Vol.3 No.4 2011

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Research paper : Development of single-crystalline diamond wafers (A. Chayahara et al.)−261−Synthesiology - English edition Vol.3 No.4 (2011) Ir, platinum Pt, and SiC. Since large diameter substrates such as the Si or SiC wafers have become available, the product realization of large single-crystal substrate using heteroepitaxial growth is being attempted, yet the improvement of crystal formation is an issue.We aimed to fabricate a large diamond crystal using the homoepitaxial growth with excellent crystallization, by using the microwave plasma CVD method that has smaller surface area than the hot filament method but has relatively faster growth time that allows prolonged synthesis.3 Diamond single-crystal synthesis by microwave plasma CVD methodThe high-speed synthesis of diamond single crystal was conducted using the general-use ASTeX microwave plasma CVD device (2.45 GHz, 5 kW; Seki Technotron Corporation) shown in Fig. 1. Refer to Reference[21] for the microwave plasma CVD device. Thinking that a high-density plasma production was needed for high-speed growth, the form of the Mo substrate holder to concentrate the plasma onto the substrate was modified[22]-[25].The {100} face of the Ib type diamond synthesized by high-temperature high-pressure method was used as the seed crystal substrate. Methane and hydrogen were used as raw material gases, with flow rate of 60 and 500 sccm respectively, pressure of 21 kPa, and substrate temperature of about 1100~1200 ºC. The substrate was heated by plasma. The luminescence from the plasma was collected by optical fibers and monitored by a spectroscope.The synthesized growth layer was evaluated by the locking curve method using precise x-ray diffraction and Raman scattering spectroscopy. The half-value width of the (400) face locking curve that clearly showed the crystal quality of the synthesized single-crystal diamond was minimum 7.6 sec so far, and this was equivalent to the high quality Ib synthesized by the high-temperature high-pressure method.3.1 Growth rateAs mentioned earlier, one of the reasons the CVD method was used to synthesize the polycrystalline diamond film but not to synthesize the bulk crystal until recently, was because of the slow growth rate. For example, compared to the liquid phase growth of 1~2 mm/min in the Si ingot pull-up method and 0.2~1 mm/h of SiC by the sublimation method, diamond growth rate of a few m/h is too slow for the bulk crystal method. To increase the growth rate of diamond in the plasma CVD method, it was necessary to raise the raw material gas pressure and to increase the amount of activated seed supplied to the growth surface by increasing the density of the plasma by applying high power.There are several reports of the increased growth rate by addition of nitrogen[22][26]. Figure 2 shows the dependency of growth rate on nitrogen flow rate. The figure shows the two types of substrate holder used for the growth. The growth rate increased in both substrate holders as the nitrogen flow rate increased. While the growth rate in conventional microwave CVD was 10 m/h or less, it reached 50~100 m/h by the combined effect of increased plasma density and nitrogen addition. Although the growth temperature differed in the two substrate holders, it was known from experiments that the change in growth rate within this range of temperature difference was small. Therefore, Fig. 2 shows the effect of nitrogen addition as well as the large change in growth rate depending on the form of the substrate holders. The degree of concentration of the microwave field changes according to the form of the substrate holder, and more the concentration the faster it is due to the increased plasma density around the substrate.3.2 Control of abnormal grain growthThe {100} face is often used as the crystal face orientation of the epitaxial growth substrate. The reasons are because the bicrystal is less likely to occur compared to other facial orientations, and because polishing of the {111} face is difficult for structural reasons. When the epitaxial growth is done on the {100} face, the abnormal growth of the abnormal Fig. 1 ASTeX type microwave plasma CVD deviceWaveguideQuartz windowMagnetron5 kW2.45 GHzSubstrate holder (Mo)Susceptor (Mo)Cooling waterCooling waterGas inletVacuum pumpOptical fiberPlasmaSpectroscopeCooling waterRadiation thermometerNitrogen flow rate (sccm)growth rate (µm/h)1220 ℃1600-2600 W1155 ℃1600-1900 W21 kPaCH4:60 sccmH2:50000213101201101009080706050403020Fig. 2 Dependency of the growth rate on nitrogen flow rate

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