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Research paper : Development of single-crystalline diamond wafers (A. Chayahara et al.)−264−Synthesiology - English edition Vol.3 No.4 (2011) Although such separation method has been studied at several research institutes[33]-[46], they were limited to small size and the etching took a long time. The maximum size reported in the literature was about 3~4 mm square, and there was a barrier of conventional technology. The authors started to consider the etching method, and as a result, found a method to form dramatically large surface areas and at higher speed. Since, in principle, there are no problems in scaling up or mass processing, it is considered to have excellent prospects in the future diamond wafer manufacturing technology.The mosaic method[47]-[54] is a method where the small single-crystal diamond plates of a few millimeter square are closely packed, and the CVD growth is formed and bonded on top. Although the junctions cannot be used as a device, this method was developed to use the entire structure as a wafer and to apply it in the semiconductor process. The junctions produced abnormal grains, but it was found that the abnormal grains at the junction could be reduced dramatically when several crystal fragments fabricated from the same seed crystals using the direct wafer technique were bonded[55]. It is thought to occur since the face orientation of the crystal fragments align automatically. We used this method to fabricate a mosaic wafer of about 1 inch, and we now have ideas for a mass production method by applying the direct wafer technology using the mosaic wafer as the seed crystal. The mosaic method can be used for size increase easily, and is a method that can fulfill the immediate demand for size increase.3.5 Smart cut and direct wafer techniqueAs one of the manufacturing method for the silicon-on-insulator (SOI) wafer that can operate the device at high speed and at low power consumption, a cutting method using the hydrogen ion implantation is used[56]. This cutting method that uses hydrogen embrittlement is called “smart cut” or “ion cut”, and is a process where the surface of the single-crystal wafer of the semiconductor such as that of silicon is peeled off at thickness of submicron to micron level (corresponding to the depth of ion implantation). The basic process for SOI wafer fabrication by the smart cut method is as follows. (1) SiO2 insulating layer is formed on the silicon wafer surface by thermal oxidation. (2) Hydrogen ion is implanted. (3) Hydrophilic treatment is done, layered with other silicon wafers, and bonded at room temperature. (4) Heat treatment is done at 400~600 ºC, and the layers are separated at several micron thickness from the wafer surface that was implanted with hydrogen ion. This is possible due to the gaps formed by aggregated hydrogen. (5) Junction boundary is treated at 1000 ºC or more. (6) Separated wafer surface is polished. The above process is called the direct bonding method. Other SOI wafer that uses ion implantation includes the “separation by implanted oxygen (SIMOX)” wafer. This is a method where an embedded oxidation film is formed from the silicon of the wafer and the implanted oxygen by high-temperature treatment after the implantation of oxygen ion to the silicon wafer, but no separation is done.From the experience of the embedded SiC layer formation using the high-temperature carbon ion implantation to the silicon wafer[57][58], we obtained the knowledge to apply the aforementioned ion implantation technique, and we considered the use of ion implantation as the method for creating the wafer from diamond, a material difficult to process. A thin film such as SOI is not necessarily needed, but to manufacture a wafer with thickness of 0.3 mm or more, epitaxial growth is necessary after ion implantation. In the smart cut, the separation may occur during the growth by heat treatment at around growth temperature of 1150 ºC. To prevent this, in the direct wafer method, the amount implanted is the amount where the implanted layer transforms into graphite, and the growth layer is separated by removing the graphite layer by etching after growth. There is a pioneering study by Marchywka et al. for the etching of the graphite layer[59].4 ConclusionWe demonstrated that it was possible to synthesize a 12 mm single-crystal and 25 mm mosaic crystal using the plasma CVD method, surpassing the maximum size of 1 cm square using the high-temperature high-pressure method. Also, it was shown that a 2-inch size was possible using polycrystals in the direct wafer technology. In the next 1~2 years we plan to fabricate a 2-inch mosaic crystal. The verification for the usability as a wafer is in progress right now, but at Fig. 7 Direct wafer technologyFig. 8 Single-crystal diamond plate fabricated using the direct wafer technology EpitaxialgrowthlayerSeparationSeedcrystalSeedcrystalIon implantationGraphitelayerIntroductionof defect layerAnnealing andgraphitization(can be omitted)EpitaxialgrowthRemoval ofgraphite layerReuseWaferuseUsable asseed crystalif grownthickEpitaxialgrowthlayerEpitaxialgrowthlayerSeedcrystalSeedcrystalSeedcrystalThickness of 0.3~0.5 mm

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