Vol.3 No.4 2011

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Research paper : Development of single-crystalline diamond wafers (A. Chayahara et al.)−260−Synthesiology - English edition Vol.3 No.4 (2011) on the substrate of about 800 ºC under decompression. Several CVD methods were developed since then, but these two methods were innovative, as they are still widely used in the CVD diamond manufacturing and R&Ds.In the general CVD method, the raw material gas such as the hydrocarbon gases are decomposed under decompression, the non-diamond substrate is nucleated, and the polycrystalline diamond film is deposited. Such polycrystalline film has excellent properties for various uses such as coating tools. The production of atomic hydrogen and the decomposition of raw material gas can be accomplished by various methods. Depending on the method of the decomposition (activation), the methods can be roughly divided into thermal CVD and plasma CVD. While the decomposition of the raw material gas in the thermal CVD process is achieved by thermal activation, it occurs by electron-molecule reaction in the plasma CVD. Thermal CVD process includes the hot filament method and the combustion flame method such as the oxygen-acetylene torch. Plasma CVD method includes the microwave plasma, DC plasma, DC plasma jet, and RF plasma methods.In the hot filament CVD method, the film can be formed on a large surface area, since large equipment can be manufactured at relatively low cost. It has already been realized as the polycrystalline diamond coating method for machine tools. However, the filament material (tungsten, tantalum, rhenium, etc.) heated to high temperature may become included in the film as impurities, and the growth rate is slow. The microwave plasma CVD has few inclusions of impurities since the electrodeless discharge is used, and it is possible to form semiconductor-quality film. Although the growth rate was slow, the speed has been increased as explained later[16], and the microwave plasma CVD is used in almost all cases of bulk single-crystal synthesis by CVD.The characteristic common in the CVD diamond synthesis methods is the high-concentration hydrogen gas. Until recently, hydrogen concentration of 99 % or higher was necessary to obtain the diamond with small graphite component. In general, it is believed that the high-concentration hydrogen produces large amount of atomic hydrogen that plays an important role in the diamond CVD process. The diamond synthesis occurs as the radicals from the gas attach to the growth surface and then detach, that is, a surface reaction process takes place. When the diamond crystal grows, the nucleation of the diamond occurs first, and it is necessary to prevent the diamond growth surface to transform into a graphite phase. To do so, it is necessary for the high-concentration hydrogen atoms to bond with all the dangling bonds present on the diamond growth surface. It has been confirmed in the heating experiment that the diamond structure is relatively stable in hydrogen. In ultrahigh vacuum, the diamond surface graphitizes at about 900 ºC, but the diamond structure is maintained up to 2200 ºC when heated in hydrogen[17]. In oxygen, the mass decrease due to oxidation starts at about 585 ºC[18], and the diamond oxidation progresses along with the graphitization of the surface. The diamond growth is explained as follows. In the process of the formation of hydrogen molecules resulting from the reaction of bonding hydrogen that covers the growth surface with hydrogen atoms in the gas, holes (dangling bonds) are formed after the hydrogen is pulled from the growth surface. Next, methyl radical CH3 produced by the decomposition of the raw material gas (the hydrogen atoms play a part in this reaction) bonds with the holes and growth occurs. Moreover, the atomic hydrogen selectively etches the graphite layer that deposits simultaneously with the diamond. This is useful in reducing the graphite component within the crystal grain boundary in the CVD polycrystalline diamond synthesis. The general procedure is to add oxygen to the raw material gas. In etching with oxygen, the selected ratio of graphite and diamond is not as high as with hydrogen, but since etching can be conducted effectively at low temperature, oxygen addition is useful in reducing the temperature of the diamond growth condition. Also, for the composition ratio of the carbon-hydrogen-oxygen in the raw material gas, the Bachman diagram[19] that shows the range of composition ratio in which the diamond growth is possible is widely known.In the diamond crystal growth technology, the formation of the diamond nucleus called the bias enhanced nucleation (BEN)[20] is important. BEN is used as a nucleation technology for the heteroepitaxial growth on different substrates and for the growth of polycrystalline diamond and nano-diamond film. In the case where the polycrystalline diamond is grown without using the nucleation by BEN, the substrate must be pretreated by mechanical or ultrasound polishing in organic solvent using diamond abrasives before the growth process. This is called the “seeding” process where fine diamond grains are embedded into the substrate, and these become the seed crystals from which growth begins. BEN is a method to replace this. The substrate is charged with negative bias in the plasma with relatively high hydrocarbon concentration. Highly dense diamond nuclei are formed. To grow diamond on these nuclei, the normal growth without bias charge is conducted following the BEN process. Only the stable area formed during the BEN process survives to continue growth.The film grown after the seeding process becomes polycrystalline where the crystal orientation is random in the growth face, while in BEN, the orientation of the seed crystals may align with the substrate, and the diamond film that grows upon this substrate will be oriented accordingly. This allows heteroepitaxial growth on single-crystal iridium

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