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Metal-doped diamond-like carbon films synthesized by filter-arc deposition
Thin Solid Films 515 (2006) 1053 – 1057 www.elsevier.com/locate/tsf
Metal-doped diamond-like carbon films synthesized by filter-arc deposition Ko-Wei Weng a,⁎, Ya-Chi Chen b , Tai-Nan Lin c , Da-Yung Wang a a
Department of Materials Science and Engineering, Mingdao University, Taiwan ROC 369-B, Wen-Hua Road, Peetow, Chang-Hwa, Taiwan, ROC b Department of Materials Engineering, National Chung Hsing University, Taiwan ROC 250, Kuo Kuang Road, Taichung 402, Taiwan, ROC c Chemical Engineering Division, Institute of Nuclear Energy Research, Longtan 325, Taiwan ROC Available online 7 September 2006
Abstract Diamond-like carbon (DLC) thin films are extensively utilized in the semiconductor, electric and cutting machine industries owing to their high hardness, high elastic modulus, low friction coefficients and high chemical stability. DLC films are prepared by ion beam-assisted deposition (BAD), sputter deposition, plasma-enhanced chemical vapor deposition (PECVD), cathodic arc evaporation (CAE), and filter arc deposition (FAD). The major drawbacks of these methods are the degraded hardness associated with the low sp3/sp2 bonding ratio, the rough surface and poor adhesion caused by the presence of particles. In this study, a self-developed filter arc deposition (FAD) system was employed to prepare metal-containing DLC films with a low particle density. The relationships between the DLC film properties, such as film structure, surface morphology and mechanical behavior, with variation of substrate bias and target current, are examined. Experimental results demonstrate that FAD-DLC films have a lower ratio, suggesting that FAD-DLC films have a greater sp3 bonding than the CAE-DLC films. FAD-DLC films also exhibit a low friction coefficient of 0.14 and half of the number of surface particles as in the CAE-DLC films. Introducing a CrN interfacial layer between the substrate and the DLC films enables the magnetic field strength of the filter to be controlled to improve the adhesion and effectively eliminate the contaminating particles. Accordingly, the FAD system improves the tribological properties of the DLC films. © 2006 Elsevier B.V. All rights reserved. Keywords: Diamond-like Carbon films (DLC); Filter-arc deposition (FAD); Cathodic arc evaporation (CAE); Tribological property
1. Introduction Various wear-resistant thin films, such as TiN, CrN and diamond-like carbon (DLC) films are used extensively owing to their favorable properties. Diamond-like carbon (DLC) films are used in a wide range of industries because they exhibit the characteristics of high mechanical hardness, high thermal conductivity, excellent tribological behavior, chemical stability and ease of control during preparation. These characteristics make DLC films excellent candidates for tribological applications. In industry, commonly used methods of deposition of DLC films include ion beam-assisted (BAD), sputter deposition, plasma-enhanced chemical vapor deposition (PECVD), cathodic arc evaporation (CAE), and filter arc deposition (FAD). According to the literature [1–9], the tribological properties vary with the method of deposition and the environment. These effects reduce the film hardness for reasons associated with the ⁎ Corresponding author. E-mail address: [email protected] (K.-W. Weng). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.07.064
low sp3/sp2 bonding ratio, the roughening of the surface by contaminating particles and poor adhesion. CAE is a lowtemperature deposition method with a better ion ratio and higher ion energy. By CAE deposition, films with strong sp3 bonding are easily produced. During deposition, ion bombardment on the substrates improves adhesion between the film and the substrate [10]. However, undesirable contaminating surface particles limit the applications of CAE-DLC films. In this study, filter arc deposition (FAD) is employed to prepare DLC thin films. FAD can effectively filter out undesired particles by control of the magnetic field in the plasma tube and thus yield a 100% purified high energy ion beam to overcome the drawbacks of conventional PVD or CAE-deposited DLC films. A conducting metal interface between the substrate and the thin film increased the adhesion of the thin film by increasing the density of the microstructure; smoothed the surface, and yielded excellent mechanical properties. Therefore, in the FAD system, the substrate bias and the target electric current were adjusted to explore their influence on the microstructure, the surface morphology and the mechanical properties of the films.
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Table 1 Experimental parameter
Target material Partial pressure (Pa) Reactive gas Cathode current (A) Substrate bias voltage(− V) Temperature (°C) Deposition time (min)
FAD
CAE
Cr 1.53 N2/C2H2 50′60′70 50′100′150 100 60
Cr 3.26 N2/C2H2 40 100 350 30
7000F) is employed to observe the surface morphology and cross-sectional images. Raman spectroscopy and X-ray photoelectron spectrometry (XPS, PHI 1600), using Mg Kα radiation (hv = 1253.6 eV), were employed to identify the binding state, the species of the compounds and the sp3/sp2 ratio of the film. Finally, mechanical property tests, involving measuring the hardness and wear resistance of each coating were performed using a Vickers's micro-hardness test device kit (Mitutoyo I MVK-GB) and a CSEM ball-on-disk tribometer. 3. Results and discussion
In the FAD system, the carbon source for the DLC films is C–H gas pyrolysis. A metal layer and then a nitride layer are deposited before the direct deposition of the DLC films. The interfacial layers can relax the internal stress and improve adhesion behavior. Also, the deposition rate and ion energy are high. Therefore, the metal-containing DLC films deposited by the FAD system should have high adhesive strength and surface smoothness, improving their hardness, anti-corrosive properties, anti-oxidation properties, wear-resistance and surface energy. 2. Experimental procedure This work discusses mainly the properties of DLC thin films deposited by CAE and FAD deposition system. In a FAD source, controlling the magnetic field of the plasma duct and the electric potential can generate 100% pure plasma and excellent coating properties. The substrates used herein are SKD-5l(870 Hv) and Si. This study investigates the relationship between processing parameters and film properties. Table 1 lists the deposition parameters. Scanning electron microscopy (SEM, JOEL JSM-
3.1. Effect of microstructure of DLC film on tribological properties Fig. 1 shows the SEM micrographs of CAE-and FAD-DLC films. In Fig. 1 (a), particles from the target are directly scattered onto the surface of the film during CAE deposition, whereas FAD-deposited films exhibit surfaces smoothed by filter-out process. Approximately 60% of the particles are filtered-out statistically. However, few particles of small energy can be deposited onto the film surface by bombarding the tube wall and this is shown in Fig. 1 (b)–(d). Fig. 2 depicts the statistics of the particles in various samples. Under a given magnification (1000×) the number of particles obtained by CAE-DLC is 374, while the number obtained by FAD-DLC is 157–170. The number of particles increases with the target current, presumably because increasing the target current from 50 A to 70 A increased the temperature. The above results indicate that the FAD system reduces the number of particles on the surface of the film. Since the surface of the film formed by FAD-DLC is smoother than that obtained by CAE-DLC, the friction coefficient tests also
Fig. 1. Surface morphology of (a) CAE-DLC and FAD-DLC at various cathode current (b) 50 A (c) 60 A (d) 70 A.
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Fig. 2. Statistical surface particle numbers of CAE-DLC and FAD-DLC films. Fig. 4. Ball-on-disk wear tests of CAE-DLC and FAD-DLC films.
reflect the differences. The measured friction coefficients are 0.2 for CAE-DLC and 0.14 for FAD-DLC films. The particles on the film's surface increase the surface roughness and thereby affect the friction coefficient. The number of particles is increased by the target electric current and the magnetic field of the filter system. As the target electric current declines, the heat energy accumulated on the target decreases, reducing the number of particles dispersed on the film surface. Combining this reduced target electric current with the filtering process can produce DLC films with smooth surfaces and low friction coefficient. Fig. 3 shows the crosssectional SEM micrographs of CAE-DLC and FAD-DLC films. The particles scattered during deposition importantly affect the deposited microstructure. The CAE-DLC film exhibits a loose structure while the FAD-DLC films exhibit a dense structure. Fig. 4 depicts the relationship between friction coefficient and sliding distance. The tests are performed at 30 m/s for a fixed sliding contact area, and the load is kept constant at 10 N. Since
the testing sample fails at a friction coefficient of 0.5, this value is assumed to represent the critical condition. From the figure, the friction coefficient of CAE-DLC film starts to increase at 50 km and the wear life of the film is approximately 80 km. The friction coefficient of FAD-DLC film starts to increase at 200 km and the wear life of the film is approximately 340 km. The friction coefficient and wear life are improved by the dense film structures and the better layer adhesion obtained using the FAD method. In the same testing environment for similar film thicknesses, the wear life of the FAD-DLC films is four times than that of the CAE-DLC film. 3.2. Effect of chemical states on hardness of DLC films Fig. 5 clearly displays DLC's characteristic Raman D and G bands, at 1350 cm− 1 and 1550 cm− 1. Fig. 6 plots the G band peak position as a function of the target electric current. The G
Fig. 3. The cross sectional microstructures of (a) CAE-DLC and FAD-DLC at various cathode current (b) 50 A (c) 60 A (d) 70 A.
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Fig. 8. sp3/sp2 ratios and microhardness of CAE-DLC and FAD-DLC films at various cathodic currents. Fig. 5. Raman spectra of CAE-DLC and FAD-DLC films.
Fig. 6. The G peak position measurements and I and ratios of CAE-DLC and FAD-DLC films at various cathodic currents.
band, which originates from the lattice vibrations shifts, is shifted to a considerably longer wavelength as the target electric current is increased, suggesting a higher internal stress. Fig. 6 also displays the Raman intensity ratio ID/IG. A smaller ID/IG ratio indicates greater sp3 bonding in the film. The ID/IG value of the CAE-DLC film significantly exceeds that of the FAD-DLC, indicating that the latter has stronger sp3 bonding.
The disorder ratio of the film structure formed by CAE-DLC is lower, and is thus responsible for the higher ID/IG. The increase in the target electric current in FAD-DLC films provides sufficient energy for the deposited atomic species to move and rearrange on the film surface. Atoms tend to move towards the lowest energy site, reducing the disorder ratio. Accordingly, for the same substrate bias, the ID/IG value is the lowest and the sp3 ratio is highest at a target electric current of 50 A. ESCA yields evidence of the chemical states of the film structures. Fig. 7 displays the ESCA spectra of the DLC thin films, including Cls (284.5 eV), O1s (531.0 eV), and Cr2p2/3 (576.9 eV). The O1s peak is attributed to the excess oxygen in the deposition chamber. The structure of diamond is tetrahedral covalent sp3 bonding while that of graphite is layered hexagonal sp2 bonding. More sp3 bonding of the DLC film makes it more diamond-like while more sp2 bonding makes it more graphitelike. Prawer [11] et al. reported that the ID/IG value, obtained by Raman spectroscopy was proportional to the sp2/sp3 ratio obtained from ESCA. Therefore, if ID/IG is lower, more sp3 bonds are present in the film structure. Cls data from the ESCA spectra can be fitted by a Gaussian function with three characteristic peaks. The peak binding energies are 284.3 eV (C_C), 285.2 eV (C–C), and 286.8 eV (C_O) [12]. In Fig. 8,
Fig. 7. ESCA spectra of CAE-DLC and FAD-DLC films.
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the calculated sp3/sp2 ratios are in the range 0.74–1.23. The value is 0.74 for the CAE-DLC film, which is consistent with the Raman results. The Raman and ESCA analyses suggest that sp3 bonding in FAD-DLC exceeds that in CAE-DLC, indicating that it is more diamond-like and, therefore, harder. Fig. 8 also plots the measured hardness. The hardness of the CAE-DLC film is 3506 Hv. Those of the FAD-DLC films are 5542 Hv for a target electric current of 50 A, 4957 Hv for 60 A, and 4055 for 70 A. Three major effects on film hardness are as follows. First, a single crystal structure exhibits more isotropic hardness than poly crystal ones. Second, a larger average grain size results in lower hardness. Third, a denser film structure is harder. Accordingly, Figs. 1 and 3 showed numerous defects in the CAE-DLC film structure. Furthermore, based on previous chemical analyses, the sp3/sp2 values are 0.742 for CAE-DLC film, 1.24 for 5OA-FAD-DLC, 1.20 for 6O A-FAD-DLC and 0.83 for 70 A-FAD-DLC films. Clearly, the hardness values of the FAD-DLC films exceeds those of CAE-DLC films because their microstructures are denser and the sp3/sp2 ratios higher. 4. Summary Filter Arc Deposition (FAD) is employed herein in this study to prepare metal-containing DLC thin films with few contaminating particles. FAD can eliminate the contaminating particles that are formed in conventional deposition methods, improving the properties of the films. The substrate bias and target electric current are varied to explore the influences of so doing on the DLC film structures, their surface morphologies and their mechanical properties. 1. FAD can effectively improve deposition by filtering out 60% of the undesired particles. It can reduce the surface roughness
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and enhance wear resistance. The friction coefficient decreases from 0.2 to 0.14. 2. When particles are filtered out by FAD, the FAD-DLC films have dense film structures. The wear life is four times that of the CAE-DLC film. 3. At a low target electric current, the disorder ratio in the FADDLC films increases, shifting to low wavelength the G band in the Raman spectra. Therefore, the DLC films have a diamond-like structure as the sp3 bonding is increased. 4. DLC thin films prepared by FAD contain more sp3 bonding, increasing their hardness. The hardness, ID/IG, and sp3/sp2 values for CAE-DLC and FAD-DLC films are 3506 Hv and 5542 Hv, 0.8 and 1.23, 1.25 and 0.75, respectively. References [1] H.R. Kauhann, J. Vac. Sci. Technol. 15 (1978) 272. [2] R. Locher, C. Wild, P. Koidl, Surf. Coat. Technol. 47 (1991) 426. [3] B. Druz, R. Ostan, S. Distefano, A. Hayes, V. Kanarov, V. Polyakov, Diamond Relat. Mater. 7 (1998) 965. [4] J.J. Cuomo, J.P. Doyle, J. Bruley, C. Liu, Appl. Phys. Lett. 58 (1991) 466. [5] M. Chhowalla, J. Robertson, C.W. Chen, S.R.P. Silva, G.A.J. Amaratunga, J. Appl. Phys. 81 (1997) 139. [6] J. Robertson, Materials Science and Engineering, R37(129–281). [7] D.Y. Wang, K.W. Weng, S.Y. Hwang, Diamond Relat. Mater. 9 (2000) 1762. [8] D.Y. Wang, K.W. Weng, C.L. Chang, X.J. Guo, Diamond Relat. Mater. 9 (2000) 831. [9] J. Robertson, Mater. Sci. Eng., R Rep. 37 (2002) 129. [10] J.E. Daalder, IEEE Trans. Power Appar. Syst. 93 (1974) 1747. [11] S. Prawer, K.W. Nugent, Y. Lifsfitz, G.D. Lempert, E. Grossman, J. Kulik, I. Avigal, R. Kalish, Diamond Relat. Mater. (1996) 433. [12] S.T. Jackson, R.G. Nuzzo, Appl. Surf. Sci. 90 (1990) 195.
Metal-doped diamond-like carbon films synthesized by filter-arc deposition Ko-Wei Weng a,⁎, Ya-Chi Chen b , Tai-Nan Lin c , Da-Yung Wang a a
Department of Materials Science and Engineering, Mingdao University, Taiwan ROC 369-B, Wen-Hua Road, Peetow, Chang-Hwa, Taiwan, ROC b Department of Materials Engineering, National Chung Hsing University, Taiwan ROC 250, Kuo Kuang Road, Taichung 402, Taiwan, ROC c Chemical Engineering Division, Institute of Nuclear Energy Research, Longtan 325, Taiwan ROC Available online 7 September 2006
Abstract Diamond-like carbon (DLC) thin films are extensively utilized in the semiconductor, electric and cutting machine industries owing to their high hardness, high elastic modulus, low friction coefficients and high chemical stability. DLC films are prepared by ion beam-assisted deposition (BAD), sputter deposition, plasma-enhanced chemical vapor deposition (PECVD), cathodic arc evaporation (CAE), and filter arc deposition (FAD). The major drawbacks of these methods are the degraded hardness associated with the low sp3/sp2 bonding ratio, the rough surface and poor adhesion caused by the presence of particles. In this study, a self-developed filter arc deposition (FAD) system was employed to prepare metal-containing DLC films with a low particle density. The relationships between the DLC film properties, such as film structure, surface morphology and mechanical behavior, with variation of substrate bias and target current, are examined. Experimental results demonstrate that FAD-DLC films have a lower ratio, suggesting that FAD-DLC films have a greater sp3 bonding than the CAE-DLC films. FAD-DLC films also exhibit a low friction coefficient of 0.14 and half of the number of surface particles as in the CAE-DLC films. Introducing a CrN interfacial layer between the substrate and the DLC films enables the magnetic field strength of the filter to be controlled to improve the adhesion and effectively eliminate the contaminating particles. Accordingly, the FAD system improves the tribological properties of the DLC films. © 2006 Elsevier B.V. All rights reserved. Keywords: Diamond-like Carbon films (DLC); Filter-arc deposition (FAD); Cathodic arc evaporation (CAE); Tribological property
1. Introduction Various wear-resistant thin films, such as TiN, CrN and diamond-like carbon (DLC) films are used extensively owing to their favorable properties. Diamond-like carbon (DLC) films are used in a wide range of industries because they exhibit the characteristics of high mechanical hardness, high thermal conductivity, excellent tribological behavior, chemical stability and ease of control during preparation. These characteristics make DLC films excellent candidates for tribological applications. In industry, commonly used methods of deposition of DLC films include ion beam-assisted (BAD), sputter deposition, plasma-enhanced chemical vapor deposition (PECVD), cathodic arc evaporation (CAE), and filter arc deposition (FAD). According to the literature [1–9], the tribological properties vary with the method of deposition and the environment. These effects reduce the film hardness for reasons associated with the ⁎ Corresponding author. E-mail address: [email protected] (K.-W. Weng). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.07.064
low sp3/sp2 bonding ratio, the roughening of the surface by contaminating particles and poor adhesion. CAE is a lowtemperature deposition method with a better ion ratio and higher ion energy. By CAE deposition, films with strong sp3 bonding are easily produced. During deposition, ion bombardment on the substrates improves adhesion between the film and the substrate [10]. However, undesirable contaminating surface particles limit the applications of CAE-DLC films. In this study, filter arc deposition (FAD) is employed to prepare DLC thin films. FAD can effectively filter out undesired particles by control of the magnetic field in the plasma tube and thus yield a 100% purified high energy ion beam to overcome the drawbacks of conventional PVD or CAE-deposited DLC films. A conducting metal interface between the substrate and the thin film increased the adhesion of the thin film by increasing the density of the microstructure; smoothed the surface, and yielded excellent mechanical properties. Therefore, in the FAD system, the substrate bias and the target electric current were adjusted to explore their influence on the microstructure, the surface morphology and the mechanical properties of the films.
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Table 1 Experimental parameter
Target material Partial pressure (Pa) Reactive gas Cathode current (A) Substrate bias voltage(− V) Temperature (°C) Deposition time (min)
FAD
CAE
Cr 1.53 N2/C2H2 50′60′70 50′100′150 100 60
Cr 3.26 N2/C2H2 40 100 350 30
7000F) is employed to observe the surface morphology and cross-sectional images. Raman spectroscopy and X-ray photoelectron spectrometry (XPS, PHI 1600), using Mg Kα radiation (hv = 1253.6 eV), were employed to identify the binding state, the species of the compounds and the sp3/sp2 ratio of the film. Finally, mechanical property tests, involving measuring the hardness and wear resistance of each coating were performed using a Vickers's micro-hardness test device kit (Mitutoyo I MVK-GB) and a CSEM ball-on-disk tribometer. 3. Results and discussion
In the FAD system, the carbon source for the DLC films is C–H gas pyrolysis. A metal layer and then a nitride layer are deposited before the direct deposition of the DLC films. The interfacial layers can relax the internal stress and improve adhesion behavior. Also, the deposition rate and ion energy are high. Therefore, the metal-containing DLC films deposited by the FAD system should have high adhesive strength and surface smoothness, improving their hardness, anti-corrosive properties, anti-oxidation properties, wear-resistance and surface energy. 2. Experimental procedure This work discusses mainly the properties of DLC thin films deposited by CAE and FAD deposition system. In a FAD source, controlling the magnetic field of the plasma duct and the electric potential can generate 100% pure plasma and excellent coating properties. The substrates used herein are SKD-5l(870 Hv) and Si. This study investigates the relationship between processing parameters and film properties. Table 1 lists the deposition parameters. Scanning electron microscopy (SEM, JOEL JSM-
3.1. Effect of microstructure of DLC film on tribological properties Fig. 1 shows the SEM micrographs of CAE-and FAD-DLC films. In Fig. 1 (a), particles from the target are directly scattered onto the surface of the film during CAE deposition, whereas FAD-deposited films exhibit surfaces smoothed by filter-out process. Approximately 60% of the particles are filtered-out statistically. However, few particles of small energy can be deposited onto the film surface by bombarding the tube wall and this is shown in Fig. 1 (b)–(d). Fig. 2 depicts the statistics of the particles in various samples. Under a given magnification (1000×) the number of particles obtained by CAE-DLC is 374, while the number obtained by FAD-DLC is 157–170. The number of particles increases with the target current, presumably because increasing the target current from 50 A to 70 A increased the temperature. The above results indicate that the FAD system reduces the number of particles on the surface of the film. Since the surface of the film formed by FAD-DLC is smoother than that obtained by CAE-DLC, the friction coefficient tests also
Fig. 1. Surface morphology of (a) CAE-DLC and FAD-DLC at various cathode current (b) 50 A (c) 60 A (d) 70 A.
K.-W. Weng et al. / Thin Solid Films 515 (2006) 1053–1057
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Fig. 2. Statistical surface particle numbers of CAE-DLC and FAD-DLC films. Fig. 4. Ball-on-disk wear tests of CAE-DLC and FAD-DLC films.
reflect the differences. The measured friction coefficients are 0.2 for CAE-DLC and 0.14 for FAD-DLC films. The particles on the film's surface increase the surface roughness and thereby affect the friction coefficient. The number of particles is increased by the target electric current and the magnetic field of the filter system. As the target electric current declines, the heat energy accumulated on the target decreases, reducing the number of particles dispersed on the film surface. Combining this reduced target electric current with the filtering process can produce DLC films with smooth surfaces and low friction coefficient. Fig. 3 shows the crosssectional SEM micrographs of CAE-DLC and FAD-DLC films. The particles scattered during deposition importantly affect the deposited microstructure. The CAE-DLC film exhibits a loose structure while the FAD-DLC films exhibit a dense structure. Fig. 4 depicts the relationship between friction coefficient and sliding distance. The tests are performed at 30 m/s for a fixed sliding contact area, and the load is kept constant at 10 N. Since
the testing sample fails at a friction coefficient of 0.5, this value is assumed to represent the critical condition. From the figure, the friction coefficient of CAE-DLC film starts to increase at 50 km and the wear life of the film is approximately 80 km. The friction coefficient of FAD-DLC film starts to increase at 200 km and the wear life of the film is approximately 340 km. The friction coefficient and wear life are improved by the dense film structures and the better layer adhesion obtained using the FAD method. In the same testing environment for similar film thicknesses, the wear life of the FAD-DLC films is four times than that of the CAE-DLC film. 3.2. Effect of chemical states on hardness of DLC films Fig. 5 clearly displays DLC's characteristic Raman D and G bands, at 1350 cm− 1 and 1550 cm− 1. Fig. 6 plots the G band peak position as a function of the target electric current. The G
Fig. 3. The cross sectional microstructures of (a) CAE-DLC and FAD-DLC at various cathode current (b) 50 A (c) 60 A (d) 70 A.
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Fig. 8. sp3/sp2 ratios and microhardness of CAE-DLC and FAD-DLC films at various cathodic currents. Fig. 5. Raman spectra of CAE-DLC and FAD-DLC films.
Fig. 6. The G peak position measurements and I and ratios of CAE-DLC and FAD-DLC films at various cathodic currents.
band, which originates from the lattice vibrations shifts, is shifted to a considerably longer wavelength as the target electric current is increased, suggesting a higher internal stress. Fig. 6 also displays the Raman intensity ratio ID/IG. A smaller ID/IG ratio indicates greater sp3 bonding in the film. The ID/IG value of the CAE-DLC film significantly exceeds that of the FAD-DLC, indicating that the latter has stronger sp3 bonding.
The disorder ratio of the film structure formed by CAE-DLC is lower, and is thus responsible for the higher ID/IG. The increase in the target electric current in FAD-DLC films provides sufficient energy for the deposited atomic species to move and rearrange on the film surface. Atoms tend to move towards the lowest energy site, reducing the disorder ratio. Accordingly, for the same substrate bias, the ID/IG value is the lowest and the sp3 ratio is highest at a target electric current of 50 A. ESCA yields evidence of the chemical states of the film structures. Fig. 7 displays the ESCA spectra of the DLC thin films, including Cls (284.5 eV), O1s (531.0 eV), and Cr2p2/3 (576.9 eV). The O1s peak is attributed to the excess oxygen in the deposition chamber. The structure of diamond is tetrahedral covalent sp3 bonding while that of graphite is layered hexagonal sp2 bonding. More sp3 bonding of the DLC film makes it more diamond-like while more sp2 bonding makes it more graphitelike. Prawer [11] et al. reported that the ID/IG value, obtained by Raman spectroscopy was proportional to the sp2/sp3 ratio obtained from ESCA. Therefore, if ID/IG is lower, more sp3 bonds are present in the film structure. Cls data from the ESCA spectra can be fitted by a Gaussian function with three characteristic peaks. The peak binding energies are 284.3 eV (C_C), 285.2 eV (C–C), and 286.8 eV (C_O) [12]. In Fig. 8,
Fig. 7. ESCA spectra of CAE-DLC and FAD-DLC films.
K.-W. Weng et al. / Thin Solid Films 515 (2006) 1053–1057
the calculated sp3/sp2 ratios are in the range 0.74–1.23. The value is 0.74 for the CAE-DLC film, which is consistent with the Raman results. The Raman and ESCA analyses suggest that sp3 bonding in FAD-DLC exceeds that in CAE-DLC, indicating that it is more diamond-like and, therefore, harder. Fig. 8 also plots the measured hardness. The hardness of the CAE-DLC film is 3506 Hv. Those of the FAD-DLC films are 5542 Hv for a target electric current of 50 A, 4957 Hv for 60 A, and 4055 for 70 A. Three major effects on film hardness are as follows. First, a single crystal structure exhibits more isotropic hardness than poly crystal ones. Second, a larger average grain size results in lower hardness. Third, a denser film structure is harder. Accordingly, Figs. 1 and 3 showed numerous defects in the CAE-DLC film structure. Furthermore, based on previous chemical analyses, the sp3/sp2 values are 0.742 for CAE-DLC film, 1.24 for 5OA-FAD-DLC, 1.20 for 6O A-FAD-DLC and 0.83 for 70 A-FAD-DLC films. Clearly, the hardness values of the FAD-DLC films exceeds those of CAE-DLC films because their microstructures are denser and the sp3/sp2 ratios higher. 4. Summary Filter Arc Deposition (FAD) is employed herein in this study to prepare metal-containing DLC thin films with few contaminating particles. FAD can eliminate the contaminating particles that are formed in conventional deposition methods, improving the properties of the films. The substrate bias and target electric current are varied to explore the influences of so doing on the DLC film structures, their surface morphologies and their mechanical properties. 1. FAD can effectively improve deposition by filtering out 60% of the undesired particles. It can reduce the surface roughness
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and enhance wear resistance. The friction coefficient decreases from 0.2 to 0.14. 2. When particles are filtered out by FAD, the FAD-DLC films have dense film structures. The wear life is four times that of the CAE-DLC film. 3. At a low target electric current, the disorder ratio in the FADDLC films increases, shifting to low wavelength the G band in the Raman spectra. Therefore, the DLC films have a diamond-like structure as the sp3 bonding is increased. 4. DLC thin films prepared by FAD contain more sp3 bonding, increasing their hardness. The hardness, ID/IG, and sp3/sp2 values for CAE-DLC and FAD-DLC films are 3506 Hv and 5542 Hv, 0.8 and 1.23, 1.25 and 0.75, respectively. References [1] H.R. Kauhann, J. Vac. Sci. Technol. 15 (1978) 272. [2] R. Locher, C. Wild, P. Koidl, Surf. Coat. Technol. 47 (1991) 426. [3] B. Druz, R. Ostan, S. Distefano, A. Hayes, V. Kanarov, V. Polyakov, Diamond Relat. Mater. 7 (1998) 965. [4] J.J. Cuomo, J.P. Doyle, J. Bruley, C. Liu, Appl. Phys. Lett. 58 (1991) 466. [5] M. Chhowalla, J. Robertson, C.W. Chen, S.R.P. Silva, G.A.J. Amaratunga, J. Appl. Phys. 81 (1997) 139. [6] J. Robertson, Materials Science and Engineering, R37(129–281). [7] D.Y. Wang, K.W. Weng, S.Y. Hwang, Diamond Relat. Mater. 9 (2000) 1762. [8] D.Y. Wang, K.W. Weng, C.L. Chang, X.J. Guo, Diamond Relat. Mater. 9 (2000) 831. [9] J. Robertson, Mater. Sci. Eng., R Rep. 37 (2002) 129. [10] J.E. Daalder, IEEE Trans. Power Appar. Syst. 93 (1974) 1747. [11] S. Prawer, K.W. Nugent, Y. Lifsfitz, G.D. Lempert, E. Grossman, J. Kulik, I. Avigal, R. Kalish, Diamond Relat. Mater. (1996) 433. [12] S.T. Jackson, R.G. Nuzzo, Appl. Surf. Sci. 90 (1990) 195.