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H2 plasma
Diamond & Related Materials 14 (2005) 279 – 282 www.elsevier.com/locate/diamond
Diamond nucleation in low electron temperature CH4/H2 plasma Satoru IizukaT, Gouki Nishimura, Reijiro Ikada, Hideyuki Yamaguchi Department of Electrical Engineering, Graduate School of Engineering, Tohoku University, Sendai 980 8579, Japan Available online 5 March 2005
Abstract Nucleation of diamond crystal is observed on a surface of fine diamond particles of micron meters attached to a nickel plate placed in methane–hydrogen plasma. The electron temperature Te in the plasma is quite important for the nucleation. When Te is reduced to ~0.4 eV, we observed an appearance of nanometer diamond nucleation on the surface of the fine diamond particles. The size and the number of nucleation cite increase with the deposition time with average growth rate of ~10 nm/h. D 2005 Elsevier B.V. All rights reserved. Keywords: Nucleation of diamond; Nano-diamond; Diamond crystal; Low electron temperature plasma
1. Introduction Diamond has big potential for the production of electronic devices because of its many superior characteristics such as large heat conductivity, maximum hardness, chemical inertness and wide energy band gap. Especially, the development of the method for the growth of single crystal diamond has been a crucial subject to be solved by many researchers. The growth of single crystal diamond gives a strong impact on the various kinds of industrial applications [1–5]. For the synthesis of diamond crystal by plasma-enhanced chemical vapor deposition using a mixture of CH4 and H2, it has been believed that CH3 radical is important precursor for the diamond growth. On the other hand, the role of H atom has been considered to act on etching of non-diamond component. When the electron temperature Te is high, many kinds of radical species except CH3, such as CH2 and CH, are generated at the same time in plasmas, which makes the reactive system being very complicated. Therefore, the control of electron energy distribution in CH4/H2 plasmas is of crucial importance. To date, we have found a novel method for the control of electron energy distribution function in order to reduce higher order dissociation of CH4 [6]. This technique is T Corresponding author. Tel.: +81 22 217 7113; fax: +81 22 263 9374. E-mail address: [email protected] (S. Iizuka). 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.01.040
applied to CH4/H2 plasma for diamond deposition [7]. We observed that high quality diamond is produced in a low Te plasma. On the other hand, when Te is high, we obtained deposition of diamond-like carbon or graphite film. In this paper, we investigate nucleation of diamond crystal growing on the surface of fine diamond particles that are attached to the nickel substrate with no heating system. It is expected that diamond crystal grows on the fine diamond particles epitaxially when they are placed in a low electron temperature CH4/H2 plasma. Experimental apparatus and methods are described in section 2, and experimental results and discussions are presented in section 3. We finally give conclusions in section 4.
2. Experimental apparatus and methods Fig. 1 shows a schematic of the experimental apparatus to observe the growth of diamond [8]. The experiment has been carried out in a parallel plate radio frequency (rf) discharge plasma. The rf frequency is 13.56 MHz and rf power can be changed up to 300 W. Plasma discharge takes place between rf electrode with diameter of 9 cm and grounded anode electrode with diameter of 30 cm, the center of which consists of a hemispherical grid that is insulated from the grounded anode. The diameter of the hemispherical grid is 5 cm and the separation distance between the rf
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Fig. 1. Experimental apparatus with a hemispherical grid for Te control.
electrode and the anode is 3 cm. This grid separates the experimental region into two parts. One is a plasma production region below the grid and the other is a processing region above the grid. The hemispherical grid is biased at 100 V to generate a low Te plasma [9]. Owing to the shielding of the rf field, we can produce a low Te plasma in the processing region. We place Ni substrate plate of 10 10 mm with thickness of 0.5 mm in the processing region, inserted from the upper chamber wall. Before starting the deposition, a few tens of fine diamond particles were attached to the surface of the Ni plate. The average size of the diamond particle is 2 Am with extremely irregular shape. The Ni surface with diamond particles is faced downward to expose the plasma. When the hemispherical grid is removed, Te is increased in the whole region. In this case, the Ni plate is directly immersed in the high Te plasma. In this way, we can compare the effect of Te on the deposition on the surface of fine diamond particles. Here, we fix the gas flow rate of CH4 and H2 at 20 and 180 sccm, respectively, under the constant total pressure of 100 mTorr. The Ni plate is not directly heated externally, but maybe heated by the plasma. First, the Ni plate with fine diamond particles is exposed in the pure H2 plasma for 30 min for the cleaning, then CH4 is introduced to start the deposition. The materials deposited on the fine diamond particles were analyzed by scanning electron microscopy (SEM) and Raman spectroscopy.
The deposition experiments are carried out when Te ~ 0.4 eV at the substrate. Fig. 2(a) shows a SEM image of the diamond particles after 4 h deposition. We observe many small protuberances on the surface as shown typically by an arrow. Maximum size of the protuberance is of the order of 100 nm and the small one is of the order of 10 nm in Fig. 2(a). Therefore, the distribution of the size is rather dispersive. On the contrary, no such protuberance appears on the nickel substrate. From a careful observation at the initial stage of the growth, the number of the protuberances is extremely reduced when the surface of fine diamond particle is different. Therefore, these protuberances seem to be created and growing randomly only on a specific surface. This implies that some selectivity regarding to the surface direction might exist for the appearance of the protuberance. Although it is quite difficult to estimate the material consisting of such protuberance from the SEM image in Fig. 2(a), we think that these protuberances consist of the diamond crystal which epitaxitially grows after the nucleation on the specific surface of fine diamond particles. Fig. 2(b) shows the Raman spectrum of the ones deposited for 4 h. We observe simple peak at 1332 cm 1 in the Raman spectrum. In the background neither signal showing the diamond-like carbon nor graphite is observed. Therefore, this structure may be created by the nucleation of diamond crystal growing on the fine diamond surface.
3. Experimental results and discussion Before the experiments we measured the plasma parameters such as electron density n e and electron temperature Te above and below the hemispherical grid, respectively. In the plasma production region at z = 5 mm, Te is ~ 2.8 eV and n e is about 6.5 108/cm for the rf power of 200 W. In the processing region at z = 20 mm, however, Te is decreased to about 0.5 eV. On the other hand, n e is increased up to 2.5 109/cm at around z = 20 mm. Here, the vertical origin of z is the position of the grid center. The substrate is placed at z = 15 mm. When the grid is removed, Te attains to ~2.8 eV at the substrate.
Fig. 2. (a) SEM image and (b) Raman spectrum of diamond particles in case of Te ~ 0.4 eV for 4 h deposition, when rf power is 200 W. Arrow in (a) shows the nuclei.
S. Iizuka et al. / Diamond & Related Materials 14 (2005) 279–282
Fig. 3 shows a SEM image for the 8 h deposition. Contrary to the image in Fig. 2(a) many small protuberances are observed, covering almost all surface of fine diamond particle. From a careful observation of the shape of single protuberance it looks like a pyramid at the beginning as shown in Fig. 2(a). Under the assumption that the base surface is directed to (1, 0, 0), the nucleation growth might occur three-dimensionally with a slant surface (1, 1, 1). These inclined surfaces may compose a pyramid structure. Such protuberances grow in time on the surface of the diamond particle. The number of the protuberances increases with the deposition time. Fig. 4(a, b, c) show the size distribution of the protuberances observed after 4, 6 and 8 h deposition, respectively. We find that the maximum size is about 180 nm and the average size is about 50 nm for the 4 h deposition as shown in Fig. 4(a). However, after 8 h deposition we find that the maximum size of protuberances grows up to about 380 nm as shown in Fig. 4(c), although the average size is about 75 nm. From these results we find that the average growth rate of the protuberance is about 10 nm/h, which is quite slow, by two orders of magnitude, compared with the growth rate of the diamond of about 1000 nm/h on a heated Ni substrate [7]. It should be emphasized that during this experiment we did not employ a heating system for the fine diamond particles attached to the Ni plate. However, the temperature of the Ni plate might be heated by the direct contact with the plasma. We may also expect heating from radiation from the powered rf electrode, although it might give very weak effect. In order to measure the temperature of the Ni plate, we put a thermocouple on the Ni surface. We have measured Ni temperature of about 250 8C for 200 W rf discharge. Therefore, the bulk temperature of the diamond particles is also expected to be ~250 8C. Although the growth rate is slow, we have observed a nucleation of diamond on the surface of the diamond particles attached to the Ni plate surface immersed in a low electron temperature plasma. We finally note that the control of electron temperature is important for the appearance of the nucleation. We observed thin diamond-like carbon film simply grown on the surface
281
Fig. 4. Size distribution of protuberances deposited on diamond particles for (a) 4 h, (b) 6 h, and (c) 8 h deposition in low Te plasma, when rf power is 200 W.
of diamond particles in a high Te (~2.8eV) plasma, when the hemispherical grid is removed. The homoepitaxial growth of diamond on the diamond surface is quite usual phenomenon. Our results show that the low electron temperature plasma is quite effective for the homoepitaxial growth of diamond.
4. Conclusions
Fig. 3. SEM image of diamond particles in case of Te ~ 0.4 eV for 8 h deposition, when rf power is 200 W.
Nucleation of diamond takes place on the surface of the fine diamond particles in a low Te (~0.4eV) plasma, even when the temperature of the fine diamond particle is ~250 8C. The average growth rate of the protuberance on the
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diamond surface is 10 nm/h. We find that the reduction of electron temperature is quite important for the nucleation and growth of nanometer size diamond crystal on the surface of fine diamond particles. In the low electron temperature plasma, the deposition of diamond-like carbon or graphite film on the fine diamond particles is suppressed markedly.
Acknowledgement The authors are indebted to Dr. T. Abe for the film analysis. The work was partially supported by a Grant-inAid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
References [1] A. Gicquel, K. Hassouni, F. Silva, J. Achard, Curr. Appl. Phys. 1 (2001) 479. [2] M. Kamo, Y. Sato, S. Matsumoto, N. Setaka, J. Cryst. Growth 62 (1983) 642. [3] S. Lee, Z. Lin, X. Jiang, Mater. Sci. Eng. 25 (1999) 127. [4] B.R. Stoner, J.T. Glass, Appl. Phys. Lett. 60 (1992) 698. [5] S.P. McGinnis, M.A. Kelly, S.B. Hagstrom, Appl. Phys. Lett. 66 (1995) 3117. [6] K. Kato, T. Shimizu, S. Iizuka, N. Sato, Appl. Phys. Lett. 76 (2000) 547. [7] T. Shimizu, S. Iizuka, K. Kato, N. Sato, Plasma Sources Sci. Technol. 12 (2003) 521. [8] G. Nishimura, S. Iizuka, T. Shimizu, N. Sato, Diam. Relat Mater. 12 (2003) 374. [9] S. Iizuka, N. Sato, Elect. Eng. Jpn. 190 (2000) 1.
Diamond nucleation in low electron temperature CH4/H2 plasma Satoru IizukaT, Gouki Nishimura, Reijiro Ikada, Hideyuki Yamaguchi Department of Electrical Engineering, Graduate School of Engineering, Tohoku University, Sendai 980 8579, Japan Available online 5 March 2005
Abstract Nucleation of diamond crystal is observed on a surface of fine diamond particles of micron meters attached to a nickel plate placed in methane–hydrogen plasma. The electron temperature Te in the plasma is quite important for the nucleation. When Te is reduced to ~0.4 eV, we observed an appearance of nanometer diamond nucleation on the surface of the fine diamond particles. The size and the number of nucleation cite increase with the deposition time with average growth rate of ~10 nm/h. D 2005 Elsevier B.V. All rights reserved. Keywords: Nucleation of diamond; Nano-diamond; Diamond crystal; Low electron temperature plasma
1. Introduction Diamond has big potential for the production of electronic devices because of its many superior characteristics such as large heat conductivity, maximum hardness, chemical inertness and wide energy band gap. Especially, the development of the method for the growth of single crystal diamond has been a crucial subject to be solved by many researchers. The growth of single crystal diamond gives a strong impact on the various kinds of industrial applications [1–5]. For the synthesis of diamond crystal by plasma-enhanced chemical vapor deposition using a mixture of CH4 and H2, it has been believed that CH3 radical is important precursor for the diamond growth. On the other hand, the role of H atom has been considered to act on etching of non-diamond component. When the electron temperature Te is high, many kinds of radical species except CH3, such as CH2 and CH, are generated at the same time in plasmas, which makes the reactive system being very complicated. Therefore, the control of electron energy distribution in CH4/H2 plasmas is of crucial importance. To date, we have found a novel method for the control of electron energy distribution function in order to reduce higher order dissociation of CH4 [6]. This technique is T Corresponding author. Tel.: +81 22 217 7113; fax: +81 22 263 9374. E-mail address: [email protected] (S. Iizuka). 0925-9635/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.01.040
applied to CH4/H2 plasma for diamond deposition [7]. We observed that high quality diamond is produced in a low Te plasma. On the other hand, when Te is high, we obtained deposition of diamond-like carbon or graphite film. In this paper, we investigate nucleation of diamond crystal growing on the surface of fine diamond particles that are attached to the nickel substrate with no heating system. It is expected that diamond crystal grows on the fine diamond particles epitaxially when they are placed in a low electron temperature CH4/H2 plasma. Experimental apparatus and methods are described in section 2, and experimental results and discussions are presented in section 3. We finally give conclusions in section 4.
2. Experimental apparatus and methods Fig. 1 shows a schematic of the experimental apparatus to observe the growth of diamond [8]. The experiment has been carried out in a parallel plate radio frequency (rf) discharge plasma. The rf frequency is 13.56 MHz and rf power can be changed up to 300 W. Plasma discharge takes place between rf electrode with diameter of 9 cm and grounded anode electrode with diameter of 30 cm, the center of which consists of a hemispherical grid that is insulated from the grounded anode. The diameter of the hemispherical grid is 5 cm and the separation distance between the rf
280
S. Iizuka et al. / Diamond & Related Materials 14 (2005) 279–282
Fig. 1. Experimental apparatus with a hemispherical grid for Te control.
electrode and the anode is 3 cm. This grid separates the experimental region into two parts. One is a plasma production region below the grid and the other is a processing region above the grid. The hemispherical grid is biased at 100 V to generate a low Te plasma [9]. Owing to the shielding of the rf field, we can produce a low Te plasma in the processing region. We place Ni substrate plate of 10 10 mm with thickness of 0.5 mm in the processing region, inserted from the upper chamber wall. Before starting the deposition, a few tens of fine diamond particles were attached to the surface of the Ni plate. The average size of the diamond particle is 2 Am with extremely irregular shape. The Ni surface with diamond particles is faced downward to expose the plasma. When the hemispherical grid is removed, Te is increased in the whole region. In this case, the Ni plate is directly immersed in the high Te plasma. In this way, we can compare the effect of Te on the deposition on the surface of fine diamond particles. Here, we fix the gas flow rate of CH4 and H2 at 20 and 180 sccm, respectively, under the constant total pressure of 100 mTorr. The Ni plate is not directly heated externally, but maybe heated by the plasma. First, the Ni plate with fine diamond particles is exposed in the pure H2 plasma for 30 min for the cleaning, then CH4 is introduced to start the deposition. The materials deposited on the fine diamond particles were analyzed by scanning electron microscopy (SEM) and Raman spectroscopy.
The deposition experiments are carried out when Te ~ 0.4 eV at the substrate. Fig. 2(a) shows a SEM image of the diamond particles after 4 h deposition. We observe many small protuberances on the surface as shown typically by an arrow. Maximum size of the protuberance is of the order of 100 nm and the small one is of the order of 10 nm in Fig. 2(a). Therefore, the distribution of the size is rather dispersive. On the contrary, no such protuberance appears on the nickel substrate. From a careful observation at the initial stage of the growth, the number of the protuberances is extremely reduced when the surface of fine diamond particle is different. Therefore, these protuberances seem to be created and growing randomly only on a specific surface. This implies that some selectivity regarding to the surface direction might exist for the appearance of the protuberance. Although it is quite difficult to estimate the material consisting of such protuberance from the SEM image in Fig. 2(a), we think that these protuberances consist of the diamond crystal which epitaxitially grows after the nucleation on the specific surface of fine diamond particles. Fig. 2(b) shows the Raman spectrum of the ones deposited for 4 h. We observe simple peak at 1332 cm 1 in the Raman spectrum. In the background neither signal showing the diamond-like carbon nor graphite is observed. Therefore, this structure may be created by the nucleation of diamond crystal growing on the fine diamond surface.
3. Experimental results and discussion Before the experiments we measured the plasma parameters such as electron density n e and electron temperature Te above and below the hemispherical grid, respectively. In the plasma production region at z = 5 mm, Te is ~ 2.8 eV and n e is about 6.5 108/cm for the rf power of 200 W. In the processing region at z = 20 mm, however, Te is decreased to about 0.5 eV. On the other hand, n e is increased up to 2.5 109/cm at around z = 20 mm. Here, the vertical origin of z is the position of the grid center. The substrate is placed at z = 15 mm. When the grid is removed, Te attains to ~2.8 eV at the substrate.
Fig. 2. (a) SEM image and (b) Raman spectrum of diamond particles in case of Te ~ 0.4 eV for 4 h deposition, when rf power is 200 W. Arrow in (a) shows the nuclei.
S. Iizuka et al. / Diamond & Related Materials 14 (2005) 279–282
Fig. 3 shows a SEM image for the 8 h deposition. Contrary to the image in Fig. 2(a) many small protuberances are observed, covering almost all surface of fine diamond particle. From a careful observation of the shape of single protuberance it looks like a pyramid at the beginning as shown in Fig. 2(a). Under the assumption that the base surface is directed to (1, 0, 0), the nucleation growth might occur three-dimensionally with a slant surface (1, 1, 1). These inclined surfaces may compose a pyramid structure. Such protuberances grow in time on the surface of the diamond particle. The number of the protuberances increases with the deposition time. Fig. 4(a, b, c) show the size distribution of the protuberances observed after 4, 6 and 8 h deposition, respectively. We find that the maximum size is about 180 nm and the average size is about 50 nm for the 4 h deposition as shown in Fig. 4(a). However, after 8 h deposition we find that the maximum size of protuberances grows up to about 380 nm as shown in Fig. 4(c), although the average size is about 75 nm. From these results we find that the average growth rate of the protuberance is about 10 nm/h, which is quite slow, by two orders of magnitude, compared with the growth rate of the diamond of about 1000 nm/h on a heated Ni substrate [7]. It should be emphasized that during this experiment we did not employ a heating system for the fine diamond particles attached to the Ni plate. However, the temperature of the Ni plate might be heated by the direct contact with the plasma. We may also expect heating from radiation from the powered rf electrode, although it might give very weak effect. In order to measure the temperature of the Ni plate, we put a thermocouple on the Ni surface. We have measured Ni temperature of about 250 8C for 200 W rf discharge. Therefore, the bulk temperature of the diamond particles is also expected to be ~250 8C. Although the growth rate is slow, we have observed a nucleation of diamond on the surface of the diamond particles attached to the Ni plate surface immersed in a low electron temperature plasma. We finally note that the control of electron temperature is important for the appearance of the nucleation. We observed thin diamond-like carbon film simply grown on the surface
281
Fig. 4. Size distribution of protuberances deposited on diamond particles for (a) 4 h, (b) 6 h, and (c) 8 h deposition in low Te plasma, when rf power is 200 W.
of diamond particles in a high Te (~2.8eV) plasma, when the hemispherical grid is removed. The homoepitaxial growth of diamond on the diamond surface is quite usual phenomenon. Our results show that the low electron temperature plasma is quite effective for the homoepitaxial growth of diamond.
4. Conclusions
Fig. 3. SEM image of diamond particles in case of Te ~ 0.4 eV for 8 h deposition, when rf power is 200 W.
Nucleation of diamond takes place on the surface of the fine diamond particles in a low Te (~0.4eV) plasma, even when the temperature of the fine diamond particle is ~250 8C. The average growth rate of the protuberance on the
282
S. Iizuka et al. / Diamond & Related Materials 14 (2005) 279–282
diamond surface is 10 nm/h. We find that the reduction of electron temperature is quite important for the nucleation and growth of nanometer size diamond crystal on the surface of fine diamond particles. In the low electron temperature plasma, the deposition of diamond-like carbon or graphite film on the fine diamond particles is suppressed markedly.
Acknowledgement The authors are indebted to Dr. T. Abe for the film analysis. The work was partially supported by a Grant-inAid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
References [1] A. Gicquel, K. Hassouni, F. Silva, J. Achard, Curr. Appl. Phys. 1 (2001) 479. [2] M. Kamo, Y. Sato, S. Matsumoto, N. Setaka, J. Cryst. Growth 62 (1983) 642. [3] S. Lee, Z. Lin, X. Jiang, Mater. Sci. Eng. 25 (1999) 127. [4] B.R. Stoner, J.T. Glass, Appl. Phys. Lett. 60 (1992) 698. [5] S.P. McGinnis, M.A. Kelly, S.B. Hagstrom, Appl. Phys. Lett. 66 (1995) 3117. [6] K. Kato, T. Shimizu, S. Iizuka, N. Sato, Appl. Phys. Lett. 76 (2000) 547. [7] T. Shimizu, S. Iizuka, K. Kato, N. Sato, Plasma Sources Sci. Technol. 12 (2003) 521. [8] G. Nishimura, S. Iizuka, T. Shimizu, N. Sato, Diam. Relat Mater. 12 (2003) 374. [9] S. Iizuka, N. Sato, Elect. Eng. Jpn. 190 (2000) 1.