- Email: [email protected]
Thin film characterization of diamond-like carbon films prepared by r.f. plasma chemical vapor deposition
Thin Solid Films 302 (1997) 5-11
ELSEVIER
Thin film characterization of diamond-like carbon films prepared by r.f. plasma chemical vapor deposition Kenji K. Hirakuri
a.
~, Takumi Minorikawa b, Gernot Friedbacher a, Manfred Grasserbauer
~
Technische Unil'ersitiit Wien. lnstitut fiir Analytische Chemie. Getreidemarkt 9/151, A-1060 Wien, Austria Department of Applied Electronic Engineering. Facullx c~fScience and Engineering. Tokyo Denki UnicersiO', l~hi:aka. Hatoyama, Saitama 350-03, Japan Received 15 February 1996: accepted 7 January 1997
Abstract
Using atomic force microscopy and infrared spectroscopy, diamond-like carbon films deposited by r.f. plasma discharge were investigated. Hardness and roughness of the films strongly depend on the bias voltage of the substrates and thus the pressure in the deposition chamber. At higher pressures, the surface roughness of the films increases with their thickness. For films deposited at medium pressures, the roughness is almost constant regardless of the film thickness. At pressures below 53 Pa decreasing roughness is observed and hard films are obtained. These findings can be explained by the predominance of different species in the plasma. At low pressure, ionic species are accelerated towards sharp tips on the film surface leading to sputter removal of weakly bond material in these regions. At higher pressure neutral species in the plasma dominate. Thus, the balancing effect of sputtering is not observed. Hydrogen content and sp3/sp 2 ratios obtained from IR spectra revealed that the lower hardness of the rough surfaces can be explained by a higher content of sp 2 species in the films deposited at high pressure. Whereas the intensity ratio of sp3/sp 2 is almost constant for the films at higher pressure preferred sputter removal of sp 2 species leads to a significant increase of that ratio at low pressure. © 1997 Elsevier Science S.A. Keywords: Diamond-like carbon film; Atomic force microscopy; Surface morphology; Infrared spectroscopy
1. Introduction
Diamond-like amorphous carbon (DLC) films have been investigated by many researchers because of their attractive properties (e.g. high mechanical hardness, high electrical resistivity, optical transparency over a wide range and uniform flat surface) which are similar to those of diamond [1-5]. The hardness of DLC films ranges between a third and half the value of diamond ( > 100 GPa). The electrical resistivity is higher than 101° 1) cm. DLC films are almost fully transparent in the visible region (300-800 nm). Moreover, flat surfaces can be produced which is important for various applications (e.g. resist material for photolithography) [6]. Compared with chemical vapor deposition (CVD) of diamond, deposition of DLC films is easier.
* Corresponding author. Tel: +81 492 96 2911 ext. 3102; lax: +81 492 96 6413; e-mail [email protected]. Permanent address: Department of Applied Electronic Engineering, Faculty of Science and Engineering, Tokyo Denki University, Ishizaka, Hatoyama. Saitama 350-03, Japan 0040-6090/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 ( 9 7 ) 0 0 0 2 2-9
Large areas can be coated at room temperature using only one hydrocarbon gas. Since hydrocarbon gases can be easily decomposed by plasma discharges, plasma CVD is the most common technique for deposition of carbon films. Especially, radio frequency (r.f.) plasma discharge has been widely used to produce amorphous carbon films, because this method can be applied not only for etching but also for deposition of insulators. The advantage of r.f. plasma discharge is the application for wide area and the stability as compared with d.c. plasma. The occurrence of the self-bias voltage is originated by the mobility difference of ions and electrons at high frequency, then the positive ions are collected to the powered electrode. However, the properties of DLC films deposited by r.f. plasma CVD strongly depend on deposition conditions such as total gas pressure, input power supplied from a generator, gas flow rate and also substrate material [5]. In our previous papers, we have reported that the hardness of DLC films is mainly determined by the number of the ionic species in the plasma and by the bias voltage, which depends on positioning and shape of the
6
K.K. Hirakuri ~l al. / Thin Solid Fihm 302 (1997) 5-11
electrodes [4,5]. Thus, the geometry of the experimental set-up is very important. In addition, the number of the ionic species accelerated to the substrate and the ratio between ionic and neutral species significantly influences the properties of DLC films. In fact, one observes lower hardness for films deposited by ion beam techniques [5]. The concentration of sp 3 components of CH bonds included in DLC films is greater than that of soft amorphous carbon films. The hardness increases with the concentration of sp 3 components in the films [4]. Jiang et al. reported that the hydrogen content of hydrogenated carbon films, deposited by the d.c. plasma technique, has influenced the nanochemical and nanotribological characteristics of their films [7]. Also the ratio between carbon and hydrogen plays an important role for the fihn properties. Optical and electrical techniques have been used to establish relationships between plasma conditions for C H , discharges and properties of the resulting films. A relationship between hardness of the DLC films and the occurrence of CH 3 radicals a n d / o r ionic species in the plasma has been obtained [8-12]. High hardness can be explained by removal of soft components in the films through etching a n d / o r sputtering by ionic and active species. Due to the high electrical resistivity morphological characterization of DLC films by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning tunnelling microscopy (STM), is difficult. Atomic force microscopy (AFM) is a powerful alternative, because insulating materials can be imaged in a straightforward manner [13]. It has been used by Jiang et al. to study amorphous carbon films on the nanometer scale [7]. Friedbacher et al. have investigated changes of the surface morphology of Si substrates during hot-filament CVD diamond growth with AFM [14]. In this work, AFM and IR spectroscopy have been used to elucidate the deposition mechanism of DLC films prepared by r.f. plasma discharge. The properties of the obtained films are discussed on the basis of the analytical characterization (morphology, sp3/sp ~- ratio, hydrogen content) by AFM and IR spectroscopy.
2. Experimental Fig. l shows a schematic diagram of the experimental set-up used for DLC film deposition. The chamber and both electrodes consist of stainless steel. In order to avoid discharges between the electrode and the wall of the chamber, metal covers are used to shield the rear and the side of the electrodes. An r.f. power generator with a 13.56 MHz oscillator was connected with each electrode through an electrical matching circuit keeping the input power constant at 100 W. Methane gas from a cylinder was introduced into the chamber through a mass flow controller. The residual gas was exhausted by the rotary pump.
I
LFlow Meter I bVa]v c
Plasma
Matching Box
. J ~ Substrate
I- Exhaust 13.56MHz I
~ R.F. Power
Fig. I. Schematic illustration of the cxperimental set-up for the deposition of Dt.C fihns.
The DLC films have been deposited on Si substrates at thicknesses of 1, 5, 10, 50, 100 and 1000 nm and at four different pressure conditions ([3, 53, 130, 1300 Pa). The thickness of the films was both estimated from the deposition rates and measured by interference microscopy and SEM. However, it must be mentioned that this procedure does not allow to obtain v e r y accurate values for the thinner films (1 nm). The hardness of the films was determined with a SHIMADZU HMV-2000 micro hardness tester. The given hardness values are mean values from three different samples pieces. The AFM images have been recorded using a Nano Scope Ill system designed by Digital Instruments Inc, Santa Barbara, CA. The root-mean-square (rms) roughness values used throughout the paper have been determined on areas of 1 lain × 1 ~xm by the Nanoscope III software. IR spectra of the DLC films were recorded with an FTIR spectrometer with a resolution of 0.4 cm ~. The hydrogen content was calculated from the integral peak area of the 1R spectrum belween 2800 and 3060 cm ~ The s p - / s p - ratio was evaluated by calculating the intensity ratio between the sp3-CH peak at 2915 cm ~ and the sp2-CH peak at 3000 cm 1.
3. Results and discussion Fig. 2 shows that the hardness of the DLC films decreases monotonously with increasing pressure (causing the negative bias voltage to decrease at the same time). This behavior was observed for various film thicknesses, although it must be mentioned that it was difficult to measure the hardness of films thinner than 100 nm. This finding shows that the hardness must be controlled by the bias voltage which is in accordance with previous work [4]. The film hardness deposited at low pressure region is comparable to that of other reports [7,15] Figs. 3 - 6 show AFM images of film topographies obtained at different pressures. In the low pressure region (13 and 53 Pa) pronounced protrusions were observed on the thinner films (1 and 5 nm). The protrusions observed at
K.K. Hirakuri et al. / Thin Solid Films 302 (1997) 5-11
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........
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}Zig. 2. Knoop hardness of the DLC fihns (thickness, 500 nm) as a (unction of the pressure. 13 Pa are sharper than those observed at 53 Pa, because the energy of the sputtering ionic species is higher at low pressure (high bias voltage). The bias voltages were - 2 5 0 V at 13 Pa and - 1 8 0 V at 53 Pa, respectively. On the thickest fihns (1 ~xm) deposited at lower pressures (Fig. 3(c) and Fig. 4(c)), such pronounced features cannot be observed. This flattening effect can be explained by sputter removal of protruding features through ions accelerated towards sharp tips in the electrical field. This process is only significant at low pressures. Whereas a decrease of the roughness with film thickness is observed at 13 Pa and 53 Pa an opposite behavior
7
is found at 130 and 1300 Pa (Fig. 7). For instance, at 130 Pa initially (1 rim) a fine grained structure (rms roughness, (1.4 nm) is observed. During the course of deposition these small features grow leading to comparatively large undulations (size 0.2 nm) at an rms roughness of 0.5 nm (Fig. 5(c)). Fig. 6 shows the topography of the film surfaces at 1300 Pa. Starting from an initially flat surface, growth of features similar to deposition at 1300 Pa is observed leading to an nns roughness of 0.6 nm at 1 txm film thickness. Fig. 7 shows the nns roughness versus film thickness for the different deposition conditions. The roughness of the film surface at lower pressures (13 and 53 Pa) decreases with the thickness until a constant value of approximately 0.2 nm is reached for thicknesses larger than 10 nm. At medium pressures (130 Pa) the roughness is almost constant. At 1300 Pa the roughness increases with film thickness. In Fig. 7 three regions can be identified: region l up to 1 nm thickness, region II between 1 nm and 10 nm and region Ill beyond 10 nm. In region ! a significant increase of the roughness can be observed for all pressure conditions due to deposition of DLC films on the plain Si substrates. The lower value for the film deposited at 1300 Pa could be explained by the fact that its actual thickness was lower than 1 nm. It should be emphasized, however, that all surfaces studied in this work were much flatter
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Fig. 3. AFM images showing the morphology of DLC films deposited on Si at 13 Pa. Film thicknesses: (a) 10 nm, (b) 100 nm and (c) 1000 nm. Image size, 1 g m × 1 Ixm; depth scale. 10 nm from black to white.
8
K.K. Hirakuri et al./Thin Solid Fihns 302 (1997) 5-11
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K.K. Hirakuri et a l . / T h i n Solid Films 302 (1997) 5 - I 1
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than the roughnesses of commonly used CVD diamond films and WC materials, which are in the order of several micrometers and several hundred nanometers, respectively. At lower pressures (13 and 53 Pa), the roughness in region II is decreasing due to predominance of sputtering by ionic species as also shown in Figs. 3 and 4. At 130 Pa, the roughness is almost constant, since sputtering and deposition is about balanced in the pressure region. At 1300 Pa the roughness increases in II region, because sputtering cannot take place at that pressure. The maximum roughness is only determined by the random deposition of neutral species on the surface. The decrease of the rms roughness from 13 Pa to 53 Pa is originated by the 1 "~
0.8
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difference (70 V) of the bias voltages at 13 Pa and 53 Pa. At low pressure condition (13 Pa), both weak and strong bonds of the film surface are sputtered and milled by the ionic species with high energy, and most of the ionic species attack to the surface without collision to any other molecules in the atmosphere. In region III, the roughness of all films is roughly constant. The roughnesses of the films deposited at 13 Pa are higher than those of the films deposited at 53 Pa, because sputtering attack is more pronounced at lower pressure changing the equilibrium towards higher roughness. Nakayama et al. reported that the magnitude of negative bias voltage influences surface flatness by the collisions of accelerated positive ions against the substrate surface [16]. At a few hundred nanometers thickness, the roughness of our films agrees with their results. The topography of hydrogenated amorphous carbon films was observed by AFM [17]. Since the power and the substrate temperature are high as compared with our deposition condition, the bias voltage on the substrate may be high. The ionic species attached to the substrates and the roughness of their samples is rougher than our samples. Next, we have investigated our DLC films by IR spectroscopy. First, the absorption of CH bonds around 3000 cm J was measured in order to obtain information oll the hydrogen content [18,19]. The hydrogen content has been evaluated by integrating the areas of all CH,, peaks. Due to
10
K.K. Hirakuri el al./Thin Solid Fihns 302 (1997) 5 11 12
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Thickness (nm) Fig. 8. Hydrogen content of the DLC films versus thickness. All values are normalized to 1 for all films with a thickness of 1 gin.
Fig. 9. Peak intensity ralio o[" sp ~/sp 2 versus film thickness.
the absorption sensitivity of the IR spectrometer, this is only possible if the peaks are not shifted and the shapes of the spectra are similar [20]. This condition was fulfilled for all but the thickest film at the lowest pressure. In the latter film the shapes of the spectra are different, since sp :~ components of CH bonds are dominant. Fig. 8 shows the hydrogen content C H (normalized at 1 for all 1000 nm thick films) as a function of the film thickness. The films with a thickness of 1 nm are not included, since IR spectra could not be recorded. At lower pressures, C n gradually decreases with thickness up to 100 rim. This can be explained by the sputtering a n d / o r etching of weakly bond sp 2 components in the films leading to decreasing peak intensities at 3000 and 3020 cm ~ At medium pressure the curve is about constant, since deposition and sputtering is balanced. At higher pressure, C H abruptly increases with thickness up to 100 nm. The lowest value of C H for the film at 1300 Pa may be affected by the uncertainly of the thickness. Next, IR spectra were evaluated in order to determine the s p 3 / s p 2 ratio. Table 1 gives an overview on relevant peaks. The peaks are identified as sp'"-CH,, vibrations by Refs. [18,19,21-23]. The most intensive peaks are at 2920 cm ~ due to CH bonds of sp:~-CH and spS-CH~. Two peaks originating from sp2-CH,, were identified at higher
wavenumber near 3010 cm ~. In order to calculate the s p S / s p 2 ratio, the intensities of the sp 3 and the sp 2 peaks were measured after waveform separation. Fig. 9 shows the intensity ratios of the spS-CH peak at 2920 c m - J and the spe-CH peak at 3000 cm ~. The ratios of the films at 130 and 1300 Pa are almost constant. At high pressure the fihns are observed to consist of polymer-like carbon containing a high portion of sp 2 components [18]. At lower pressure, the ratio increases with the thickness. This can be explained by sputter removal of weakly bond sp 2 components.
"Fable 1 C - H absorption bands for DLC films
sp~-CH: (symmetrical) sp~-CH~ (symmetrical) sp3-CH sp~-CH ~_(asymmetrical) sP 2-CH 2 (olefinic) sp3-CH 3 (asymmetrical) spe-CH (olefinic) sp 2 CH: (olefinic) sp:-CH (aromatic)
Predicted frequency (cm I )
Observed frequency (cm I)
2855 2870 2915 2925 2950 2960 3000 3020 3050
2850 2870 2915 2920 2960 3005 3020
4. Conclusions A F M and IR spectroscopy have been used successfully to study the mechanism of DLC thin film deposition on silicon substrates by r.f. plasma discharge of methane gas. Generally, the surfaces were much flatter than other materials (e.g. diamond films) frequently used for hard coating. A F M results revealed that the surface morphology of the films is strongly affected by the deposition conditions, i.e. the bias voltage between electrode and substrate holder which depends on the pressure in the chamber. The flattest films with the highest hardness are obtained at lower pressures. Those films also exhibit the highest content of sp ~ components. IR absorption spectra were used to determine hydrogen content and s p ; / s p e ratio of the films. Combining IR and AFM results, relations between film thickness, hardness, hydrogen content and s p 3 / s p -~ ratios could be established for different deposition conditions. It can be concluded that film growth and properties are controlled by the predominance of different species in the plasma leading to sputtering through acceleration in the electric field. Therefore the pressure, and thus the bias voltage must be optimized in order to obtain fiat films with sufficient hardness. In the shown investigations this optimum was found to be around a pressure of 53 Pa.
K.K. Hirakuri et al./Thin Solid Fihns 302 (1997) 5-11
Acknowledgements This work was supported by the Centre for Research, Tokyo Denki University. Financial support of this work by 1he Austrian Science Foundation, project P11015-OPY is gratefully acknowledged.
References [1] S. Berg and L.P. Andersson, Thin Solid Films, 58 (1979) l l7. [2] H. Vora and T.J. Moravec, J. Appl. Phys., 52 (1981) 6151. [3] L.P. Andersson, S. Berg, H. Norslrom. R. Olaison and S. Towta, Thin Solid Films, 63 (1979) 155. [4] K. Kobayashi, N. Mutsukura and Y. Machi, Thin Solid Fihns, 158 (1988) 233. [5] K.K. Hirakuri, N. Mutsukura and Y. Machi, Vacuum. 46 (1995) 57. [6] J. Seth. S.V. Babu, V.G. Ralehenko, T.V. Kononenko, V.P. Ageev and V.E. Strelnitsky~ Thin Solid Fih,s, 254 (1995) 92. [7] Z. Jiang, C.-J. Lu, D.B. Bogy, C.S. Bhatia, T. Miyamoto. Thin Solid Films, 258 (1995) 75. [8] S. Naito, N. Ito, T. Hattori and T. Golo, .Ipn..I. Appl. Phys., 34 (1995) 302. [9] E. Gogolides, D. Mary, A. Rhallabi and G. Turban, Jpn. J. Appl. Phys., 34 (1995) 261.
11
[10] K. Tachibana, T. Mukai, A. Yuuki. Y. Matsui and H. Harima, Jpn. J. Appl. Phys.. 29 (1990) 215. [11] N. Mutsukura, S. lnoue and Y. Machi, J. Appl. Phys.. 72 (1992) 43. [121 H. Toyoda, H. Kojima and H. Sugai, Appl. Phys. Lett., 55 (1989) 1292. [ 13] G. Friedbacher, T. Prohaska and M. Grasserbauer, Mikrochim. Acta. 113 (1994) 179. [14] G. Friedbacher, E. Bouveressc, (;. Fuchs, M. Grasserbauer, D. Schwarzbach, R. Haubner and B. Lux, Appl. Surf] Sci., 84 (1995) 133. [15] H. J. Scheibe. D. Drescher and A. Mensch, Fres. J. Anal. Chem.. 353 (1995) 690. [16] M. Nakayama. A. Tsuyoshi, M. Shibahara, K. Maruyama and K. Kamata, J. Vac. Sci. Technol., 13 (1995) 195. [17] Y. Nakamura. Y. Watanabe and Y. Suefuji, J. Amer. Ceram. Soc., 77 (1994) 1385. [18] B. Dishler, A. Bubenzer and P. Koidl, Solid State Commun.. 48 (2) (1983) 105. [19] P. Couderec and Y. Catherine. Thin Solid Fihns. 146 (1987) 93. [20] K. Kobayashi, K. Yamamoto, N. Mutsukura and Y. Machi, Thin Solid t:ihn.~, 185 (1990) 71. [21] J.W. Zou, K. Schmidt. K. Reichelt and B. Dischler, J. Appl. Phys.. 67 (1990) 487. [22] A. Hol'fman, A. Fayer. A. Laikhtman and R. Brener, J. Appl. Phys., 77 (1995) 3126. [23] H.D. Bint. S. Bhargava and M. Aslam..I. Raman Spectrosc., 25 (1994) 67.
ELSEVIER
Thin film characterization of diamond-like carbon films prepared by r.f. plasma chemical vapor deposition Kenji K. Hirakuri
a.
~, Takumi Minorikawa b, Gernot Friedbacher a, Manfred Grasserbauer
~
Technische Unil'ersitiit Wien. lnstitut fiir Analytische Chemie. Getreidemarkt 9/151, A-1060 Wien, Austria Department of Applied Electronic Engineering. Facullx c~fScience and Engineering. Tokyo Denki UnicersiO', l~hi:aka. Hatoyama, Saitama 350-03, Japan Received 15 February 1996: accepted 7 January 1997
Abstract
Using atomic force microscopy and infrared spectroscopy, diamond-like carbon films deposited by r.f. plasma discharge were investigated. Hardness and roughness of the films strongly depend on the bias voltage of the substrates and thus the pressure in the deposition chamber. At higher pressures, the surface roughness of the films increases with their thickness. For films deposited at medium pressures, the roughness is almost constant regardless of the film thickness. At pressures below 53 Pa decreasing roughness is observed and hard films are obtained. These findings can be explained by the predominance of different species in the plasma. At low pressure, ionic species are accelerated towards sharp tips on the film surface leading to sputter removal of weakly bond material in these regions. At higher pressure neutral species in the plasma dominate. Thus, the balancing effect of sputtering is not observed. Hydrogen content and sp3/sp 2 ratios obtained from IR spectra revealed that the lower hardness of the rough surfaces can be explained by a higher content of sp 2 species in the films deposited at high pressure. Whereas the intensity ratio of sp3/sp 2 is almost constant for the films at higher pressure preferred sputter removal of sp 2 species leads to a significant increase of that ratio at low pressure. © 1997 Elsevier Science S.A. Keywords: Diamond-like carbon film; Atomic force microscopy; Surface morphology; Infrared spectroscopy
1. Introduction
Diamond-like amorphous carbon (DLC) films have been investigated by many researchers because of their attractive properties (e.g. high mechanical hardness, high electrical resistivity, optical transparency over a wide range and uniform flat surface) which are similar to those of diamond [1-5]. The hardness of DLC films ranges between a third and half the value of diamond ( > 100 GPa). The electrical resistivity is higher than 101° 1) cm. DLC films are almost fully transparent in the visible region (300-800 nm). Moreover, flat surfaces can be produced which is important for various applications (e.g. resist material for photolithography) [6]. Compared with chemical vapor deposition (CVD) of diamond, deposition of DLC films is easier.
* Corresponding author. Tel: +81 492 96 2911 ext. 3102; lax: +81 492 96 6413; e-mail [email protected]. Permanent address: Department of Applied Electronic Engineering, Faculty of Science and Engineering, Tokyo Denki University, Ishizaka, Hatoyama. Saitama 350-03, Japan 0040-6090/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 ( 9 7 ) 0 0 0 2 2-9
Large areas can be coated at room temperature using only one hydrocarbon gas. Since hydrocarbon gases can be easily decomposed by plasma discharges, plasma CVD is the most common technique for deposition of carbon films. Especially, radio frequency (r.f.) plasma discharge has been widely used to produce amorphous carbon films, because this method can be applied not only for etching but also for deposition of insulators. The advantage of r.f. plasma discharge is the application for wide area and the stability as compared with d.c. plasma. The occurrence of the self-bias voltage is originated by the mobility difference of ions and electrons at high frequency, then the positive ions are collected to the powered electrode. However, the properties of DLC films deposited by r.f. plasma CVD strongly depend on deposition conditions such as total gas pressure, input power supplied from a generator, gas flow rate and also substrate material [5]. In our previous papers, we have reported that the hardness of DLC films is mainly determined by the number of the ionic species in the plasma and by the bias voltage, which depends on positioning and shape of the
6
K.K. Hirakuri ~l al. / Thin Solid Fihm 302 (1997) 5-11
electrodes [4,5]. Thus, the geometry of the experimental set-up is very important. In addition, the number of the ionic species accelerated to the substrate and the ratio between ionic and neutral species significantly influences the properties of DLC films. In fact, one observes lower hardness for films deposited by ion beam techniques [5]. The concentration of sp 3 components of CH bonds included in DLC films is greater than that of soft amorphous carbon films. The hardness increases with the concentration of sp 3 components in the films [4]. Jiang et al. reported that the hydrogen content of hydrogenated carbon films, deposited by the d.c. plasma technique, has influenced the nanochemical and nanotribological characteristics of their films [7]. Also the ratio between carbon and hydrogen plays an important role for the fihn properties. Optical and electrical techniques have been used to establish relationships between plasma conditions for C H , discharges and properties of the resulting films. A relationship between hardness of the DLC films and the occurrence of CH 3 radicals a n d / o r ionic species in the plasma has been obtained [8-12]. High hardness can be explained by removal of soft components in the films through etching a n d / o r sputtering by ionic and active species. Due to the high electrical resistivity morphological characterization of DLC films by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning tunnelling microscopy (STM), is difficult. Atomic force microscopy (AFM) is a powerful alternative, because insulating materials can be imaged in a straightforward manner [13]. It has been used by Jiang et al. to study amorphous carbon films on the nanometer scale [7]. Friedbacher et al. have investigated changes of the surface morphology of Si substrates during hot-filament CVD diamond growth with AFM [14]. In this work, AFM and IR spectroscopy have been used to elucidate the deposition mechanism of DLC films prepared by r.f. plasma discharge. The properties of the obtained films are discussed on the basis of the analytical characterization (morphology, sp3/sp ~- ratio, hydrogen content) by AFM and IR spectroscopy.
2. Experimental Fig. l shows a schematic diagram of the experimental set-up used for DLC film deposition. The chamber and both electrodes consist of stainless steel. In order to avoid discharges between the electrode and the wall of the chamber, metal covers are used to shield the rear and the side of the electrodes. An r.f. power generator with a 13.56 MHz oscillator was connected with each electrode through an electrical matching circuit keeping the input power constant at 100 W. Methane gas from a cylinder was introduced into the chamber through a mass flow controller. The residual gas was exhausted by the rotary pump.
I
LFlow Meter I bVa]v c
Plasma
Matching Box
. J ~ Substrate
I- Exhaust 13.56MHz I
~ R.F. Power
Fig. I. Schematic illustration of the cxperimental set-up for the deposition of Dt.C fihns.
The DLC films have been deposited on Si substrates at thicknesses of 1, 5, 10, 50, 100 and 1000 nm and at four different pressure conditions ([3, 53, 130, 1300 Pa). The thickness of the films was both estimated from the deposition rates and measured by interference microscopy and SEM. However, it must be mentioned that this procedure does not allow to obtain v e r y accurate values for the thinner films (1 nm). The hardness of the films was determined with a SHIMADZU HMV-2000 micro hardness tester. The given hardness values are mean values from three different samples pieces. The AFM images have been recorded using a Nano Scope Ill system designed by Digital Instruments Inc, Santa Barbara, CA. The root-mean-square (rms) roughness values used throughout the paper have been determined on areas of 1 lain × 1 ~xm by the Nanoscope III software. IR spectra of the DLC films were recorded with an FTIR spectrometer with a resolution of 0.4 cm ~. The hydrogen content was calculated from the integral peak area of the 1R spectrum belween 2800 and 3060 cm ~ The s p - / s p - ratio was evaluated by calculating the intensity ratio between the sp3-CH peak at 2915 cm ~ and the sp2-CH peak at 3000 cm 1.
3. Results and discussion Fig. 2 shows that the hardness of the DLC films decreases monotonously with increasing pressure (causing the negative bias voltage to decrease at the same time). This behavior was observed for various film thicknesses, although it must be mentioned that it was difficult to measure the hardness of films thinner than 100 nm. This finding shows that the hardness must be controlled by the bias voltage which is in accordance with previous work [4]. The film hardness deposited at low pressure region is comparable to that of other reports [7,15] Figs. 3 - 6 show AFM images of film topographies obtained at different pressures. In the low pressure region (13 and 53 Pa) pronounced protrusions were observed on the thinner films (1 and 5 nm). The protrusions observed at
K.K. Hirakuri et al. / Thin Solid Films 302 (1997) 5-11
30
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........ 10
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........
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Pressure (Pa)
}Zig. 2. Knoop hardness of the DLC fihns (thickness, 500 nm) as a (unction of the pressure. 13 Pa are sharper than those observed at 53 Pa, because the energy of the sputtering ionic species is higher at low pressure (high bias voltage). The bias voltages were - 2 5 0 V at 13 Pa and - 1 8 0 V at 53 Pa, respectively. On the thickest fihns (1 ~xm) deposited at lower pressures (Fig. 3(c) and Fig. 4(c)), such pronounced features cannot be observed. This flattening effect can be explained by sputter removal of protruding features through ions accelerated towards sharp tips in the electrical field. This process is only significant at low pressures. Whereas a decrease of the roughness with film thickness is observed at 13 Pa and 53 Pa an opposite behavior
7
is found at 130 and 1300 Pa (Fig. 7). For instance, at 130 Pa initially (1 rim) a fine grained structure (rms roughness, (1.4 nm) is observed. During the course of deposition these small features grow leading to comparatively large undulations (size 0.2 nm) at an rms roughness of 0.5 nm (Fig. 5(c)). Fig. 6 shows the topography of the film surfaces at 1300 Pa. Starting from an initially flat surface, growth of features similar to deposition at 1300 Pa is observed leading to an nns roughness of 0.6 nm at 1 txm film thickness. Fig. 7 shows the nns roughness versus film thickness for the different deposition conditions. The roughness of the film surface at lower pressures (13 and 53 Pa) decreases with the thickness until a constant value of approximately 0.2 nm is reached for thicknesses larger than 10 nm. At medium pressures (130 Pa) the roughness is almost constant. At 1300 Pa the roughness increases with film thickness. In Fig. 7 three regions can be identified: region l up to 1 nm thickness, region II between 1 nm and 10 nm and region Ill beyond 10 nm. In region ! a significant increase of the roughness can be observed for all pressure conditions due to deposition of DLC films on the plain Si substrates. The lower value for the film deposited at 1300 Pa could be explained by the fact that its actual thickness was lower than 1 nm. It should be emphasized, however, that all surfaces studied in this work were much flatter
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Fig. 3. AFM images showing the morphology of DLC films deposited on Si at 13 Pa. Film thicknesses: (a) 10 nm, (b) 100 nm and (c) 1000 nm. Image size, 1 g m × 1 Ixm; depth scale. 10 nm from black to white.
8
K.K. Hirakuri et al./Thin Solid Fihns 302 (1997) 5-11
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K.K. Hirakuri et a l . / T h i n Solid Films 302 (1997) 5 - I 1
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than the roughnesses of commonly used CVD diamond films and WC materials, which are in the order of several micrometers and several hundred nanometers, respectively. At lower pressures (13 and 53 Pa), the roughness in region II is decreasing due to predominance of sputtering by ionic species as also shown in Figs. 3 and 4. At 130 Pa, the roughness is almost constant, since sputtering and deposition is about balanced in the pressure region. At 1300 Pa the roughness increases in II region, because sputtering cannot take place at that pressure. The maximum roughness is only determined by the random deposition of neutral species on the surface. The decrease of the rms roughness from 13 Pa to 53 Pa is originated by the 1 "~
0.8
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difference (70 V) of the bias voltages at 13 Pa and 53 Pa. At low pressure condition (13 Pa), both weak and strong bonds of the film surface are sputtered and milled by the ionic species with high energy, and most of the ionic species attack to the surface without collision to any other molecules in the atmosphere. In region III, the roughness of all films is roughly constant. The roughnesses of the films deposited at 13 Pa are higher than those of the films deposited at 53 Pa, because sputtering attack is more pronounced at lower pressure changing the equilibrium towards higher roughness. Nakayama et al. reported that the magnitude of negative bias voltage influences surface flatness by the collisions of accelerated positive ions against the substrate surface [16]. At a few hundred nanometers thickness, the roughness of our films agrees with their results. The topography of hydrogenated amorphous carbon films was observed by AFM [17]. Since the power and the substrate temperature are high as compared with our deposition condition, the bias voltage on the substrate may be high. The ionic species attached to the substrates and the roughness of their samples is rougher than our samples. Next, we have investigated our DLC films by IR spectroscopy. First, the absorption of CH bonds around 3000 cm J was measured in order to obtain information oll the hydrogen content [18,19]. The hydrogen content has been evaluated by integrating the areas of all CH,, peaks. Due to
10
K.K. Hirakuri el al./Thin Solid Fihns 302 (1997) 5 11 12
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1000
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Thickness (nm)
Thickness (nm) Fig. 8. Hydrogen content of the DLC films versus thickness. All values are normalized to 1 for all films with a thickness of 1 gin.
Fig. 9. Peak intensity ralio o[" sp ~/sp 2 versus film thickness.
the absorption sensitivity of the IR spectrometer, this is only possible if the peaks are not shifted and the shapes of the spectra are similar [20]. This condition was fulfilled for all but the thickest film at the lowest pressure. In the latter film the shapes of the spectra are different, since sp :~ components of CH bonds are dominant. Fig. 8 shows the hydrogen content C H (normalized at 1 for all 1000 nm thick films) as a function of the film thickness. The films with a thickness of 1 nm are not included, since IR spectra could not be recorded. At lower pressures, C n gradually decreases with thickness up to 100 rim. This can be explained by the sputtering a n d / o r etching of weakly bond sp 2 components in the films leading to decreasing peak intensities at 3000 and 3020 cm ~ At medium pressure the curve is about constant, since deposition and sputtering is balanced. At higher pressure, C H abruptly increases with thickness up to 100 nm. The lowest value of C H for the film at 1300 Pa may be affected by the uncertainly of the thickness. Next, IR spectra were evaluated in order to determine the s p 3 / s p 2 ratio. Table 1 gives an overview on relevant peaks. The peaks are identified as sp'"-CH,, vibrations by Refs. [18,19,21-23]. The most intensive peaks are at 2920 cm ~ due to CH bonds of sp:~-CH and spS-CH~. Two peaks originating from sp2-CH,, were identified at higher
wavenumber near 3010 cm ~. In order to calculate the s p S / s p 2 ratio, the intensities of the sp 3 and the sp 2 peaks were measured after waveform separation. Fig. 9 shows the intensity ratios of the spS-CH peak at 2920 c m - J and the spe-CH peak at 3000 cm ~. The ratios of the films at 130 and 1300 Pa are almost constant. At high pressure the fihns are observed to consist of polymer-like carbon containing a high portion of sp 2 components [18]. At lower pressure, the ratio increases with the thickness. This can be explained by sputter removal of weakly bond sp 2 components.
"Fable 1 C - H absorption bands for DLC films
sp~-CH: (symmetrical) sp~-CH~ (symmetrical) sp3-CH sp~-CH ~_(asymmetrical) sP 2-CH 2 (olefinic) sp3-CH 3 (asymmetrical) spe-CH (olefinic) sp 2 CH: (olefinic) sp:-CH (aromatic)
Predicted frequency (cm I )
Observed frequency (cm I)
2855 2870 2915 2925 2950 2960 3000 3020 3050
2850 2870 2915 2920 2960 3005 3020
4. Conclusions A F M and IR spectroscopy have been used successfully to study the mechanism of DLC thin film deposition on silicon substrates by r.f. plasma discharge of methane gas. Generally, the surfaces were much flatter than other materials (e.g. diamond films) frequently used for hard coating. A F M results revealed that the surface morphology of the films is strongly affected by the deposition conditions, i.e. the bias voltage between electrode and substrate holder which depends on the pressure in the chamber. The flattest films with the highest hardness are obtained at lower pressures. Those films also exhibit the highest content of sp ~ components. IR absorption spectra were used to determine hydrogen content and s p ; / s p e ratio of the films. Combining IR and AFM results, relations between film thickness, hardness, hydrogen content and s p 3 / s p -~ ratios could be established for different deposition conditions. It can be concluded that film growth and properties are controlled by the predominance of different species in the plasma leading to sputtering through acceleration in the electric field. Therefore the pressure, and thus the bias voltage must be optimized in order to obtain fiat films with sufficient hardness. In the shown investigations this optimum was found to be around a pressure of 53 Pa.
K.K. Hirakuri et al./Thin Solid Fihns 302 (1997) 5-11
Acknowledgements This work was supported by the Centre for Research, Tokyo Denki University. Financial support of this work by 1he Austrian Science Foundation, project P11015-OPY is gratefully acknowledged.
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[10] K. Tachibana, T. Mukai, A. Yuuki. Y. Matsui and H. Harima, Jpn. J. Appl. Phys.. 29 (1990) 215. [11] N. Mutsukura, S. lnoue and Y. Machi, J. Appl. Phys.. 72 (1992) 43. [121 H. Toyoda, H. Kojima and H. Sugai, Appl. Phys. Lett., 55 (1989) 1292. [ 13] G. Friedbacher, T. Prohaska and M. Grasserbauer, Mikrochim. Acta. 113 (1994) 179. [14] G. Friedbacher, E. Bouveressc, (;. Fuchs, M. Grasserbauer, D. Schwarzbach, R. Haubner and B. Lux, Appl. Surf] Sci., 84 (1995) 133. [15] H. J. Scheibe. D. Drescher and A. Mensch, Fres. J. Anal. Chem.. 353 (1995) 690. [16] M. Nakayama. A. Tsuyoshi, M. Shibahara, K. Maruyama and K. Kamata, J. Vac. Sci. Technol., 13 (1995) 195. [17] Y. Nakamura. Y. Watanabe and Y. Suefuji, J. Amer. Ceram. Soc., 77 (1994) 1385. [18] B. Dishler, A. Bubenzer and P. Koidl, Solid State Commun.. 48 (2) (1983) 105. [19] P. Couderec and Y. Catherine. Thin Solid Fihns. 146 (1987) 93. [20] K. Kobayashi, K. Yamamoto, N. Mutsukura and Y. Machi, Thin Solid t:ihn.~, 185 (1990) 71. [21] J.W. Zou, K. Schmidt. K. Reichelt and B. Dischler, J. Appl. Phys.. 67 (1990) 487. [22] A. Hol'fman, A. Fayer. A. Laikhtman and R. Brener, J. Appl. Phys., 77 (1995) 3126. [23] H.D. Bint. S. Bhargava and M. Aslam..I. Raman Spectrosc., 25 (1994) 67.