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Microwave plasma generated in a narrow gap to achieve high power efficiency during diamond growth
Diamond & Related Materials 18 (2009) 117–120
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Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Microwave plasma generated in a narrow gap to achieve high power efficiency during diamond growth Hideaki Yamada ⁎, Akiyoshi Chayahara, Yoshiaki Mokuno, Shin-ichi Shikata Diamond Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Midorigaoka 1-8-31, ikeda, osaka 563-8577, Japan
a r t i c l e
i n f o
Available online 3 November 2008 Keywords: Microwave plasma CVD Diamond Simulation
a b s t r a c t Single crystal diamonds were synthesized using a conventional configuration and a proposed configuration of reactors for microwave plasma chemical vapor deposition. The distributions of growth rate and substrate temperature were compared. Using relatively high gas pressure and moderate microwave input power the proposed configuration gave improved distributions. Secondary ion mass spectroscopy analysis did not indicate inclusion of metals which compose the boundaries of the discharge region of the proposed configuration. The power efficiency was estimated by the growth rate and the input power. Comparison with other systems implies that the proposed configuration gives similar or improved power efficiency. © 2008 Elsevier B.V. All rights reserved.
1. Introduction To realize industrial use of single crystalline diamond (SCD) as a semiconductor material for high performance electronic devices, diamond wafers with a diameter of several inches need to be efficiently produced. Recently, a relatively high growth rate was reported for substrates with a of 10 mm2 area by using microwave plasma chemical vapor deposition (MWPCVD) at 2.45 GHz [1]. In this case, under relatively high gas pressure, plasma was concentrated around the tip of the substrate holder with a diameter of ≈10 mm. The plasma ball generated was relatively small and is thus not suitable for substrates with a larger area. One way to increase the deposition area is to decrease the frequency of the MW. Reactors that use the 915 MHz frequency have been proposed [2–4]. A polycrystalline diamond (PCD) with 200 mm diameter has been reported [4]. A common characteristic of the MW plasma used for diamond growth is that the plasma boundary is (hemi-) spherical, i.e. the ratio of horizontal-to-vertical extent of the plasma-boundary (aspect ratio) is 1–2. It would be difficult to change the discharge character for reactors, especially those in which plasma ignition occurs at the tip of the electrode [1,5,6]. Power loss in regions away from the substrate surface is expected to lower the efficiency of diamond growth. Planner plasma is expected to be desirable for efficient use of input power, i.e. improved “power efficiency”. The utilization of millimeter waves to generate and maintain plasma with a 10 kW gyrotron at 30 GHz has been reported and a planner plasma boundary was achieved [7,8]. This method was applied to diamond growth over areas with diameters of 60–90 mm. The gyrotron, however, is a large apparatus. The discharge region of the surface wave ⁎ Corresponding author. Tel.: +81 72 751 9531; fax: +81 72 751 9631. E-mail address: [email protected] (H. Yamada). 0925-9635/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2008.10.038
plasma is normally thin [9–11] but this method cannot avoid damaging the dielectric window, which results in contamination of the substrate by the window materials. This contamination limits the potential experimental growth time. Reactors with dielectric tube side walls have a similar problem [12–15]. To optimize the MWPCVD reactor, several researchers have analyzed the electromagnetic field in vacuum, i.e. without plasma [14,16]. A phenomenological model was proposed in which the plasma density is proportional to the field strength [3]. To some extent, these techniques are suitable for smaller and/or sparse plasmas [3,17]. These techniques are not applicable to high density plasmas with large volume. A rigorous model [18] would require immense computational resources and time to calculate various types of configurations. We have, therefore, developed a simple MWP model for the analyses of dense plasma over a large area [19]. This model takes into account the effects of plasma, while the number of equations to be solved numerically is reduced. Using this model, we have proposed a configuration where the plasma should be generated in a narrow gap that is much smaller than the wavelength. Predictions from the numerical simulations were confirmed experimentally and the fundamental characteristics of the discharge were studied [20,21]. This article compares the results obtained for the proposed and the conventional configuration. Secondary ion mass spectroscopy (SIMS) analysis was conducted to verify if any of the narrow gap materials were included in the CVD layer. From these results, the power efficiency was roughly estimated and compared with the power efficiency of other types of reactors. 2. Experiments As stated in the previous section, we have proposed a tentative configuration of a reactor to generate microwave plasma within a
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Fig. 1. Schematic cross-sectional views of (a) the conventional and (d) the proposed configurations for which the simulation and experiments were conducted. The contours represent numerically calculated distributions of electron number density where their magnitudes are large (small) in region with the red (purple) contours. Photographs given in this figure were taken during discharge for the corresponding configurations and conditions. Figures (b) and (c) [(e)–(j)] are the results for the conventional [proposed] configuration, where the plasma region is magnified.
narrow gap. A customized conventional reactor, AX6550 [Fig. 1(a)] was used for this configuration [Fig. 1(d)] by inserting a Cu cylinder and a substrate holder. In the proposed configuration [Fig. 1(d)] plasma is generated in a ≃10 mm narrow gap between the bottom surface of the Cu cylinder and the top surface of the substrate holder as shown in Fig. 1(d). The temperature of this Cu cylinder, in which coolant flows during the experiment, was measured by a thermocouple that was inserted into the cylinder. The temperature measurements were taken at 2 mm and 30 mm from the side and the bottom surface of the cylinder, respectively. The temperature was less than 60 °C during growth. We conducted a simulation in which the bottom surface of the cylinder was placed on a heated wall, the temperature of which was varied up to 4000 °C. In this simulation, the bottom surface of the cylinder was less than 100 °C. In this article, a substrate holder with 38–50 mm (1.5–2 in.) diameters was used for the proposed configuration [Fig. 1 (d)]. We compared the results obtained for this proposed configuration with those obtained for the conventional configuration without the Cu cylinder [Fig. 1 (d)]. For the conventional configuration, a standard holder of 64 mm (2.5 in.) in diameter was used. For both the investigated configurations, we estimated the growth rate distributions and compared them by those obtained for small SCD samples that were discretely arranged in the radial direction. (100) oriented SCDs with 3–4.5 mm edge lengths were used as samples. Before arranging the samples on the substrate holder, they are cleaned in an ultrasonic bath with isopropyl alcohol for 10 min. The temperature of the substrates was measured with an optical pyrometer. The temperature varied from 1000–1200 °C during the growth, depending on the configuration and the position of the samples. The temperature can be controlled by adjusting the height of the substrate, if necessary [22]. All sample temperatures were kept between 1100 ± 100 °C by adjusting the height to within 0.1–0.5 mm. It was difficult to raise the substrate temperature for the conventional reactor (substrate at r = 1.5 in.) but inserting Mo foil between the substrate and the substrate holder to thermally insulate them worked well.
3. Results The contours given in Fig. 1 numerically represent the calculated distributions of electron number density where their magnitudes are large (small) in regions with the red (purple) contours. The photographs in Fig. 1 were taken during discharge for the corresponding configurations and conditions. Fig. 1(b) and (c) [1(e)–(j)] show results for the conventional [proposed] configuration, and the plasma region has been magnified. Numerical predictions of the electron number densities reproduce those of the luminescent regions from the experiments very well. As mentioned previously, the plasma is fairly spherical for the conventional configuration [Fig. 1 (b) and (c)]. The plasma that was confined by the narrow gap for the proposed configuration [Figs. 1 (e) and 2 (f)] seems to be more ellipsoidal. Fig. 2 (g)–(j) show a magnified view of the edge of the bottom surface of the cylinder. When the input power is moderate, the plasma is confined to
Fig. 2. Radial profiles of the growth rates (experiments) and the electron number density (simulations). The symbol shapes correspond to the configuration; squares (circles) for the conventional (proposed) configuration. Gas pressure and input power used to obtain the open and the filled symbols are 16 kPa/3.2 kW and 20 kPa/4 kW, respectively.
H. Yamada et al. / Diamond & Related Materials 18 (2009) 117–120
the central region, and thus is difficult to observe from the edge [Fig. 1(g) and (h)]. The discharge, however, tends to start at the edge when excessive input power is applied [Fig. 1(i) and (j)]. Both numerical prediction and experimental observation confirm these characteristics of the discharge for the proposed configuration. The model and method used for the numerical simulation is thus useful for predicting the position and extent of the plasma. Filled and open circles (squares) in Fig. 2 represent horizontal profiles of the growth rates, obtained from the proposed (conventional) configurations, where the left hand side vertical and the horizontal axes represent the growth rate and the radial coordinates, respectively. The samples were placed at the positions indicated by the symbols. Results represented by filled symbols were obtained under 4 kW of input MW power and 20 kPa (150 Torr) pressure while open symbols correspond to those obtained for 3.2 kW power and 16 kPa (120 Torr) pressure. The source gas was H2: CH4: N2: He = 300 : 40 : 0.6 : 100 sccm in all cases. For both configurations the input power, pressure and gas composition was fixed. At elevated pressure [20 kPa (150 Torr)/4 kW], the conventional configuration gives a very high growth rate of 86 μm/h. at the central region [see filled square]. The growth rate at the edge, r = 19 mm (=1.5/2 in.) is 43 μm/h which is only 50% of the central region’s magnitude. Substrate temperatures at the center (r = 0) and edge (r = 19 mm = 1.5/2 in.) were 1165 °C and 1110 °C, respectively. For the conventional configuration, we had to insert Mo foil, as mentioned previously, to elevate the temperature of the sample when placed at the edge. For the proposed configuration, the difference in the growth rate at the edge (1.5 in. diameter) was 26% of the maximum magnitude at the center as indicated by the filled circles. Substrate temperatures of the center and edge (r = 19 mm = 1.5/2 in.) for this case were 1190 °C and 1000 °C, respectively. The samples were set directly upon the substrate holder. The profile indicated by the solid line with small gray dots represents the electron number density distribution calculated for the MW power at 4 kW and a pressure of 20 kPa (150 Torr) for the proposed configuration. The right side vertical axis is its magnitude. The number density profile has a convex distribution in the horizontal direction. At a lower pressure of 16 kPa (120 Torr) with an input power of 3.2 kW the growth rate distributions are more uniform than for the elevated pressure experiments. The growth rates for both configurations are roughly 30–40 μm/h over the 1.5 in. diameter range. The substrate height was controlled by embedding the substrate into a holder where the maximum difference in height was ≈250 μm for both cases. SIMS analysis was used to determine the amounts of impurities in the synthesized crystals. The samples grown under 1) 20 kPa (150 Torr), 3.5 kW, H2 : CH4 : N2 = 950 : 60 : 2 sccm ([N2]/[CH4] = 3%) in the proposed configuration and 2) 15 kPa (110 Torr), 4.3–5.4 kW, H2 : CH4 : N2 = 890 : 107 : 3 sccm ([N2]/[CH4] = 3%) in the conventional configuration were analyzed. For reference a Ib-type HTPH substrate without the CVD layer was also studied. The depth profiles of nitrogen measured for these samples are shown in Fig. 3a. In the HTHP sample, approximately 100 ppm of nitrogen was measured, as indicated by the black line. Both samples synthesized using the proposed and the conventional configurations included approximately 10 ppm nitrogen, as indicated by the red and blue lines. This result is similar to the
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Fig. 3. Depth profiles of (a) nitrogen and (b) copper/molybdenum/iron analysis for the CVD layer grown under the both the proposed and the conventional configurations. For reference, the result for HPHT nitrogen concentration is also shown.
nitrogen level reported elsewhere [22]. Fig. 3b shows the measured depth profiles of the Mo, Fe and Cu obtained from the proposed configuration experiment. None of these elements were detected in the CVD layer (detection limits are ≈1015 atom/cm3 = 10 ppb). The results obtained for the conventional configuration sample and the HTHP sample show the same profiles as the profile for the proposed configuration shown in Fig. 3b. The growth rate may be estimated by dividing the volume by the input power, i.e. γ × S / Pin where γ, S and Pin are the growth rate, growth area and input power, respectively. This index may be used to estimate the power efficiency of the growth. This power efficiency has been calculated for the proposed configurations reported here and elsewhere [2,3,7,23]. The values used for the calculation are summarized in Table 1. Reactors that use the 915 MHz frequency [2,3] have power efficiencies of around 5 mm3/h kW. The growth rate γ [μm/h] may be enhanced by concentrating the plasma around the tip of the substrate holder [23]. However, as mentioned in Section 1, the deposition area is relatively small in these experiments and the power efficiency is much smaller. By utilizing a millimeter scale wave, planner plasma can be generated and the power efficiency can be increased [7]. Plasma generated under the proposed configuration gives a similar or improved power efficiency compared with the conventional configuration. 4. Discussions
Table 1 Power efficiencies [Section 3] estimated for various types of reactors Type
Pin [kW]
S [mm2]
γ [μm/h hr.]
Power efficiency [mm3/h kW]
References
Dome-type chamber Ellipsoidal chamber Customized holder Millimeter wave Present work
8.0 60.0 1.7 20.0 4.0
3.07 × 103 1.82 × 104 7.07 1.77 × 104 1.14 × 103
10 15 100 15 66
3.8 4.6 0.4 13.3 18.8
[2] [3] [24] [7]
Comparing the two configurations reveals that the proposed configuration improves the growth rate uniformity and the substrate temperature uniformity at elevated pressure. At pressures less than 16 kPa (120 Torr), the uniformities observed in the proposed and the conventional configurations are similar, while the growth rates are less than those observed at the higher pressure range that gave a high growth rate. Common problems for “high speed growth” at the higher pressure range include controlling the substrate temperature uniformity, as well as the growth rate uniformity. For the conventional
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configuration, the substrate temperature at the periphery rapidly decreases when the pressure is increased. This horizontal temperature variation causes thermal distortion, which is critical for crystals of large area. The temperature distribution is expected to improve to a certain extent if a SCD wafer with 1.5 in. diameter is used. The results from this work suggest that the growth rate obtained for the proposed configuration is still more uniform than that obtained for the conventional configuration. These results imply that the proposed configuration is effective for uniform growth over a large area under high pressure, i.e. with high growth rates. In the proposed configuration, the plasma seems to be more sensitive to the distance between the substrate (holder) and the upper boundary of the gap in which the plasma is confined. The shape of the substrate holder and the shape of the upper boundary of the gap can be used to control plasma shape and improve its uniformity. These factors influence the distribution of the growth rate and the substrate temperature. SIMS analysis did not show metallic impurities from the materials that form the narrow gap, i.e. Cu and Mo. In all HPHT and CVD samples synthesized by the conventional and proposed configurations these metal concentrations are less than several tens of ppb. These levels are lower than those of Ni observed in type-Ib samples reported elsewhere [24] which are present at levels of several hundred ppb. While we could not identify impurity differences among the samples, further precise measurements, such as neutron activation analysis, would be effective for verification. The proposed configuration gives similar or improved power efficiency for the growth. It may be possible to achieve a high growth rate over a large area without increasing the input power. The “efficiency” index is, however, not only determined by the power efficiency, as proposed in this article, but also depends on the continuous operation time, stability, compactness, etc. The proposed configuration does not require a large waveguide or gyrotron (MW at 2.45 GHz) to generate or maintain the plasma. The proposed configuration does not necessarily require the AX6550 as its basic configuration. Further comprehensive analysis and improvement of the estimation efficiency are required to optimize configurations. 5. Summary SCDs were synthesized under conventional and proposed configurations for MWPCVD-reactors and their growth rate distributions as well as substrate temperature distributions were compared. The proposed configuration was found to be effective for uniform growth over a large area under high pressure, i.e. with high growth rates. Despite the plasma touching the narrow gap materials, SIMS analysis did not show significant impurity inclusion within the CVD. The power efficiency was estimated by introducing a simple index and suggested
that the proposed configuration gives similar or improved efficiency compared with the conventional configuration. Further optimization of the lower and upper boundaries of the proposed configuration may provide further improvement of uniformities and power efficiency. A numerical simulation (and developed model) was used to predict possible configurations and several qualitative characteristics of the experimental results were successfully predicted. This was an example that showed the utility of the model and also the method used to design a reactor configuration. Further comparisons between numerical predictions and experimental observations will be reported in the near future. References [1] [2] [3] [4] [5] [6]
[7]
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
Y. Mokuno, A. Chayahara, H. Yamada, Diamond Relat. Mater. 17 (2008) 415. E. Sevillano, B. Williams, Diam. Films Technol. 8 (1998) 73. M. Füner, C. Wild, P. Koidl, Surf. Coat. Technol. 116–119 (1999) 853. D. King, M.K. Yaran, T. Schuelke, T.A. Grotjohn, D.K. Reinhard, J. Asmussen, Diamond Relat. Mater. 17 (2008) 520. N. Taniyama, M. Kudo, O. Matsumoto, H. Kawarada, Jpn. J. Appl. Phys. 40 (2001) L698. O. Ariyada, S. Sato, H. Suzuki, “Chemical vapor deposition apparatus for semiconductor device manufacture, has substrate holder mounted on front end of coaxial antenna, which positions at center of discharge chamber”, Jpn. Pat. No. JP2006083405-A. A.L. Vikharev, A.M. Gorbachev, A.V. Kozlov, V.A. Koldanov, A.G. Litvak, N.M. Ovechkin, D.B. Radishev, Yu.V. Bykov, M. Caplan, Diamond Relat. Mater. 15 (2006) 502. A.L. Vikharev, A.M. Gorbachev, A.V. Kozlov, D.B. Radishev, A.B. Muchnikov, Diamond Relat. Mater. 17 (2008) 1055. Y. Hayashi, H. Nakamura, M. Nagahiro, D. Nishino, Jpn. J. Appl. Phys. 38 (1999) 4508. T.J. Wu, C.S. Kou, Rev. Sci. Inst. 70 (1999) 2331. J. Kim, M. Katsurai, J. Appl. Phys. 101 (2007) 023301. M. Kamo, Y. Sato, S. Matsumoto, N. Setaka, J. Cryst. Growth 62 (1983) 642. M. Walter, D. Korzec, M. Hütten, J. Engemann, Jpn. J. Appl. Phys. 36 (1997) 4777. A. Pfuch, R. Chihar, Surf. Coat. Technol. 183 (2004) 134. C.F.M. Borges, M. Moisan, A. Gicquel, Diamond Relat. Matter 4 (1995) 149. M. Graf, E. Rauchle, H. Urban, M. Kaiser, L. Alberts, R. Emmerich, P. Elsner, Surf. Coat. Technol. 200 (2005) 904. H. Yamada, A. Chayahara, Y. Mokuno, Y. Horino, S. Shikata, Diamond Relat. Mater. 15 (2006) 1383. G. Lombardi, K. Hassouni, G.-D. Stancu, L. Mechold, J. Röpcke, A. Gicquel, J. Appl. Phys. 98 (2005) 053303. H. Yamada, A. Chayahara, Y. Mokuno, J. Appl. Phys. 101 (2007) 063302. H. Yamada, A. Chayaharaa, Y. Mokunoa, S. Shikata, Diamond Relat. Mater. 17 (2008) 494. H. Yamada, A. Chayahara, Y. Mokunoa, S. Shikata, Diamond Relat. Mater. 17 (2008) 1062. Y. Mokuno, A. Chayahara, Y. Soda, H. Yamada, Y. Horino, N. Fujimori, Diamond Relat. Mater. 15 (2006) 455. A. Chayahara, Y. Mokuno, Y. Horino, Y. Takasu, H. Kato, H. Yoshikawa, N. Fujimori, Diamond Relat. Mater. 13 (2004) 1954. J. Kaneko, C. Yonezawa, Y. Kasugai, H. Sumiya, T. Nishitani, Diamond Relat. Matter. 9 (2000) 2019.
Contents lists available at ScienceDirect
Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d
Microwave plasma generated in a narrow gap to achieve high power efficiency during diamond growth Hideaki Yamada ⁎, Akiyoshi Chayahara, Yoshiaki Mokuno, Shin-ichi Shikata Diamond Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Midorigaoka 1-8-31, ikeda, osaka 563-8577, Japan
a r t i c l e
i n f o
Available online 3 November 2008 Keywords: Microwave plasma CVD Diamond Simulation
a b s t r a c t Single crystal diamonds were synthesized using a conventional configuration and a proposed configuration of reactors for microwave plasma chemical vapor deposition. The distributions of growth rate and substrate temperature were compared. Using relatively high gas pressure and moderate microwave input power the proposed configuration gave improved distributions. Secondary ion mass spectroscopy analysis did not indicate inclusion of metals which compose the boundaries of the discharge region of the proposed configuration. The power efficiency was estimated by the growth rate and the input power. Comparison with other systems implies that the proposed configuration gives similar or improved power efficiency. © 2008 Elsevier B.V. All rights reserved.
1. Introduction To realize industrial use of single crystalline diamond (SCD) as a semiconductor material for high performance electronic devices, diamond wafers with a diameter of several inches need to be efficiently produced. Recently, a relatively high growth rate was reported for substrates with a of 10 mm2 area by using microwave plasma chemical vapor deposition (MWPCVD) at 2.45 GHz [1]. In this case, under relatively high gas pressure, plasma was concentrated around the tip of the substrate holder with a diameter of ≈10 mm. The plasma ball generated was relatively small and is thus not suitable for substrates with a larger area. One way to increase the deposition area is to decrease the frequency of the MW. Reactors that use the 915 MHz frequency have been proposed [2–4]. A polycrystalline diamond (PCD) with 200 mm diameter has been reported [4]. A common characteristic of the MW plasma used for diamond growth is that the plasma boundary is (hemi-) spherical, i.e. the ratio of horizontal-to-vertical extent of the plasma-boundary (aspect ratio) is 1–2. It would be difficult to change the discharge character for reactors, especially those in which plasma ignition occurs at the tip of the electrode [1,5,6]. Power loss in regions away from the substrate surface is expected to lower the efficiency of diamond growth. Planner plasma is expected to be desirable for efficient use of input power, i.e. improved “power efficiency”. The utilization of millimeter waves to generate and maintain plasma with a 10 kW gyrotron at 30 GHz has been reported and a planner plasma boundary was achieved [7,8]. This method was applied to diamond growth over areas with diameters of 60–90 mm. The gyrotron, however, is a large apparatus. The discharge region of the surface wave ⁎ Corresponding author. Tel.: +81 72 751 9531; fax: +81 72 751 9631. E-mail address: [email protected] (H. Yamada). 0925-9635/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2008.10.038
plasma is normally thin [9–11] but this method cannot avoid damaging the dielectric window, which results in contamination of the substrate by the window materials. This contamination limits the potential experimental growth time. Reactors with dielectric tube side walls have a similar problem [12–15]. To optimize the MWPCVD reactor, several researchers have analyzed the electromagnetic field in vacuum, i.e. without plasma [14,16]. A phenomenological model was proposed in which the plasma density is proportional to the field strength [3]. To some extent, these techniques are suitable for smaller and/or sparse plasmas [3,17]. These techniques are not applicable to high density plasmas with large volume. A rigorous model [18] would require immense computational resources and time to calculate various types of configurations. We have, therefore, developed a simple MWP model for the analyses of dense plasma over a large area [19]. This model takes into account the effects of plasma, while the number of equations to be solved numerically is reduced. Using this model, we have proposed a configuration where the plasma should be generated in a narrow gap that is much smaller than the wavelength. Predictions from the numerical simulations were confirmed experimentally and the fundamental characteristics of the discharge were studied [20,21]. This article compares the results obtained for the proposed and the conventional configuration. Secondary ion mass spectroscopy (SIMS) analysis was conducted to verify if any of the narrow gap materials were included in the CVD layer. From these results, the power efficiency was roughly estimated and compared with the power efficiency of other types of reactors. 2. Experiments As stated in the previous section, we have proposed a tentative configuration of a reactor to generate microwave plasma within a
118
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Fig. 1. Schematic cross-sectional views of (a) the conventional and (d) the proposed configurations for which the simulation and experiments were conducted. The contours represent numerically calculated distributions of electron number density where their magnitudes are large (small) in region with the red (purple) contours. Photographs given in this figure were taken during discharge for the corresponding configurations and conditions. Figures (b) and (c) [(e)–(j)] are the results for the conventional [proposed] configuration, where the plasma region is magnified.
narrow gap. A customized conventional reactor, AX6550 [Fig. 1(a)] was used for this configuration [Fig. 1(d)] by inserting a Cu cylinder and a substrate holder. In the proposed configuration [Fig. 1(d)] plasma is generated in a ≃10 mm narrow gap between the bottom surface of the Cu cylinder and the top surface of the substrate holder as shown in Fig. 1(d). The temperature of this Cu cylinder, in which coolant flows during the experiment, was measured by a thermocouple that was inserted into the cylinder. The temperature measurements were taken at 2 mm and 30 mm from the side and the bottom surface of the cylinder, respectively. The temperature was less than 60 °C during growth. We conducted a simulation in which the bottom surface of the cylinder was placed on a heated wall, the temperature of which was varied up to 4000 °C. In this simulation, the bottom surface of the cylinder was less than 100 °C. In this article, a substrate holder with 38–50 mm (1.5–2 in.) diameters was used for the proposed configuration [Fig. 1 (d)]. We compared the results obtained for this proposed configuration with those obtained for the conventional configuration without the Cu cylinder [Fig. 1 (d)]. For the conventional configuration, a standard holder of 64 mm (2.5 in.) in diameter was used. For both the investigated configurations, we estimated the growth rate distributions and compared them by those obtained for small SCD samples that were discretely arranged in the radial direction. (100) oriented SCDs with 3–4.5 mm edge lengths were used as samples. Before arranging the samples on the substrate holder, they are cleaned in an ultrasonic bath with isopropyl alcohol for 10 min. The temperature of the substrates was measured with an optical pyrometer. The temperature varied from 1000–1200 °C during the growth, depending on the configuration and the position of the samples. The temperature can be controlled by adjusting the height of the substrate, if necessary [22]. All sample temperatures were kept between 1100 ± 100 °C by adjusting the height to within 0.1–0.5 mm. It was difficult to raise the substrate temperature for the conventional reactor (substrate at r = 1.5 in.) but inserting Mo foil between the substrate and the substrate holder to thermally insulate them worked well.
3. Results The contours given in Fig. 1 numerically represent the calculated distributions of electron number density where their magnitudes are large (small) in regions with the red (purple) contours. The photographs in Fig. 1 were taken during discharge for the corresponding configurations and conditions. Fig. 1(b) and (c) [1(e)–(j)] show results for the conventional [proposed] configuration, and the plasma region has been magnified. Numerical predictions of the electron number densities reproduce those of the luminescent regions from the experiments very well. As mentioned previously, the plasma is fairly spherical for the conventional configuration [Fig. 1 (b) and (c)]. The plasma that was confined by the narrow gap for the proposed configuration [Figs. 1 (e) and 2 (f)] seems to be more ellipsoidal. Fig. 2 (g)–(j) show a magnified view of the edge of the bottom surface of the cylinder. When the input power is moderate, the plasma is confined to
Fig. 2. Radial profiles of the growth rates (experiments) and the electron number density (simulations). The symbol shapes correspond to the configuration; squares (circles) for the conventional (proposed) configuration. Gas pressure and input power used to obtain the open and the filled symbols are 16 kPa/3.2 kW and 20 kPa/4 kW, respectively.
H. Yamada et al. / Diamond & Related Materials 18 (2009) 117–120
the central region, and thus is difficult to observe from the edge [Fig. 1(g) and (h)]. The discharge, however, tends to start at the edge when excessive input power is applied [Fig. 1(i) and (j)]. Both numerical prediction and experimental observation confirm these characteristics of the discharge for the proposed configuration. The model and method used for the numerical simulation is thus useful for predicting the position and extent of the plasma. Filled and open circles (squares) in Fig. 2 represent horizontal profiles of the growth rates, obtained from the proposed (conventional) configurations, where the left hand side vertical and the horizontal axes represent the growth rate and the radial coordinates, respectively. The samples were placed at the positions indicated by the symbols. Results represented by filled symbols were obtained under 4 kW of input MW power and 20 kPa (150 Torr) pressure while open symbols correspond to those obtained for 3.2 kW power and 16 kPa (120 Torr) pressure. The source gas was H2: CH4: N2: He = 300 : 40 : 0.6 : 100 sccm in all cases. For both configurations the input power, pressure and gas composition was fixed. At elevated pressure [20 kPa (150 Torr)/4 kW], the conventional configuration gives a very high growth rate of 86 μm/h. at the central region [see filled square]. The growth rate at the edge, r = 19 mm (=1.5/2 in.) is 43 μm/h which is only 50% of the central region’s magnitude. Substrate temperatures at the center (r = 0) and edge (r = 19 mm = 1.5/2 in.) were 1165 °C and 1110 °C, respectively. For the conventional configuration, we had to insert Mo foil, as mentioned previously, to elevate the temperature of the sample when placed at the edge. For the proposed configuration, the difference in the growth rate at the edge (1.5 in. diameter) was 26% of the maximum magnitude at the center as indicated by the filled circles. Substrate temperatures of the center and edge (r = 19 mm = 1.5/2 in.) for this case were 1190 °C and 1000 °C, respectively. The samples were set directly upon the substrate holder. The profile indicated by the solid line with small gray dots represents the electron number density distribution calculated for the MW power at 4 kW and a pressure of 20 kPa (150 Torr) for the proposed configuration. The right side vertical axis is its magnitude. The number density profile has a convex distribution in the horizontal direction. At a lower pressure of 16 kPa (120 Torr) with an input power of 3.2 kW the growth rate distributions are more uniform than for the elevated pressure experiments. The growth rates for both configurations are roughly 30–40 μm/h over the 1.5 in. diameter range. The substrate height was controlled by embedding the substrate into a holder where the maximum difference in height was ≈250 μm for both cases. SIMS analysis was used to determine the amounts of impurities in the synthesized crystals. The samples grown under 1) 20 kPa (150 Torr), 3.5 kW, H2 : CH4 : N2 = 950 : 60 : 2 sccm ([N2]/[CH4] = 3%) in the proposed configuration and 2) 15 kPa (110 Torr), 4.3–5.4 kW, H2 : CH4 : N2 = 890 : 107 : 3 sccm ([N2]/[CH4] = 3%) in the conventional configuration were analyzed. For reference a Ib-type HTPH substrate without the CVD layer was also studied. The depth profiles of nitrogen measured for these samples are shown in Fig. 3a. In the HTHP sample, approximately 100 ppm of nitrogen was measured, as indicated by the black line. Both samples synthesized using the proposed and the conventional configurations included approximately 10 ppm nitrogen, as indicated by the red and blue lines. This result is similar to the
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Fig. 3. Depth profiles of (a) nitrogen and (b) copper/molybdenum/iron analysis for the CVD layer grown under the both the proposed and the conventional configurations. For reference, the result for HPHT nitrogen concentration is also shown.
nitrogen level reported elsewhere [22]. Fig. 3b shows the measured depth profiles of the Mo, Fe and Cu obtained from the proposed configuration experiment. None of these elements were detected in the CVD layer (detection limits are ≈1015 atom/cm3 = 10 ppb). The results obtained for the conventional configuration sample and the HTHP sample show the same profiles as the profile for the proposed configuration shown in Fig. 3b. The growth rate may be estimated by dividing the volume by the input power, i.e. γ × S / Pin where γ, S and Pin are the growth rate, growth area and input power, respectively. This index may be used to estimate the power efficiency of the growth. This power efficiency has been calculated for the proposed configurations reported here and elsewhere [2,3,7,23]. The values used for the calculation are summarized in Table 1. Reactors that use the 915 MHz frequency [2,3] have power efficiencies of around 5 mm3/h kW. The growth rate γ [μm/h] may be enhanced by concentrating the plasma around the tip of the substrate holder [23]. However, as mentioned in Section 1, the deposition area is relatively small in these experiments and the power efficiency is much smaller. By utilizing a millimeter scale wave, planner plasma can be generated and the power efficiency can be increased [7]. Plasma generated under the proposed configuration gives a similar or improved power efficiency compared with the conventional configuration. 4. Discussions
Table 1 Power efficiencies [Section 3] estimated for various types of reactors Type
Pin [kW]
S [mm2]
γ [μm/h hr.]
Power efficiency [mm3/h kW]
References
Dome-type chamber Ellipsoidal chamber Customized holder Millimeter wave Present work
8.0 60.0 1.7 20.0 4.0
3.07 × 103 1.82 × 104 7.07 1.77 × 104 1.14 × 103
10 15 100 15 66
3.8 4.6 0.4 13.3 18.8
[2] [3] [24] [7]
Comparing the two configurations reveals that the proposed configuration improves the growth rate uniformity and the substrate temperature uniformity at elevated pressure. At pressures less than 16 kPa (120 Torr), the uniformities observed in the proposed and the conventional configurations are similar, while the growth rates are less than those observed at the higher pressure range that gave a high growth rate. Common problems for “high speed growth” at the higher pressure range include controlling the substrate temperature uniformity, as well as the growth rate uniformity. For the conventional
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configuration, the substrate temperature at the periphery rapidly decreases when the pressure is increased. This horizontal temperature variation causes thermal distortion, which is critical for crystals of large area. The temperature distribution is expected to improve to a certain extent if a SCD wafer with 1.5 in. diameter is used. The results from this work suggest that the growth rate obtained for the proposed configuration is still more uniform than that obtained for the conventional configuration. These results imply that the proposed configuration is effective for uniform growth over a large area under high pressure, i.e. with high growth rates. In the proposed configuration, the plasma seems to be more sensitive to the distance between the substrate (holder) and the upper boundary of the gap in which the plasma is confined. The shape of the substrate holder and the shape of the upper boundary of the gap can be used to control plasma shape and improve its uniformity. These factors influence the distribution of the growth rate and the substrate temperature. SIMS analysis did not show metallic impurities from the materials that form the narrow gap, i.e. Cu and Mo. In all HPHT and CVD samples synthesized by the conventional and proposed configurations these metal concentrations are less than several tens of ppb. These levels are lower than those of Ni observed in type-Ib samples reported elsewhere [24] which are present at levels of several hundred ppb. While we could not identify impurity differences among the samples, further precise measurements, such as neutron activation analysis, would be effective for verification. The proposed configuration gives similar or improved power efficiency for the growth. It may be possible to achieve a high growth rate over a large area without increasing the input power. The “efficiency” index is, however, not only determined by the power efficiency, as proposed in this article, but also depends on the continuous operation time, stability, compactness, etc. The proposed configuration does not require a large waveguide or gyrotron (MW at 2.45 GHz) to generate or maintain the plasma. The proposed configuration does not necessarily require the AX6550 as its basic configuration. Further comprehensive analysis and improvement of the estimation efficiency are required to optimize configurations. 5. Summary SCDs were synthesized under conventional and proposed configurations for MWPCVD-reactors and their growth rate distributions as well as substrate temperature distributions were compared. The proposed configuration was found to be effective for uniform growth over a large area under high pressure, i.e. with high growth rates. Despite the plasma touching the narrow gap materials, SIMS analysis did not show significant impurity inclusion within the CVD. The power efficiency was estimated by introducing a simple index and suggested
that the proposed configuration gives similar or improved efficiency compared with the conventional configuration. Further optimization of the lower and upper boundaries of the proposed configuration may provide further improvement of uniformities and power efficiency. A numerical simulation (and developed model) was used to predict possible configurations and several qualitative characteristics of the experimental results were successfully predicted. This was an example that showed the utility of the model and also the method used to design a reactor configuration. Further comparisons between numerical predictions and experimental observations will be reported in the near future. References [1] [2] [3] [4] [5] [6]
[7]
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
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