Vol.3 No.4 2011

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Research paper : Development of single-crystalline diamond wafers (A. Chayahara et al.)−263−Synthesiology - English edition Vol.3 No.4 (2011) in practical use as power semiconductor in the near future. In conventional SiC sublimation method, the growth takes place along the c axis <0001>, whereas in the RAF method, the dislocation structure change by the {1120} and {1100} faces called the a-face are used to reduce the dislocational defects. Since the growth is repeated toward the a-face direction, it is called the repeated a-face (RAF) growth method. From the ingot grown in the c-face direction, (1) the {1100} face crystal is cut out and the growth is done on that face. Next, (2) {1120} face crystal is cut out, and growth is done on that face. After repeating (1) and (2) several times, the seed crystal of the c-face is cut out from the ingot, and this is used for the c-face growth. The conventional c-face grown ingot has several dislocations, but the dislocation of the seed crystals is reduced by the a-face growth. Majority of the dislocations are present parallel to the seed crystal face, and it is thought that the defects parallel to the crystal face are not carried over during the a-face growth.The motivation for developing the RAF growth method for the SiC single crystal was the reduction of crystal defects such as micropipes. In contrast, the {100} face repeated growth method in the CVD diamond was primarily done to obtain a larger diameter. In the growth condition of ultralow volume nitrogen addition, the growth was only in the <100> direction, and an increase of the crystal diameter by growth could not be expected. There are dramatic differences in the diameter and quality of the SiC and diamond wafers. Although SiC is more than 20 years ahead, it is interesting that the lateral repeated growth was proposed at almost the same time.3.4 Wafer fabricationSince diamond is the hardest material, it is not easy to process. Therefore, its formation into a wafer form necessary for the manufacture of semiconductor devices is very difficult. Laser cutting is used to process industrial diamonds, but the losses due to cutting reserve and processing time are issues. Also, flatness and low defects are required in the precision polishing technology. “Direct wafer technology” and mosaic method have been developed as methods for wafer fabrication.In the direct wafer technology[32], ion is implanted to the single-crystal diamond that will be used as a seed prior to the growth, and a defect layer is introduced immediately beneath the surface. After vapor deposition, the defect layer assumes a graphite structure, and can be removed by electrochemical etching. The seed crystal and the growth layer have diamond structures and are very stable chemically, and are not etched chemically. The seed crystal and the growth layer are separated to form a plate diamond (Figs. 7, 8). Some small parts of the seed crystal will be lost during the separation, but this thickness is only about 1 m or the depth of the ion implantation. Therefore, the ordinary seed crystal can be used repeatedly, and the separated crystals can also be used as seed crystals.Fig. 5 Size increase of crystal by repeated lateral growth Step 1Step 3Step 2(100) face growth of thick CVD diamond by repeated growthRepeated growth on the (010) face perpendicular to the CVD growth on face of Step 1Step 2 repeatedSeed crystalSeed crystalSeed crystalCVD diamondCVD diamond10 µm0.5 sccmN2: 0 sccm1 sccm2 sccm3 sccm3 mmFig. 3 Dependency of the surface form on nitrogen flow rate observed by a differential interference microscopeTop: Macro image (low magnification). Bottom: micro image (high magnification).10 mm Fig. 4 CVD single-crystal diamondWeight: 2 g (10 carat) Seed crystalStep 2Step 110 mmFig. 6 Example of lateral growth

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