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Hardness and surface roughness of hydrogenated amorphous carbon films synthesized by atmospheric pressure plasma enhanced CVD method with various pulse frequencies
Surface & Coatings Technology 215 (2013) 460–464
Contents lists available at SciVerse ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Hardness and surface roughness of hydrogenated amorphous carbon films synthesized by atmospheric pressure plasma enhanced CVD method with various pulse frequencies T. Sakurai ⁎, M. Noborisaka, T. Hirako, A. Shirakura, T. Suzuki Center for Science of Environment, Resources and Energy, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan
a r t i c l e
i n f o
Available online 6 November 2012 Keywords: Hydrogenated amorphous carbon Atmospheric pressure CVD Hardness Surface roughness
a b s t r a c t Atmospheric pressure plasma enhanced chemical vapor deposition (AP-PECVD) has attracted much attention for its cost-effectiveness owing to eliminate the use of vacuum devices. We synthesized hydrogenated amorphous carbon (a-C:H) films under atmospheric pressure from C2H2 gas diluted with N2 with varying pulse frequency of plasma source. We investigated the effect of surface texture and chemical bonding structure of the films on hardness. The hardness, surface roughness and chemical content ratio were analyzed by tribo scope nano-mechanical indentation tester, atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS), respectively. As the pulse frequency decreased from 10 to 2 kHz, the hardness increased from 0.35 to 0.92 GPa and the surface roughness decreased from 49.8 to 14.3 nm. From the result of XPS analysis, the N/C molar ratio increased from 0.022 to 0.094 with increasing the pulse frequency. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Diamond-like carbon (DLC) films have various excellent properties such as high hardness, high electrical resistivity, low friction coefficients, low wear rates chemical inertness and low gas permeability [1–3]. Therefore, DLC films have been used for cutting instruments, sliding parts, wear resistant parts [1,2] and food packaging [3]. In conventional, DLC films are synthesized by plasma-enhanced chemical vapor deposition (PECVD) process under vacuum condition less than 100 Pa [4]. However, this synthesis process has some problems such as high equipment cost, long deposition time and limited synthetic area. Atmospheric pressure-plasma enhanced CVD (AP-PECVD) process using dielectric barrier discharge (DBD) serves as one approach to overcoming the aforementioned problems. In 1988, Yokoyama et al. reported that a homogeneous DBD which is called an atmospheric pressure glow discharge (APGD) can be obtained. APGD needs the following three treatments by preventing the undesired transition from glow to arc discharge [5]. 1) using a power source frequency over 1 kHz; 2) inserting dielectric plates between metal electrodes and 3) using helium or neon as a dilution gas. From our previous research [6], we synthesized the hydrogenated amorphous carbon (a-C:H) films whose hardness is under 10 GPa [7] under atmospheric pressure. Sakata et al. synthesized a-C:H films at atmospheric pressure with He dilution and improved their hardness ⁎ Corresponding author at: Center for Science of Environment, Resources and Energy, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan. Tel.: +81 45 563 1141x42070; fax: +81 45 563 5945. E-mail address: [email protected] (T. Sakurai). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2012.09.058
by increasing substrate-temperature up to 200 °C [6]. However, low-temperature process is desired in the case of synthesizing a-C:H films on heat-sensitive materials such as commodity polymers. Furthermore, another type of DBD using molecular gasses such as nitrogen instead of He, which is called atmospheric pressure Townsend-like discharge (APTD), has been actively studied with a view to low-cost process [8,9]. Therefore, in this study, we synthesized a-C:H films under atmospheric pressure at low-temperature with the use of N2 as a dilution gas. We focused on the frequency of the pulsed power supply used for atmospheric-pressure plasma chemical vapor deposition (AP-PECVD) apparatus to improve hardness of the a-C:H films. Sugiyama et al. reported that power supply frequency has an influence on hardness of a-C:H films synthesized under low pressure [10], while no results have been reported to date on the effect of hardness of a-C:H films synthesized under atmospheric pressure on the pulse frequency. Moreover, we investigated the effect of surface texture and chemical content ratio on the hardness of a-C:H films. 2. Experimental 2.1. Apparatus The schematic diagram of the line type atmospheric-pressure plasma CVD apparatus is illustrated in Fig. 1. The plasma was evenly produced and sustained between the upper and lower electrode, whose sizes are 60 × 120 × 50 mm 3 and 300 × 120 × 1 mm 3, respectively. The upper electrode was connected to the electric power supply and the other was connected to the earth. The upper electrode
T. Sakurai et al. / Surface & Coatings Technology 215 (2013) 460–464
461
Power source
Blower
Process gas
Cu electrodes
Blower
Dielectric plates
Si substrate
Plasma
movable
Fig. 1. Schematic diagram of a line type atmospheric-pressure plasma CVD apparatus.
was covered with a dielectric plate (Al2O3) to prevent the transition from glow to arc discharge. As the substrates, Si wafers with a thickness of 100 μm were set on the lower electrode moving at the speed range from 0.1 to 10 mm/s in parallel to the upper one. The distance between upper and lower electrode was 1 mm.
2.2. Experimental procedure The C2H2 gas diluted with N2 gas was supplied between the upper and lower electrodes. The films were prepared at the following gas mixture ratios: [C2H2/(C2H2 + N2)] × 100 (%) = 2, with a total gas flow rate of 1 l/min. We used the power generator (SPD1PGU1, SK Medical Electronics Co., Ltd., Japan). The schematic of pulse wave form is shown in Fig. 2. The pulse width and voltage between electrodes were fixed at 5 μs and 18 kV, respectively. We investigated the effect of pulse frequency on physical properties of a-C:H films synthesized with variation of pulse frequencies at 2, 4, 6, 8, 10 kHz. The power of the power generator increases from 40 W to 220 W in direct proportion to pulse frequency from 2 kHz to 10 kHz.
2.3. Characterization of a-C:H film The thickness of a-C:H films was measured by cross-sectional observation by scanning electron microscope (SEM: S-4700, Hitachi High Technologies Corp., Japan). The measurements were performed at 10 different locations and the results are expressed as the mean of 10 replicates and the corresponding standard deviation. The hardness of the films was measured by tribo scope nano-mechanical indentation tester (TI 900 TriboIndenter, HYSITRON Inc., USA) with a Berkovich diamond indenter. The sample thickness for nano-hardness measurement was approximately 1 μm. The penetration depth was about 100 nm and the load–displacement curve was analyzed using the Oliver and Pharr method [11]. The measurements of the hardness were performed at 5 different surface locations and the results are expressed as the mean of 3 replicates and the corresponding standard deviation. The surface roughness was measured by atomic force microscope (AFM: SPM-9600, Shimadzu Corp., Japan). The scanning area was 5 × 5 μm2. The measurements of the surface roughness were performed at 4 different surface locations and the results are expressed as the mean of 3 replicates and the corresponding standard deviation. The chemical content ratio of a-C:H films were analyzed by X-ray photoelectron spectroscopy (XPS: JPS-9000MC, JEOL Ltd., Japan). The energy resolution was 0.65 eV. For elemental analysis, Ar ion sputtering was applied immediately before measurement to remove surface contaminants and oxide.
7.7
0 time (s)
-10.3
Deposition rate (µm/min)
Voltage (kV)
1.2 1.0 0.8 0.6 0.4 0.2 0.0 5 µs Fig. 2. Schematic of pulse wave form.
0
2
4
6
8
10
Pulse frequency (kHz) Fig. 3. Deposition rate of a-C:H films as a function of pulse frequency.
12
T. Sakurai et al. / Surface & Coatings Technology 215 (2013) 460–464
1.2
60
1
50
0.8
40
0.6
30
0.4
20
0.2 0
Hardness Surface Roughness RMS 0
2
4
6
8
10
10
Surface roughness RMS (nm)
Hardness (GPa)
462
0 12
Pulse frequency (kHz) Fig. 4. Hardness and Surface roughness of a-C:H films as a function of pulse frequency.
The structure of the films was determined by Raman spectroscopy (STR300, Seki Technotron Corp., Japan). Unpolarized Raman spectra were acquired at 532 nm. The spectral resolution was 0.75 cm−1.
Table 1 The relative atomic concentration of a-C:H film surfaces. Pulse frequency
C (at.%)
N (at.%)
O (at.%)
2 kHz 4 kHz 6 kHz 8 kHz 10 kHz
91.3 89.7 87.3 85.7 85.7
2.0 4.6 5.5 6.5 8.1
6.7 5.7 7.2 7.8 6.2
3. Results and discussion 3.1. Deposition rate The effect of pulse frequency on the deposition rate of the film is shown in Fig. 3. The deposition rates increased from 0.22 to 0.98 μm/min as the pulse frequency increased from 2 to 10 kHz. The correlation between the pulse frequency and deposition rate was linear. This result shows that the carbon and hydrocarbon ions are generated, collide with each other and consequently deposited on the substrate within one pulse and an interval time. Therefore, one cycle of deposition process is completed within one pulse and an interval time. If the carbon and hydrocarbon ions generated at one
(a)
(b)
(c)
(d)
1 µm
(e)
Fig. 5. SEM image of the surface views of a-C:H films deposited at (a) 2 kHz, (b) 4 kHz, (c) 6 kHz, (d) 8 kHz and (e) 10 kHz.
T. Sakurai et al. / Surface & Coatings Technology 215 (2013) 460–464
1
Molar ratio
0.08
0.8
0.06 N/C Hardness
0.6
0.04 0.4
0.02 0
Hardness (GPa)
0.1
0
2
4
6
8
10
12
0.2
Pulse frequency (kHz) Fig. 6. Hardness and N/C molar ratio of a-C:H films as a function of pulse frequency.
pulse are deposited on the substrate by the next pulse, the deposition rate increases in an exponential relationship with increasing pulse frequency [12]. The deposition rate obtained in this study was faster than that of low-pressure plasma CVD (0.0030–0.13 μm/min [13,14]). Niu et al. reported that increasing pressure leads to a decrease of electron temperature and ion energy, which indicates that more energy may be used to excite hydrocarbon molecules and produce high-density activate hydrocarbon species [15]. Typical ion energies in atmospheric-pressure and low-pressure plasma CVD are about 0.1 eV and tens of eV, respectively [16,17]. Therefore, such “activate” hydrocarbon species may contribute to the rapid film deposition at high pressure.
3.2. Hardness The deposition time was adjusted to obtain the film thickness of approximately 1 μm at various pulse frequencies for measurements of hardness and surface roughness. Fig. 4 shows the nano-hardness as a function of pulse frequency. The hardness of the films decreased from 0.92 to 0.35 GPa with increasing pulse frequency from 2 to 6 kHz. On the other hand the films synthesized with 6, 8, 10 kHz were not so different in hardness. The film synthesized with 2 kHz was about 3 times harder than that synthesized at 10 kHz.
463
Fig. 5 shows the surface SEM images of the films synthesized at various pulse frequencies. All the films were composed of particles. The shape of particles as for the films synthesized at 2 and 4 kHz was different from those deposited at over 6 kHz. The films synthesized at 2 and 4 kHz were densely packed with particles, while those synthesized at over 6 kHz existed spaces between particles, which would lead to a rough surface. Fig. 4 shows the root-mean-square (RMS) surface roughness data of the films synthesized at various pulse frequencies. The RMS surface roughness increased from 14.3 to 49.8 nm with increasing pulse frequency from 2 to 10 kHz. The reason of decreasing surface roughness is to stimulate ‘surface reaction’ and ‘reaction in the film’. In the surface reaction, the activate species migrate on the surface to expend their excessive energy and enter into chemical combinations at the energetically stable positions [18]. Decreasing deposition rate increased the time of the surface migration of activate species which led to low surface roughness. Taken together with hardness data, it can be inferred that the smooth surface texture would lead to an improvement of hardness. From the XPS result of quantification in all experiment conditions, three kinds of element such as carbon (C), nitrogen (N) and oxygen (O) are detected. The relative atomic concentration of the film surface is listed in Table 1. The data were presented after background subtraction by the Shirley method and using Gaussian peak fitting [19]. Fig. 6 shows hardness and chemical content ratio of a-C:H films that were analyzed by XPS. N/C molar ratio increases from 0.022 to 0.094 with increasing the pulse frequency from 2 to 10 kHz. On the other hand O/C molar ratio wasn't changed by pulse frequency. The addition of O is due to oxygen gas and moisture from the deposition atmosphere. Yen et al. reported that the nitrogen-doped amorphous carbon film synthesized under low pressure is softer than the amorphous carbon films [20]. They also reported that the addition of N causes the sp2 carbon content in the a-C:H films, which result in films with lower density and hardness [20]. Fig. 7 shows 532 nm Raman spectra of a-C:H films deposited at 2 kHz and 10 kHz which is performed by background subtraction. The bands in the 2500–3300 cm−1 wavelength range are obtained which resulted from deuteration as expected for a CHx stretching [21]. On the other hand, DLC-specific G band (1540 cm−1) and D band (1380 cm−1) were not obtained in all conditions. The featureless spectrum on the G band (1540 cm−1) and D band (1380 cm−1) means the excess H concentration [22] which resulted in low hardness less than 1 GPa in the all conditions.
Intensity (a.u.)
10 kHz
2 kHz
3000
2500
2000
1500
1000
Wave number (cm-1) Fig. 7. 532 nm Raman spectra of a-C:H films deposited at 2 kHz and 10 kHz.
500
464
T. Sakurai et al. / Surface & Coatings Technology 215 (2013) 460–464
4. Conclusion In this study, a-C:H films were synthesized by AP-PECVD with the use of C2H2 as a source gas and N2 as a dilution gas at various pulse frequencies and improved their hardness in low-temperature process. The main results obtained from this study are summarized as follows; 1) The hardness increased from 0.35 to 0.92 GPa with decreasing pulse frequency from 6 to 2 kHz. The RMS surface roughness decreased from 49.8 to 14.3 nm with decreasing pulse frequency from 10 to 2 kHz. Surface texture of a-C:H films was correlated with their hardness. A smooth-surface film densely packed with particles led to an improvement in hardness 2) From the Raman spectra, a-C:H films deposited at various pulse frequencies obtained the excess H concentration. N/C molar ratio decreased from 0.094 to 0.022 with decreasing the pulse frequency from 10 to 2 kHz. Excess H concentration and N contents obtained a-C:H films led to low hardness. References [1] A. Grill, B.S. Meyerson, V.V. Patel, IBM J. Res. Dev. 34 (1990) 849. [2] Y. Lifshitz, Diamond Relat. Mater. 8 (1999) 1659. [3] A. Shirakura, M. Nakaya, Y. Koga, H. Kodama, T. Hasebe, T. Suzuki, Thin Solid Films 494 (2006) 84.
[4] S. Yamamoto, H. Kodama, T. Hasebe, A. Shirakura, T. Suzuki, Diamond Relat. Mater. 14 (2005) 1112. [5] T. Yokoyama, M. Kogoma, T. Moriwaki, S. Okazaki, J. Phys. D: Appl. Phys. 23 (1990) 1125. [6] T. Sakata, H. Kodama, H. Hayashi, T. Shimo, T. Suzuki, Surf. Coat.Technol. 205 (2010) S414. [7] J. Robertson, Mater. Sci. Eng. R37 (2002) 130. [8] H. Kodama, S. Iizumi, M. Nakaya, A. Shirakura, A. Hotta, T. Suzuki, J. Photopolym. Sci. Technol. 19 (2006) 673. [9] T. Suzuki, H. Kodama, Diamond Relat. Mater. 18 (2009) 990. [10] S. Sugiyama, K. Hirakuri, Diamond Symp. 11 (1999) (Japan). [11] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (6) (1992) 1564. [12] N. Ohtake, T. Saito, Y. Kondo, S. Hosono, Y. Nakamura, Y. Imanishi, Jpn. J. Appl. Phys. 43 (2004) L1406. [13] S. Kumar, C.M.S. Rauthan, P.N. Dixit, K.M.K. Srivatsa, M.Y. Khan, R. Bhattacharyya, Vacuum 63 (2001) 433. [14] G. Fedosenko, A. Schwabedissen, J. Engemann, E. Braca, L. Valentini, J.M. Kenny, Diamond Relat. Mater. 11 (2002) 1047. [15] J. Niu, L. Zhang, Z. Zhang, D. Liu, Y. Liu, Z. Feng, Appl. Surf. Sci. 256 (2010) 6887. [16] T. Nozaki, K. Ohnishi, K. Okazaki, U. Kortshagen, Carbon 45 (2007) 364. [17] A. von Keudell, W. Jacob, Prog. Surf. Sci. 76 (2004) 21. [18] P.W. May, J.N. Harvey, N.L. Allan, J.C. Richley, Yu.A. Mankelevich, J. Appl. Phys. 108 (2010) 014905. [19] A. Jablonski, Surf. Interface Anal. 23 (1995) 29. [20] B.K. Yen, J. Thiele, M. Geisler, P.H. Kasai, R.L. White, B.R. York, H. Zadoori, A.J. Kellock, W.C. Tang, T.W. Wu, M.F. Toney, B. Marchon, IEEE Trans. Magn. 37 (2001) 1786. [21] A.C. Ferrari, Diamond Relat. Mater. 11 (2002) 1053. [22] C. Casiraghi, F. Piazza, A.C. Ferrari, D. Grambole, J. Robertson, Diamond Relat. Mater. 14 (2005) 1098.
Contents lists available at SciVerse ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Hardness and surface roughness of hydrogenated amorphous carbon films synthesized by atmospheric pressure plasma enhanced CVD method with various pulse frequencies T. Sakurai ⁎, M. Noborisaka, T. Hirako, A. Shirakura, T. Suzuki Center for Science of Environment, Resources and Energy, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan
a r t i c l e
i n f o
Available online 6 November 2012 Keywords: Hydrogenated amorphous carbon Atmospheric pressure CVD Hardness Surface roughness
a b s t r a c t Atmospheric pressure plasma enhanced chemical vapor deposition (AP-PECVD) has attracted much attention for its cost-effectiveness owing to eliminate the use of vacuum devices. We synthesized hydrogenated amorphous carbon (a-C:H) films under atmospheric pressure from C2H2 gas diluted with N2 with varying pulse frequency of plasma source. We investigated the effect of surface texture and chemical bonding structure of the films on hardness. The hardness, surface roughness and chemical content ratio were analyzed by tribo scope nano-mechanical indentation tester, atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS), respectively. As the pulse frequency decreased from 10 to 2 kHz, the hardness increased from 0.35 to 0.92 GPa and the surface roughness decreased from 49.8 to 14.3 nm. From the result of XPS analysis, the N/C molar ratio increased from 0.022 to 0.094 with increasing the pulse frequency. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Diamond-like carbon (DLC) films have various excellent properties such as high hardness, high electrical resistivity, low friction coefficients, low wear rates chemical inertness and low gas permeability [1–3]. Therefore, DLC films have been used for cutting instruments, sliding parts, wear resistant parts [1,2] and food packaging [3]. In conventional, DLC films are synthesized by plasma-enhanced chemical vapor deposition (PECVD) process under vacuum condition less than 100 Pa [4]. However, this synthesis process has some problems such as high equipment cost, long deposition time and limited synthetic area. Atmospheric pressure-plasma enhanced CVD (AP-PECVD) process using dielectric barrier discharge (DBD) serves as one approach to overcoming the aforementioned problems. In 1988, Yokoyama et al. reported that a homogeneous DBD which is called an atmospheric pressure glow discharge (APGD) can be obtained. APGD needs the following three treatments by preventing the undesired transition from glow to arc discharge [5]. 1) using a power source frequency over 1 kHz; 2) inserting dielectric plates between metal electrodes and 3) using helium or neon as a dilution gas. From our previous research [6], we synthesized the hydrogenated amorphous carbon (a-C:H) films whose hardness is under 10 GPa [7] under atmospheric pressure. Sakata et al. synthesized a-C:H films at atmospheric pressure with He dilution and improved their hardness ⁎ Corresponding author at: Center for Science of Environment, Resources and Energy, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan. Tel.: +81 45 563 1141x42070; fax: +81 45 563 5945. E-mail address: [email protected] (T. Sakurai). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2012.09.058
by increasing substrate-temperature up to 200 °C [6]. However, low-temperature process is desired in the case of synthesizing a-C:H films on heat-sensitive materials such as commodity polymers. Furthermore, another type of DBD using molecular gasses such as nitrogen instead of He, which is called atmospheric pressure Townsend-like discharge (APTD), has been actively studied with a view to low-cost process [8,9]. Therefore, in this study, we synthesized a-C:H films under atmospheric pressure at low-temperature with the use of N2 as a dilution gas. We focused on the frequency of the pulsed power supply used for atmospheric-pressure plasma chemical vapor deposition (AP-PECVD) apparatus to improve hardness of the a-C:H films. Sugiyama et al. reported that power supply frequency has an influence on hardness of a-C:H films synthesized under low pressure [10], while no results have been reported to date on the effect of hardness of a-C:H films synthesized under atmospheric pressure on the pulse frequency. Moreover, we investigated the effect of surface texture and chemical content ratio on the hardness of a-C:H films. 2. Experimental 2.1. Apparatus The schematic diagram of the line type atmospheric-pressure plasma CVD apparatus is illustrated in Fig. 1. The plasma was evenly produced and sustained between the upper and lower electrode, whose sizes are 60 × 120 × 50 mm 3 and 300 × 120 × 1 mm 3, respectively. The upper electrode was connected to the electric power supply and the other was connected to the earth. The upper electrode
T. Sakurai et al. / Surface & Coatings Technology 215 (2013) 460–464
461
Power source
Blower
Process gas
Cu electrodes
Blower
Dielectric plates
Si substrate
Plasma
movable
Fig. 1. Schematic diagram of a line type atmospheric-pressure plasma CVD apparatus.
was covered with a dielectric plate (Al2O3) to prevent the transition from glow to arc discharge. As the substrates, Si wafers with a thickness of 100 μm were set on the lower electrode moving at the speed range from 0.1 to 10 mm/s in parallel to the upper one. The distance between upper and lower electrode was 1 mm.
2.2. Experimental procedure The C2H2 gas diluted with N2 gas was supplied between the upper and lower electrodes. The films were prepared at the following gas mixture ratios: [C2H2/(C2H2 + N2)] × 100 (%) = 2, with a total gas flow rate of 1 l/min. We used the power generator (SPD1PGU1, SK Medical Electronics Co., Ltd., Japan). The schematic of pulse wave form is shown in Fig. 2. The pulse width and voltage between electrodes were fixed at 5 μs and 18 kV, respectively. We investigated the effect of pulse frequency on physical properties of a-C:H films synthesized with variation of pulse frequencies at 2, 4, 6, 8, 10 kHz. The power of the power generator increases from 40 W to 220 W in direct proportion to pulse frequency from 2 kHz to 10 kHz.
2.3. Characterization of a-C:H film The thickness of a-C:H films was measured by cross-sectional observation by scanning electron microscope (SEM: S-4700, Hitachi High Technologies Corp., Japan). The measurements were performed at 10 different locations and the results are expressed as the mean of 10 replicates and the corresponding standard deviation. The hardness of the films was measured by tribo scope nano-mechanical indentation tester (TI 900 TriboIndenter, HYSITRON Inc., USA) with a Berkovich diamond indenter. The sample thickness for nano-hardness measurement was approximately 1 μm. The penetration depth was about 100 nm and the load–displacement curve was analyzed using the Oliver and Pharr method [11]. The measurements of the hardness were performed at 5 different surface locations and the results are expressed as the mean of 3 replicates and the corresponding standard deviation. The surface roughness was measured by atomic force microscope (AFM: SPM-9600, Shimadzu Corp., Japan). The scanning area was 5 × 5 μm2. The measurements of the surface roughness were performed at 4 different surface locations and the results are expressed as the mean of 3 replicates and the corresponding standard deviation. The chemical content ratio of a-C:H films were analyzed by X-ray photoelectron spectroscopy (XPS: JPS-9000MC, JEOL Ltd., Japan). The energy resolution was 0.65 eV. For elemental analysis, Ar ion sputtering was applied immediately before measurement to remove surface contaminants and oxide.
7.7
0 time (s)
-10.3
Deposition rate (µm/min)
Voltage (kV)
1.2 1.0 0.8 0.6 0.4 0.2 0.0 5 µs Fig. 2. Schematic of pulse wave form.
0
2
4
6
8
10
Pulse frequency (kHz) Fig. 3. Deposition rate of a-C:H films as a function of pulse frequency.
12
T. Sakurai et al. / Surface & Coatings Technology 215 (2013) 460–464
1.2
60
1
50
0.8
40
0.6
30
0.4
20
0.2 0
Hardness Surface Roughness RMS 0
2
4
6
8
10
10
Surface roughness RMS (nm)
Hardness (GPa)
462
0 12
Pulse frequency (kHz) Fig. 4. Hardness and Surface roughness of a-C:H films as a function of pulse frequency.
The structure of the films was determined by Raman spectroscopy (STR300, Seki Technotron Corp., Japan). Unpolarized Raman spectra were acquired at 532 nm. The spectral resolution was 0.75 cm−1.
Table 1 The relative atomic concentration of a-C:H film surfaces. Pulse frequency
C (at.%)
N (at.%)
O (at.%)
2 kHz 4 kHz 6 kHz 8 kHz 10 kHz
91.3 89.7 87.3 85.7 85.7
2.0 4.6 5.5 6.5 8.1
6.7 5.7 7.2 7.8 6.2
3. Results and discussion 3.1. Deposition rate The effect of pulse frequency on the deposition rate of the film is shown in Fig. 3. The deposition rates increased from 0.22 to 0.98 μm/min as the pulse frequency increased from 2 to 10 kHz. The correlation between the pulse frequency and deposition rate was linear. This result shows that the carbon and hydrocarbon ions are generated, collide with each other and consequently deposited on the substrate within one pulse and an interval time. Therefore, one cycle of deposition process is completed within one pulse and an interval time. If the carbon and hydrocarbon ions generated at one
(a)
(b)
(c)
(d)
1 µm
(e)
Fig. 5. SEM image of the surface views of a-C:H films deposited at (a) 2 kHz, (b) 4 kHz, (c) 6 kHz, (d) 8 kHz and (e) 10 kHz.
T. Sakurai et al. / Surface & Coatings Technology 215 (2013) 460–464
1
Molar ratio
0.08
0.8
0.06 N/C Hardness
0.6
0.04 0.4
0.02 0
Hardness (GPa)
0.1
0
2
4
6
8
10
12
0.2
Pulse frequency (kHz) Fig. 6. Hardness and N/C molar ratio of a-C:H films as a function of pulse frequency.
pulse are deposited on the substrate by the next pulse, the deposition rate increases in an exponential relationship with increasing pulse frequency [12]. The deposition rate obtained in this study was faster than that of low-pressure plasma CVD (0.0030–0.13 μm/min [13,14]). Niu et al. reported that increasing pressure leads to a decrease of electron temperature and ion energy, which indicates that more energy may be used to excite hydrocarbon molecules and produce high-density activate hydrocarbon species [15]. Typical ion energies in atmospheric-pressure and low-pressure plasma CVD are about 0.1 eV and tens of eV, respectively [16,17]. Therefore, such “activate” hydrocarbon species may contribute to the rapid film deposition at high pressure.
3.2. Hardness The deposition time was adjusted to obtain the film thickness of approximately 1 μm at various pulse frequencies for measurements of hardness and surface roughness. Fig. 4 shows the nano-hardness as a function of pulse frequency. The hardness of the films decreased from 0.92 to 0.35 GPa with increasing pulse frequency from 2 to 6 kHz. On the other hand the films synthesized with 6, 8, 10 kHz were not so different in hardness. The film synthesized with 2 kHz was about 3 times harder than that synthesized at 10 kHz.
463
Fig. 5 shows the surface SEM images of the films synthesized at various pulse frequencies. All the films were composed of particles. The shape of particles as for the films synthesized at 2 and 4 kHz was different from those deposited at over 6 kHz. The films synthesized at 2 and 4 kHz were densely packed with particles, while those synthesized at over 6 kHz existed spaces between particles, which would lead to a rough surface. Fig. 4 shows the root-mean-square (RMS) surface roughness data of the films synthesized at various pulse frequencies. The RMS surface roughness increased from 14.3 to 49.8 nm with increasing pulse frequency from 2 to 10 kHz. The reason of decreasing surface roughness is to stimulate ‘surface reaction’ and ‘reaction in the film’. In the surface reaction, the activate species migrate on the surface to expend their excessive energy and enter into chemical combinations at the energetically stable positions [18]. Decreasing deposition rate increased the time of the surface migration of activate species which led to low surface roughness. Taken together with hardness data, it can be inferred that the smooth surface texture would lead to an improvement of hardness. From the XPS result of quantification in all experiment conditions, three kinds of element such as carbon (C), nitrogen (N) and oxygen (O) are detected. The relative atomic concentration of the film surface is listed in Table 1. The data were presented after background subtraction by the Shirley method and using Gaussian peak fitting [19]. Fig. 6 shows hardness and chemical content ratio of a-C:H films that were analyzed by XPS. N/C molar ratio increases from 0.022 to 0.094 with increasing the pulse frequency from 2 to 10 kHz. On the other hand O/C molar ratio wasn't changed by pulse frequency. The addition of O is due to oxygen gas and moisture from the deposition atmosphere. Yen et al. reported that the nitrogen-doped amorphous carbon film synthesized under low pressure is softer than the amorphous carbon films [20]. They also reported that the addition of N causes the sp2 carbon content in the a-C:H films, which result in films with lower density and hardness [20]. Fig. 7 shows 532 nm Raman spectra of a-C:H films deposited at 2 kHz and 10 kHz which is performed by background subtraction. The bands in the 2500–3300 cm−1 wavelength range are obtained which resulted from deuteration as expected for a CHx stretching [21]. On the other hand, DLC-specific G band (1540 cm−1) and D band (1380 cm−1) were not obtained in all conditions. The featureless spectrum on the G band (1540 cm−1) and D band (1380 cm−1) means the excess H concentration [22] which resulted in low hardness less than 1 GPa in the all conditions.
Intensity (a.u.)
10 kHz
2 kHz
3000
2500
2000
1500
1000
Wave number (cm-1) Fig. 7. 532 nm Raman spectra of a-C:H films deposited at 2 kHz and 10 kHz.
500
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4. Conclusion In this study, a-C:H films were synthesized by AP-PECVD with the use of C2H2 as a source gas and N2 as a dilution gas at various pulse frequencies and improved their hardness in low-temperature process. The main results obtained from this study are summarized as follows; 1) The hardness increased from 0.35 to 0.92 GPa with decreasing pulse frequency from 6 to 2 kHz. The RMS surface roughness decreased from 49.8 to 14.3 nm with decreasing pulse frequency from 10 to 2 kHz. Surface texture of a-C:H films was correlated with their hardness. A smooth-surface film densely packed with particles led to an improvement in hardness 2) From the Raman spectra, a-C:H films deposited at various pulse frequencies obtained the excess H concentration. N/C molar ratio decreased from 0.094 to 0.022 with decreasing the pulse frequency from 10 to 2 kHz. Excess H concentration and N contents obtained a-C:H films led to low hardness. References [1] A. Grill, B.S. Meyerson, V.V. Patel, IBM J. Res. Dev. 34 (1990) 849. [2] Y. Lifshitz, Diamond Relat. Mater. 8 (1999) 1659. [3] A. Shirakura, M. Nakaya, Y. Koga, H. Kodama, T. Hasebe, T. Suzuki, Thin Solid Films 494 (2006) 84.
[4] S. Yamamoto, H. Kodama, T. Hasebe, A. Shirakura, T. Suzuki, Diamond Relat. Mater. 14 (2005) 1112. [5] T. Yokoyama, M. Kogoma, T. Moriwaki, S. Okazaki, J. Phys. D: Appl. Phys. 23 (1990) 1125. [6] T. Sakata, H. Kodama, H. Hayashi, T. Shimo, T. Suzuki, Surf. Coat.Technol. 205 (2010) S414. [7] J. Robertson, Mater. Sci. Eng. R37 (2002) 130. [8] H. Kodama, S. Iizumi, M. Nakaya, A. Shirakura, A. Hotta, T. Suzuki, J. Photopolym. Sci. Technol. 19 (2006) 673. [9] T. Suzuki, H. Kodama, Diamond Relat. Mater. 18 (2009) 990. [10] S. Sugiyama, K. Hirakuri, Diamond Symp. 11 (1999) (Japan). [11] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (6) (1992) 1564. [12] N. Ohtake, T. Saito, Y. Kondo, S. Hosono, Y. Nakamura, Y. Imanishi, Jpn. J. Appl. Phys. 43 (2004) L1406. [13] S. Kumar, C.M.S. Rauthan, P.N. Dixit, K.M.K. Srivatsa, M.Y. Khan, R. Bhattacharyya, Vacuum 63 (2001) 433. [14] G. Fedosenko, A. Schwabedissen, J. Engemann, E. Braca, L. Valentini, J.M. Kenny, Diamond Relat. Mater. 11 (2002) 1047. [15] J. Niu, L. Zhang, Z. Zhang, D. Liu, Y. Liu, Z. Feng, Appl. Surf. Sci. 256 (2010) 6887. [16] T. Nozaki, K. Ohnishi, K. Okazaki, U. Kortshagen, Carbon 45 (2007) 364. [17] A. von Keudell, W. Jacob, Prog. Surf. Sci. 76 (2004) 21. [18] P.W. May, J.N. Harvey, N.L. Allan, J.C. Richley, Yu.A. Mankelevich, J. Appl. Phys. 108 (2010) 014905. [19] A. Jablonski, Surf. Interface Anal. 23 (1995) 29. [20] B.K. Yen, J. Thiele, M. Geisler, P.H. Kasai, R.L. White, B.R. York, H. Zadoori, A.J. Kellock, W.C. Tang, T.W. Wu, M.F. Toney, B. Marchon, IEEE Trans. Magn. 37 (2001) 1786. [21] A.C. Ferrari, Diamond Relat. Mater. 11 (2002) 1053. [22] C. Casiraghi, F. Piazza, A.C. Ferrari, D. Grambole, J. Robertson, Diamond Relat. Mater. 14 (2005) 1098.