Download Surface Polymerisation (Polymer) 1 Plasma Enhanced Chemical

513.020 Experimentelles Praktikum, 3LU, 4ECTS
Pflichtlehrveranstaltung im Masterstudium Technische Physik
513.011 Forschungslabor 1, 3LU, 4ECTS
Wahlfach im Masterstudium Technische Physik
513.014 Forschungslabor 2, 3LU, 4ECTS
Wahlfach im Masterstudium Technische Physik
* PHY.005 Fortgeschrittenenpraktikum Nanophysik, 6LU, 6 ECTS
Pflichtfach im Masterstudium „Nanophysik“,
* in Zusammenarbeit mit der KFU Graz, jeweiliger Anteil von 50% in der
Durchführung
Surface Polymerisation (Polymer)
During the exercise, two Chemical Vapor Deposition (CVD) methods (PECVD and
iCVD) will be used to deposit thin film coatings on different substrates. The thickness
of the coatings will be measured by interferometry, and the deposition rate will be
calculated. The surface energy of the coating will be evaluated by water contact angle
measurements. Finally, the chemical composition will be investigated by infrared
spectroscopy. The objective of the experiments is to get familiar with CVD
techniques and understanding the advantages and disadvantages of the different
methods.
1 Plasma Enhanced Chemical Vapor Deposition (PECVD).
Plasma generalities. A plasma is a partially ionized gas, electrically neutral,
containing neutral molecules of the gas (atoms, radicals), and charged fragments
(ions), electrons and some by-products. These species can exist in different states of
excitation, depending on the energy of the system.[1]
When the plasma is ignited and the gas receives energy from the electric or magnetic
field, the electrons (the charged particles with the highest mobility) transfer the
acquired energy to the other species through elastic and inelastic collisions, causing
ionization and fragmentation of the precursors. At low pressure, the frequency of
collisions is too low to get a complete thermalization in the degrees of freedom of the
system. This means that the electrons remain the particles with the highest energy,
and create through collisions with the neutral species a large number of reactive
species, while the gas temperature remains low (in general lower than 100°C).
Therefore, a low-pressure plasma is also called cold plasma, or glow discharge for the
light-emission of the excited species.
The non-equilibrium plasmas can be generated applying an RF (13.56 MHz) electric
field to the gas. In general, the reactor contains two parallel flat electrodes with the
same or different dimensions, one connected to the RF generator through a matching
network, and the other connected to the ground or to a second generator. If the second
electrode is also connected to a generator, the configuration of the reactor is named
triode, and the reactor walls are grounded. This kind of configuration is useful when a
bias potential needs to be added to have a certain influence on the structural, optical,
and electrical properties of the deposited film.
Because of the high mobility of the electrons, each surface in contact with the plasma
is negatively charged, and assumes a negative potential with respect to the plasma.
This potential rejects the electrons and negative ions towards the centre of the
discharge and accelerates the positive ions on the surface (ion bombardment). If the
fraction of electrons with energy higher than ionization threshold is not null, then the
plasma remains ignited, and the condition of electron-neutrality is respected
(electronic flux equal to ionic flux).
The quantity of the reactive species, their energy, and in turn the properties of the
plasma, are determined by the external parameters, such as the pressure, the applied
voltage, the geometry of the reactor, the nature and the flow of gas.
There are mainly three plasma processes involving surface modification, and
changing the external parameters they can co-exist or one can be predominant on the
others.
− Etching: this process is based on the ablation of the substrate through the
formation of volatile compounds through heterogeneous chemical reactions
between the plasma active species and the materials.
− Plasma treatments: with this term one includes the grafting of functional
groups on the surface of the materials, through the formation of covalent
bonds after the interaction with a plasma of reactive gas (i.e. NH3, H2O, O2,
etc.) or surface reticulation (cross-linking) and ramification (branching)
which are accomplished through the use of non-reactive plasma (i.e. Ar, He,
etc.).
− Plasma Enhanced Chemical Vapour deposition (PECVD): this process is
called also plasma polymerization and brings to the formation of thin films
(from 50 Å to 2 µm) on the treated surface, through the linkage of polymer
fragments, i.e. radicals acting as building blocks for the construction of
polymer layers.
Deposition of thin films. PECVD is an interesting technique of deposition leading to
the formation of thin polymeric films on surfaces, by means of the reactive species
formed inside the plasma. In the plasma polymerization the film formation starts
because of the high-energy electron collisions, which break the film precursor (called
also monomer) creating reactive small fragments, i.e. radicals or ions. An important
difference between PECVD and the conventional polymerization techniques regards
the propagation step.[2] In the conventional process, propagation proceeds via chaingrowth reactions, leading to the formation of a linear macromolecule containing
regularly repeating structural units. In plasma polymerization, instead, the high rate of
initiation reactions (due to the great number of electron collisions) leads to a much
higher concentration of polymerization building blocks than in the conventional
processes and initiation and chain termination reactions dominated here over chain
propagation. Moreover termination does not necessary interrupt the plasma
polymerization, since the neutrals formed in the termination step can undergo reinitiation and propagation reactions due to further electron or ion collisions.
Therefore, the resulting chemical structure of plasma polymer is irregular.
The chemical composition and structure of the plasma deposited materials can be
easily modified through the deposition parameters and gas mixtures. In fact, the
deposition condition tuning allows changing the fragmentation of the monomer,
which is responsible for the film chemistry. Another method to control the monomer
fragmentation and the ion bombardment consists in pulsing the discharge.
Because of the potential distribution in a discharge, the positively charged fragments
are accelerated towards the substrate surface, which has a negative potential. The
energy of the ion bombardment depends on the external parameters and affects the
properties of the deposited films. Low-energy ion bombardment creates distortions
and dangling bonds on the substrate surface, so that the fragments coming from the
plasma phase beyond being physically adsorbed can also chemically react with the
surface itself, creating covalent bonds, responsible for the high adhesion characteristic
of the plasma coatings.[ 3 ] Generally the films deposited under energetic ion
bombardment are more dense and compact than those deposited in milder conditions,
although too dense structure may be characterized by high internal stress.[4]
Plasma polymerized organosilicon polymers are deposited through homogeneous and
heterogeneous steps between the film precursors and the other species generated in
the glow (e.g. charged particles, etchants). Typical organosilicon building blocks are
produced by electron-impact dissociation of the Si-C, Si-H or C-H bonds of the
precursor. Examples of activated fragments created in the plasma phase are
represented in Figure 1.
SiMe
SiMe
SiMe
e
Si۠۠
.
+ Me .
.
e
.
Si۠۠CH2 + H
e
.
Si۠۠=CH2 + Me + H
.
Figure 1: Examples of initiation steps in plasma discharge for a methyl containing silane monomer.
The monomer fragments can react homogeneously, e.g. with the other gases added in
the plasma feed (typically H2, O2, N2, Ar, He) or heterogeneously, that is with the
surface. Homogeneous reactions between fragments may result also in powder
formation.[5] The final film composition is a result also of heterogeneous reaction
which takes place on the growing film. In fact it continues in experiencing ion
bombardment and reactions, with release of labile groups or water or other oxidized
compounds.
The advantages of the plasma deposition are mainly that:
• it modifies the surface, maintaining intact the bulk properties of the materials,
(e.g. weight, flexibility).
• the depositions of thin films can be made on substrates of different materials
(metal, polymers, paper, textile, glasses, etc.).
• it is multi-directional, since a coating is deposited on all areas where the
plasma contacts a surface. This is especially advantageous when coating
complex shapes.
• due to the fact that the plasma feeding gases are activated by the collisions
with the electrons, this technique allows the production of polymers also from
unfunctionalyzed monomers (e.g. methane, tetramethylsilane) which could not
be utilized in conventional polymer synthesis in order to yield coatings with
new physical and chemical properties.
The main drawbacks of the plasma polymerization are the irregular structure of
plasma polymers and the interaction of the growing polymer with the energetic ions
or electrons which can damage the films creating dangling bonds or inhomogeneities
due to shadowing in sites of high substrate roughness.
2 initiated Chemical Vapor Deposition (iCVD)
The initiated CVD (iCVD) has been successfully used for the deposition of
hydrogels,[ 6 ] antimicrobial surface coatings,[ 7 ] fluoropolymers,[ 8 ] insulating
materials[9] and random/alternating copolymers[10] on flat or structured surfaces. The
deposition of films with well-defined chemical structures with side groups that retain
full functionality can be achieved due to the possibility of highly control the reaction
pathways.
The iCVD is a variation of the classical hot-wire CVD (HWCVD) in which the
precursor decomposition is accomplished thermally by an array of resistively heated
filaments (T>800°C).[11] In the iCVD, instead, free-radical initiating species are used
to start the polymerization.[12] An initiator is a species containing a labile bond such as
azo- or peroxy- linkage, whose thermal decomposition occurs at relatively low T
(200-400°C) generating free radicals. The concept was first introduced by Lewis et al.
[12]
, who noticed that adding as initiator the perfluorooctane sulfonyl fluoride (PFOS),
forming CF3(CF2)6CF2 radicals, dramatically enhanced the deposition rate of
polythetrafluoroethylene. The initiator radicals attack selectively the unsatured bonds
of the monomer, creating monomer radicals which polymerize. Without the initiator,
the polymerization occurs but less rapidly and less controllably because of the thermal
decomposition of the monomer and the other background species in the chamber.
In terms of polymerization steps, iCVD is very similar to the conventional free radical
polymerization, but with the absence of any liquid-step. The backside-cooled
substrate is maintained at a temperature lower than 150oC (often near ambient
temperature) to promote the adsorption of the monomer. The use of wires as heating
elements limits the view factors for radiative heat transfer to the substrate. Also,
employing low pressures limit the heat transfer to substrate by conduction through the
gas phase.
Figure 2: Schematics of the polymerization steps in iCVD process. (g) and (ad) refers to the gas or
adsorbed phase, respectively, in which the relative species participates to the polymerization. The
species are indicated as I = initiator, M = monomer, R = radical and P = polymer.
The free initiator radicals attack the monomer molecules absorbed on the surface,
initiating the polymerization. Propagation step involves the addition of the monomer
units to the polymer chains. Termination of the polymer chains may occur either by
addition of a free radical to the end of the chain or by bonding of two polymer chains.
The propagation and the termination steps of the polymerization take place on the
surface. The schematics of the iCVD process is reported in Figure 2.[ 13 ] The
decomposition in the gas phase of the initiator is the key to rapid iCVD deposition.
Moreover, to avoid unwanted crosslinking reactions and achieve full retention of the
functionality, iCVD monomers must be stable at the conditions required for the
decomposition of the initiator.
Previous studies show that in iCVD the reaction kinetics are governed by the surface
monomer concentration ( θ M ) and the concentration of initiator radicals (I.), as
expressed in Equation 1.[14]
rdep ~ k dep [ I ⋅][θ M ]
1
where rdep is the deposition rate and kdep the rate constant for the deposition process.
The kdep is favoured by the substrate temperature following the Arrhenius law. It has
been demonstrated that the quantity of monomer absorbed on the surface can be
quantified through the ratio between the monomer’s partial pressure (PM) and the
saturation pressure (Psat), which depends on the deposition conditions through the
following relations:
Φ
2
PM = M Ptot
Φ tot
Psat = A exp[
− ΔH vap
3
].
RTsub
The iCVD growth rate increases with PM/Psat, suggesting that rate-controlling step for
film deposition occurs at the surface. Low substrate temperature enhances the
adsorption on the surface, hence, when the deposition process is adsorption-limited,
lowering the substrate temperature the deposition rate increases. In other cases,
mainly dealing with the organosilicon polymerization, the iCVD processes are under
kinetics-control, meaning that the rate limiting step are the chain growth reactions. In
these cases, higher substrate temperatures, which increase the rate constant, are
required to enhance the polymerization. If the limiting reactions are known, the
system parameters can be tuned to achieve higher deposition rates and better
conformal coverage of 3D structures and the deposition processes can be scaled up
over large substrate areas for the production of industrial size samples.
The dryness of the process, the low temperature involved, and the possibility of
control the reaction pathways are the mayors advantages when using iCVD, over
other deposition techniques. These factors make this technique compatible with
virtually any substrate, including paper and membrane, which can be, instead,
dissolved, swelled, or degraded by solvents in liquid-phase-based polymerization.
One of the mayor drawbacks of the iCVD is in general the poor adhesion of the
coatings to the substrate. Strategies have been developed to create covalent linkages
between the film and the underlying substrate.[15] One of these is a two step substrate
pre-treatment: first an oxygen plasma is used to graft OH groups on the substrate
surface.[16] Then, the substrate is exposed to the vapor of a coupling agent such as
trichlorovinylsilane, creating surface vinyl terminations available to be initiated by the
radicals of the iCVD process.
References
[1] R. d’Agostino, F. Fracassi, P. Favia, “Plasma processing of polymers”, 1997, R.
d’Agostino et al. Eds, Kluwer Academic Publishers.
[2] A.M. Wrobel, M. R. Wertheimer in “Plasma Deposition, Treatment, and Etching
of Polymers”, 1990, R. d’Agostino Editor, Academic Press, San Diego, CA, p. 163.
[3] P. Favia, R. d’Agostino, F. Fracassi, Pure & Appl. Chem., 1994, 66, 1373.
[4] A. Lefèvre, L. J. Lewis, L. Martinu, M. R. Wertheimer, Phys. Rev. B, 2001, 64,
115429.
[5] N. Morosoff, in “Plasma Deposition, Treatment, and Etching of Polymers”,
1990,R. d’Agostino Editor, Academic Press, San Diego, CA, p. 1.
[6] K. Chan, L. E. Kostun, W. E. Tenhaeff, K. K. Gleason, Polymer, 2006, 47, 6941.
[7] T. P. Martin, K. K. Gleason, Chem. Vap. Deposition, 2006, 12, 685.
[8] M. Gupta, K. K. Gleason, Langmuir 2006, 22, 10047.
[9] W. S. O’Shaughnessy, M. L. Gao, K. K. Gleason, Langmuir, 2006, 22, 7021.
[10] W. E. Tenhaeff, K. K. Gleason, Langmuir, 2007, 23, 6624.
[11] K. K. S. Lau, J. A. Caulfield, K. K. Gleason, Chem. Mater., 2000, 12, 3032.
[12] H. G. P. Lewis, J. A. Caulfield, K. K. Gleason, Langmuir, 2001, 17, 7652.
[13]K. K. S. Lau, K. K. Gleason, Macromolecules 2006, 39, 3695.
[14]K. K. S. Lau, K. K. Gleason, Macromolecules 2006, 39, 3688.
[15] Im S. G., P. J. Yoo, P. T. Hammond, K. K. Gleason, Adv. Mater., 2007, 19, 2863.
[16] S. W. Choi, W. B. Choi, Y. H. Lee, B. K. Ju, M. Y. Sung, B. H. Kim, J.
Electrochem. Soc. 2002, 149, G8.