Download CHAPTER 1 INTRODUCTION TO DIAMOND LIKE CARBON FILMS

1 CHAPTER 1
INTRODUCTION TO DIAMOND LIKE CARBON FILMS AND ITS
DEPOSITION TECHNIQUES
1.1
CARBON
Carbon is an unique and abundant chemical element in nature and
also proven to be one of the most fascinating elements. Carbon has an
outstanding ability to form different hybridizations (sp3, sp2 and sp1).
Depending on the hybridization, carbon can form structures of various
geometries with different fractions of sp3 and sp2 bonding in both crystalline
and non-crystalline forms. Diamond and graphite are the well known
crystalline forms of carbon, for which the structure and the properties are well
understood. Recently, very important and great advances in the science of
carbon have been developed in nanoscience and nanotechnology like CVD of
diamond [1-3], discovery of C60 and carbon nanotubes [4].
In Parallel to the crystalline carbon there was an equivalent
development in the field of non-crystalline carbon and their deposition
techniques. Glassy carbon, DLC film, carbon fibers, etc., are the noncrystalline forms of carbon, which are amorphous containing a mixture of sp3
and sp2 bonded carbon and has the properties between diamond and graphite.
Properties exhibited by various forms of carbon were consolidated by
Robertson [5] are listed in Table 1.1.
2 Table 1.1: Properties of various forms of carbon
Materials
Density
BandGap
(eV)
Hardness
(GPa)
SP3
H(%)
Reference
Diamond
Graphite
Glassy Carbon
Evaporated C
Sputtered C
ta-C
3.515
2.267
1.3-1.55
1.9
2.2
3.1
5.5
0
0.01
0.4 - 0.7
0.5
2.5
100
3
3
1
80
100
0
0
0
5
80 - 88
[2]
Hard a-C:H
1.6-2.2
1.1 - 1.7
10-20
40
Soft a-C:H
1.2-1.6
1.7 - 4.0
<10
60
0
0
0
0
0
0
30 40
40-50
2.4
2.0-2.5
50
70
30
[10]
ta-C:H
1.2
[6]
[7]
[7]
[8]
[8]
[9]
[9]
DLC FILMS - AN OVERVIEW
DLC film is a metastable form of amorphous carbon containing a
significant amount of sp3 bonds [5]. It can be deposited as thin films over a
range of surfaces using various techniques rather than any other forms. The
basic classification, properties, and applications of the DLC films have been
detailed out in this section.
In 1970, Aisenberg and Chabot [11] produced the first insulating
carbon films using the ion beam deposition (IBD) technique and it was shown
that these carbon films had similar properties to natural diamond but the films
were predominantly amorphous in nature. In 1971, Aisenberg and Chabot [12]
named this material as ‘Diamond-Like Carbon’ (DLC) to describe the new
form of amorphous carbon. Since then, the research had started to investigate
the various properties and applications of the DLC films.
3 1.2.1
Classification of Amorphous Carbon
In general, amorphous carbon is a mixture of sp3, sp2 and sp1 bonds
[13] in various proportions. The hydrogen content as well as sp2 to sp3 ratio
determines the structure and properties of the DLC films obtained and this
depends on the parameters of the deposition method employed. DLC films can
be broadly classified into two broad categories as hydrogen free amorphous
carbon (a-C) and hydrogenated amorphous carbon (a-C:H) based on the
carbon source used for their deposition [14,15].
(i) DLC films formed only using solid carbon sources are termed as
hydrogen-free DLC or non-hydrogenated DLC. Within this category a film
rich in sp3 bonds (typically >70%) is denoted as tetrahedral amorphous carbon
(ta-C) and this is obtained when the deposition densities are around 3 g/cm2.
At lower densities (< 2 g/ cm2), the film has predominantly sp2 bonding [16]
and it is termed as amorphous carbon (a-C).
(ii) The second category of DLC is obtained when the deposition is
carried out using hydrocarbons precursors and they are termed as
hydrogenated amorphous carbon or hydrogenated DLC (DLC:H), which
contain a significant amount of hydrogen (approximately upto 50 atomic
percent). These films are further classified into four major groups as follows
[17];
(1) a-C:H films with the highest H content (40–50%) are termed as
polymer-like a-C:H (PLCH). These films can have up to ~60% sp3. However,
most of the sp3 bonds are hydrogen terminated and these materials are soft
(low density).
4 (2) a-C:H films with intermediate H content (20–40%) are termed
as diamond-like a-C:H (DLC:H). Even if these films have lower sp3 content,
they form more C-C sp3 bonds when compared with PLCH. Thus, they have
better mechanical properties.
(3) Hydrogenated tetrahedral amorphous carbon films (ta-C:H).
ta-C:H films are a class of DLC:H for which the C-C sp3 content can be
increased while keeping a fixed H content. Thus, most films defined in
literature as ta-C:H are just DLC:Hs. However, the ta-C:H films with the
highest sp3 content (~70%) and ~25 atm.% H content do really fall in a
different category.
(4) a-C:H with low H content (less than 20%) are termed as
graphite-like a-C:H (GLCH) and they have high sp2 content and sp2 clustering.
Figure 1.1 Ternary phase diagram of bonding in amorphous carbonhydrogen alloys
5 These categories do not have any sharp boundaries and
furthermore, the overall structure is not necessarily homogeneous. It is more
convenient to represent the different amorphous carbons on a ternary phase
diagram (Figure 1.1) [18,5], which shows the relation between the three
parameters (hydrogen content, sp² and sp³ carbon). The lower left corner
represents 100 % sp2 hybridised carbon and is made up of many forms of
evaporated a-C with disordered graphitic ordering. On moving along the sp2sp3 axis, sputtered a-C followed by ta-C is found. The central portion of the
diagram constitutes the hydrogenated amorphous carbon films, a-C:H and taC:H. The hydrogen-rich side of the diagram is characterized by long chain
polymeric phases such as polyethylene and polyacetylene beyond which it is
not possible to form continuous C-C networks to obtain stable films. The
softer types of a-C and a-C: H are found in the bottom half of the triangle,
while the harder ta-C and ta-C:H are found in the top half of the diagram.
1.2.2
Structure of DLC Films
The detailed bonding structure of DLC is not completely
determined and various models have been proposed with ambiguity in each of
them. McKenzie et al., [19] described DLC as nano crystalline two-phase
structure consisting of polycyclic aromatic hydrocarbon interconnected by
tetrahedral carbon. Angus et al., [20] proposed their model based on a random
covalent network (RCN). It assumes that RCN is completely constrained when
the number of constraints per atom is just equal to the number of mechanical
degrees of freedom per atom. They also found that the covalent network
consists of sp3 and sp2 carbon sites, and optimal ratio of this coordination is a
function of atomic fraction of hydrogen in the film. This model was well
supported by the experimental observations. As per Robertson, [21] the
structure of DLC is a network of covalently bonded carbon atoms in different
hybridization, with a substantial degree of medium range order of 1 nm scale.
6 Tamor and Wu [22] proposed a defected graphite (DG) model. The
model assumed two dimensional graphitic structure with randomly distributed
non aromatic defects at which the π-electron density is zero. When the defect
density is low, the remaining π-electrons are delocalized over the entire sp2
network and the structure remains metallic. However, at some critical density
of defects, the region over which the π-electrons may delocalize becomes
disconnected, conduction electrons are confined to an “archipelago” of small
aromatic domains, and material becomes insulating. π-bonded clusters, or
graphitic or aromatic domains, are defined as fused clusters of closed six fold
rings of sp2 coordinated carbon.
Most of the models discussed above are based on graphite structure
with some distortion or defects. Jager et al., [23] proposed a simple model,
which is non-graphitic. It consists of hydrogen distributed sp3 and sp2 carbon
network and short chains of CH2 and CH groups separated by a layer of nonhydrogenated sp2 carbons. These chains of CH2 and CH groups were
distributed in carbon network with olefinic rather than aromatic carbons.
1.2.3
Physical Properties of DLC Films
The physical properties of DLC films could be determined from the
hydrogen concentration along with the relative ratio of hybridized carbon
bonds, namely tetrahedral sp3 bonds and trigonal sp2 bonds. Hydrogenated
amorphous carbon with low hydrogen concentration is often called as ‘hard’ aC:H due to its high hardness, as shown in Table 1.1. This hard a-C:H contains
a considerable amount of sp2 carbon in addition to sp3 carbon. Hydrogenated
amorphous carbon with high hydrogen concentration is often called as ‘soft’
a-C:H. The low hardness of these films is due to the monovalent hydrogen
which serves only as a terminating atom on the carbon skeletal network. Most
of the excess hydrogen is bonded to the film in the sp3 configuration, which
7 results in soft a-C:H having a high percentage of sp3 bonding. In contrast,
films with a low hydrogen concentration and high percentage of sp3 carbon are
referred to as tetrahedral a-C:H (ta-C:H).
1.2.4
Applications of DLC Films
The potential applications of DLC films are due to their unique
combination of specific properties, such as high mechanical hardness,
chemical inertness, low friction, infrared transparency, tunable optical
coefficients, low electron affinity, room temperature photoluminescence,
biocompatibility, etc.
The combination of chemical inertness, low friction and high
mechanical hardness makes DLC films very suitable for protective coatings in
various fields like in tribology [24-28], magnetic storage disks and their
read/write heads [29-33].
DLC exhibits ultra smoothness (surface roughness less than 1 nm)
[34] because it is amorphous and has low surface energy. Gillette company
[35] alone has invested over $200 million in one year to develop DLC
coatings on razor blades. The DLC films coated blades have been proven to be
more comfortable than the uncoated blades. The DLC film coated on the edges
of the razor’s can retains its sharpness and also improves the frictional
properties.
DLC films have been used as surface protection coatings in infrared
multilayer optics due to its high transmittivity in the infra red range. Tunable
optical properties of DLC are also favorable for various applications.
Depending on preparation conditions, the energy gap of DLC films ranges
from 0.5 to 3.0 eV, while the refractive index can be varies from 1.5 to 2.5.
8 These features can be utilized for anti-wear and anti-reflection protective
coatings for optics [36].
DLC films are used as biocompatible outer layer for medical
implants such as prosthetic heart valves [37] and as wear resistant coatings for
joint replacements [38]. DLC films as a scratch resistant and UV protection
layer for lenses are now well established. DLC films are now also been coated
as a gas membrane barrier on PET bottles used for drinks and food stuffs [3943].
1.3
DEPOSITION TECHNIQUES OF DLC FILMS - AN
OVERVIEW
Initially, the focus of the researchers were mainly on the properties
of DLC thin films produced by the ion beam technique and they were aware of
the limitation of the ion beam technique, which generally results in low
deposition rates [44]. Later it was found that higher deposition rate is obtained
from the discharge of hydrocarbon gases, such as methane or acetylene [45].
Chemical Vapour Deposition (CVD) technique had been used in
earlier works but the films had to be deposited at high temperature exceeding
1000ºC [46] and this can be overcome by creating an electric discharge or
glow discharge plasma. In glow discharge, the electrical conduction through
gases produces a large number of free radicals and ionic species. This
technique is known as Plasma Enhanced Chemical Vapour Deposition
(PECVD).
In course of time, several other deposition techniques have been
developed to produce DLC thin films. For example, sputtering, pulsed laser
deposition, cathodic vacuum arc discharge method, etc.
9 In general, the most popular deposition techniques can be
categorized into five groups; Ion Beam Deposition (IBD), Cathodic Vacuum
Arc Deposition, Sputtering Deposition, Pulsed Laser Deposition (PLD) and
Plasma Enhanced Chemical Vapour Deposition (PECVD). Some of these
techniques are used only in laboratories, whereas others have also been
employed in industry. The schematic diagram of various deposition techniques
have shown in Figure 1.2. However the technique used in this work to deposit
DLC films is the well known RF-PECVD.
1.3.1
Ion Beam Deposition
The first ever DLC films were deposited using the IBD method by
Aisenberg et al., in 1971 [12]. Since the ion beam production and the
deposition process are independent of each other [48], separate control and
optimization of the deposition parameters is therefore facilitated [49].
Although the films produced were relatively pure, they did contain some
contamination from the Ar or Cs ions used for sputtering. Moreover the
limited availability of carbon atoms within the ion source plasma resulted in
lower deposition rates [48]. Kaufmann [45] used a carbon-containing gas e.g.
a hydrocarbon like methane to increase the deposition rates and obtained DLC
film.
In IBD there are two methods that use different ion sources, one is
created from the gas phase and the other is obtained by the sputtering of a
solid target. In the case of DLC a suitable gas phase ion source would be from
the ionization of methane. Under a bias the ions are then extracted into a
beam. A suitable solid phase ion source would be a graphite target. Ions are
liberated by collision with Ar ions and then extracted into a beam, which is
then accelerated onto a substrate surface to deposit the DLC film.
10 Figure 1.2 Schematic diagram of various deposition techniques (a) Ion
Deposition (b) Ion Assisted Sputtering (c) Sputtering (d) Pulsed Laser
Deposition (e) Plasma Deposition (f) Cathodic Vacuum Arc
11 A variation of this method is the mass selected ion beam (MSIB)
deposition, where the beam is passed through a magnetic field and only ions
within a certain mass and energy range are used for deposition. IBD is usually
used for research purposes as deposition is slow and apparatus is expensive.
1.3.2
Cathodic Vacuum Arc Deposition
This technique was first demonstrated by Askenov et al., [50]
suitable for both laboratory and industrial applications in which highly
tetrahedral DLC films can be deposited. By creating a high current (> 40 A)
and low voltage (< 30 V) at the surface of the consumable graphite cathode, a
pure carbon plasma is generated in vacuum arc discharge [51]. High and pure
carbon fluxes of the order of 1017-1018 atoms /cm3/sec can be obtained in this
process facilitating high deposition rates. However, the carbon plasma may
contain carbon ions with different charge states, neutral atoms and
macroscopic carbon particles (0.1 to 1 μm in diameter) resulting in non
homogeneous deposition, thereby necessitating the elimination of the
macroparticles. Therefore a new method that has been employed to eliminate
the macro particles is the FCVA [51]. In which a curved magnetic filter
removes
the
micro-particles
and
hence
eliminates
the
associated
contamination. However, the submicron particles may not be totally removed
and the filtering is not sufficient for some applications. Nevertheless, the
advantages of this process are the possibility of doping and the production of
neutral plasma facilitating deposition onto the insulating substrates like
polymers without the need of an adhesion layer.
1.3.3
Sputtering Deposition
A common technique used for many coating processes, in which
the high energetic argon ions bombards the graphite electrodes to deposit DLC
films [52,53]. Plasma is generated by using either a DC or RF power. Pure
12 carbon plasma with a broad energy distribution is produced by the
impingement of the energetic ions on the graphitic target. A combination of
hydrogen or CH4 plasma with the Ar plasma results in hydrogenated DLC (aC:H), whereas for nitrogenated DLC nitrogen replaces either hydrogen or Ar.
The drawbacks of this process, such as low deposition rates, low ion
efficiencies in the plasma and the high substrate heating effects, can be
overcome by the magnetron sputtering process. Here the magnets placed
behind the target increases the path length of the electrons by giving them a
spiral motion, thereby increasing the degree of ionisation of the plasma and
the resultant yield. The unbalanced magnetron and ion assisted deposition
processes are the improvements of the sputtering techniques for obtaining high
density films.
1.3.4
Pulsed Laser Deposition
Marquadt et al., [54] deposited DLC films with properties varying
from graphite-like to diamond-like using laser ablation method, also called as
PLD. In laser ablation, carbon particles will be ejected from the graphite
target, when a high energy laser beam struck it in vacuum. These carbon
particles condense on the substrate to form the amorphous carbon film [55].
Properties of the deposited film depend on laser fluence, vacuum environment
and substrate condition [49]. Like the cathodic arc process, a highly ionised
plasma containing macro-particles is generated [51]. However, filtering is not
employed in this process.
1.3.5
Radio Frequency-Plasma Enhanced Chemical Vapour
Deposition
The Chemical Vapour Deposition (CVD) method involves the
deposition of gaseous reactants on a heated substrate surface. In CVD, the
process often requires high temperatures for a chemical reaction to take place
13 on the surface of the substrate [56]. This major limitation of CVD has been
eliminated by creating an electric discharge or glow discharge plasma in
which the deposition can be done even at the room temperature. A glow
discharge is a manifestation of the electrical conduction through the reactant
gases [57], which produces a large number of free radicals and ionic species.
This technique is known as RF-PECVD. The PECVD process is based on the
decomposition of a gaseous compound into radicals, atoms and ions near the
substrate surface. The decomposition of hydrocarbon gases is achieved in
glow discharge plasma either by applying radio frequency (RF) or microwave
(MW) or direct current (DC) power supply.
Holland and Ojha [47] produced hydrogen containing DLC by
using RF-PECVD. The electrode configuration for RF glow discharge is that
the RF power is capacitively coupled between the asymmetric electrodes on
which the substrate is mounted on the smaller one and the other electrode
(larger) is earthed [5,49,58]. The RF power produces plasma between the
electrodes (substrate holder and gas shower head). The normal operating
frequency for RF glow discharge deposition is 13.56 MHz (frequency allotted
by International communications authorities at which one can radiate a certain
amount of energy without interfering with communications). Only electrons
can follow the variation in the field polarity due to the large mass difference
between electrons and ions at this frequency. Therefore, the plasma can be
described as an electron gas which moves back and forth in a sea of relatively
stationary ions. As the electron cloud approaches one electrode, the ions are
exposed to the other electrode, forming a positive sheath where the most of the
voltage drops occurs. In the sheath region, the ions are accelerated and
bombard the electrodes. Although RF-PECVD is technically expensive and
difficult to set up with impedance matching problems, this technique is widely
used because RF is more efficient in promoting ionization and sustaining the
discharge since electrons gain higher energies as they follow oscillatory paths
14 between the electrodes. RF also gives the ability to bombard insulating
surfaces since the oscillating electrons do not reach the electrodes and no real
current flows through the circuit.
1.4
GROWTH MECHANISM OF HYDROGENATED DLC FILM
IN PECVD – AN OVERVIEW
Any growth mechanisms of the DLC films should account mainly
for the process of initial nucleation, high sp3 content, high hardness and high
stress. There have been number of mechanism proposed for the growth of both
a-C and a-C:H films. However, in order to tailor the film properties in a
controlled way, a fundamental understanding of the microscopic deposition
process is necessary. The development of the growth mechanism according to
the real time situation has been discussed in this section.
The growth of a-C films have been explained by various
mechanisms like subplantation [59-61], thermal spike [62], atomic penning
[63], radiation enhanced diffusion [64] etc. All these mechanisms mainly
focused on the reaction taking place at the surface and subsurface of the film
depending upon the energy of the carbon species.
As per the growth of a-C:H film is concerned different mechanisms
are likely to be operative depending on the deposition conditions. Usually
experiments are performed in unlike conditions like different growth
parameters, various source gases (CH4, C2H2, C6H6) used and therefore
different growth species (CH3, C2H2, C, C2, and CH) [65,66] have been
proposed. Thus the results are often odds and it appears quite difficult to
achieve definite conclusions. The deposition process is the result of a complex
chain, which includes various factors such as plasma chemistry, gas-surface
interactions, surface chemistry, etching and bombardment effects etc. There is
15 a consensus about the fact that the ion bombardment during plasma deposition
plays an important role in defining the film properties. The subplantation
model, developed to describe film deposition by carbon-ion beams, is able to
describe the ion-induced sp3 site formation by the densification due to the
incorporation of incident energetic species at interstitial sites beneath the
surface [59]. There is some experimental evidence that this model also applies
to the deposition of a-C:H films from plasma precursor gases [67].
Nevertheless, in the case of plasma-assisted film growth from hydrocarbon
precursor gases, not only does carbon ion bombardment modify the material,
but also hydrogen ions as well as CXHY radicals simultaneously interact with
the growing film surface [68].
In that way, many attempts have been made to obtain the basic
understanding of the deposition process of a-C:H in the PECVD on the basis
of theoretical and experimental considerations. However, some of the models
[69-72] deal with the discharge process and some of them include the surface
processes that preclude any simulation of the deposition rates. In general, there
are three major stages in the plasma deposition; the reaction in the plasma
(dissociation, ionization, etc.), the plasma-surface interaction and the
subsurface reactions in the film.
Kline et al., [73], Kersten et al., [74] and Reinke et al., [75]
proposed the most popular adsorbed layer model in which it is assumed that
depending on the number of surface sites and the surface temperature, CH3
radicals from the plasma were adsorbed at the surface in a physisorbed state.
Depending on the substrate temperature and the deposition energy some of
these physisorbed methyl radicals can return to the plasma by thermal
desorption and part of these radicals can be transferred to the chemisorbed
state by impact of energetic particles. Reactive carbon species are assumed to
be incorporated with a sticking coefficient of 1.0. In Kline studies it was
16 proposed that the effective sticking coefficients are used to describe the
growth of the films directly, where as Keudell and Moller [76] proposed a
more complex plasma surface model in which both the deposition processes
and etching of carbon from the surface by atomic hydrogen has taken into
account. The neutral CH3 molecules would be the dominant contribution to
this adsorption process and this model explains the deposition rate and the
composition of the deposited films based on the temperature and gas flow
successfully. This model successfully describes the temperature and gas flow
dependent on the deposition rate and the composition of the deposited films.
Rhallabi and Catherine [77] developed a transport and reaction model of a
low-pressure, high frequency CH4 plasma used for diamond like carbon
deposition. A simple surface-model based on the hydrogen coverage of
surface and ion flux and energy at the substrate surface were also established.
Moller et al., [78] have shown that the adsorbed layer model is applicable and
can equally well describe the experimentally determined deposition rates.
However, von Keudell and Jacob [79] disagreed the adsorbed layer model on
the basis of in-site investigations. Later, Jacob [80] proposed ‘ion-assisted
chemical erosion’ model in which the net deposition rate is a competition
between a temperature-independent deposition process and the temperaturedependent erosion by atomic hydrogen.
However, in the models mentioned above the growth of
hydrogenated DLC films has considered only from CH4 plasma in PECVD.
These models were developed without the participation of Ar ions. To
synthesize DLC film by PECVD, the use of CH4/Ar plasma is of great interest
due to the growth mechanism and the active species in the CH4/Ar plasma
were slightly different from the pure CH4 plasma.
Nasser et al., [81] have underlined that Ar can be an important
additional source of active species, for its metastable state contributes to gas
17 phase processes such as the charge transfer reaction and Penning ionization
[82]. Raveh et al., [83] have found that the addition of Ar enhances CH4
fragmentation. Ar can be added to the hydrocarbon precursor to enhance the
diamond-like properties of a-C:H films by excluding sp2 structures and
therefore improving the sp3/sp2 ratio [84].
Catherine's [85] work showed the variation of the PECVD
deposited a-C:H films when one diluted the hydrocarbon gas (CH4) in a noble
gas (Ar). As the ionization energies of noble gases such as Ar and He are
significantly higher than the various radicals of the CH4 molecules, it is
expected that the whole energy distribution of the RF discharge is shifted to
higher energies. This also results in a higher dissociation rate as well as higher
electron temperatures within the plasma. In the experiments of Catherine, it
was shown that the emission intensities of CH, H, and H2 species all increased
linearly with increasing Ar concentration within the plasma. The variation of a
number of physical properties of a-C:H when deposited using a hydrocarbon
in the
presence of
noble gas have been
studied by many researchers
thereafter.
Amaratunga et al., [86] discussed the variation of the properties of
hard DLC films giving rise to a deposition-etch process when a CH4 gas is
mixed with Ar during the deposition. Silva et al., [87] followed this work by
showing that a large controllable variation in the material properties can be
obtained by using Ar dilution in PECVD plasmas. They indicated that when
the DC self-bias is gradually increased, the a-C:H films deposition undergo
transmission from polymer like (PLCH) to diamond like (DLC) transition and
at very high bias this converts back to a high sp2 film due to extensive
bombardment of the growing surface. They showed that the behavior of the
deposited film when diluted with a noble gas is different than when using only
a hydrocarbon source gas.
18 McCauley et al., [88] suggested a new growth process based on C2
dimers acting in highly diluted Ar/CH4 plasmas. Riccardi [89] confirmed the
above discussion in their results by showing the transition of growth species
from CH3 to C2 in CH4/Ar plasmas as a function of the Ar percentage, (i.e)
the transition from a CH3-rich plasma in a pure CH4 discharge, to a C2 rich
plasma in a discharge of CH4, which is highly diluted by Ar. This proposal
resolves outstanding discrepancies between experimental observations. As an
example, it has been reported that the deposition rate decreases as a function
of the temperature substrate in pure CH4 plasmas [90], while it increases in
CH4/Ar plasmas [88]. This can now be rationalized by considering that in one
case the mechanism is based on physisorbed CH3 radicals, while in the other
case the growth process is based on the insertion of C2 carbon dimmers [89].
1.5
SUMMARY AND OBJECTIVE OF THE RESEARCH
DLC films can be coated by various techniques ranging from IBD
to RF-PECVD. RF plasma discharge has been widely used to produce DLC
films, because this method can be applied not only for etching but also for
deposition on insulators. The advantage of RF plasma discharge is its wide
area of application and its stability when compared with DC plasma. The DLC
film is generally deposited using various source gases (methane, ethane,
ethylene, acetylene) along with hydrogen and/or argon gas by RF-PECVD.
Films grown in source gases with higher hydrogen-to-carbon ratios has much
lower friction coefficients and wear rate than films derived from source gases
with lower hydrogen to carbon ratios [91].
The properties of the DLC films can be changed by varying the
deposition parameters. De Martino et al., [92] have studied and verified that
the physical characteristics of DLC films are independent of the deposition
19 methods. For this reason, the films are differentiated based on their properties
and not by their deposition methods. The properties of DLC films deposited
by RF-PECVD strongly depends on deposition conditions such as deposition
pressure, substrate temperature, deposition energy of the hydrocarbon or
discharge power, application of bias voltage, reactive gases used and dilution
of reactive gases. Thus the growth parameters in PECVD play a critical role,
since they all affect the average impact energy and greatly influence the
hydrogen content incorporated into the DLC films during the deposition
process and the way its atomic orbitals are hybridized when making chemical
bonds [93-94].
However there is a need of more thorough understanding of the
complicated effects of deposition parameters on DLC properties [95], thus lot
of work had been done to study the effect of growth parameters on the growth
and physical properties of these films such as band gap, optical constants,
refractive index, sp3/sp2 ratio, film density, internal stress, growth rate,
tribological studies, etc, but it is equally important to study the surface
morphology of the DLC film [96,97] under various conditions.
Morphological studies gives the information about the continuous
coating of the film, surface grain size, roughness of the film, stability of the
film on the substrate, etc., which are all plays an important role in deciding the
performance and efficiency of the DLC film on various technological
applications. For example, there is a strong need for ultra-smooth, thin and
hard films for functional layers or protective overcoats in the field of microdevices and magnetic storage media. On the other hand, rough surfaces
generally display an enhanced hydrophobicity, which might be a prerequisite
in many biomedical applications. However, we have seen DLC film
morphological studies data in the literature, there have been only fewer studies
performed on the sequential variation of morphology of the DLC film during
20 their growth under various conditions of RF-PECVD. However, the term DLC
represents hydrogenated DLC hereafter in this thesis.
The main objective of this work is to study the sequential variation
in the surface morphology of capacitively coupled RF-PECVD grown DLC
films on silicon (100) substrate under three phases namely,
™ During the growth.
™ During the variation in the major RF-PECVD parameters
(plasma pressure and deposition temperature).
™ During the post deposition treatment (annealing at high
temperature).
All the DLC films grown in this work utilizes the self bias voltage
created due to the plasma sheath potential on the electrodes of RF-PECVD
without using any external negative bias voltage. The RF power of 200 W
were used for the growth of all DLC films and thus the self bias voltage
created on the electrodes of the RF-PECVD were measured to be in the range
of -15 to -20 V, which slightly varies in the given range depends on the
pressure and temperature of the chamber. Under this low negative self bias
voltage the surface diffusion process is the dominant mechanism in governing
the growth of the DLC film on the silicon substrate. CH4 and Ar were used as
the precursor gases to grow DLC film.
21 The grown DLC films have been characterized by various
techniques to study the properties of the films.
™ Surface topographical studies like 2D and 3D surface
morphology, surface roughness, grain size, etc., of the grown
DLC film were conducted by contact mode AFM.
™ Long range uniform coating of the DLC films over the
substrate was studied by SEM.
™ Bond nature and the bond hybridization of the DLC films
were studied by Raman spectroscopy.