CHAPTER 1 INTRODUCTION TO DIAMOND LIKE CARBON FILMS AND ITS DEPOSITION TECHNIQUES

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 (sp 3, sp 2 and sp 1 ). Depending on the hybridization, carbon can form structures of various geometries with different fractions of sp 3 and sp 2 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 C 60 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 sp 3 and sp 2 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) SP 3 H(%) Reference Diamond 3.515 5.5 100 100 0 [2] Graphite 2.267 0-0 0 [6] Glassy Carbon 1.3-1.55 0.01 3 0 0 [7] Evaporated C 1.9 0.4-0.7 3 0 0 [7] Sputtered C 2.2 0.5 1 5 0 [8] ta-c 3.1 2.5 80 80-88 0 [8] Hard a-c:h 1.6-2.2 1.1-1.7 10-20 40 30-40 [9] Soft a-c:h 1.2-1.6 1.7-4.0 <10 60 40-50 [9] ta-c:h 2.4 2.0-2.5 50 70 30 [10] 1.2 DLC FILMS - AN OVERVIEW DLC film is a metastable form of amorphous carbon containing a significant amount of sp 3 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 sp 3, sp 2 and sp 1 bonds [13] in various proportions. The hydrogen content as well as sp 2 to sp 3 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 sp 3 bonds (typically >70%) is denoted as tetrahedral amorphous carbon (ta-c) and this is obtained when the deposition densities are around 3 g/cm 2. At lower densities (< 2 g/ cm 2 ), the film has predominantly sp 2 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% sp 3. However, most of the sp 3 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 sp 3 content, they form more C-C sp 3 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 sp 3 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 sp 3 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 sp 2 content and sp 2 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 % sp 2 hybridised carbon and is made up of many forms of evaporated a-c with disordered graphitic ordering. On moving along the sp 2 - sp 3 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 ta- C: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 sp 3 and sp 2 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 sp 2 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 sp 2 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 sp 3 and sp 2 carbon network and short chains of CH 2 and CH groups separated by a layer of nonhydrogenated sp 2 carbons. These chains of CH 2 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 sp 3 bonds and trigonal sp 2 bonds. Hydrogenated amorphous carbon with low hydrogen concentration is often called as hard a- C:H due to its high hardness, as shown in Table 1.1. This hard a-c:h contains a considerable amount of sp 2 carbon in addition to sp 3 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 sp 3 configuration, which

7 results in soft a-c:h having a high percentage of sp 3 bonding. In contrast, films with a low hydrogen concentration and high percentage of sp 3 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 [39-43]. 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 10 17-10 18 atoms /cm 3 /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 CH 4 plasma with the Ar plasma results in hydrogenated DLC (a- C: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 sp 3 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 (CH 4, C 2 H 2, C 6 H 6 ) used and therefore different growth species (CH 3, C 2 H 2, C, C 2, 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 sp 3 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 C X H Y 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, CH 3 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 CH 3 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 CH 4 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 CH 4 plasma in PECVD. These models were developed without the participation of Ar ions. To synthesize DLC film by PECVD, the use of CH 4 /Ar plasma is of great interest due to the growth mechanism and the active species in the CH 4 /Ar plasma were slightly different from the pure CH 4 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 CH 4 fragmentation. Ar can be added to the hydrocarbon precursor to enhance the diamond-like properties of a-c:h films by excluding sp 2 structures and therefore improving the sp 3 /sp 2 ratio [84]. Catherine's [85] work showed the variation of the PECVD deposited a-c:h films when one diluted the hydrocarbon gas (CH 4 ) 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 CH 4 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 H 2 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 CH 4 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 sp 2 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 C 2 dimers acting in highly diluted Ar/CH 4 plasmas. Riccardi [89] confirmed the above discussion in their results by showing the transition of growth species from CH 3 to C 2 in CH 4 /Ar plasmas as a function of the Ar percentage, (i.e) the transition from a CH 3 -rich plasma in a pure CH 4 discharge, to a C 2 rich plasma in a discharge of CH 4, 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 CH 4 plasmas [90], while it increases in CH 4 /Ar plasmas [88]. This can now be rationalized by considering that in one case the mechanism is based on physisorbed CH 3 radicals, while in the other case the growth process is based on the insertion of C 2 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, sp 3 /sp 2 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. CH 4 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.