Development of High-Quality Large-Size Synthetic Diamond Crystals

NEW MATERIALS

Development of High-quality Large-size Synthetic Diamond Crystals

Hitoshi SUMIYA, Naohiro TODA and Shuichi SATOH

High-quality, large-size type Ⅱa diamond crystals up to 8 carats (up to 10 mm in diameter) have been successfully synthesized by using the temperature gradient method at high pressure and high temperature. The concentration of chemical impurities in synthetic diamond crystal, such as N, B and Ni, was reduced to the levels lower than 0.1 ppm by using a high-purity carbon source, employing a Fe-Co solvent, and adding Ti to the solvent as a nitrogen getter. Incorporation of Cu into the solvent permitted a substantial decrease of inclusions in the diamond crystals. In addition, crystalline quality was further improved by using a high quality seed crystal. Furthermore, the optimization of metal compositions and additive amounts in the solvent and the high precision control of growth conditions enabled the growth of high-quality, large-size type Ⅱa diamond crystals of 7-8 carats (10 mm in diameter) at a high growth rate of 6-7 mg/hr. The synthetic diamonds show no absorptions due to impurities and have very high crystalline quality. These salient characteristics of the large- size high-quality synthetic type IIa diamond permit the application to a considerably wide range of industrial and scientific uses such as monochromators for synchrotron X-ray beams, high-pressure anvils, optical parts, semiconductor substrates, radiation detectors, and so on.

1. Introduction high pressure and high temperature (1), (2). In 1985, Sumitomo Electric developed a mass-production process Diamond is a typical covalent crystal in which car- of diamond crystals based on the temperature-gradient bon atoms are bonded together very strongly. This struc- method. This enabled the commercialization of typeⅠb tural feature leads to the highest hardness among all diamonds (containing dispersed nitrogen impurities of materials and superior thermal conductivity, as well as several tens or hundreds of ppm) of 1-2 carats (or 5-6 excellent chemical stability and transparency. Owing to mm) in diameter. Sumitomo Electric applied the syn- these outstanding properties, diamond has been theticⅠb diamonds (SUMICRYSTAL) in heat sinks, wire employed in a wide range of uses. Large diamond crys- drawing dies, cutting tools for high-precision machin- tals of several millimeters in size are used for high preci- ing, and other products (3). After ten years, Sumitomo sion cutting tools, wire-drawing dies, heat sinks, optical Electric succeeded in consistently synthesizing high- windows, surgical blades, and so on. In many fields of purity type Ⅱa diamonds which had formerly been scientific research such as space, high pressure and radi- impossibly difficult (4), (5). The synthetic typeⅡa diamonds ation, large diamond crystals are the essential compo- (SUMICRYSTAL TYPE Ⅱ) were applied to the optical nents of research equipment, and will become more parts for FT-IR spectroscopy. The synthetic typeⅡa dia- essential to a wider variety of commercial and scientific monds are transparent over a wide range of wavelengths purposes in future. from ultraviolet to far infrared regions, showing no It is very difficult, however, to find large and high- absorptions due to impurities. They also have high crys- quality crystals among natural diamonds. Most natural talline quality with fewer crystal defects, less internal diamonds contain many nitrogen impurities (typeⅠa) strain, and less variations in defects among crystals than and almost all natural diamonds have a number of those of natural diamonds or conventional synthetic defects and much strain in the crystals because of their typeⅠb diamonds (6). The outstanding characteristics of complicated and varying growth processes that occurred synthetic type Ⅱa diamond permit application to new in the earth's interior. This is true even in high-purity nat- industrial and scientific uses such as high-pressure ural diamonds with impurity less than 1 ppm (typeⅡa). anvils, monochromators, semiconductor substrates and On the other hand, synthetic diamonds, which are radiation detectors (7). grown under the controlled conditions of high pressure Recently, the authors made it possible to grow and high temperature in a thermodynamically stable much large high-quality type Ⅱa diamonds of 7-8 ct region, have invariable quality. In addition to this, other (about 10 mm across) with prolonged high precision aspects such as solvent and growth rate can be con- temperature control and adequate selection of solvent trolled to a high precision, possibly leading to the con- metal and additives (8). In this paper, the authors pro- sistent synthesis of high-quality diamonds. vide a summary of the research concerning high-quality Among the various methods of diamond synthesis, large synthetic diamonds. the most effective one to grow large diamond single crystals is the temperature gradient method under static

10 · Development of High-quality Large-size Synthetic Diamond Crystals 2. Temperature gradient method under high rities less than 0.1 ppm even at a high growth rate of 3-4 pressure mg/hr (4), (5). This success enabled the commercial production of typeⅡa diamond crystals. The following is an As it is well known, there are various methods for outline of the diamond growth technique. diamond synthesis such as temperature gradient (differ- Figure 1 shows the growth method of the developed ence) method or solubility difference method under sta- high-quality Ⅱa diamond crystal. It is well known that N, tic high pressure, dynamic high pressure (shock wave) B and Ni are easily incorporated into the synthetic dia- method, and chemical vapor deposition method. mond as chemical impurities. This method was Among these methods, using the temperature gradient designed to eliminate the impurities. A high-purity car- method under static high pressure is thought to be most bon material containing less than 1 ppm of boron is effective to grow large, good quality diamond crystals of used as the carbon source. High-purity Fe-Co alloy is several millimeters. used as the solvent metal, and Ti is added into the sol- In this method, a carbon source is placed at the hot- vent as the nitrogen getter. Here, when Ti is added, a ter part above the solvent (made from ferrous metals, large amount of TiC is formed in the solvent. The for- such as Fe, Ni and Co) and the seed crystal is positioned mation of TiC inhibits the transport and diffusion of at the cooler part under the solvent in the high-pressure carbon, thus reducing the amount of carbon supplied to reaction cell. The driving force for crystallization arises the crystal growth surface and delaying the lateral from the difference in the solubility of diamond in the growth on crystal surface. This leads to the trapping of solvent that is caused by the temperature gradient in the metal inclusions. TiC particles themselves are also reaction cell. In 1971, General Electric Research Center trapped into the grown crystal as fine inclusions. It is reported that they succeeded in growing diamond crys- known that TiC decomposes in the molten group Ⅰb tals up to 1 carat using this method (1), (2). The process, metals (Cu, Ag, Au, etc.) (11). Based on this fact, besides however, was considered too costly to be translated into adding Ti as nitrogen getter, addition of Cu to the sol- commercial production at that time, because the growth vent was also considered to suppress the formation of rate must be restricted to very slow in order to grow carbide (TiC). good quality diamond. When attempts were made to Table 1 shows the experimental results of diamond grow crystals at a growth rate faster than a certain limit growth with varying amounts of Ti and Cu. Nitrogen (hereinafter referred to as the “limit growth rate”), impurities are found to be reduced to less than 0.1 ppm many solvent metals would be included in the grown crystals or polycrystallization or skeletonization would occur. At that time, it was reported that the limited High-purity Nitrogen getter growth rate of a type Ⅰb diamond containing nitrogen carbon source Solvent metal (Fe-Co +Ti, Cu) impurities of 10-100 ppm is about 2.5 mg/hr, and that Carbide of a typeⅡa diamond is less than 1.5 mg/hr. Carbide die piston TiC formation In 1985, Sumitomo Electric enabled the commer- inhibitor cialization of type Ⅰb diamond crystals of about 1 ct, by Graphite heater increasing the limit growth rate to up to 4 mg/hr (High temperature region) Temperature through the optimization of solvent metal and growth difference (20-50˚C) conditions as well as the development of a mass-produc- Pressure (Low temperature region) tion process using the temperature gradient method. medium Grown crystal Furthermore, in 1990, larger-size type Ⅰb diamond crys- Seed crystal Insulator tals weighing 9 ct (12 mm across) were grown at a Graphite heater High-quality diamond growth rate as high as 15 mg/hr using the large seed (100), up to 0.5 mm method (9), (10). Thus, large yellow type Ⅰb diamond crystals could be grown at a high growth rate and produced Fig. 1. Growth method of high-quality typeⅡa diamond crystals commercially. It had been impossible, however, to pro- (Temperature gradient method under high pressure) duce high-purity type Ⅱa diamond crystals commercially because of the difficulty in increasing the limit growth rate. The entrapment of inclusions in crystal tends to be Table 1. Quality of typeⅡa diamonds with different amounts of Ti and facilitated when nitrogen getter is added to the solvent Cu added to the solvent metal. metal for synthesizing a type Ⅱa diamond. To avoid the Added amount(wt%) Quality of grown diamonds occurrence of inclusions, growth rates had to be kept Nitrogen Inclusions (*) very slow such as 1 mg/hr. Ti Cu (ppm) Metal Fine 0.5 0.5 2.8 – – 1.0 – 0.3 ++ + 1.0 1.0 0.3 – – 3. Elimination of impurities and increase of limit 1.5 – <0.1 ++ + growth rate 1.5 1.5 <0.1 – – 2.0 – <0.1 ++ ++ In 1994, Sumitomo Electric succeeded in growing 2.0 3.0 <0.1 + – high-quality typeⅡa diamond crystals that contain impu- *Degree of inclusions : –, non; +, a few; ++, many.

SEI TECHNICAL REVIEW · NUMBER 60 · JUNE 2005 · 11 by adding Ti of more than 1.5 wt%. The experimental diamond crystals, respectively. Figures 3 shows the case results also indicate that, by the addition of Cu, the for- where large (100) crystals are used as seeds (large seed mation of TiC particles is suppressed. This is accompa- method). High-quality type Ⅰb diamond crystals can be nied by the decreases in metal and fine inclusions, so obtained in region A. When the large seed method is good-quality type Ⅱa diamond crystals can be synthe- used, the region becomes narrower (region B) because sized. However, a large excess of Cu (3 wt%) causes the crystal must be grown in a low temperature growth metal inclusions to occur. The optimum added amount ({100} dominant) region to prevent the occurrence of of Cu appears to be 1-2 wt%. inclusions (9), (10). As for typeⅡa diamond crystals (Fig. 4), Figure 2 shows the conditions of diamond forma- the region is considerably narrower (region C). The tion for different Co contents of Fe-Co solvent. Good width of the allowable temperature for A, B and C at 5.5 quality diamond crystals are obtained in Co 40-60 wt %. GPa is about 40˚C, 20˚C and 10˚C, respectively. When When the Co content is either too low or too high, high- the temperature condition is higher than these valid quality diamond crystals cannot be obtained because of regions, many inclusions will be contained in the grown the formation of inclusions or skeleton crystals. Even crystals. At lower temperature, well-formed diamond with a suitable Fe-Co composition, the valid growth tem- crystals cannot be obtained because of the formation of perature region is very narrow. The width of synthesis skeleton crystals. This means that a high precision tem- temperature region for high-quality diamond crystal is perature control technique is required for growing less than 10˚C. high-quality typeⅡa diamond crystals. The reason is that Figures 3 and 4 show the pressure-temperature the lower limitation of growth temperature becomes 20- regions for growing high-quality type Ⅰb and type Ⅱa 30˚C higher by adding the nitrogen getter such as Ti to the solvent. Figure 5 shows the photos of (a) high-quality type Ⅱa diamond crystals obtained at region C, (b)

1450 skeleton crystals grown at low temperature region, and Pressure: 5.5 GPa (c) crystals containing metal inclusions grown at high Growth rate: 2 mg/hr temperature region. On the other hand, a well-formed ▲ 1400 ▲▲ type Ⅱa diamond crystal cannot be obtained at a {100} ▲ cubic morphology (low temperature) region, because of × ▲● ▲ × ▲ the formation of skeleton crystals at lower temperatures. 1350 × ●● ●● ● ▲ ●● This indicates that unlike the case of type Ⅰb diamond, ▲ ▲▲ ▲ ▲ the large seed method is not applicable for growing a emperature (ûC) × T ▲ 1300 type Ⅱa diamond crystal on the (100) seed. Thus, the allowable temperature region for growing a good-quality type Ⅱa diamond is considerably narrow. Therefore, a 1250 020406080100 high precision temperature control technique is

Co content in Fe-Co solvent (wt%) required for growing high-quality typeⅡa diamond. Based on these experimental results, the authors Fig. 2. Conditions of type Ⅱa diamond formation with different Co tried to grow several type Ⅱa diamond crystals of 1-2 contents of Fe-Co solvent: ({) high quality diamond, (△) dia- carats using Fe-40Co with adding 1.5 wt% Ti and 1.5 wt% mond with many inclusions, (▲) skeleton crystal, (✕) no diamond formation. Cu at 1340-1350˚C for 70 hours. The growth rates were

A B {100} {100} + {111} C (Skeleton) (Skeleton) {100} 6 {100} + {111} {111} 6 {111} 5.5GPa (Inclusion) 5.5GPa (Inclusion) Pressure (GPa)

5 Pressure (GPa) Diamond/graphite 5 equilibrium line Diamond/graphite equilibrium line Solvent/carbon eutectic line Solvent/carbon eutectic line 1300 1350 1400 1300 1350 1400 Temperature (˚C) Temperature (˚C) Fig. 3. Growth region of high-quality typeⅠb diamond crystal: (A) small seed (0.5 mm), (B) large seed (5 mm). Fig. 4. Growth region of high-quality typeⅡa diamond crystal (C).

12 · Development of High-quality Large-size Synthetic Diamond Crystals over a wide range from the ultraviolet region to the far infrared region. In addition, the diamond has high crystalline quality. Table 2 shows the evaluation results of crystalline perfection of diamonds (6). The results show that the synthetic type Ⅱa diamond has excellent crystalline quality with considerably less residual strain and fewer crystal defects than natural diamonds or conventional synthetic diamonds. It was also demonstrated that the synthetic type Ⅱa diamond has excellent mechanical (a) Good-quality diamond crystals properties because the diamond has few crystal defects and little internal strains which may be the origins of destruction or plastic deformation (12).

Table 2. Crystalline quality of various diamonds.

SyntheticⅡa SyntheticⅠb NaturalⅡa NaturalⅠa

– x 1-2 up to 98 Natural abundance (%) Nitrogen content (ppm) up to 100 up to 1000 <0.1 <1 (dispersed) (aggregated) (b) Skeleton diamond crystals Internal strain observed much much much (grown at low temperature region) none or little by polarizing microscopy (radial, stripe) (tatami) (radial, stripe) Defects observed by x-ray some many very many many topography (line, plane) (line, stripe) (tatami) (line, stripe) FWHM of 004 rocking curve 4-6 6-20 200-2500 7-60 (CuKα1, arcsec) FWHM of Raman spectra 1.6-1.8 1.8-2.6 2.0-2.5 3.2-3.8 (cm-1)

Fig. 5. Synthetic typeⅡa diamond crystals. As mentioned above, the synthetic Ⅱa diamond has higher crystalline quality than natural diamonds or con- varied by changing the temperature difference. The ventional syntheticⅠb diamond. results of the growth test showed that good-quality type However, the X-ray topography experiments reveal Ⅱa diamond crystals of 1-2 carats could be obtained even that the syntheticⅡa diamond has many line defects that at growth rates as high as 3 mg/hr as shown in Fig. 6. appear to radiate outward from the seed crystal as The technical development mentioned above has shown in Fig. 7(a). Such defects are normally present in enabled the commercial production of synthetic type Ⅱa conventional synthetic Ⅰb diamonds. The origins of the diamond crystal of about 1 carat. This synthetic type Ⅱa line defects observed in synthetic Ⅱa diamond crystals diamond contains very few impurities, being transparent are the crystal imperfections present in the seed crystals. The synthetic diamond crystals described above were grown on the diamond grits synthesized by the phase- 5.5 GPa, 1340-1350˚C, up to 70hr (Fe-40Co) difference method. It is known that diamond grits have 4 many crystal defects, and the amounts of inclusions and ▲�1wt%Al 3.5 ◆�1wt%Ti strains in synthetic diamond grits are also much larger ■�2wt%Ti ■� 3 ●�1.5wt%Ti+1.5wt%Cu than those in the synthetic diamond crystals grown by 2.5 ◆� the temperature-gradient method. From the investiga- ◆� ■� ●� 2 tion, it was found out that the FWHM of diamond grits ◆� α ■� ●� (25-85 arcseconds under MoK 1 radiation) is much 1.5 ▲� ■� wider than that of the synthetic Ⅱa diamond crystals 1 ▲�▲� ◆�◆� grown by the temperature-gradient method (2-6 arcsec- ▲� ◆� 0.5 ◆� Metal inclusion content (wt%) ●� ●�●� ●�●� ●� ●� onds). Then, the authors attempted to synthesizeⅡa dia- ◆� ◆� ●�●� ●�●� ●� 0 ●� ●� mond crystals with much fewer defects by using the dia- 012345 mond seeds of high crystalline quality. High-quality Growth rate (mg/hr) seeds (0.5 × 0.5 × 0.3 mm) were cut from the large synthetic Ⅱa crystals grown by the method mentioned Fig. 6. Metal inclusion content in synthetic Ⅱa diamond crystals plotted against growth rate. The dotted line indicates the limit growth rate above, and some new synthetic Ⅱa diamonds of about 1 for good-quality diamond. carat were grown on the seeds as shown in Fig. 7. An X-

SEI TECHNICAL REVIEW · NUMBER 60 · JUNE 2005 · 13 ray topographic image of an as-grown synthetic Ⅱa dia- the calculated one (1.0 arcseconds for the intrinsic mond crystal grown on the high-quality seed is shown in width). This indicates that the synthetic diamond crystal Fig. 7(b). It is apparent that there are very few line is very close to a perfect crystal. defects in the crystals. These results indicate that the line defects in grown crystals can be reduced and crystalline quality can be improved by using high-quality diamond as the seed crystal. Figures 8 and 9 show high-res- SR Beam. λ=0.76Å 7000 ● olution rocking curves and topographic images of the c 6000 synthetic type Ⅱa diamond crystals grown on a high- FWHM : 1.2 arcsec quality seed and on a diamond grit seed, respectively (13). 5000

It is clearly seen in Fig. 8 that the broadening of the 4000 rocking curve of the synthetic diamond is mainly due to 3000 line defects. The width of the rocking curve of the syn- 2000 ● d thetic type Ⅱa diamond crystal grown on a high-quality INTENSITY (a.u.) b● 1000 seed (FWHM = 1.2 arcseconds, Fig. 9) is close to that of a ● 0 -10 -5 0 5 10

ANGLE (arcsec)

Line defects (dislocation Seed bundles)

1mm Diamond grit seed (synthesized by phase-difference method) a bdc (a) Synthetic type Ⅱa diamond grown on diamond grit seed Fig. 9. X-ray rocking curve and topographs of synthetic type Ⅱa diamond grown on a high-quality seed for (004) reflection under synchrotron radiation.

1mm High-quality diamond seed 5. Growth of large-size (1 cm class) high-quality (cut-out from large Ⅱa crystal grown by TGM) diamond (b) Synthetic type Ⅱa diamond grown on high-quality diamond seed Further growth tests of much larger-size high-quali- Fig. 7. X-ray topographs of as-grown synthetic type Ⅱa diamond crystals. (220 reflections) ty type Ⅱa diamond crystals (up to 8 ct, or up to 10 mm across) were carried out using Fe-40Co+1.5Ti+1.5Cu solvent and high-quality seed crystal and conducting precision temperature control at 1340-1350˚C over a pro- SR Beam. λ=0.76Å 14000 longed time (up to 200 hours). The volume of the sol- c● 12000 vent were made larger and the temperature gradient FWHM : 4.4 arcsec 10000 was adjusted by changing the heater shape to grow the larger-size crystals. 8000 b Figure 10 shows the weight of the crystals grown at ● 6000 various growth rates and growth times. In the early stage 4000 ● d INTENSITY (a.u.) (growth time less than 60-70 hours), the limit growth a ● 2000 rate (maximum stable growth rate) for high quality dia- e ● mond without inclusions is 3-4 mg/hr as mentioned 0 -10 -8 -6 -4 -2 0246810 above. When the growth time is over 100 hours, high- ANGLE (arcsec) quality diamond crystals can be obtained even at a high growth rate of 6-7 mg/hr. Occasionally, a slight change or variation in growth temperature leads to the formation of inclusions. It is apparent that the limit growth Seed rate increases as the crystal grows. This tendency is also observed when growing type Ⅰb diamond crystal. While the deposition area (surface area) of a grown crystal a b c d enlarges as the growth time increases, the deposition Fig. 8. X-ray rocking curve and topographs of synthetic type Ⅱa diamond rate per unit area (mm/hr) which influences the quality grown on a diamond grit seed for (004) reflection under synchrotron of crystal is unchanged. This seems to be the reason why radiation. the limit growth rate per crystal (mg/hr) increases as

14 · Development of High-quality Large-size Synthetic Diamond Crystals 2000 6. Conclusions 1800 × ▲ × × 1600 ● 6mg/hr The authors have succeeded in synthesizing high- × ▲ ● 1400 quality, large-size typeⅡa diamond crystals up to 8 carats ● ● ● 1200 ▲ ● ▲ (up to 10 mm across) at a high growth rate of 6-7 mg/hr ▲ ● ● ● ● ● ▲ 1000 ● by the temperature gradient method under high pres- ● ● × ● ● 3mg/hr 800 ▲ ● sure and high temperature. The synthetic diamonds ▲ ● ● 600 ▲ ● ●▲ show no absorptions due to impurities and have very Crystal weight (mg) ● ● × ●▲ high crystalline quality. These salient characteristics of 400 ×● ● ● ● ● ▲● ▲●▲ 200 ▲▲ large-size high-quality synthetic type Ⅱa diamond permit ● ▲● 0 the application to a considerably wide range of industrial 050100 150 200 250 and scientific uses such as monochromators for synchro- Growth time (hr) tron X-ray beam, high-pressure anvils, optical parts, semiconductor substrates, radiation detectors, and so on. Fig. 10. Growth rate and degree of inclusions of large-size synthetic type Ⅱa diamond crystals. The dotted line shows the limit growth rate for avoiding metal inclusions. References (1) R. H. Wentorf, Jr., J. Phys. Chem., 75, 1833 (1971). the crystal grows. (2) H. M. Strong and R. M. Chrenko, J. Phys. Chem., 75, 1838 (1971). Thus, large high-quality typeⅡa diamond crystals of (3) A. Hara, Seimitsukikai, 51, 1497 (1985) [in Japanese]. 7-8 ct (about 10 mm across) can be grown at a high (4) H. Sumiya, S. Satoh, Y. Nishibayashi and Y. Goda, Sumitomo growth rate of 6-7 mg/hr through the high precision Electric Tech. Rev., 39, 69 (1995). (5) H. Sumiya, S. Satoh, Diamond and Related Materials, 5, 1359 temperature control over a prolonged time with an ade- (1996). quate selection of solvent metal and additives. The (6) H. Sumiya, N. Toda, Y. Nishibayashi, S. Satoh, J. Crystal growth, growth rate is considerably higher than those reported 178, 485 (1997). (2), (14) previously (1.5-1.8 mg/hr ). The high-quality large- (7) H. Sumiya, S. Satoh, S. Yazu, Rev. High Pressure Sci. Technol., 7, size type Ⅱa diamond crystals of 7-8 ct have the same 960 (1998). properties as those of 1-2 ct mentioned in the previous (8) H. Sumiya, N. Toda, S. Satoh, J. Crystal Growth, 237-239, 1281 section, which means the large-size type Ⅱa diamond (2002). crystals contain very few impurities and have excellent (9) H. Sumiya, S. Satoh, K. Tsuji and S. Yazu, 31st High Pressure crystalline quality. Conf. Japan, programme and abstracts, pp. 48-49 (1990) [in Figure 11 shows the large diamond plates (10 × 10 × Japanese]. (10) S. Satoh and H. Sumiya, The Review of High Pressure Science 1 mm3) prepared from the 8 ct typeⅡa diamond crystals and Technology, 2, 315 (1993) [in Japanese]. obtained in this study. The diamond plates are already (11) M. Wakatsuki, Japan. J. Appl. Phys., 5, 337 (1966). used for the monochromators of synchrotron radiation (12) H. Sumiya, N. Toda, S. Satoh, Diamond and Related Materials, 6, (15) beams . 1841 (1997). (13) H. Yamaoka, K. Otomo, D. Hirata and T. Ishikawa, SR Sci. and Technol. Inform., 5, 6 (1995). (14) R. C. Burns, J. O. Hansen, R. A. Spits, M. Sibanda, C. M. Welbourn, D. L. Welch, Diamond and Related Materials 8, 1433 (1999). (15) M. Yamamoto, T. Kumasaka, T. Ishikawa, New Diamond, 58, 16 (2000) [in Japanese].

Fig. 11. Large-size synthetic diamond plates prepared from 7-8 ct type Ⅱa diamond crystals.

SEI TECHNICAL REVIEW · NUMBER 60 · JUNE 2005 · 15 Contributors H. SUMIYA • Dr. Eng., Assistant General Manager, Advanced Materials R&D Department, Electronics & Materials R&D Laboratories N. TODA • Manager, Advanced Materials R & D Department, Electronics & Materials R&D Laboratories S. SATOH • Dr. Eng., Senior Assistant General Manager, Fine Ceramics Research Association

16 · Development of High-quality Large-size Synthetic Diamond Crystals