In order to evaluate in a more quantitative way the increase of the organic
character of the deposits, the ratios of the areas of the peaks of interest in
the spectrum have been analyzed. To this end, a deconvolution process of
the spectra has been performed using Lorentzian function as basis [136]. In
Figure 7.7 is represented the resulting fit for a 0.05% HMDSO deposit. Not
all the peaks are considered: the areas of interest are marked with pattern
fill and named out. A quantitative analysis from an infrared spectra can only
be performed by calculating ratios of the areas internal to a single measure
[136].
To this end two ratios have been evaluated as a function of HMDSO
102
DEPOSITION PROCESS OF ORGANOSILICON THIN FILMS
Figure 7.7: peaks areas recognition with Lorentzian functions from the FTIR
spectra of 0.05% HMDSO concentration. The areas of interest are marked with
pattern fills and named.
concentration. The first is the ratio between the areas at 1260 cm−1 relative to Si-CH3 symmetric bending and the area of the band at 10001150 cm−1 relative to Si-O-Si asymmetric stretching mode [137]. This ratio gives some information on the organic character of the deposits and
in particular on the methyl group retention. The second ratio considered
is the CH3 rocking in Si-(CH3 )2 at 800 cm−1 and the Si-CH3 rocking in
Si-(CH3 )3 at 840 cm−1 . The presence of three methyl groups attached to
silicon means a termination of the polymeric chain or cross-linked structure.
Two methyl groups attached to silicon are related to compounds of the type
Me − (Me2 SiO)n − SiMe3 (or ramifications of them). Compounds of this
kind have been observed with gas chromatography analysis of the exhaust
of process gases [23, 138]. In Figure (7.8) (left scale) it is plotted the value
of the first ratio as a function of the HMDSO concentration. We can observe
that the organic part of the thin film grows rapidly up to a saturation value
of ∼0.24. This means the retention of methyl groups cannot grow beyond a
certain value which possibly represents the limit of stability of the deposit.
This is confirmed by the saturation behaviour of the second ratio which
represents somehow the degree of polymerization and, thus, the stability of
the deposit. This chemical analysis confirms the observation of a softer and
7.3 CHARACTERIZATION OF THE DEPOSITION PROCESS
103
Figure 7.8: Ratio of the area at 1260 cm−1 relative to Si-CH3 and the area at
1000-1150 cm−1 relative to Si-O-Si asymmetric stretching mode(left scale). CH3
rocking in Si-(CH3 )2 and the Si-CH3 rocking in Si-(CH3 )3 at 840 cm−1 (right scale).
Figure 7.9: Mass deposition rate as a function of the HMDSO concentration.
smoother deposit at higher concentrations.
The same behaviour is visible in the mass deposition rate plotted in
Figure 7.9. The reaching of a saturation value of ∼0.8 µg/mm2 suggests
the growth of the polymer is not simply limited by the quantity of monomer
present in the gas phase but it depends on the complex chemistry both in the
plasma phase and on the surface. Indeed, this saturation is reached with
104
DEPOSITION PROCESS OF ORGANOSILICON THIN FILMS
Figure 7.10: Advancing and receding contact angle with water as a function of
HMDSO concentration. The error bars indicate the statistical errors on 5 independent measures. Horizontal dashed lines are, in the same colour the advancing and
receding contact angle of untreated PE surface.
the increase in monomer concentration while the power is kept constant,
and could be interpreted as that the deposition process reaches a power
deficient regime in which not enough fragmentation is achieved in plasma
phase [139, 140]. However the deposition process here seems much more
complicated by the presence of two phases (plasma and neutral atmosphere)
alternate during the treatment because of the electrode configuration.
The retention of methyl groups can be evaluated by a macroscopic measurement of water contact angles. Using the technique described in Section
4.3.3 it is possible to gather information on the microscopic chemical heterogeneity of the deposit and evaluate the achieve degree of hydrophobicity
of the surfaces. In Figure 7.10 are showed the advancing and receding water
contact angle measured on treated PE surfaces. Advancing angle, which is
connected to the presence of non-polar groups on the surface have a saturation behaviour similar to other quantities observed before, indicating that
a limiting retention degree of the initial methyl groups has been reached.
The observed receding angles lay always below the value of untreated PE.
This behaviour can be connected, on one side to the presence of inorganic
SiOx compounds in the deposit which are characterized by a high wetta-
7.4 CONCLUDING REMARKS
105
bility, on the other side can be connected to the presence on the surface of
polar compounds affine to water [141] which are due to a plasma activation
of the surface and the subsequent reaction with atmospheric oxygen and
water [16] (which brings to the creation of polar compounds like hydroxyl,
carboxyl or carbonyl groups). This surface effect suggests that together with
the deposition process an activation process due to nitrogen is also present.
7.4 Concluding remarks
The deposition process of organosilicon thin films with plasma of nitrogen
with small admixtures of HMDSO vapour has been characterized. Analyzing
the behaviour of several quantities as a function of the HMDSO concentration we have found the deposition mode changes with increasing concentration. For lower values the deposition strongly depends on the concentration
itself, while after some threshold value it remains most independent. This
behaviour has been observed in several quantities relative both to the plasma
phase and to the resulting deposits. The retention of organic compounds in
the deposits have been studied at a microscopic and macroscopic level. It
has been found that the retention of initial monomer methyl groups saturate
with concentration and so does the hydrophobic character of the resulting
surface. Stability issues of the resulting deposit will be discussed in Chapter
9 dealing with the application of this process to cellulosic materials (paper).
106
DEPOSITION PROCESS OF ORGANOSILICON THIN FILMS
CHAPTER
8
Fluorination of polymer surfaces
8.1 Introduction
In this chapter a grafting process of fluorine atoms on polyethylene (PE)
surface is investigated at atmospheric pressure as a method to obtain and
control hydrophobicity and oil-repellency of materials surface.
Plasma induced modifications of materials surface with fluorination (grafting of fluorine atoms) processes and deposition of fluorocarbon thin films
have been studied for several applications because of the unique properties
that can be attained. For example, fluorination of the polymer surfaces produces hydrophobic surfaces, at the same time, preserving the bulk properties
of the materials [142, 143, 144, 145]. Fluorocarbon thin films have been studied for the creation of barrier layer against solvent and fuel permeation [146],
adhesion of carbon nano-tubes in composite material [147], bio-compatible
materials production [148]. Most of the processes are performed at low
pressure even if few reports of atmospheric pressure processes are present
107
108
FLUORINATION OF POLYMER SURFACES
[149, 26, 7]. However, at atmospheric pressure, the process is rather far from
being well understood both when dealing with the plasma-phase properties
and the induced surface modifications. Moreover, experiments on fluorination at atmospheric pressure have never been performed in continuous-mode
plasma reactors such as the one used here (see Section 3.1).
Plasma processes based on Sulfur hexafluoride (SF6 ) are an effective
source of fluorine radicals and fluorination of materials surface can be successfully realized [26, 142, 144, 143]. Fluorination process is a grafting process (see Section 2.4.2) which involves the substitution of an hydrogen atom,
bound to carbon, with a fluorine. The basic reaction scheme is the following:
hν,e −
F•
−CH −−−−→ −C• −−→ −CF,
Ion,F•
(8.1)
where the surface can be activated by ions, electrons, photons and other active species generated in the plasma (see Section 2.4.2) and, then, a fluorine
radical adsorbed to the surface reacts with a carbon radical forming a stable chemical bond. SF6 is a highly electronegative gas which posses a high
dielectric strength (i.e. the minimum electric field strength for breakdown)
and is usually used as electrical insulator in high-voltage circuit breaker
[150]. For this reason, in the present experiment SF6 is only added in small
quantities to argon to obtain stable discharges. Thus, materials surface is
exposed to both the effect of argon carrier and SF6 . It must be recalled
that when polymer surfaces are brought into contact with chemically inert
plasmas (like argon), activation (hydrogen abstraction) and etching of low
molecular weight molecules are the main processes occurring at the polymerplasma interface, leading to the formation of radical species on the treated
surface. In this case a chemical modification of the treated surface is a consequence to the exposure of the treated specimens to the atmosphere [16],
mainly because reacts with oxygen and water forming peroxide and hydroperoxide radicals which then form hydroxyl, carboxyl or carbonyl groups
[151]. When polymer surfaces are brought into contact with reactive plasmas (like Ar/SF6 mixtures), grafting of chemical species, simultaneous to
surface activation or etching occur leading to surface functionalization. This
means that the balance of these two competitive processes will determine
the final surface properties. To this end, the key parameter that controls
8.2 EXPERIMENTAL, DIAGNOSTICS AND METHODS
109
this equilibrium is the concentration of SF6 in the gas mixture. In the following, both the plasma-phase characteristic and surface properties will be
studied as a function of SF6 concentration.
8.2 Experimental, diagnostics and methods
The plasma source used for the experiments is the DBD described in Section
3.1 which works in a roller configuration and is able to simulate continuous
treatments of material surfaces and operates in a controlled atmosphere.
Small sheets (10x10 cm2 ) of low density polyethylene (PE) 0.2 mm thick,
are washed in acetone and attached to the grounded rotating electrode with
tape. The inter-electrode gap distance is kept fixed at 2.5 mm. Gas mixture
of Ar and SF6 are created with the mixing unit described in Section 3.1.2.
The vacuum chamber is initially evacuated with the rotary pump P1 (Figure
3.1) down to 5 · 10−3 mbar to avoid contaminations, then calibrated fluxes
from the injection system fill the chamber up to a working pressure of 900
mbar. After the working pressure is reached the dry pump P2 is used to
balance the inlet fluxes and keeps the pressure stable. The total gas flux
is maintained at 10 ln /min. The experiments are implemented at constant
power of 155 W injected in the system. The specimens undergoing the
treatment are exposed to the plasma at the tangent speed of 1 m/min. for
8 times. The experiments are performed with a corona dose (see Section
7.2) D = 354.3 kJ/m2 . An estimate of the residence time (see Section 7.2)
is around 11.2 seconds which is lower with respect to low pressure plasma
processes to obtain the same results. Treatment times have been chosen
rather higher than those needed to obtain a good fluorination effect in order
to allow a more meaningful comparison of the experiments at different SF6
concentrations.
The plasma discharge is characterized by current-voltage measurements
and by the acquisition of optical emission spectra (see Sections 3.1, 4.1 and
4.2). The materials surface properties are characterized at a microscopic
level by measuring the morphology of the treated PE using the atomic force
microscope (AFM) (see Section 4.3.2), and at a macroscopic level by measuring the dynamical contact angle with water (bi-distilled, de-ionized) and
α-bromonaphthalene (from Aldrich) and by calculating the resulting surface
110
FLUORINATION OF POLYMER SURFACES
energy with its polar and dispersive components (see Section 4.3.3).
8.3 Characterization of the fluorine grafting process
The substitution of hydrogen atoms with fluorine can give unique properties
to material surfaces. The CF groups show a strong repulsion of hydrogenbond forming molecules like water and other compounds containing hydroxyl, carboxyl or carbonyl groups. This characteristic gives a high hydrophobic property to the surface along with the resistance to organic polar
compounds (some oils and solvents). Moreover, the polarity of the CF group
also shows repulsion with non-polar compounds like organic molecules and
some other oils. This characteristics are the basis of the high chemical stability, and unique properties of fluorocarbon-based polymers. The attainment
of these properties on polymer surfaces is the aim of the present research. It
is worth to mention that the fluorination process of equation (8.1) involves
only few atomic layers of the substrate. This means that a little quantity of
fluorine on the surface is needed to obtain the desired properties. For this
reason the fluorination process is somehow preferred to deposition processes
of fluorocarbon thin films which require longer times and greater quantities
of reactive fluorocarbon gases.
8.3.1 Plasma-phase characterization
Important information on plasma discharges can be obtained by the analysis of current-voltage (I-V) characteristic. Figure 8.1 shows the current
and voltage waveforms for discharges in Ar/SF6 mixtures at various concentrations in comparison with pure Ar discharge1 . In Figure 8.1(a) the pure
argon discharge I-V characteristic is shown. By observing the discharge
current Idisch , calculated according to equation (5.2), it seems that a diffuse discharge mode is achieved [152]. However, a visual observation reveals
the presence of really bright diffuse zones in proximity of the dielectrics connected by wide plasma channels. Anyway, the discharge is rather uniform on
the electrode surfaces as also confirmed by the uniformity of surface proper1
The pure argon discharge has a slightly lower power level because the low breakdown
voltage may cause the discharge to happen outside the discharge gap. This is a limitation
of the present experimental setup.
8.3 CHARACTERIZATION OF THE FLUORINE GRAFTING PROCESS
(a) Discharge in pure argon, 88W
111
(b) Discharge in argon and 0.5% SF6 , 155W
(c) Discharge in argon and 3% SF6 , 155W (d) Discharge in argon and 13.4% SF6 , 155W
Figure 8.1: Current-voltage characteristic for discharges at various concentrations
of SF6 in comparison with a pure argon discharge. Lower panel: applied voltage
and total current. Upper panel: applied voltage and discharge current according to
equation (5.2).
ties of the treated PE. Even small admixtures of SF6 (Figure 8.1(b)) changes
drastically the discharge which goes into a fully developed streamer regime
composed by numerous and thin plasma channels. This is in agreement
with the increase of the derivative of reduced effective ionization coefficient
with respect to reduced field in Ar/ SF6 mixtures [153] which brings to the
reduction of streamer radius (see Section 2.3.2). In comparison with air and
nitrogen atmosphere (Figure 6.1), it is evident the streamer have a shorter
duration.
It is interesting to observe the behaviour of root mean square (rms) quanrms increase in the same way because the I rms
tities (Figure 8.2). V rms and Itot
tot
is dominated by the displacement component. However, by observing the
rms
and
two components of current and considering their rms values Idisplacement
rms
, it is evident a decrease of the latter in comparison to the former.
Idischarge
112
FLUORINATION OF POLYMER SURFACES
(a) RMS Voltage (left scale) and total cur- (b) RMS displacement current (left scale)
rent (right scale) as a function of SF6 con- and discharge current (right scale) as a funccentration.
tion of SF6 concentration.
Figure 8.2: Behaviour of rms quantities as a function of SF6 concentration at
constant injected power.
This means that for the same power level, at higher concentrations of SF6 a
lesser part of the current is effectively due to the plasma discharge. Possibly,
the SF6 tends to shorten the lifetime of the plasma channel (streamer) reducing the current flow and so the total charge transferred during the discharge
rms
varying from 230 nC for
process which has the same behaviour as Idischarge
the pure argon discharge to 56 nC for the Ar/SF6 13.4% discharge.
Concomitant with the decrease of the discharge current is the decrease
of the brightness of the discharge as it can be observed by the absolute intensity recorded with the spectrometer. In Figure 8.3 the emission spectra
of a discharge in argon with 1% of SF6 between 600 nm and 860 nm because
outside this region the emission lines are absent or too weak. The spectrum is dominated by emission lines of the argon and no contributions of
the brightest emission lines of the fluorine are visible. In the inset of Figure
8.3 are compared the spectra for various SF6 concentrations normalized on
the 772.5 nm line of the argon. A drastic change in the distribution of the
intensities between all the argon emission lines is evident (as it is also in
other parts of the spectra which are not shown). It is known the relative
intensities of the lines of the argon emission can be connected through a
collisional-radiative model to the electron temperature in the plasma discharge [154, 155, 156] even if a complete validation of these models for the
streamer regime has to be found. It is possible to suggest that the quenching of electrons in the presence of SF6 leads to a decrease of the electron
8.3 CHARACTERIZATION OF THE FLUORINE GRAFTING PROCESS
113
Figure 8.3: Emission spectrum of a discharge in argon with 1% of SF6 . Emission
is dominated by argon lines and no emission line of fluorine are present. Inset:
Comparison between spectra for various SF6 concentrations normalized on the 772.4
nm emission line of the Argon.
temperature with increase of SF6 concentration. A lower electron temperature means less effectiveness of the plasma creating radicals both in plasma
phase and on the surface. This is possibly a key effect for the less effectiveness of the process at higher SF6 concentrations, as it will be shown in the
following.
8.3.2 Material surface characterization
A complete characterization of the materials surface treated with a fluorination process is more difficult than the characterization of the deposition
process described in Chapter 7. This is because, even if other modifications
can be introduced by concurrent etching process due to argon activity, a
grafting process involves only few atomic layers on the surface and the presence of fluorine cannot be easily detected. Here the characterization has
been performed at a microscopic level with the AFM to measure eventual
modification of the morphology of PE surfaces, while the fluorination has
114
FLUORINATION OF POLYMER SURFACES
a
b
c
d
Figure 8.4: AFM images of PE surfaces exposed to plasma treatment for various
concentrations of SF6 in argon and compared with the untreated PE. (a): untreated
PE. (b): pure argon plasma. (c):argon with 1% SF6 . (d): argon with 6.7% SF6
been evaluated by indirect measurements of the macroscopic properties of
the surface with the techniques described in Section 4.3.3.
In Figure 8.4 are compared images of the deposits at different concentration of SF6 in comparison with the untreated PE surface (a). It can be
observed in Figure 8.4 that the exposure to pure argon treatments changes
the surface morphology to some extent. However, the measurements of
roughness does not show significant changes. The root mean square (RMS)
of the heights of the surfaces has been measured for different image sizes
down to 1x1 µm2 but appreciable changes has not been found. Possibly, the
morphological intrinsic roughness of the PE covers other eventual effects.
Moreover, the ionic activity is low and the effect of the argon treatment
8.3 CHARACTERIZATION OF THE FLUORINE GRAFTING PROCESS
mN m
Water
α-bromonaphthalene
Surface Energy
72.1
44.0
Polar
52.2
0.0
115
Dispersive
19.9
44.0
Table 8.1: Surface tension with polar and dispersive components according to
Ref. [157].
is at most an activation of the surface that reacts with oxygen and water
when exposed to the ambient atmosphere. Another interesting observation
is the presence on the surfaces treated with Ar/SF6 plasma, of spot-like
structures. These structures tend to reduce in dimension and vanish with
increasing SF6 concentration.
The presence of fluorine bounded to carbon on the surface has been evaluated indirectly by measuring the macroscopic effect that the presence of
fluorocarbons or polar groups generates. These effect are the repellency or
affinity of the surface to water and non polar liquid compounds. By measuring the advancing and receding contact angles (see Section 4.3.3) with
liquids of different polar and dispersive character it is possible to evaluate
the presence of fluorine or other polar compounds bound to the surface. In
fact, Figure 8.4 and the measure of RMS guarantees that roughness does
not change significantly between the different experiments and the change
in advancing and receding angles is then only connected to the chemical
heterogeneity of the surface. The two chosen liquids are water for its high
polar components and α-bromonaphthalene which is completely lacking of
polar groups and posses only a dispersive component. The use of pure liquids also avoids complications with adsorption kinetics which can influence
the dynamic of wetting and de-wetting phenomena [59]. In Table 8.1 are
reassumed the surface tensions of the used liquids according to Ref. [157].
The measurement of advancing and receding contact angles can give informations on the presence of affinity or repellency to a liquid deposited
on a surface [59, 158, 159, 160]. It is possible to interpret the wetting or
de-wetting process as an irreversible process in which some surface energy
is dissipated as heat to the environment [161], i.e. some potential energy
between atoms or molecules is dissipated as vibration (heat) as bonds are
formed or snapped in the process. With this assumption a phenomeno-
116
FLUORINATION OF POLYMER SURFACES
Figure 8.5: Advancing and receding contact angle of PE surfaces with water as a
function of SF6 concentration in argon. Dotted lines represent in the same colours
the advancing and receding angles of untreated PE.
logical interpretation of the advancing and receding contact angles can be
stressed. When a contact line is de-pinned from the surface (receding), stable bonds between liquid and solid must be broken, so receding angle (θr )
can be connected to the affinity of components between the solid and liquid
phases (for example, a low receding angle with water means a high presence
of polar groups on the surface forming hydrogen-bonds with water). On the
contrary, when the contact line tries to advance, it remains pinned to the
surface (advancing) because the liquid must overcome the energy barriers
due to repulsion (for example, a high advancing angle θa with water means
an high presence of non-polar groups on the surface which repel the highly
polar water molecules). A similar kinetic interpretation is given for water on
hydro-repellent surfaces in [141], here the concept is extended to a general
interaction scheme between liquid and solid phase.
Figure 8.5 shows the measured advancing and receding water contact
angle of treated and untreated PE surfaces as a function of SF6 concentration. It can be observed that a pure argon treatment lowers both θa and θr
indicating that an activation process brings, after exposure to oxygen and
water of ambient air, to the grafting of polar groups on the originally nonpolar surface. For higher concentrations the effect of fluorination is evident
8.3 CHARACTERIZATION OF THE FLUORINE GRAFTING PROCESS
117
Figure 8.6: Advancing and receding contact angle of PE surfaces with α-bromonaphthalene as a function of SF6 concentration in argon. Dotted lines represent in
the same colours the advancing and receding angles of untreated PE.
the increase of θa which, as explained above, is sensible to repulsive interactions (in this case between water and fluorocarbon groups). It is interesting
to observe that for SF6 concentrations below 1% θr remains under the untreated PE value indicating that probably the activation process is more
effective than the fluorination, thus, the surface still undergoes grafting of
polar groups when exposed to ambient air. Another interesting aspect is the
decrease of θr for high SF6 concentrations. Possibly, this can be explained
with the reduced activity of the discharge due to the SF6 (see Section 8.3.1).
In Figure 8.6 are shown the measured advancing and receding α-bromonaphthalene contact angle of treated and untreated PE surfaces as a function
of SF6 concentration. As for water contact angles, a pure argon treatment
slightly lowers the repelling properties of the surface. The presence of fluorocarbon groups on the surface can be seen even at low concentrations of SF6
as both the advancing and receding contact angles are abruptly increased.
This is because both untreated PE and α-bromonaphthalene have only a
non-polar character and this affinity cause the liquid to wet very well the
surface while the presence of fluorocarbon groups introduce immediately a
strong repulsive effect. A far more glaring dependence on SF6 concentration
118
FLUORINATION OF POLYMER SURFACES
Figure 8.7: Calculated surface energy of PE surfaces as a function of SF6 concentration in argon. Total, polar and dispersive components are plotted. Dotted lines
represent in the same colours the surface energy and its components of untreated
PE.
of the treatment effectiveness is evident in comparison with water contact
angles (Figure 8.5). In particular, the receding θr angle, which is bound to
the presence of affine (non-polar) groups on the surface, most return to the
value of untreated PE for high concentration. This confirms that an effective fluorination of surface is not reached for concentrations of SF6 too high.
The analysis of advancing and receding contact angles for the chosen liquids
shows clearly that there exists an optimal concentration of SF6 reactive gas
for the maximum effectiveness of fluorination process with respect to the
competitive activation process.
From the contact angle measurements with two different liquids it is
possible to determine the surface energy components of the treated PE substrates (see Section 4.3.3). The determination of surface energy is based on
the measure of the equilibrium contact angle of the liquid with the surface.
However, an equilibrium contact angle is even hard to define and a generally
accepted definition still lacks [162, 163, 164]. It must be stressed that the
system can be prepared with an apparent stable contact angle θs with the
restriction θr < θs < θa . Usually the advancing contact angle is chosen for
the determination of the surface energy [162] as it is done here. In Figure
8.4 CONCLUDING REMARKS
119
8.7 are shown the calculated surface energies of PE surfaces as a function of
SF6 concentration in comparison with the untreated PE. Total, polar and
dispersive components are plotted. It can be seen that the surface energy
remains higher than untreated PE for concentrations below 0.5% SF6 most
because a high polar character is created by the activation process. At concentrations of 1% SF6 the polar component most vanishes indicating a high
hydro-repellency has been reached. After experiencing a minimum value
around 3% SF6 the surface energy increases indicating that the reduction of
the dispersive component is less effective at high concentrations.
8.4 Concluding remarks
The fluorination process of polymer surfaces with mixtures of Ar/SF6 at
atmospheric pressure has been studied. The presence of SF6 strongly modifies the discharge properties converting the diffuse discharge regime of pure
argon to a streamer regime. The increase of SF6 concentration also reduces
the effective current flowing through the plasma possibly quenching the electrons in th discharge. By analyzing the surface morphology it is found that
modifications are introduced by the treatment on the surface on the hundred nanometer scale but these alterations do not affect substantially the
roughness of the substrates. The effectiveness of the fluorination process
has been evaluated through the analysis of the macroscopic surface properties with dynamical and static measurements of contact angles with water
and a non-polar liquid. It is found the effective existence of two competitive processes: an activation process which brings to the grafting of polar
groups from the atmosphere and a fluorination process. The former process
is more important at low concentrations of SF6 and tends to disappear for
higher concentrations. The fluorination process is more effective in a concentration interval also reducing it effectiveness at higher concentrations.
The combination of these two effects determine the presence of an optimal
SF6 concentration where the surface energy is lower. This process is then
interesting for the modification of the properties of organic materials like
polymers, fabrics, paper, leather and others in order to obtain resistance to
water and oils.
120
FLUORINATION OF POLYMER SURFACES
CHAPTER
9
Plasma Application for modification
of paper surfaces
9.1 Introduction
In this chapter some applications of the process studied in the previous
chapters are discussed. Some results obtained with treatments of cellulosic
surfaces (paper) are showed.
9.1.1 Cellulose and paper
Cellulose is a natural polymer of vegetal origin which is found in wood
and plants (for example cotton). Cellulose is a polysaccharide with formula
(C6 H10 O5 )n (Figure 9.1 (a)) and it is usually found in plants as microfibrils
2-20 nm diameter and 100-40000 nm long (Figure 9.1 (c)). Cellulose is a
linear polymer stabilized by intra- and inter-molecular hydrogen bonding
121
122
PLASMA APPLICATION FOR MODIFICATION OF PAPER SURFACES
(Figure 9.1 (b)) which minimizes its flexibility. Cellulose tends to form
crystal structures allowing the more hydrophobic ribbon faces to stack. Each
residue is oriented 180◦ to the next with the chain synthesized two residues
at a time. Although individual strand of cellulose are intrinsically no less
hydrophilic, or no more hydrophobic, this tendency to form crystals utilizing
extensive intra- and inter-molecular hydrogen bonding makes it completely
insoluble in normal aqueous solutions.
Figure 9.1: Cellulose: (C6 H10 O5 )n monomer structure (a), polymer structure and
hydrogen bond linkage (b), SEM image [165] of cellulose fibres (c).
Paper is the most important utilization of cellulose which is typically obtained from wood after removing lignin. The applications of paper products
are numerous and do not include only the production of printing paper, but
also applications in packaging, filtering, biomedical, construction and more.
Paper of pure cellulose is rarely used and, besides the fibres, it may contain
fillers such as chalk or china clay, which improve the characteristics of the
paper for printing or writing. Also coatings may be applied to the paper web
later in the manufacturing process in order to attain, for example, water or
oil and grease resistance.
Plasma based technologies are an interesting alternative for cellulose and
paper modification to the standard chemical treatments. The main advantages of cold plasma technologies are several. Without heating which would
result in damage of the soft cellulosic materials, the typical energies of active
species are comparable with the values of most common bond energies of organic molecules. Consequently gas-phase and surface-phase plasma-induced
9.2 DEPOSITION OF ORGANIC SILICON COMPOUNDS FOR HYDROPHOBICITY
123
reaction mechanisms can conveniently be developed. The plasma-generated
surface modification reactions involve only a thin layer (tenth of nanometers) of the substrates leaving the bulk properties mostly unchanged. Due to
the ubiquitous thin-layer nature of the modifications, very small amounts of
starting materials are required for the surface modification processes. This
is also extremely important for the environmental issues of the modern restrictive regulations which limit the use of some reagents and require the
treatment of waste-products. These characteristics allow to give the desired
properties to surface layers of paper, depending on the nature of plasma
gases and plasma parameters.
In the following some applicative results are presented focusing on the
achievement of the desired properties rather than discussing the processes
in detail. In Section 9.2 the deposition of thin organosilicon films on paper
surfaces is used to achieve hydrophobicity. In Section 9.3 a fluorination
process is used to obtain oil-repellent paper surfaces.
9.2 Deposition of organic silicon compounds for hydrophobicity
In most part of the applications of paper a certain grade of hydrophobicity
is required. The hydrophilic character of cellulose is then a problem for
applications like liquid recipients, printing and packaging. As a matter of
fact, in environments with 50% relative humidity, cellulose adsorbs about
5% of its own weight of water [166]. Due to its fibre network structure,
paper is a porous material, and can be covered by polymer films in order
to make it impermeable to water. In some applications however, it is desirable to combine permeability to air and water repellency. Currently, water
repellency is obtained using solvents and organic reagents which can cause
environmental problems. Plasma-based technologies have all the characteristics to solve many of these problems and are, thus, an extremely interesting
alternative to conventional methods. The application of plasma to modify
cellulosic materials have been studied starting from different active precursors. Plasma polymerization of hydrocarbon monomers [167], fluorocarbon
compounds [168, 165, 169] and organic silicon compounds [166, 170]. However, all these experiments have been conducted in low pressure plasma
124
PLASMA APPLICATION FOR MODIFICATION OF PAPER SURFACES
reactors and vacuum technology is unadaptable to paper processing that is
usually continuous. The use of atmospheric pressure plasmas is indeed a
new challenge in the field of paper surface treatment. The aim is to explore
the possibility to transfer processes already known in a low pressure environment the high pressure plasma regime and to the continuous processing
mode which is typical for web materials treatment. Comparison with low
pressure treatment will be shown also in the following.
9.2.1 Experimental setup and diagnostics
The experimental setup is described in Section 3.1 and a discussion on the
deposition process can be found in Chapter 7. Here the discussion will
be mostly on the applicative results on paper surfaces. The process used
to modify the properties of paper surfaces is a plasma deposition of thin
organosilicon films using nitrogen gas with small admixtures of hexamethyldisiloxane (HMDSO) with the aim of achieve a high retention of the initial
organic component of HMDSO and obtain hydro-repellent surfaces. Methods are also similar to those described in Section 7.2 even if the treatment
times are greatly reduced. The specimens utilized cover a good range of
kind of papers. Have been chosen: a collated low weight paper for food
packaging (Paper Type A), a medium weight printing paper (Paper Type
B), a low weight filter paper (Filter Paper) and a high weight packaging
paper (Packaging paper A). Hydrophobicity is tested directly on the treated
surfaces using different methods. The water static contact angle is measured
with the optical goniometer (Dataphysics OCA20) described in Section 4.3.3.
This instrument is also used to determine water adsorption rate of a water
droplet (3 µl) by measuring its dynamical behaviour on the surface. From
the geometrical properties it is possible to determine the volume lessening
of the drop over time. It has been verified that the adsorption rate is constant over the measurement time and the interface base diameter remains
constant without changing too much for different kinds of paper surfaces
analyzed. Also the evaporation effect of water is negligible and does not
influence the measurement. Thus, this method can give information on the
adsorption degree of different paper surfaces. Cobb60 measurement methods
is anyway used because it is a more standardized value for paper industries.
It consists of exposing the paper surface to a 1 cm thick layer of water for
9.2 DEPOSITION OF ORGANIC SILICON COMPOUNDS FOR HYDROPHOBICITY
125
60 seconds and measuring by weighting, before and after the exposure, the
mass of water that has been adsorbed by the paper surface. It is expressed
in g/m2 .
9.2.2 Hydrophobicity of treated paper surface
As already shown in Chapter 7 the concentration of hexamethyldisiloxane
is a key parameter in determining the retention of the organic character
of HMDSO. The non-polar character of the methyl groups (CH3 ) present
in the original monomer is fundamental for the repellency of water which
possesses, instead, a high polar character. In Figure 9.2(a) the behaviour
(a) Static contact angle.
(b) Water drop adsorption rate.
Figure 9.2: Effect of variation of HMDSO concentration for various type of paper
surfaces and comparison with a low pressure deposition process. The 0 concentration is the value for untreated surfaces. When the untreated value is not indicated
the adsorption is mostly instantaneous and measurements cannot be performed.
of water contact angle and water adsorption is showed as a function of the
HMDSO concentration. It is evident that even at very low concentrations
the contact angle grows to very high values and it is much independent
on the concentration. This is possibly due to the roughness of the paper
surface which greatly increases the water contact angle1 to a sort of saturation value and, thus, make it dependent on the surface roughness more
than on its chemical heterogeneity. On the contrary, the behaviour of water
adsorption rate (showed in Figure 9.2(b)) is more sensible to the variation
1
It is known that the roughness of a surface will increase the apparent contact angle if
the equilibrium contact angle θe > 90◦ , while it is decreased if θe < 90◦ [59]
126
PLASMA APPLICATION FOR MODIFICATION OF PAPER SURFACES
in HMDSO concentrations. It is interesting to observe that the hydrophobic properties of surfaces are dependent on the kind of paper only below
a certain saturation value of the HMDSO concentration. In Figure 9.3 the
(a) Static contact angle
(b) Water drop adsorption
Figure 9.3: Effect of tangent speed variation for various type of paper surfaces
and comparison with a low pressure deposition process [137].
behaviour of water contact angle and water adsorption is showed as a function of the tangent speed of the web treated which is inversely proportional
to the residence time of the specimens in the plasma discharge area. The
results show that the measure of the contact angle is mostly independent
on speed while the adsorption measure can detect the different behaviour
for different speeds. Also for this parameter exists a saturation value which
cancels the differences between different papers. It is interesting to compare the behaviour of the papers versus HMDSO concentration and speed.
For example, Paper Type B properties strongly depend on HMDSO concentration while is most independent on speed (residence time) variations.
The Packaging paper A shows, instead, an opposite behavior being more
dependent on speed and less dependent on concentration. This suggests
that exists a parameter region in which the behavior of the different substrates is dissimilar. Developing processes in this region would require the
optimization of parameter for each specimen. However, exist a region of
the parameters in which the behaviour of the different substrates is almost
the same. The possibility to find a parameter combination which makes
the treatment independent on the paper specimen type is confirmed also by
the adsorption measurements with Cobb60 methods showed in Figure 9.4,
9.2 DEPOSITION OF ORGANIC SILICON COMPOUNDS FOR HYDROPHOBICITY
127
Figure 9.4: Water adsorption of different kind of paper surfaces measured with
Cobb60 method. A comparison with a low pressure deposition process [137].
where the specimens have been treated ”beyond” the saturation discussed
before. it is evident that conditions exist where the water-repellency effect is
really similar and does not depend too much on the kind of paper surfaces.
Another interesting result showed in Figure 9.4 is the comparison with a
treatment with HMDSO plasma in a low pressure device [137]. It is possible
to see that almost the same adsorption value are achieved even if the typical
treatment times are greatly reduced at atmospheric pressure.
In the applications of plasma treatment to industrial processes, of great
importance is the stability of the properties of treated surfaces. To this
end, have been performed aging experiments on the treated paper surfaces.
Specimens have been exposed after a plasma treatment to an atmosphere
at 80 ◦ C and 65% relative humidity for 7 days. The results are showed in
Table 9.1. It is evident that the thin film deposited is extremely stable and
has a good adhesion to paper surfaces. Multiple specimens have been tested
also to verify uniformity of plasma treatments in the continuous mode.
In conclusion, results for a deposition process of organosilicon thin films
on paper surfaces have been shown. There exist regions of the treatment
parameters where the water-repellency effect strongly depend on the paper
128
PLASMA APPLICATION FOR MODIFICATION OF PAPER SURFACES
Specimen
Paper Type A
Packaging paper A
After treatment
13.6±1
4.5±1
After aging
11.5±1
5.1±1
Table 9.1: Cobb60 measurements [g/m2 ] of paper foils after plasma treatment
and after the aging process. The errors are statistical standard deviations on five
different specimens.
type but it is possible to find a region where this dependency vanishes.
Also, the best results achieved with low pressure treatment have been well
reproduced with atmospheric pressure treatments. It has also been shown
that the resulting water-repellency is extremely stable and is not affected by
aging. These results show that atmospheric pressure plasmas are extremely
interesting for the development of new industrial applications.
9.3 Fluorination process for oil repellency
In many applications of paper water repellency is not sufficient. Specifically,
in food packaging applications, also a great resistance to oil and grease is
required. Usually these properties are achieved using expensive fluorine
based coatings and standard wet chemistry processes. Costs in terms of
reagents and dangerous waste by-products disposal are really high and even
more restrictive directives of the governments make the use of these traditional methods everyday more difficult. The search for different methods
to achieve the same property becomes then a necessity. Plasma treatments
have potentially the lowest environmental impact due to the little quantities of reagents needed and the nanometer scale character of the processes.
Obtain oil-repellency is not a simple task. While water, which possesses a
high surface tension due to strong polar component, can be easily repelled
by surfaces with non polar character, oils have usually a low surface tension
and cannot be easily repelled. Moreover, oils can be both polar or nonpolar and a choice at a microscopic level for the surface modification is not
enough. It is known that fluorine containing compound are able to give oil
and grease repellency to surfaces, together with a water repellency due to the
repulsion of fluorocarbon to form hydrogen bonds with water. Experiments
for the modification of paper surfaces with compounds containing fluorine
129
9.3 FLUORINATION PROCESS FOR OIL REPELLENCY
have been performed using CF4 [171, 168, 172], perfluoro-methylcyclohexane
[165] or fluorotrimethylsilane [169]. However, as for water repellency (see
Section 9.2), all these experiments have been performed with low pressure
plasma devices which are not suited for applications in paper industry.
9.3.1 Experimental setup and diagnostics
The experimental setup is the same described in preceding Section 9.2. The
methods are similar to those described in Chapter 8 and the treatments have
been optimized for different kind of papers. The fluorination is obtained in a
mixture of argon and sulfur hexafluoride (SF6 ). Here are shown the results
regarding only a single kind of paper. and compared with a different kind
of low pressure plasma process process [172]. Oil repellency is evaluated
measuring the surface energy of paper surfaces. This is achieved with the
method described in Section subsec:advrec. A typical method to estimate
the oil repellency is the so called Kit Test [173]. It consist of exposing the
surface to a drop of a mixture of oils an solvents for determined amount of
time. There exist two kinds of Kit Test: non-polar Kit Test which is made
up of mixtures of castor oil, toluene and heptane which are all non-polar
compounds. Polar Kit Test is made up of mixture of water and isopropyl
alcohol which are polar compounds. For each mixture is assigned a number
and increasing numbers indicate lower surface tensions. The test is passed
if the mixture is not adsorbed by the analyzed surface.
9.3.2 Oil repellency of paper surfaces
Fluorination process is a grafting process (see Section 2.4.2) which is typical
for plasma discharges in mixtures containing fluorinated gases and involves
the substitution of an hydrogen atom, bound to carbon, with a fluorine atom
following the reaction scheme:
hν,e −
F•
−CH −−−−→ −C• −−→ −CF,
Ion,F•
(9.1)
where a fluorine radical adsorbed to the surface reacts with a carbon radical
activated by the plasma on the surfaces. X-ray photoelectron spectroscopy
(XPS) can be used to determine the degree of fluorination achieved in the
process. The F/C ratio increases from 0 to 0.45 while the O/C ratio de-
130
PLASMA APPLICATION FOR MODIFICATION OF PAPER SURFACES
(a) Untreated paper surface
(b) Fluorinated paper surface
(c) Comparison of the area relative to carbon carbon
bonds for the two energy spectra above
(d) Deconvolution of area for treated surface.
Figure 9.5: XPS analysis of treated and untreated paper surfaces. The grafting
of fluorine atoms to carbon is evidenced.
creases from 0.53 to 0.46 indicating that fluorine not only replace hydrogen
atoms but also oxygen containing groups which are possibly removed from
9.3 FLUORINATION PROCESS FOR OIL REPELLENCY
131
the surface. In Figure 9.6 is shown the Kit Test values for a paper surface
Figure 9.6: Kit Test value as a function of time for an atmospheric pressure
fluorination plasma process based on SF6 containing gas mixture and a low pressure
plasma process with CF4 gas.
as a function of time. Two processes are compared: an atmospheric pressure fluorination plasma process based on SF6 containing gas mixture and
a low pressure plasma process with CF4 gas. The main difference in the
two processes is that the CF4 gas can activate both the fluorination mechanism of equation (9.1) and a deposition process of fluorocarbon compounds.
By adjusting the plasma parameters the process will create a thin film of
teflon-like polymer deposited on the paper surface. It is evident that, if the
fluorine is replaced directly on the cellulose fibres, a fast aging effect cancels
completely the oleo-phobic properties of the surface. However, if a deposition process is added the aging effect is not present. Possibly the aging
effect is due to the presence of extensive intra- and intermolecular hydrogen
bonding which form the cellulose structure and fibres. When fluorine atoms
replace the hydrogen atoms the possibility to realize hydrogen bonds no
longer exist and the cellulose structure near the surface becomes extremely
unstable. This new mobility allows the fibres to move more freely to find
the configuration which minimize surface energy. Possibly, thus, because of
the repulsion of fluorocarbon to form hydrogen bonds with water, the fluorine containing fibres rotate to the inside of the surface leaving conventional
132
PLASMA APPLICATION FOR MODIFICATION OF PAPER SURFACES
cellulose to be exposed to the atmospheric humidity. As shown in Figure 9.6
Figure 9.7: Kit Test value as a function of time for an atmospheric pressure
fluorination plasma process based on SF6 containing gas mixture and the same
process used on papers previously treated with a deposition process of organosilicon
films.
for the low pressure CF4 process, the presence of a deposition mechanism
can stabilize the treatment. Thus, a solution to the problem can be found
in the utilization of a deposition process which is able to create a stable,
well adherent thin film on paper surfaces. To this end, the fluorination process has been used on paper surfaces previously treated with the deposition
process described in preceding Section 9.2, which has shown (see Table 9.1)
to be extremely stable to aging effects. The result are shown in Figure 9.7.
The deposition of organosilicon films has two advantages: it is extremely
stable, blocking the cellulose fibres from turning and has a high number of
CH groups exposed to the atmosphere (because of the methyl groups retention) being a good basis for the grafting process of fluorine atoms. It is
evident that the double treatment is stable and removes the aging effect.
In conclusion, it has been shown that an atmospheric pressure plasma
can produce oil-repellent paper surfaces using a fluorination process based
on SF6 containing gas mixtures. A specific problem with the molecular
structure of cellulosic materials generate a fast aging effect which removes
completely the attained properties. It has been shown that by combining a
9.3 FLUORINATION PROCESS FOR OIL REPELLENCY
133
deposition process of thin organosilicon films with the fluorination process
can remove the aging effect giving stable properties over time. These results
show that atmospheric pressure plasmas are extremely interesting for the
development of new industrial applications for the substitution of traditional
coating processes.
134
PLASMA APPLICATION FOR MODIFICATION OF PAPER SURFACES
CHAPTER
10
Conclusions
The great interest in cold atmospheric pressure plasmas, in dielectric barrier
discharges and their potential for the development of plasma processes is one
of the motivations of this thesis. DBDs are rapidly growing as one of the
the best choice for atmospheric plasma applications, but the field is still new
and unexplored in many aspects both regarding the discharge processes and
their applications, for example, in material surface modifications. Another
motivation is the lack of a clear understanding of the discharge regimes that
can develop in DBDs. The streamer regime, which was the first observed
(and utilized) back in 1857 Siemens’s DBD ozonizer, still presents some unclear phenomena particularly when dealing with the interactions between
the micro-discharges. In this thesis (Chapter 5) the streamer regime of a
DBD in air has been characterized by means of the statistical analysis of
the discharge current. The typical time scales of micro-discharges (tens of
nanoseconds in this experimental setup) made compulsory the development
of suitable diagnostics based on home-made current probes (Rogowski coils)
135
136
CONCLUSIONS
able to catch the fast current pulses due to streamers (Section 4.2.1). It
has been found that the interaction between micro-discharges determines
the presence of two different discharge regimes, depending on the applied
voltage, which has been observed in several quantities both regarding the
statistical properties of the current intensity and its temporal behavior. One
of the great issues of DBDs is the so called memory effect which is due to
the presence of the dielectric layers and tends to promote the formation of
a streamer in the same spot of the preceding half-cycle. This effect could
bring to pattern formation which could affect performance of DBD treatment in application where spatial uniformity is required. To understand if
this ”memory” is present in the apparently random streamer regime, the
presence of correlations between discharge processes and within the single
discharge process has been studied. With the help of a surrogate model it has
been shown that the observed residual cross-correlations between half-cycles
are only an effect of the intrinsic non-stationarity of the signal, indicating
that no memory persistence is present in the temporal structure of the discharge. However, by analyzing the current signal inside the half-cycle, it
is found that on time scales of the order of hundreds of nanoseconds (i.e.,
within a single current burst, in which the streamers develop sufficiently
close in time), strong correlations exist which also reveal a peculiar ordered
temporal structure of the discharge current signal. This temporal structure
has been studied using newly defined correlation functions, which reveal
the existence of a characteristic frequency in the occurrence of streamers.
This frequency suggests the existence of an excitation mechanism between
the streamers which connects their development in the gap. These findings
reveal very interesting aspect of the cooperative behaviour of the streamer
regime and suggest the possibility to carry on the research maybe using also
fast optical diagnostics in order to obtain a clearer picture of the spatiotemporal behaviour of streamers.
Atmospheric pressure plasma processing is a leading thematic in development of plasma applications. In the last decades great efforts have been done
by many research groups in this field, however still lacks a clear knowledge
of the plasma discharge properties and of their interaction with surfaces. In
this work a new DBD device has been developed (Chapter 3) which is able to
operate continuous treatment of web materials in a wide range of pressures
137
and compositions of the gas mixture. The plasma discharge has been initially characterized in nitrogen (Chapter 6) which is often chosen as a basis
of discharges for the development of plasma processes for applications. The
capabilities of the new DBD device have been explored varying the control
parameters and finding that it can work in a wide range of conditions.
Another motivation of this work is the study and development of previously known low pressure processes at atmospheric pressure. The deposition
process of organosilicon thin films with plasma of nitrogen with small admixtures of HMDSO vapour has been characterized (Chapter 7). It has
been found that concentration is a key parameter in controlling the organic/inorganic character of the resulting deposit. Analyzing the behavior
of several quantities as a function of the HMDSO concentration we have
found the deposition mode changes with increasing concentration. It has
been found that the retention of initial monomer methyl groups saturate
with concentration and so does the hydrophobic character of the resulting
surface. It has also been found that the DBD device is able to create uniform
and smooth deposits even if working in a full developed streamer regime.
This kind of process is able to create highly hydrophobic surfaces with lower
treatment times in comparison with similar low pressure processes.
The fluorination process of polymer surfaces with mixtures of Ar/SF6 at
atmospheric pressure has been studied (Chapter 8) and it has been found
that the presence of SF6 strongly modifies the discharge properties converting the diffuse discharge regime of pure argon to a streamer regime.
It is found the effective existence of two competitive processes: an etching/activation process which brings, after the treatment, to the grafting of
polar groups from the ambient air, and a fluorination process in which grafting of fluorine atoms to carbon in the backbone of polymers is realized. The
balance of these two processes is controlled by the SF6 concentration. By
analyzing the surface morphology it is found that modifications are introduced by the treatment on the surface on the hundred nanometer scale but
these alterations do not affect substantially the roughness of the substrates.
The effectiveness of the fluorination process has been evaluated through the
analysis of the macroscopic surface properties with dynamical measurements
of contact angles with water and a non-polar liquid. The result is that this
process is effective in the modification of the properties of organic materials
138
CONCLUSIONS
making the surface highly hydrophobic and oleophobic. These properties
are needed in order to obtain resistance to water and oils.
Finally, the studied atmospheric plasma processes have been employed
for the modification of cellulosic materials (paper). Some aspects of this
research are discussed in Chapter 9. It is found the deposition process of
thin organosilicon films is able to produce effective hydro-repellent paper
surface and that exists a deposition condition in which very different substrate kinds assume the same surface properties. It has also been found
that the resulting water-repellency is extremely stable and is not affected by
aging. These results show that atmospheric pressure plasmas are extremely
interesting for the development of new industrial applications. It has been
found that an atmospheric pressure plasma can produce oil-repellent paper
surfaces using a fluorination process based on SF6 containing gas mixtures.
A disturbance of the fluorination process with the molecular structure of
cellulosic materials generate a fast aging effect which removes completely
the attained properties. It has been found that by combining a deposition
process of thin organosilicon films with the fluorination process it is possible to remove the aging effect giving stable properties over time. These
results show that atmospheric pressure plasmas are extremely interesting
for the development of new industrial applications for the substitution of
traditional oil-repellent coating processes.
In conclusion, it has been shown that suitable diagnostics and experimental and statistical characterization can lead to unveil part of the puzzling
aspects concerning dielectric barrier discharges. Also it has been shown, using a newly developed DBD device, that a better understanding of discharge
conditions allows to investigate suitable plasma processes to give new surface
properties to natural and artificial polymers. The results of this thesis work
can be the basis for researches focused on the study of dielectric barrier
discharge dynamics and on the development of new atmospheric pressure
plasma processes for surface modifications.
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