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Free-standing diamond films prepared by a dc-plasma method above the ethylene glycol solution
Diamond & Related Materials 16 (2007) 570 – 575 www.elsevier.com/locate/diamond
Free-standing diamond films prepared by a dc-plasma method above the ethylene glycol solution Yuta Matsushima ⁎, Tsutomu Yamazaki, Kazuyuki Maeda, Tatsuo Noma, Takeyuki Suzuki Division of Advanced Materials Science and Technology, Graduate School of Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho, Koganei-shi, Tokyo 184-8588, Japan Received 16 June 2006; received in revised form 11 September 2006; accepted 13 November 2006 Available online 18 December 2006
Abstract Free-standing diamond films were prepared using a plasma chemical vapor deposition method above the liquid surface through two routes. Diamond was deposited on the surface of a tungsten anode under a dc-plasma regime. The electric field near the anode surface was flattened by placing a sub-electrode and it brought uniform film deposition. The growth rate was 5 μm h− 1 and the thickness increased with the deposition time up to 12 μm. A free-standing film removed from the tungsten anode showed translucency. A glassy carbon layer with a thickness of 100 nm existed between the diamond film and the anode surface, and it partly remained on the back side of the removed diamond film. Under a plasma-jet regime, diamond was deposited on a silicon substrate brown with a plasma jet expelled from a nozzle exit. A high growth rate of 100 μm h− 1 was attained at the maximum with increasing discharge power and carbon concentration, but the thickness profile was quite uneven. The removed film was elliptical and was larger than the nozzle size. A 3C–SiC layer was formed on the back side of the removed film. © 2006 Elsevier B.V. All rights reserved. Keywords: Diamond; Plasma CVD; Optical emission spectroscopy; Free-standing film; Ethylene glycol
1. Introduction Diamond has high thermal conductivity, high mechanical strength and optical transparency. A free-standing film is expected as a window material for electromagnetic waves and a heat sink substrate for high power electric devices [1–6]. Chemical vapor deposition (CVD) methods involve less restriction on the film dimensions, compared to the high pressure and high temperature method. Free-standing diamond films are obtained by removing the substrates. In the view point of the cost, the high growth rate is desirable. Among the CVD methods, the arc-jet and the dc-plasma methods provide relatively high deposition rate. For example, Zhong et al. [7] and Lu et al. [8] reported free-standing films with the growth rate of 8–40 μm h− 1 in the arc-jet method. Although the growth rate was not shown, a free-standing diamond film was heteroepitaxially grown on iridium with the dc-plasma method [9].
⁎ Corresponding author. Tel./fax: +81 42 388 7921. E-mail address: [email protected] (Y. Matsushima). 0925-9635/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2006.11.049
The authors have reported the novel CVD process above organic solutions such as ethylene glycol for the diamond synthesis, where the plasma is generated above the liquid surface [10–15]. The liquid concurrently works as a cathode and a carbon source. The molecules of the liquid are taken into the plasma from the surface and are decomposed into radicals to deposit as diamond on the substrate. This method has an advantage that there is a small possibility of metal contamination with cathode sputtering under a dc-plasma regime, because the cathode is the liquid carbon source. To date, diamond films were deposited through two routes using plasma above the liquid surface. One was a dc-plasma regime, where diamond was deposited on a metal anode opposite the liquid surface. The other was a plasma-jet regime, where diamond was deposited on a silicon substrate blown by the plasma jet expelled from a nozzle exit. Under both regimes, diamond was synthesized under semiatmospheric pressures between 26.7 and 53.3 kPa. Plasma spectroscopy revealed the existence of C2 and OH, which were somewhat uncommon in the plasma CVD processes for the diamond synthesis [15]. The crystallinity of the deposited film was relatively high in terms of Raman spectroscopy. It was
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surface of a tungsten anode (substrate) (Fig. 1(a)). The dimensions of the tungsten rod were ϕ5 × 5 mm. The substrate was sustained with a water-cooled copper-holder. The apparatus was similar to that previously reported in Ref. [10] but a subelectrode was equipped to flatten the electric field near the substrate edge. The sub-electrode was made of a copper ring with the inner and outer diameters of 7 and 11 mm. The separation between the sub-electrode and the liquid surface was 3.0 mm. The substrate was placed upper than the sub-electrode by 0.5 mm, which was experimentally determined. The substrate and the sub-electrode were electrically grounded. The plasma was generated under atmospheric pressure (101.3 kPa). The solution was 75 mol% ethylene glycol aqueous solution. The corresponding C/H/O ratio was 0.182/0.606/0.212. The electric currents flowing to the substrate and to the sub-electrode were 400 and 20 mA, which were adjusted with a variable resistance between the substrate and the ground terminal. The substrate temperature was kept at 1123 K during the deposition. Under the plasma-jet regime, diamond was deposited on a silicon substrate placed above a nozzle exit. The apparatus was similar to that reported in Ref. [15] and is reviewed in Fig. 1(b). The substrate was blown with the plasma jet expelled from the nozzle exit. The nozzle diameter was 2 mm. The substrate was inclined to measure the substrate temperature at the surface with a two-color radiation thermometer (Chino, IR-CAQ) through
Fig. 1. Schematic illustrations of apparatuses used under the dc-plasma regime (a), and the plasma-jet regime (b).
probably due to the existence of oxidizing species, such as O and OH, which were more effective than hydrogen in removing nondiamond carbon impurities [15,16]. It is known that the transparency and the thermal conductivity of diamond films are affected by quality [17]. Our method is expected to be applied to the fabrication of high quality diamond films. However, the dc-plasma regime had a problem in uniformity of the deposited film [10]. Diamond was synthesized on the edge of the substrate, or anode. At the center of the substrate, only glassy carbon was deposited. It resulted from the plasma convergence at the edge. Under the plasma-jet regime, the uniformity of the film was improved in the previous report but the growth rate was modest and was 10–20 μm h− 1 [15]. To obtain free-standing diamond films, the high growth rate is advantageous and our plasma-jet method has the potential for it. In this work, the fabrication of free-standing diamond films was demonstrated by overcoming the problems previously reported. Uniform diamond films were prepared by improving the distribution of the electric field under the dc-plasma regime. The growth rate was enhanced by increasing discharge power and carbon concentration under the plasma-jet regime. 2. Experimental The apparatuses were schematically illustrated in Fig. 1. Under the dc-plasma regime, diamond was deposited on the
Fig. 2. SEM images of deposits obtained under the dc-plasma regime (a) without, and (b) with the sub-electrode. Left hand side of the images shows the deposits at the substrate center and right shows those at the edge.
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Fig. 4. Film thickness with the deposition time under the dc-plasma regime.
Fig. 3. Raman spectra of deposits obtained under the dc-plasma regime (a) without, and (b) with the sub-electrode. The spectra compare the deposits at the center and the edge.
the chamber wall of quartz glass from the outside. The separation between the nozzle exit and the substrate surface was adjusted with the inclination. The substrate temperature was kept at 1173 K. To enhance the plasma density, the plasma was generated under a semi-atmospheric pressure about 80 kPa. The surface level of the liquid was kept by overflowing the liquid from glass tubes for both apparatuses. The glass tubes had inner diameters of 7 and 13 mm for the dc-plasma and the plasma-jet apparatuses, respectively. The electric conductivity of the liquid was adjusted to be 1.2–1.5 mS cm− 1 by adding hydrochloric acid. The liquid was circulated with rotary pumps for cooling. The deposited films were characterized with microRaman spectroscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD) with Cu Kα radiation.
the substrate center and the edge. Semispherical particles were observed at the center (Fig. 2(a) left) and they showed broad peaks specific to glassy carbon at 1350 and 1580 cm− 1 (Fig. 3 (a)). At the edge, where the plasma converged, large grain diamond was deposited (Fig. 2(a) right). It exhibited the sharp diamond peak at 1330 cm− 1 in the Raman spectrum (Fig. 3(a)). To prevent the convergence of the plasma at the edge, the subelectrode was equipped. Previous to the experiment, the effect of the sub-electrode on flattening the electric field was confirmed with the electric field simulation program (POISSON) in Consortium for Upper-level Physics Software (CUPS) [18]. With the sub-electrode, the uniformity of the film was much improved. The film showed similar morphology over the whole area. Fig. 2(b) compares the surface morphologies at the center (left) and the edge (right). There was substantially no difference in the quality between the center and the edge (Fig. 3(b)). Fig. 4 shows the change in the film thickness with the deposition time. The thickness linearly increases with the time up to 12 μm. The deposition rate was about 5 μm h− 1. Beyond 12 μm, the cracks of the film were observed. That is due to the
3. Results and discussion 3.1. Dc-plasma regime Figs. 2 and 3 show SEM images and Raman spectra, respectively, in order to compare the deposits on the substrates without and with the sub-electrode. Without the sub-electrode (Figs. 2(a) and 3(a)), the deposits were quite different between
Fig. 5. Plasma spectra under the plasma-jet regime with the conditions of the previous work (C/H/O = 0.186/0.604/0.210 with 600 W), and this work (C/H/ O = 0.203/0.607/0.190 with 1500 W).
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higher and C/H/O = 0.203/0.607/0.190, where n-propanol was added in the ratio of 0.82 to ethylene glycol to adjust the carbon concentration. This difference is crucial for the diamond synthesis, because it is known that the synthetic region of diamond extends over a narrow area near the C:O = 1:1 line [19]. Both concentrations were included in the region. The plasma spectra changed with the discharge power. Fig. 5 compares the plasma spectra of the previous condition (C/H/ O = 0.186/0.604/0.210 with 600 W) and those of this work (C/ H/O = 0.203/0.607/0.190 with 1500 W). The species observed in the optical spectroscopy were basically the same but the intensity ratio was varied. The relative intensities of OH and CO decreased with the discharge power, although those of H and C2 increased. The emission from O rose at 1500 W. Probably, the reduction of OH intensity and the increment of H and O intensities are due to the dissociation reaction of OH → O + H in the plasma with the higher discharge power. The plasma temperature evaluated using the line pair method with the Hα
Fig. 6. SEM images of cross sections of the films with different nozzle–substrate separation. (a) 1.0, (b) 2.0, and (c) 3.0 mm.
high electric resistance of the deposited film, which interrupts the current flow to maintain the plasma discharge. 3.2. Plasma-jet regime The previous report about the plasma-jet method using ethylene glycol solution showed the modest growth rate of 10– 20 μm h− 1. The increase of the discharge power was effective in enhancing the growth rate. There existed the correlation between carbon concentration and the discharge power on the diamond film deposition. At the discharge power of 600 W (the previous work), the carbon concentration was C/H/O = 0.186/ 0.604/0.210. With the increase of the discharge power to 1500 W, the appropriate carbon concentration was slightly
Fig. 7. Photographs of free-standing diamond films obtained under the dcplasma regime (a), and the plasma-jet regime (b).
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and Hβ lines was 5500 K. The value was comparable to that estimated for the arc jet, where almost hydrogen was assumed to be dissociated to atomic hydrogen [7]. Fig. 6 shows the cross sections of the diamond films after 1 h with the various distances from the nozzle exit. The deposition rate was highest at 1.0 mm and was 100 μm h− 1 at the maximum. It decreased to 20 μm h− 1 at 3.0 mm. The film prepared with the 1.0 mm separation was inhomogeneous in thickness and the uniformity was improved for the film with the 3.0 mm separation. However, all the films showed cross sections in the form of a hillock. 3.3. Free-standing diamond films The diamond films prepared under both regimes were removed from the substrates by chemically dissolving the substrates in diluted solutions of nitric and fluoric acid for tungsten, and fluoric acid for silicon. Fig. 5 shows the photographs of the diamond films. The film under the dcplasma regime shows translucency (Fig. 7(a)). On the other hand, the film under the plasma-jet regime is opaque (Fig. 7(b)). Although the size of the nozzle exit was 2 mm, the film extends over an elliptical area with the major axis of 8 mm. Fig. 8 shows the XRD patterns of the substrate side (back side) of the films. No other peaks than diamond are shown in Fig. 8. The film obtained under the plasma-jet regime slightly shows the preferred orientation along b110N. Fig. 9 shows the Raman spectra of those films on the substrate side and the growth side (front side) exposed to plasma. Glassy carbon was observed on the substrate side of the film under the dc-plasma regime, as shown in Fig. 9(a). The glassy carbon layer was recognized in SEM as a thin layer of ∼ 100 nm thickness, which indicated the formation of an intermediate layer. The film came off at the layer from the substrate. The insufficient mechanical strength of the glassy carbon layer was also a reason for the cracks of the films beyond 12 μm of thickness. Because the chemical inertness of the glassy carbon to acid is similar to diamond, the layer is difficult
Fig. 9. Raman spectra of the free-standing films on the substrate sides (back sides) and the growth side (front side). (a) dc-plasma regime and (b) plasma-jet regime. Peaks indicated by the mark ▾ are 3C–SiC.
to be chemically removed. The opaque part at the center of the film in Fig. 7(a) is due to the partly remained glassy carbon layer. The growth rate of 5 μm h− 1 was comparable to that which the transparency was obtained in the arc-jet method [8,9]. The full width at half maximum (FWHM) of the diamond Raman peak was 4.0–5.2 cm− 1. According to Yang et al. [17], there is a correlation between the quality of diamond films and the thermal conductivity. Their translucent film with the FWHM of 5.2 cm− 1 had the thermal conductivity of 18.3 ± 0.2 W cm− 1 K− 1. If the effect of grain scattering is not taken into account, the film prepared under the dc-plasma regime is expected to have the thermal conductivity of a comparable degree. On the other hand, the intermediate SiC layer was formed on the substrate side under the plasma-jet regime (Fig. 9(b)). The layer was thin enough that peaks from 3C–SiC were not observed in XRD (Fig. 8). 4. Summary
Fig. 8. XRD patterns of the films obtained under the dc-plasma and the plasmajet regimes. The films were mounted with the substrate side (back side) irradiated with X-rays.
Free-standing diamond films were prepared using the plasma CVD method above the liquid surface under the dc-plasma regime and the plasma-jet regime. Free-standing films removed from the substrates showed translucency under the dc-plasma regime. The growth rate of 5 μm h− 1 was comparable to that which the transparency was obtained with the arc-jet method
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[7,8]. Because the maximum thickness was 12 μm without a discernible crack, it was difficult to mechanically remove the residual glassy carbon layer. Under the plasma-jet regime, the opaque film was deposited on the larger area than that of the nozzle exit. The growth rate as high as 100 μm h− 1 was attained at the maximum. There remained a problem in the uniformity of the thickness. The intermediate 3C–SiC layer was formed on the silicon substrate. There was no crack in the films beyond tens of micrometers. Acknowledgements The authors thank Mr. K. Ishida and Ms. S. Kawashima for the experimental support. This research was financially supported in part by the Fellowship to Researchers and Support to Academic Circles from The Association for the Progress of New Chemistry and by Saneyoshi Scholarship Foundation. References [1] K. Takahashi, S. Illy, R. Heidinger, A. Kasugai, R. Minami, K. Sakamoto, M. Thumm, T. Imai, Fusion Eng. Des. 74 (2005) 305. [2] Y. Tzuk, A. Tal, S. Goldring, Y. Glick, E. Lebiush, G. Kaufman, R. Lavi, IEEE J. Quantum Electron. 40 (2004) 262. [3] Y. Kawano, S. Chiba, A. Inoue, Rev. Sci. Instrum. 75 (2004) 279. [4] X.-T. Ying, J.-L. Luo, P.-N. Wang, M.-Q. Cui, Y.-D. Zhao, G. Li, P.-P. Zhu, Diamond Relat. Mater. 12 (2003) 719.
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[5] K. Takahashi, K. Sakamoto, A. Kasugai, T. Imai, J.R. Brandon, R.S. Sussmann, Rev. Sci. Instrum. 71 (2000) 4139. [6] B. Fiegl, R. Kuhnert, H. Schwarzbauer, F. Koch, Diamond Relat. Mater. 3 (1994) 658. [7] G.-F. Zhong, F.-Z. Shen, W.-Z. Tang, F.-X. Lu, Diamond Relat. Mater. 9 (2000) 1678. [8] F.-X. Lu, W.-Z. Tang, G.-F. Zhong, T.-B. Huang, J.-M. Liu, G.-H. Li, T.-L. Lo, Y.-G. Zhang, Z.-L. Sun, S.-M. Du, Q.-Y. He, S.-I. Wang, Diamond Relat. Mater. 9 (2000) 1655. [9] K. Suzuki, H. Fukuda, T. Yamada, A. Sawabe, Diamond Relat. Mater. 10 (2001) 2153. [10] T. Suzuki, T. Ishihara, T. Yamazaki, S. Wada, Jpn. J. Appl. Phys. 36 (1997) L504. [11] T. Suzuki, T. Ishihara, T. Shimosato, T. Yamazaki, S. Wada, J. Eur. Ceram. Soc. 18 (1998) 141. [12] T. Suzuki, M. Tsukada, T. Yamazaki, S. Wada, T. Noma, J. Mater. Sci. 34 (1999) 4761. [13] T. Suzuki, S. Wada, M. Suzuki, T. Yamazaki, T. Noma, J. Mater. Sci. Lett. 19 (2000) 1899. [14] T. Suzuki, S. Wada, M. Tsukada, T. Yamazaki, T. Noma, J. Eur. Ceram. Soc. 21 (2001) 331. [15] Y. Matsushima, T. Yamazaki, K. Maeda, T. Noma, T. Suzuki, Diamond Relat. Mater. 15 (2006) 1314. [16] K.V. Ravi, C.A. Koch, D.S. Olson, P. Choong, J.W. Vandersande, L.D. Zoltan, Appl. Phys. Lett. 64 (1994) 2229. [17] J.-X. Yang, X.-F. Duan, F.-X. Lu, C.-M. Li, T.-C. Zuo, F.-L. Wang, Diamond Relat. Mater. 14 (2005) 1583. [18] R. Ehrlich, J. Tuszynski, L. Roelofs, R. Stoner, Electricity and Magnetism Simulations: The Consortium for Upper-Level Physics Software, John Wiley & Sons, Inc., New York, 1994. [19] P.K. Bachmann, D. Leers, H. Lydtin, Diamond Relat. Mater. 1 (1991) 1.
Free-standing diamond films prepared by a dc-plasma method above the ethylene glycol solution Yuta Matsushima ⁎, Tsutomu Yamazaki, Kazuyuki Maeda, Tatsuo Noma, Takeyuki Suzuki Division of Advanced Materials Science and Technology, Graduate School of Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho, Koganei-shi, Tokyo 184-8588, Japan Received 16 June 2006; received in revised form 11 September 2006; accepted 13 November 2006 Available online 18 December 2006
Abstract Free-standing diamond films were prepared using a plasma chemical vapor deposition method above the liquid surface through two routes. Diamond was deposited on the surface of a tungsten anode under a dc-plasma regime. The electric field near the anode surface was flattened by placing a sub-electrode and it brought uniform film deposition. The growth rate was 5 μm h− 1 and the thickness increased with the deposition time up to 12 μm. A free-standing film removed from the tungsten anode showed translucency. A glassy carbon layer with a thickness of 100 nm existed between the diamond film and the anode surface, and it partly remained on the back side of the removed diamond film. Under a plasma-jet regime, diamond was deposited on a silicon substrate brown with a plasma jet expelled from a nozzle exit. A high growth rate of 100 μm h− 1 was attained at the maximum with increasing discharge power and carbon concentration, but the thickness profile was quite uneven. The removed film was elliptical and was larger than the nozzle size. A 3C–SiC layer was formed on the back side of the removed film. © 2006 Elsevier B.V. All rights reserved. Keywords: Diamond; Plasma CVD; Optical emission spectroscopy; Free-standing film; Ethylene glycol
1. Introduction Diamond has high thermal conductivity, high mechanical strength and optical transparency. A free-standing film is expected as a window material for electromagnetic waves and a heat sink substrate for high power electric devices [1–6]. Chemical vapor deposition (CVD) methods involve less restriction on the film dimensions, compared to the high pressure and high temperature method. Free-standing diamond films are obtained by removing the substrates. In the view point of the cost, the high growth rate is desirable. Among the CVD methods, the arc-jet and the dc-plasma methods provide relatively high deposition rate. For example, Zhong et al. [7] and Lu et al. [8] reported free-standing films with the growth rate of 8–40 μm h− 1 in the arc-jet method. Although the growth rate was not shown, a free-standing diamond film was heteroepitaxially grown on iridium with the dc-plasma method [9].
⁎ Corresponding author. Tel./fax: +81 42 388 7921. E-mail address: [email protected] (Y. Matsushima). 0925-9635/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2006.11.049
The authors have reported the novel CVD process above organic solutions such as ethylene glycol for the diamond synthesis, where the plasma is generated above the liquid surface [10–15]. The liquid concurrently works as a cathode and a carbon source. The molecules of the liquid are taken into the plasma from the surface and are decomposed into radicals to deposit as diamond on the substrate. This method has an advantage that there is a small possibility of metal contamination with cathode sputtering under a dc-plasma regime, because the cathode is the liquid carbon source. To date, diamond films were deposited through two routes using plasma above the liquid surface. One was a dc-plasma regime, where diamond was deposited on a metal anode opposite the liquid surface. The other was a plasma-jet regime, where diamond was deposited on a silicon substrate blown by the plasma jet expelled from a nozzle exit. Under both regimes, diamond was synthesized under semiatmospheric pressures between 26.7 and 53.3 kPa. Plasma spectroscopy revealed the existence of C2 and OH, which were somewhat uncommon in the plasma CVD processes for the diamond synthesis [15]. The crystallinity of the deposited film was relatively high in terms of Raman spectroscopy. It was
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surface of a tungsten anode (substrate) (Fig. 1(a)). The dimensions of the tungsten rod were ϕ5 × 5 mm. The substrate was sustained with a water-cooled copper-holder. The apparatus was similar to that previously reported in Ref. [10] but a subelectrode was equipped to flatten the electric field near the substrate edge. The sub-electrode was made of a copper ring with the inner and outer diameters of 7 and 11 mm. The separation between the sub-electrode and the liquid surface was 3.0 mm. The substrate was placed upper than the sub-electrode by 0.5 mm, which was experimentally determined. The substrate and the sub-electrode were electrically grounded. The plasma was generated under atmospheric pressure (101.3 kPa). The solution was 75 mol% ethylene glycol aqueous solution. The corresponding C/H/O ratio was 0.182/0.606/0.212. The electric currents flowing to the substrate and to the sub-electrode were 400 and 20 mA, which were adjusted with a variable resistance between the substrate and the ground terminal. The substrate temperature was kept at 1123 K during the deposition. Under the plasma-jet regime, diamond was deposited on a silicon substrate placed above a nozzle exit. The apparatus was similar to that reported in Ref. [15] and is reviewed in Fig. 1(b). The substrate was blown with the plasma jet expelled from the nozzle exit. The nozzle diameter was 2 mm. The substrate was inclined to measure the substrate temperature at the surface with a two-color radiation thermometer (Chino, IR-CAQ) through
Fig. 1. Schematic illustrations of apparatuses used under the dc-plasma regime (a), and the plasma-jet regime (b).
probably due to the existence of oxidizing species, such as O and OH, which were more effective than hydrogen in removing nondiamond carbon impurities [15,16]. It is known that the transparency and the thermal conductivity of diamond films are affected by quality [17]. Our method is expected to be applied to the fabrication of high quality diamond films. However, the dc-plasma regime had a problem in uniformity of the deposited film [10]. Diamond was synthesized on the edge of the substrate, or anode. At the center of the substrate, only glassy carbon was deposited. It resulted from the plasma convergence at the edge. Under the plasma-jet regime, the uniformity of the film was improved in the previous report but the growth rate was modest and was 10–20 μm h− 1 [15]. To obtain free-standing diamond films, the high growth rate is advantageous and our plasma-jet method has the potential for it. In this work, the fabrication of free-standing diamond films was demonstrated by overcoming the problems previously reported. Uniform diamond films were prepared by improving the distribution of the electric field under the dc-plasma regime. The growth rate was enhanced by increasing discharge power and carbon concentration under the plasma-jet regime. 2. Experimental The apparatuses were schematically illustrated in Fig. 1. Under the dc-plasma regime, diamond was deposited on the
Fig. 2. SEM images of deposits obtained under the dc-plasma regime (a) without, and (b) with the sub-electrode. Left hand side of the images shows the deposits at the substrate center and right shows those at the edge.
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Fig. 4. Film thickness with the deposition time under the dc-plasma regime.
Fig. 3. Raman spectra of deposits obtained under the dc-plasma regime (a) without, and (b) with the sub-electrode. The spectra compare the deposits at the center and the edge.
the chamber wall of quartz glass from the outside. The separation between the nozzle exit and the substrate surface was adjusted with the inclination. The substrate temperature was kept at 1173 K. To enhance the plasma density, the plasma was generated under a semi-atmospheric pressure about 80 kPa. The surface level of the liquid was kept by overflowing the liquid from glass tubes for both apparatuses. The glass tubes had inner diameters of 7 and 13 mm for the dc-plasma and the plasma-jet apparatuses, respectively. The electric conductivity of the liquid was adjusted to be 1.2–1.5 mS cm− 1 by adding hydrochloric acid. The liquid was circulated with rotary pumps for cooling. The deposited films were characterized with microRaman spectroscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD) with Cu Kα radiation.
the substrate center and the edge. Semispherical particles were observed at the center (Fig. 2(a) left) and they showed broad peaks specific to glassy carbon at 1350 and 1580 cm− 1 (Fig. 3 (a)). At the edge, where the plasma converged, large grain diamond was deposited (Fig. 2(a) right). It exhibited the sharp diamond peak at 1330 cm− 1 in the Raman spectrum (Fig. 3(a)). To prevent the convergence of the plasma at the edge, the subelectrode was equipped. Previous to the experiment, the effect of the sub-electrode on flattening the electric field was confirmed with the electric field simulation program (POISSON) in Consortium for Upper-level Physics Software (CUPS) [18]. With the sub-electrode, the uniformity of the film was much improved. The film showed similar morphology over the whole area. Fig. 2(b) compares the surface morphologies at the center (left) and the edge (right). There was substantially no difference in the quality between the center and the edge (Fig. 3(b)). Fig. 4 shows the change in the film thickness with the deposition time. The thickness linearly increases with the time up to 12 μm. The deposition rate was about 5 μm h− 1. Beyond 12 μm, the cracks of the film were observed. That is due to the
3. Results and discussion 3.1. Dc-plasma regime Figs. 2 and 3 show SEM images and Raman spectra, respectively, in order to compare the deposits on the substrates without and with the sub-electrode. Without the sub-electrode (Figs. 2(a) and 3(a)), the deposits were quite different between
Fig. 5. Plasma spectra under the plasma-jet regime with the conditions of the previous work (C/H/O = 0.186/0.604/0.210 with 600 W), and this work (C/H/ O = 0.203/0.607/0.190 with 1500 W).
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higher and C/H/O = 0.203/0.607/0.190, where n-propanol was added in the ratio of 0.82 to ethylene glycol to adjust the carbon concentration. This difference is crucial for the diamond synthesis, because it is known that the synthetic region of diamond extends over a narrow area near the C:O = 1:1 line [19]. Both concentrations were included in the region. The plasma spectra changed with the discharge power. Fig. 5 compares the plasma spectra of the previous condition (C/H/ O = 0.186/0.604/0.210 with 600 W) and those of this work (C/ H/O = 0.203/0.607/0.190 with 1500 W). The species observed in the optical spectroscopy were basically the same but the intensity ratio was varied. The relative intensities of OH and CO decreased with the discharge power, although those of H and C2 increased. The emission from O rose at 1500 W. Probably, the reduction of OH intensity and the increment of H and O intensities are due to the dissociation reaction of OH → O + H in the plasma with the higher discharge power. The plasma temperature evaluated using the line pair method with the Hα
Fig. 6. SEM images of cross sections of the films with different nozzle–substrate separation. (a) 1.0, (b) 2.0, and (c) 3.0 mm.
high electric resistance of the deposited film, which interrupts the current flow to maintain the plasma discharge. 3.2. Plasma-jet regime The previous report about the plasma-jet method using ethylene glycol solution showed the modest growth rate of 10– 20 μm h− 1. The increase of the discharge power was effective in enhancing the growth rate. There existed the correlation between carbon concentration and the discharge power on the diamond film deposition. At the discharge power of 600 W (the previous work), the carbon concentration was C/H/O = 0.186/ 0.604/0.210. With the increase of the discharge power to 1500 W, the appropriate carbon concentration was slightly
Fig. 7. Photographs of free-standing diamond films obtained under the dcplasma regime (a), and the plasma-jet regime (b).
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and Hβ lines was 5500 K. The value was comparable to that estimated for the arc jet, where almost hydrogen was assumed to be dissociated to atomic hydrogen [7]. Fig. 6 shows the cross sections of the diamond films after 1 h with the various distances from the nozzle exit. The deposition rate was highest at 1.0 mm and was 100 μm h− 1 at the maximum. It decreased to 20 μm h− 1 at 3.0 mm. The film prepared with the 1.0 mm separation was inhomogeneous in thickness and the uniformity was improved for the film with the 3.0 mm separation. However, all the films showed cross sections in the form of a hillock. 3.3. Free-standing diamond films The diamond films prepared under both regimes were removed from the substrates by chemically dissolving the substrates in diluted solutions of nitric and fluoric acid for tungsten, and fluoric acid for silicon. Fig. 5 shows the photographs of the diamond films. The film under the dcplasma regime shows translucency (Fig. 7(a)). On the other hand, the film under the plasma-jet regime is opaque (Fig. 7(b)). Although the size of the nozzle exit was 2 mm, the film extends over an elliptical area with the major axis of 8 mm. Fig. 8 shows the XRD patterns of the substrate side (back side) of the films. No other peaks than diamond are shown in Fig. 8. The film obtained under the plasma-jet regime slightly shows the preferred orientation along b110N. Fig. 9 shows the Raman spectra of those films on the substrate side and the growth side (front side) exposed to plasma. Glassy carbon was observed on the substrate side of the film under the dc-plasma regime, as shown in Fig. 9(a). The glassy carbon layer was recognized in SEM as a thin layer of ∼ 100 nm thickness, which indicated the formation of an intermediate layer. The film came off at the layer from the substrate. The insufficient mechanical strength of the glassy carbon layer was also a reason for the cracks of the films beyond 12 μm of thickness. Because the chemical inertness of the glassy carbon to acid is similar to diamond, the layer is difficult
Fig. 9. Raman spectra of the free-standing films on the substrate sides (back sides) and the growth side (front side). (a) dc-plasma regime and (b) plasma-jet regime. Peaks indicated by the mark ▾ are 3C–SiC.
to be chemically removed. The opaque part at the center of the film in Fig. 7(a) is due to the partly remained glassy carbon layer. The growth rate of 5 μm h− 1 was comparable to that which the transparency was obtained in the arc-jet method [8,9]. The full width at half maximum (FWHM) of the diamond Raman peak was 4.0–5.2 cm− 1. According to Yang et al. [17], there is a correlation between the quality of diamond films and the thermal conductivity. Their translucent film with the FWHM of 5.2 cm− 1 had the thermal conductivity of 18.3 ± 0.2 W cm− 1 K− 1. If the effect of grain scattering is not taken into account, the film prepared under the dc-plasma regime is expected to have the thermal conductivity of a comparable degree. On the other hand, the intermediate SiC layer was formed on the substrate side under the plasma-jet regime (Fig. 9(b)). The layer was thin enough that peaks from 3C–SiC were not observed in XRD (Fig. 8). 4. Summary
Fig. 8. XRD patterns of the films obtained under the dc-plasma and the plasmajet regimes. The films were mounted with the substrate side (back side) irradiated with X-rays.
Free-standing diamond films were prepared using the plasma CVD method above the liquid surface under the dc-plasma regime and the plasma-jet regime. Free-standing films removed from the substrates showed translucency under the dc-plasma regime. The growth rate of 5 μm h− 1 was comparable to that which the transparency was obtained with the arc-jet method
Y. Matsushima et al. / Diamond & Related Materials 16 (2007) 570–575
[7,8]. Because the maximum thickness was 12 μm without a discernible crack, it was difficult to mechanically remove the residual glassy carbon layer. Under the plasma-jet regime, the opaque film was deposited on the larger area than that of the nozzle exit. The growth rate as high as 100 μm h− 1 was attained at the maximum. There remained a problem in the uniformity of the thickness. The intermediate 3C–SiC layer was formed on the silicon substrate. There was no crack in the films beyond tens of micrometers. Acknowledgements The authors thank Mr. K. Ishida and Ms. S. Kawashima for the experimental support. This research was financially supported in part by the Fellowship to Researchers and Support to Academic Circles from The Association for the Progress of New Chemistry and by Saneyoshi Scholarship Foundation. References [1] K. Takahashi, S. Illy, R. Heidinger, A. Kasugai, R. Minami, K. Sakamoto, M. Thumm, T. Imai, Fusion Eng. Des. 74 (2005) 305. [2] Y. Tzuk, A. Tal, S. Goldring, Y. Glick, E. Lebiush, G. Kaufman, R. Lavi, IEEE J. Quantum Electron. 40 (2004) 262. [3] Y. Kawano, S. Chiba, A. Inoue, Rev. Sci. Instrum. 75 (2004) 279. [4] X.-T. Ying, J.-L. Luo, P.-N. Wang, M.-Q. Cui, Y.-D. Zhao, G. Li, P.-P. Zhu, Diamond Relat. Mater. 12 (2003) 719.
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