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CH4 microwave plasma
Thin Solid Films 515 (2007) 4258 – 4261 www.elsevier.com/locate/tsf
Characteristics of nano-crystalline diamond films prepared in Ar/H2/CH4 microwave plasma Masato Miyake, Akihisa Ogino, Masaaki Nagatsu ⁎ Graduate School of Science and Engineering, Shizuoka University, 3-5-1 Johoku, Hamamatsu 432-8561, Japan Available online 6 March 2006
Abstract Characteristics of nano-crystalline diamond (NCD) thin films prepared with microwave plasma chemical vapor deposition (CVD) were studied in Ar/H2/CH4 gas mixture with a CH4 gas ratio of 1–10% and H2 gas ratio of 0–15%. From the Raman measurements, a pair of peaks at 1140 cm− 1 and 1473 cm− 1 related to the trans-polyacetylene components peculiar to nano-crystalline diamond films was clearly observed when the H2 gas ratio of 5% was added in Ar/H2/CH4 mixture. With an increase of H2 gas content up to 15%, their peaks decreased, while a G-peak at roughly 1556 cm− 1 significantly increased. The degradation of NCD film quality strongly correlates with the decrease of C2 optical emission intensity with the increase of hydrogen gas contents. From the surface analysis with atomic force microscopy (AFM), it was found that grain sizes of NCD films were typically of 10–100 nm in case of 5% H2 gas addition. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanocrystalline diamond; Microwave plasma; CVD; Raman spectrum
1. Introduction Recently, nano-crystalline diamond (NCD) thin films having grain sizes of about 5–100 nm attract much attention as a new functional material in the various industrial fields. The NCD thin films have an elaborateness of the film due to smaller crystal grain sizes, and a lower friction characteristic and a higher transparency due to a smoother surface flatness compared with the conventional microcrystalline diamond (MCD) thin film. Furthermore, since they have the inherent characters of diamond films, such as high hardness, negative electron affinity (NEA), and so on, many practical applications such as protective coating, surface flattening and field emission devices are expected [1–3]. The growth of NCD thin films using Ar-rich hydrocarbon gas mixture plasma is very sensitive to the contribution of C2 species produced in the plasma [4–7]. In addition, the transition from MCD to NCD films grown from Ar/H2/CH4 microwave plasmas has been investigated by Zhou et al. [8], in which the transition of microstructure is presumably due to a change in growth ⁎ Corresponding author. Faculty of Engineering, Shizuoka University, 3-5-1 Johoku Hamamatsu, 432-8561, Japan. Tel./fax: +81 53 478 1081. E-mail address: [email protected] (M. Nagatsu). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.02.048
mechanism from CH3 in high hydrogen content to C2 as a growth species in low hydrogen content plasmas. The growth mechanism of the diamond films depends strongly on the ratio of H2 to Ar in the reactant gases. In this study, we investigate the effect of hydrogen gas addition in Ar/CH4 gas mixture on the characteristics of NCD films prepared with microwave plasma CVD. We added H2 gas up to 15% in volume percentage to Ar/CH4 gas mixture. To discuss the relation between the film quality and C2 precursors for NCD film growth, we studied the film characteristics using Raman spectroscopy and the emission intensities of C2 radicals using the optical emission spectroscopy (OES). 2. Experimental A schematic drawing of the microwave plasma CVD reactor for diamond film synthesis is shown in Fig. 1 [9]. The inner diameter and the height of SUS cylindrical chamber are 16 and 18 cm, respectively. The whole vacuum chamber was cooled by the water cooling system. The substrate stage of 8 cm in diameter was set approximately 6 cm under the circular metal plate attached just below the quartz plate, as shown in Fig. 1. Silicon substrates were scratched using diamond powders with 2–4 μm of particles sizes in the ultrasonic bath beforehand. After
M. Miyake et al. / Thin Solid Films 515 (2007) 4258–4261
Water Inlet 2.45GHz
Short Plunger
Microwave Quartz Plate Plasma Vacuum
To Pump
Chamber Substrate Stage (Water Cooling)
8 cm 16 cm Fig. 1. Schematic drawing of the experimental setup for diamond film synthesis using microwave plasma CVD.
pumping down the chamber, Ar gas was filled at a pressure of approximately 20 kPa. The 2.45 GHz microwave launched from the magnetron oscillator was fed to the chamber through the quartz ring plate in the top of chamber. A greenish spherical plasma discharge was generated and attached on the substrate stage. The silicon substrates set on the stage were heated by the plasma alone. Optical emission spectra from the greenish plasmas show Swan band emission related to C2 radicals. The increases of C2 radicals with increasing argon atoms and ions were detected with OES in microwave plasma CVD [10,11] and in laser-ablated plumes for thin carbon film depositions [12]. With more C2 radicals, it is reported that the well-faceted MCD and NCD films grow at higher growth rates [13,14]. In the NCD film synthesis, Ar/H2/CH4 mixture gas was used with H2 gas ratio of 0–15% and CH4 gas ratio of 1–10%, the total gas flow rate of 200 or 220 sccm and the gas pressure of approximately 20 kPa. The film deposition was performed with 1.3–1.5 kW of microwave power for about 8 h. The morphologies of the diamond films were characterized by the field emission type scanning electron microscope (FESEM) and an atomic force microscope (AFM) using SEIKO SPI3700. Raman spectrum measurements were carried out at ambient temperature using JASCO NR-1800, with the argon laser line at 488 or 514.5 nm. 3. Results and discussion The SEM image and the Raman spectrum of diamond film prepared with Ar/CH4 plasma CVD at the incident power of 1.4 kW are shown in Fig. 2. The gas flow rates were Ar/ CH4 = 200/10 sccm and the total gas pressure was 14.7 kPa. The round-shaped deposits having approximately 1.3 μm in diameter were formed on the substrate shown in SEM image. A broadened spectrum with several peaks in the neighborhood of sp3diamond bonding centered around 1334 cm− 1 are observed. Here, the wavelength of 488 nm argon laser was used in this
Raman spectrum measurement. The large-sized round-shape grains are kinds of microcrystalline diamond embedded in graphitic and amorphous carbon with a broad spectrum centered at 1334 cm− 1. As for a pair of peaks centered around 1155 and 1496 cm− 1 in Raman spectra, it is considered that they are assigned to trans-polyacetylene lying in grain boundaries, and are not possibly related to C–C sp3 vibrations [15]. These peaks are peculiarly observed in the NCD films [16–18]. From our results of Raman spectrum, we also believe that NCD components are contained in the present diamond thin films. The effects of adding H2 gas in Ar/CH4 mixture gas on the OES and the film characteristic are investigated. Fig. 3 shows SEM image of samples with addition of 10% H2 and Raman spectrum of diamond film prepared with Ar/H2/CH4 plasma CVD at different mixture ratio of H2 to Ar, where the wavelength of Ar laser was 514.5 nm in this measurement. An additive of 5– 15% H2 in the Ar/CH4 made the formation of a plasma on the substrate much more easily. The granular segments having 10– 100 nm in diameter were observed in the film shown in Fig. 3(a). Typical grain size is obviously smaller than that shown in Fig. 2(a). Typical NCD features are seen for all the samples, these spectra have mainly four distinctive peaks at 1139, 1350, 1475 and 1553 cm− 1, as shown in Fig. 3(b). The peaks at 1350 and 1553 cm− 1 are assigned for the D and G bands of disordered sp2carbon. The peak centered around 1140 cm− 1 of trans-
(a)
1μm
5,000 5,
(b)
Intensity (arb.units)
Gas Inlet
4259
1334cm 1155cm
1000
-1
1200
-1
1496cm
1400
-1
1600
1800
Raman shift (cm-1) Fig. 2. (a) SEM image and (b) the Raman spectrum of diamond thin film prepared with Ar/H2/CH4 microwave plasma CVD at gas flow rates of Ar/ CH4 = 200/10 sccm.
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(a) 10% H22
100nm 100nm
(b) 1556cm -1 1198cm -1
(2) 10% H2
1474cm -1 1139cm -1
1000
1200
1351cm -1
1553cm -1 (1) 5% H2
(a) 1400
1600
1800
Raman shift (cm-1) Fig. 3. (a) SEM image and (b) Raman spectra of diamond thin film prepared with Ar/H2/CH4 microwave plasma CVD, under the conditions that gas flow rates of Ar/H2/CH4, pressure, and microwave power are (1) 188/10/2 sccm, 14.7– 16.7 kPa, 1.35 kW, (2) 178/20/2 sccm, 17.3–20 kPa, 1.44 kW and (3) 168/30/ 2 sccm, 16.3–16.8 kPa, 1.3 kW, respectively.
polyacetylene components has been attributed to NCD film, it was always accompanied with the peak around 1475 cm− 1. By adding a small amount of H2 gas in Ar/CH4 mixture gas, a peak centered at 1140 cm− 1 is more clearly observed than that at 1155 cm− 1 in case of Ar/CH4 mixture gas shown in Fig. 2(b). In the experiments of the film depositions, the microwave power was 1.35 kW at the total gas pressure between 14.7 kPa and 16.7 kPa. These results of Raman spectra show that the NCD components are contained together with amorphous carbon in the film. Furthermore, when H2 mixture ratio was 10% at gas flow rates of Ar/H2/CH4 = 178/ 20/2 sccm, total gas pressure of 17.3–20 kPa and microwave power of 1.44 kW, stronger peaks centered around 1140 and 1475 cm− 1 peculiar to NCD film have been observed. On the other hand, the intensity of G band has weakened. This result does not mean that the NCD components are more dominant in the film, since the Raman cross-section relating with trans-polyacetylene is large [19]. In the case of mixture gas ratio of 84% Ar and 15% H2 at Ar/H2/CH4 = 168/30/2 sccm, total gas pressure of 16.3–16.8 kPa and microwave power of 1.3 kW, a pair of peaks of trans-polyacetylene components, especially a peak around 1475 cm− 1 became broader. In addition, peaks around 1198 and
Emission Intensity (arb.units)
Intensity (arb.units)
(3) 15% H2
1 10 5 8 10
Ar 357.2nm Ar 389.5nm C2 473.7nm C2 516.5nm Hα 656.3nm
4
6 10 4 4 10 4 2 10 4 0 0
5
10
15
20
25
30
H2 /(Ar+CH4+H2) (%)
(b) Emission Intensity (arb.units)
1144cm
-1
1550 cm− 1 appeared clearly. A peak centered around 1198 cm− 1 is due to the deformation vibration of –CgC– bonding. It seems that the C2 is lost through collisions with H2. To confirm the change of C2 radical density, the OES in Ar/H2/CH4 plasma was carried out. From the optical emission measurements, it has been also confirmed that the emission intensity of C2 radicals at 473.7 and 516.5 nm, for instance, similarly decreased when the contents of H2 gas was raised from 5% to 15%, as shown in Fig. 4(a). From the normalized emission intensities of ArI, C2 and Hα as shown in Fig. 4(b), it is clearly seen that the emission intensities of C2 radicals and ArI decreased more significantly than that of Hα. It is generally considered that atomic hydrogen plays a number of roles of preferential etching of non-diamond carbon species and termination of carbon dangling bonds. In addition to atomic hydrogen, hydrogen molecules also play important roles in a number of chemical reactions with Ar atoms, hydrocarbon species, C2 dimers and so on. As described above, emission intensities of both the ArI and C2 Swan system steeply dropped with the increase of hydrogen gas addition, as shown in Fig. 4. On the other hand, emission intensity of Hα emission gradually decreased compared with those of ArI and C2 Swan system. Similar results that C2 emission intensity decreased with the hydrogen addition have been reported in Ref. [8]. It is considered
1.2 Ar 357.2nm Ar 389.5nm C2 473.7nm C2 516.5nm Hα 656.3nm
H
1 0.8 0.6
Ar
0.4 0.2 0
C 0
5
2 10
15
20
25
30
H2 /(Ar+CH4+H2) (%) Fig. 4. (a) Absolute intensities and (b) normalized intensities of ArI (357.2 nm, 389.5 nm), C2 (473.7 nm, 516.5 nm) and Hα (656.3 nm) emission lines versus H2 gas mixture ratio of H2 / (Ar + H2 + CH4).
M. Miyake et al. / Thin Solid Films 515 (2007) 4258–4261
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was shown that the spectra of diamond films depend strongly on the ratio of H2 to Ar, in which the peaks of trans-polyacetylene components peculiar to the NCD films have been clearly observed in diamond thin film prepared by the microwave plasma in Ar/CH4 gas mixture. Optical emission measurements showed the strong correlation with the degradation of NCD film quality, since the C2 radicals decreased rapidly with H2 gas addition. AFM analysis showed that the grain sizes were roughly between 10 and 100 nm and the surface roughness of the film was less than 20 nm. A smoother NCD film was obtained with H2 additive. Acknowledgements Fig. 5. AFM image of the surface morphology of diamond film with Ar/H2/CH4 plasma CVD when the gas flow rates were Ar/H2/CH4 = 188/10/2 sccm.
that C2 radicals might be lost through collision with hydrogen molecules, via C2 + H2 → C2H + H [4,20]. Another possible loss mechanism of C2 is Penning ionization with metastable Ar atoms, that is, C2 + Ar⁎ → C2+ + Ar + e. Ar atoms excited by electron impacts collide with hydrogen molecules and change to ArH⁎, that is, Ar⁎ + H2 → ArH⁎ + H. Furthermore, we can also expect that C2 dimers decrease with the decrease of Ar gas content or hydrogen gas addition, because the main C2 production reaction is presented by C2H2 + Ar → C2 + H2 + Ar. From these expectations, we can deduce that ArI and C2 emission intensity decreased with the increase of hydrogen gas content. In Ref. [20], Lombardi et al. noticed that C2H2 is also important in NCD growth in addition to C2 dimer or atomic carbon C. Moreover, it is considered that contribution of C2H species on NCD growth is also important in Ar/H2/CH4 gas mixture [4]. C2H species are produced via C2 + H2 → C2H + H or C2⁎ + H2 → C2H + H. Therefore, with the increase of hydrogen gas, C2H will increase at the expense of C2 loss according to these reactions. Therefore, it is needed to investigate NCD growth rate, by considering not only C2 species but also C2H2 or C2H species. Lastly, Fig. 5 shows a typical AFM image of the surface morphology of diamond films prepared with Ar/H2/CH4 plasma CVD, when the gas flow rates were Ar/H2/CH4 = 188/10/2 sccm. From surface analysis using the AFM, we found that the granular segments having 10–100 nm in diameter were contained in the film. The grain sizes of Ar/H2/CH4 became smaller samples than that of Ar/CH4 ones. As shown in Fig. 5, the surface of diamond film is quite flat and the surface roughness is approximately 10–20 nm. 4. Conclusion In conclusion, the synthesis of nano-crystalline diamond film using Ar/H2/CH4 microwave plasma CVD was carried out, and the effects of addition of H2 gas on the film characteristics of NCD films were investigated. From Raman measurements, it
This work has been performed under the 21st Century COE Program “Research and Education Center of Nanovision Science” by the Japan Society for the Promotion of Science and partly supported by Faculty of Engineering of Shizuoka University as one of advance research projects. The authors would like to thank associate professor K. Murakami and Mr. W. Tomoda of Shizuoka University for the FE-SEM, AFM and Raman spectroscopy analyses. References [1] S.P. Hong, H. Yoshikawa, K. Wazumi, Y. Koga, Diamond Relat. Mater. 11 (2002) 877. [2] S. Gupta, B.R. Weiner, G. Morell, Diamond Relat. Mater. 11 (2002) 799. [3] T.D. Corrigan, D.M. Gruen, A.R. Krauss, P. Zapol, Diamond Relat. Mater. 11 (2002) 43. [4] F.J.G. Vazquez, J.M. Albella, J. Appl. Phys. 94 (2003) 6085. [5] D.M. Gruen, S. Liu, A.R. Krauss, J. Luo, X. Pan, Appl. Phys. Lett. 64 (1994) 1502. [6] D.M. Gruen, Annu. Rev. Mater. Sci. 29 (1999) 211. [7] T.H. Chein, J. Wei, Y. Tzeng, Diamond Relat. Mater. 8 (1999) 1686. [8] D. Zhou, D.M. Gruen, L.C. Qin, T.G. McCauley, A.R. Krauss, J. Appl. Phys. 84 (1998) 1981. [9] M. Nagatsu, M. Makino, M. Tanga, H. Sugai, Diamond Relat. Mater. 11 (2002) 562. [10] O. Matsumoto, T. Katagiri, Thin Solid Films 146 (1987) 283. [11] Y.K. Liu, Y. Tzeng, C. Liu, P. Tso, I.N. Lin, Diamond Relat. Mater. 13 (2004) 1859. [12] R.K. Thareja, R.K. Dwivedi, Abbilasha, Phys. Rev., B 55 (1997) 2600. [13] Tsan-Heui Chein, Jin Wei, Yonhua Tzeng, Diamond Relat. Mater. 8 (1999) 1686. [14] A.N. Goyette, J.E. Lawler, L.W. Anderson, D.M. Gruen, T.G. McCauley, J. Phys., D 31 (1998) 1975. [15] T. López-Ríos, É. Sandré, S. Leclercq, É. Sauvain, Phys. Rev. Lett. 76 (1996) 4935. [16] R. Pfeiffer, H. Kuzmany, P. Knoll, S. Bokova, N. Salk, B. Günther, Diamond Relat. Mater. 12 (2003) 268. [17] S.M. Leeds, T.J. Davis, P.W. May, C.D.O. Pickard, M.N.R. Ashfold, Diamond Relat. Mater. 7 (1998) 233. [18] Y.K. Liu, C. Liu, Y. Chen, Y. Tzeng, P.L. Tso, I.N. Lin, Diamond Relat. Mater. 13 (2004) 671. [19] P.A. Temple, C.E. Hathaway, Phys. Rev., B 7 (1973) 3685. [20] G. Lombardi, K. Hassouni, F. Benedic, F. Mohasseb, J. Ropcke, A. Gicquel, J. Appl. Phys. 96 (2004) 6739.
Characteristics of nano-crystalline diamond films prepared in Ar/H2/CH4 microwave plasma Masato Miyake, Akihisa Ogino, Masaaki Nagatsu ⁎ Graduate School of Science and Engineering, Shizuoka University, 3-5-1 Johoku, Hamamatsu 432-8561, Japan Available online 6 March 2006
Abstract Characteristics of nano-crystalline diamond (NCD) thin films prepared with microwave plasma chemical vapor deposition (CVD) were studied in Ar/H2/CH4 gas mixture with a CH4 gas ratio of 1–10% and H2 gas ratio of 0–15%. From the Raman measurements, a pair of peaks at 1140 cm− 1 and 1473 cm− 1 related to the trans-polyacetylene components peculiar to nano-crystalline diamond films was clearly observed when the H2 gas ratio of 5% was added in Ar/H2/CH4 mixture. With an increase of H2 gas content up to 15%, their peaks decreased, while a G-peak at roughly 1556 cm− 1 significantly increased. The degradation of NCD film quality strongly correlates with the decrease of C2 optical emission intensity with the increase of hydrogen gas contents. From the surface analysis with atomic force microscopy (AFM), it was found that grain sizes of NCD films were typically of 10–100 nm in case of 5% H2 gas addition. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanocrystalline diamond; Microwave plasma; CVD; Raman spectrum
1. Introduction Recently, nano-crystalline diamond (NCD) thin films having grain sizes of about 5–100 nm attract much attention as a new functional material in the various industrial fields. The NCD thin films have an elaborateness of the film due to smaller crystal grain sizes, and a lower friction characteristic and a higher transparency due to a smoother surface flatness compared with the conventional microcrystalline diamond (MCD) thin film. Furthermore, since they have the inherent characters of diamond films, such as high hardness, negative electron affinity (NEA), and so on, many practical applications such as protective coating, surface flattening and field emission devices are expected [1–3]. The growth of NCD thin films using Ar-rich hydrocarbon gas mixture plasma is very sensitive to the contribution of C2 species produced in the plasma [4–7]. In addition, the transition from MCD to NCD films grown from Ar/H2/CH4 microwave plasmas has been investigated by Zhou et al. [8], in which the transition of microstructure is presumably due to a change in growth ⁎ Corresponding author. Faculty of Engineering, Shizuoka University, 3-5-1 Johoku Hamamatsu, 432-8561, Japan. Tel./fax: +81 53 478 1081. E-mail address: [email protected] (M. Nagatsu). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.02.048
mechanism from CH3 in high hydrogen content to C2 as a growth species in low hydrogen content plasmas. The growth mechanism of the diamond films depends strongly on the ratio of H2 to Ar in the reactant gases. In this study, we investigate the effect of hydrogen gas addition in Ar/CH4 gas mixture on the characteristics of NCD films prepared with microwave plasma CVD. We added H2 gas up to 15% in volume percentage to Ar/CH4 gas mixture. To discuss the relation between the film quality and C2 precursors for NCD film growth, we studied the film characteristics using Raman spectroscopy and the emission intensities of C2 radicals using the optical emission spectroscopy (OES). 2. Experimental A schematic drawing of the microwave plasma CVD reactor for diamond film synthesis is shown in Fig. 1 [9]. The inner diameter and the height of SUS cylindrical chamber are 16 and 18 cm, respectively. The whole vacuum chamber was cooled by the water cooling system. The substrate stage of 8 cm in diameter was set approximately 6 cm under the circular metal plate attached just below the quartz plate, as shown in Fig. 1. Silicon substrates were scratched using diamond powders with 2–4 μm of particles sizes in the ultrasonic bath beforehand. After
M. Miyake et al. / Thin Solid Films 515 (2007) 4258–4261
Water Inlet 2.45GHz
Short Plunger
Microwave Quartz Plate Plasma Vacuum
To Pump
Chamber Substrate Stage (Water Cooling)
8 cm 16 cm Fig. 1. Schematic drawing of the experimental setup for diamond film synthesis using microwave plasma CVD.
pumping down the chamber, Ar gas was filled at a pressure of approximately 20 kPa. The 2.45 GHz microwave launched from the magnetron oscillator was fed to the chamber through the quartz ring plate in the top of chamber. A greenish spherical plasma discharge was generated and attached on the substrate stage. The silicon substrates set on the stage were heated by the plasma alone. Optical emission spectra from the greenish plasmas show Swan band emission related to C2 radicals. The increases of C2 radicals with increasing argon atoms and ions were detected with OES in microwave plasma CVD [10,11] and in laser-ablated plumes for thin carbon film depositions [12]. With more C2 radicals, it is reported that the well-faceted MCD and NCD films grow at higher growth rates [13,14]. In the NCD film synthesis, Ar/H2/CH4 mixture gas was used with H2 gas ratio of 0–15% and CH4 gas ratio of 1–10%, the total gas flow rate of 200 or 220 sccm and the gas pressure of approximately 20 kPa. The film deposition was performed with 1.3–1.5 kW of microwave power for about 8 h. The morphologies of the diamond films were characterized by the field emission type scanning electron microscope (FESEM) and an atomic force microscope (AFM) using SEIKO SPI3700. Raman spectrum measurements were carried out at ambient temperature using JASCO NR-1800, with the argon laser line at 488 or 514.5 nm. 3. Results and discussion The SEM image and the Raman spectrum of diamond film prepared with Ar/CH4 plasma CVD at the incident power of 1.4 kW are shown in Fig. 2. The gas flow rates were Ar/ CH4 = 200/10 sccm and the total gas pressure was 14.7 kPa. The round-shaped deposits having approximately 1.3 μm in diameter were formed on the substrate shown in SEM image. A broadened spectrum with several peaks in the neighborhood of sp3diamond bonding centered around 1334 cm− 1 are observed. Here, the wavelength of 488 nm argon laser was used in this
Raman spectrum measurement. The large-sized round-shape grains are kinds of microcrystalline diamond embedded in graphitic and amorphous carbon with a broad spectrum centered at 1334 cm− 1. As for a pair of peaks centered around 1155 and 1496 cm− 1 in Raman spectra, it is considered that they are assigned to trans-polyacetylene lying in grain boundaries, and are not possibly related to C–C sp3 vibrations [15]. These peaks are peculiarly observed in the NCD films [16–18]. From our results of Raman spectrum, we also believe that NCD components are contained in the present diamond thin films. The effects of adding H2 gas in Ar/CH4 mixture gas on the OES and the film characteristic are investigated. Fig. 3 shows SEM image of samples with addition of 10% H2 and Raman spectrum of diamond film prepared with Ar/H2/CH4 plasma CVD at different mixture ratio of H2 to Ar, where the wavelength of Ar laser was 514.5 nm in this measurement. An additive of 5– 15% H2 in the Ar/CH4 made the formation of a plasma on the substrate much more easily. The granular segments having 10– 100 nm in diameter were observed in the film shown in Fig. 3(a). Typical grain size is obviously smaller than that shown in Fig. 2(a). Typical NCD features are seen for all the samples, these spectra have mainly four distinctive peaks at 1139, 1350, 1475 and 1553 cm− 1, as shown in Fig. 3(b). The peaks at 1350 and 1553 cm− 1 are assigned for the D and G bands of disordered sp2carbon. The peak centered around 1140 cm− 1 of trans-
(a)
1μm
5,000 5,
(b)
Intensity (arb.units)
Gas Inlet
4259
1334cm 1155cm
1000
-1
1200
-1
1496cm
1400
-1
1600
1800
Raman shift (cm-1) Fig. 2. (a) SEM image and (b) the Raman spectrum of diamond thin film prepared with Ar/H2/CH4 microwave plasma CVD at gas flow rates of Ar/ CH4 = 200/10 sccm.
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(a) 10% H22
100nm 100nm
(b) 1556cm -1 1198cm -1
(2) 10% H2
1474cm -1 1139cm -1
1000
1200
1351cm -1
1553cm -1 (1) 5% H2
(a) 1400
1600
1800
Raman shift (cm-1) Fig. 3. (a) SEM image and (b) Raman spectra of diamond thin film prepared with Ar/H2/CH4 microwave plasma CVD, under the conditions that gas flow rates of Ar/H2/CH4, pressure, and microwave power are (1) 188/10/2 sccm, 14.7– 16.7 kPa, 1.35 kW, (2) 178/20/2 sccm, 17.3–20 kPa, 1.44 kW and (3) 168/30/ 2 sccm, 16.3–16.8 kPa, 1.3 kW, respectively.
polyacetylene components has been attributed to NCD film, it was always accompanied with the peak around 1475 cm− 1. By adding a small amount of H2 gas in Ar/CH4 mixture gas, a peak centered at 1140 cm− 1 is more clearly observed than that at 1155 cm− 1 in case of Ar/CH4 mixture gas shown in Fig. 2(b). In the experiments of the film depositions, the microwave power was 1.35 kW at the total gas pressure between 14.7 kPa and 16.7 kPa. These results of Raman spectra show that the NCD components are contained together with amorphous carbon in the film. Furthermore, when H2 mixture ratio was 10% at gas flow rates of Ar/H2/CH4 = 178/ 20/2 sccm, total gas pressure of 17.3–20 kPa and microwave power of 1.44 kW, stronger peaks centered around 1140 and 1475 cm− 1 peculiar to NCD film have been observed. On the other hand, the intensity of G band has weakened. This result does not mean that the NCD components are more dominant in the film, since the Raman cross-section relating with trans-polyacetylene is large [19]. In the case of mixture gas ratio of 84% Ar and 15% H2 at Ar/H2/CH4 = 168/30/2 sccm, total gas pressure of 16.3–16.8 kPa and microwave power of 1.3 kW, a pair of peaks of trans-polyacetylene components, especially a peak around 1475 cm− 1 became broader. In addition, peaks around 1198 and
Emission Intensity (arb.units)
Intensity (arb.units)
(3) 15% H2
1 10 5 8 10
Ar 357.2nm Ar 389.5nm C2 473.7nm C2 516.5nm Hα 656.3nm
4
6 10 4 4 10 4 2 10 4 0 0
5
10
15
20
25
30
H2 /(Ar+CH4+H2) (%)
(b) Emission Intensity (arb.units)
1144cm
-1
1550 cm− 1 appeared clearly. A peak centered around 1198 cm− 1 is due to the deformation vibration of –CgC– bonding. It seems that the C2 is lost through collisions with H2. To confirm the change of C2 radical density, the OES in Ar/H2/CH4 plasma was carried out. From the optical emission measurements, it has been also confirmed that the emission intensity of C2 radicals at 473.7 and 516.5 nm, for instance, similarly decreased when the contents of H2 gas was raised from 5% to 15%, as shown in Fig. 4(a). From the normalized emission intensities of ArI, C2 and Hα as shown in Fig. 4(b), it is clearly seen that the emission intensities of C2 radicals and ArI decreased more significantly than that of Hα. It is generally considered that atomic hydrogen plays a number of roles of preferential etching of non-diamond carbon species and termination of carbon dangling bonds. In addition to atomic hydrogen, hydrogen molecules also play important roles in a number of chemical reactions with Ar atoms, hydrocarbon species, C2 dimers and so on. As described above, emission intensities of both the ArI and C2 Swan system steeply dropped with the increase of hydrogen gas addition, as shown in Fig. 4. On the other hand, emission intensity of Hα emission gradually decreased compared with those of ArI and C2 Swan system. Similar results that C2 emission intensity decreased with the hydrogen addition have been reported in Ref. [8]. It is considered
1.2 Ar 357.2nm Ar 389.5nm C2 473.7nm C2 516.5nm Hα 656.3nm
H
1 0.8 0.6
Ar
0.4 0.2 0
C 0
5
2 10
15
20
25
30
H2 /(Ar+CH4+H2) (%) Fig. 4. (a) Absolute intensities and (b) normalized intensities of ArI (357.2 nm, 389.5 nm), C2 (473.7 nm, 516.5 nm) and Hα (656.3 nm) emission lines versus H2 gas mixture ratio of H2 / (Ar + H2 + CH4).
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was shown that the spectra of diamond films depend strongly on the ratio of H2 to Ar, in which the peaks of trans-polyacetylene components peculiar to the NCD films have been clearly observed in diamond thin film prepared by the microwave plasma in Ar/CH4 gas mixture. Optical emission measurements showed the strong correlation with the degradation of NCD film quality, since the C2 radicals decreased rapidly with H2 gas addition. AFM analysis showed that the grain sizes were roughly between 10 and 100 nm and the surface roughness of the film was less than 20 nm. A smoother NCD film was obtained with H2 additive. Acknowledgements Fig. 5. AFM image of the surface morphology of diamond film with Ar/H2/CH4 plasma CVD when the gas flow rates were Ar/H2/CH4 = 188/10/2 sccm.
that C2 radicals might be lost through collision with hydrogen molecules, via C2 + H2 → C2H + H [4,20]. Another possible loss mechanism of C2 is Penning ionization with metastable Ar atoms, that is, C2 + Ar⁎ → C2+ + Ar + e. Ar atoms excited by electron impacts collide with hydrogen molecules and change to ArH⁎, that is, Ar⁎ + H2 → ArH⁎ + H. Furthermore, we can also expect that C2 dimers decrease with the decrease of Ar gas content or hydrogen gas addition, because the main C2 production reaction is presented by C2H2 + Ar → C2 + H2 + Ar. From these expectations, we can deduce that ArI and C2 emission intensity decreased with the increase of hydrogen gas content. In Ref. [20], Lombardi et al. noticed that C2H2 is also important in NCD growth in addition to C2 dimer or atomic carbon C. Moreover, it is considered that contribution of C2H species on NCD growth is also important in Ar/H2/CH4 gas mixture [4]. C2H species are produced via C2 + H2 → C2H + H or C2⁎ + H2 → C2H + H. Therefore, with the increase of hydrogen gas, C2H will increase at the expense of C2 loss according to these reactions. Therefore, it is needed to investigate NCD growth rate, by considering not only C2 species but also C2H2 or C2H species. Lastly, Fig. 5 shows a typical AFM image of the surface morphology of diamond films prepared with Ar/H2/CH4 plasma CVD, when the gas flow rates were Ar/H2/CH4 = 188/10/2 sccm. From surface analysis using the AFM, we found that the granular segments having 10–100 nm in diameter were contained in the film. The grain sizes of Ar/H2/CH4 became smaller samples than that of Ar/CH4 ones. As shown in Fig. 5, the surface of diamond film is quite flat and the surface roughness is approximately 10–20 nm. 4. Conclusion In conclusion, the synthesis of nano-crystalline diamond film using Ar/H2/CH4 microwave plasma CVD was carried out, and the effects of addition of H2 gas on the film characteristics of NCD films were investigated. From Raman measurements, it
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