Akihisa Kubota

Polishing characteristics of silicon carbide by plasma chemical vaporization machining

Silicon carbide (SiC) is expected to be a promising semiconductor material for high-temperature, high-frequency, high-power and energy-saving applications. However, it is so hard and so chemically stable that there is no efficient method of machining it without causing damage to the machined surface. Plasma chemical vaporization machining (PCVM) is a gas-phase chemical etching method in which reactive species generated in atmospheric-pressure plasma are used. PCVM has a high removal rate equivalent to those of conventional machining methods such as grinding and lapping, because the radical density in atmospheric-pressure plasma is much higher than that in normal low-pressure plasma. In this paper, the polishing characteristics of silicon carbide by PCVM are described. As a result, a high machining rate (approximately 0.18 mm/min) and a very smooth surface (below 2 nm peak-to-valley in a 500 nm square area) are achieved.

Image quality improvement in a hard X-ray projection microscope using total reflection mirror optics

A new figure correction method has been applied in order to fabricate an elliptical mirror to realize a one-dimensionally diverging X-ray beam having high image quality. Mutual relations between figure errors and intensity uniformities of diverging X-ray beams have also been investigated using a wave-optical simulator and indicate that figure errors in relatively short spatial wavelength ranges lead to high-contrast interference fringes. By using a microstitching interferometer and elastic emission machining, figure correction of an elliptical mirror with a lateral resolution close to 0.1 mm was carried out. A one-dimensional diverging X-ray obtained using the fabricated mirror was observed at SPring-8 and evaluated to have a sufficiently flat intensity distribution.

Effect of Particle Morphology on Removal Rate and Surface Topography in Elastic Emission Machining

Elastic emission machining is a surface preparation technique utilizing chemical reactions between the work piece surface and the powder particle surfaces. In this process, an atomically flat surface with an extremely high figure accuracy in the nanometer range is realized. However, the surface removal rate is extremely low. To enhance removal rate, the silica powder particles having a larger surface area than the ordinarily used particles are applied to smoothen a Si(001) surface. Experimental results indicate that removal rate improved significantly by about two orders of magnitude higher than the conventional removal rate due to the increase in the contact areas between the surfaces of the particles and the work piece. Moreover, the topography of the processed surfaces was found to improve in comparison with the initial surface from power spectral density analysis.

Defect-Free Planarization of 4H–SiC(0001) Substrate Using Reference Plate

In this paper, a new defect-free planarization technique for 4H–SiC(0001) substrate is described. This technique uses hydroxyl radicals (OH radicals) that are generated on an Fe metal surface in a hydrogen peroxide (H2O2) solution. First, the oxidation of a 4H–SiC substrate by OH radicals is investigated by X-ray photoelectron spectroscopy (XPS) analysis. Next, the planarization of the 4H–SiC substrate is conducted. A very flat and smooth surface without any scratches and etch pits is obtained. The planarized surface has a step-terrace structure.

Damage-Free Planarization of 4H-SiC (0001) by Catalyst-Referred Etching

We report the damage-free planarization of 4H-SiC (0001) wafers using a new planarization technique we named CAtalyst-Referred Etching (CARE). The CARE setup equipped with a polishing pad made of a catalyst is almost the same as a lapping setup. Since the catalyst generates reactive species that activate only when they are next to the catalyst surface, SiC can be chemically removed in contact with the catalyst surface with a pressure noticeably lower than that in a conventional polishing process. The processed surfaces were observed by optical interferometry and AFM. These observations presented a marked reduction in surface roughness. A step-terrace structure was observed with a step height of approximately 3み, corresponding to one-bilayer thickness of Si and C, in the AFM images. To estimate the crystallographic properties of the CARE-processed surface, the surfaces were observed by cross-sectional TEM. The TEM images showed that a more crystallographically well-ordered surface was realized in comparison with the conventional CMP-processed surface.

Catalyst-referred etching of silicon

Abstract A Si wafer and polysilicon deposited on a Si wafer were planarized using catalyst-referred etching (CARE). Two apparatuses were produced for local etching and for planarization. The local etching apparatus was used to planarize polysilicon and the planarization apparatus was used to planarize Si wafers. Platinum and hydrofluoric acid were used as the catalytic plate and the source of reactive species, respectively. The processed surfaces were observed by optical interferometry, atomic force microscopy (AFM) and scanning electron microscopy (SEM). The results indicate that the CARE-processed surface is flat and undamaged.

Atomic-Scale Planarization of Single Crystal Diamond Substrates by Ultraviolet Rays Assisted Machining

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.

Novel Abrasive-free Planarization of Si and SiC using Catalyst

We propose a new chemical planarization method using a catalyst as a polishing plate. A sample is placed on the polishing plate in a solution that is a source of reactive species. Since the catalyst generates reactive species that activate only next to the catalyst surface, this method can efficiently planarize. This processed surface is not damaged by chemical removal. We named this method CAtalyst-Referred Etching (CARE). CARE was applied to SiC planarization. The processed surfaces were observed by atomic force microscopy and optical interferometry. These observations presented a marked reduction in surface roughness.

New chemical planarization of SiC and GaN using an Fe plate in H2O2 solution

A novel chemical planarization method was developed for silicon carbide (SiC) and gallium nitride (GaN). This method uses catalytically generated hydroxyl radicals (OH*) to oxidize the wafer surface. OH* are generated by the reductive decomposition of hydrogen peroxide (H2O2) on the surface of the iron reference plate. An extremely flat surface without pits or scratches was obtained. Atomic force microscopy (AFM) revealed that the planarized surface had an atomic step-terrace structure, in which the step height corresponded to a single bilayer of 4H-SiC and GaN. Low electron energy diffraction (LEED) and cathodeluminescence spectroscopy showed that there was no crystallographic damage on the planarized surface.

Atomic-scale evaluation of Si(111) surfaces finished by the planarization process utilizing SiO2 particles mixed with water

Flattening performance is evaluated on the atomic scale of Si(lll) surfaces finished by a precise surface-preparation method utilizing fine SiO 2 particles mixed with ultrapure water. An atomically flat Si( 111) surface with periodic steps, which is obtained by dipping into an NH 4 F solution, is processed by the surfacing method. The low-energy electron diffraction image of the processed surface exhibits a 1 X 1 pattern. Atomic force microscopy observations show that the periodic steps formed by the NH 4 F treatment are completely removed, and highly resolved scanning tunneling microscopy images reveal that the processed surface is composed of nanometer-scaled small terraces. From these results, it is speculated that fine powder particles in ultrapure water remove microbumps and apparent steps to flatten work surfaces, although they can etch some surface atoms inside a terrace.