Papers by Author: Mutsumi Touge

Abstract: The polycrystalline diamond (PCD) is made of sintered diamond fine powder with binder material, and shows clear isotropic characteristics and high toughness without frequent cleavage. As the PCD cutting tools require relatively low tool cost compared with single crystal diamond tools, they have wide applications as the precision cutting tools with the high abrasion resistance. The more sharp or complex shapes of PCD cutting tool are deeply expected even though they have the machining difficulties. In this study, the ultraviolet-irradiation polishing of single crystal diamond substrates developed in our laboratory (abbreviated as UV-polishing in this paper) was applied to realize advanced PCD tools with ultra-sharp or chamfered cutting edges. Firstly, the UV-polishing properties of PCD substrate were investigated, and high-quality polished surface with 2.6 nmRa was obtained. Secondly, the UV-polishing was applied to the precision polishing of the flank face of PCD cutting tool, and an ultra-sharp cutting edge was finally achieved. The chamfered cutting edge with desired angle and width was additionally formed by the UV-polishing to suppress the chipping left on the sharpened cutting edge. The tool wear of chamfered PCD cutting tools could be reduced almost by half to the sharpened tool under high-speed cutting of high silicon-aluminum alloy.

Abstract: In this study, we investigated the possibility of smoothing a GaN substrate utilizing ultraviolet (UV) assisted polishing method in potassium hydroxide (KOH) solution. In this polishing method, GaN substrate was excited by an UV radiation, and then an oxide layer on the GaN substrate was formed by photochemical reaction. Simultaneously, generated oxide layer was removed by synthetic quartz tool and chemically etched by KOH solution. Finally, smoothed GaN surface could be realized. The surface quality and removal depth were measured and evaluated using a scanning white light interferometer and Normalski type differential interference microscopy. Obtained results show that the surface morphology and the removal rate are strongly dependent on the existence of the UV irradiation. Moreover, the processed surface has revealed that many scratches on the preprocessed GaN surface could be completely removed. The microroughness of the processed GaN surface profile was improved to be 0.18 nm (Rms), 1.06 nm (Rz).

Abstract: We have developed a novel polishing technique by scanning a small magnetic tool in hydrogen peroxide solution for smoothing a 2-inch SiC wafer. Obtained results show that the surface roughness in almost areas on the 2-inch SiC wafer is improved markedly. Our proposed method effects a dramatic improvement in a surface microroughness from 0.699 nm Rms to 0.079 nm Rms.

Abstract: In this study, we investigated the possibility of removing and smoothing a single-crystal silicon carbide (SiC) surface under ultraviolet (UV) irradiation in hydrogen peroxide (H₂O₂) solution. In this method, a SiC substrate was excited by UV irradiation that transmitted synthetic quartz, and then an oxide layer on the SiC substrate was formed by photochemical reaction. Simultaneously, hydroxyl radical (OH*) was generated by the decomposition of H₂O₂ solution by UV irradiation. OH* plays an important role of oxidation of SiC surface. With these chemical reactions, oxide layer was effectively formed on the SiC surface. Finally, the oxide layer generated on a SiC substrate was chemically and/or mechanically removed by synthetic quartz and solutions. The polishing characteristics of this method were investigated by controlling the process parameters. Additionally, surface quality and removal depth were measured and evaluated by a phase-shift interference microscopy. Obtained results show that the surface morphology and the removal rate are strongly dependent on the existence of the UV irradiation. Moreover, it is shown that the removal characteristics of the SiC substrate depend on the process parameters such as the process time, reciprocating speed, and contact load. The processed surface has revealed that many scratches on the preprocessed surface was completely removed. The microroughness of the processed surface was improved to 0.15 nm (Rms) and 1.62 nm (p-v), respectively. These results provide useful information for obtaining an atomically smooth SiC surface.

Abstract: Ultra smooth and defect-free 4H-SiC wafers are strongly demanded in the next-generation power semiconductor devices. However, such SiC substrates are relatively difficult to machine because of their mechanical hardness and marked chemical inertness. In this study, we attempt to polish 2-inch 4H-SiC wafers by our proposed method, which utilizes Fe particles and a hydrogen peroxide solution. The processed surface was observed by phase shift interferometric microscopy, Nomarski differential interference contrast microscopy and atomic force microscopy. These observational results show that the surface roughness was improved over the entire 2-inch wafer by our proposed method. These results offer useful information for preparing a smooth SiC wafer.

Abstract: We have developed advanced lapping and polishing methods for silicon carbide (SiC) substrates using an Fe abrasive particles and hydrogen peroxide (H2O2) solution. In this method, a SiC surface is oxidized by hydroxyl radicals (OH*), which was generated by Fe catalyst reactions, and the oxide layer on the SiC is mechanically and/or chemically removed by Fe abrasive particles and solution　[1-4]. In this study, we applied this planarization method for lapping and polishing SiC surface, in which catalytically generated hydroxyl radicals were utilized to oxidize the surface of a SiC wafer. The processed surfaces were observed by optical interferometric microscope, Nomarski differential interference contrast. These observations showed that surface roughness and flatness of a SiC substrate were markedly improved and scratch-free SiC surface was obtained. These results provide useful information for preparing a high-efficiency and high-accuracy SiC substrate.

Abstract: The ultra-precision polishing assisted by the ultraviolet rays irradiation was performed to achieve the atomic-scale planarization of the single crystal diamond substrates. This polishing method is a novel and simple polishing method characterizing by a quartz disk and an ultraviolet irradiation device. The principle three crystal planes of the diamond substrate were polished by this method. The polished surfaces were evaluated by an optical interferometric profilers (Wyko), an atom force microscope (AFM) and LEED (low-energy electron diffraction). The surface roughness of the polished diamond substrates was evaluated as 0.2 ~ 0.4 nmRa in (100), (110) and (111) crystal planes. The LEED (low-energy electron diffraction) patterns indicated the almost perfect crystallographic structure without the residual processed strain beneath the polished surface. In this paper, the optimum polishing condition to achieve the atomic-scale planarization of the diamond substrates has been investigated by the evaluation of LEED patterns, Wyko and AFM images. The mechanismof the ultraviolet rays assisted polishing is discussed in detail.

Abstract: Silicon carbide (SiC) single crystal has many advantages comparing with silicon single crystal, such as wide band-gap, hardness and various stable physical properties at high temperature and severe chemical environments. SiC semiconductor substrate is expected to be applied to high power devices and sensor devices in the severe environments. The polishing process under ultraviolet-ray irradiation has been developed in our laboratory to achieve the mirror-finish process of single-crystal SiC. In this paper, after the confirmation of the elastic deformation behavior of SiC single crystal using the nano-indentation tester, pre-processings of SiC by lapping and constant-pressure grinding were performed to obtain good surface without brittle fracture region. The indentation tests indicate the single-crystal SiC shows a very high elastic recovery rate. SiC substrate was processed by the lapping and the constant-pressure grinding using the constant-pressure processing to avoid the cumulative residual stock removal. As many experimental results, the constant-pressure grinding is found to be suitable for the pre-processing of SiC substrates. Additionally, it is clarified that good surface roughness on the ground surface is achieved by using a diamond wheel with the same protrusion height of abrasive grains.

Abstract: Polycrystalline diamond (PCD) has been widely used for various cutting tools and die components making use of its hardness and wear resistance properties. The polishing method of a single crystal diamond substrate and SiC using ultraviolet irradiation was newly developed to obtain mirror-finished surfaces. Due to the long polishing time in this method, a better pre-machined surface is required to shorten the total processing time. In this work, the constant-pressure grinding was performed using a cup type metal-bonded diamond wheel and a constant pressure device. After the good constant-pressure grinding, the PCD was finished by the polishing under the ultraviolet irradiation, and the microroughness was reached to be 0.71 nmRa.

Abstract: The ultraviolet irradiation-assisted ultra precision polishing was performed on single crystal diamond substrates. This polishing method has been newly developed in our laboratory. The change of polishing performances was investigated by the presence of the UV irradiation. The polished surfaces were evaluated by the observation with WYKO. The experimental results are as follows; Surface roughness of diamond substrates polished under UV irradiation has become clearly smoother than that without UV irradiation. The surface roughness by this polishing method was reached to be 0.19 nm Ra on (100) surface of single crystal diamond. The equivalent surface was obtained on (110) surface by the UV-polishing.