Papers by Author: Kazuya Yamamura

Abstract: A sapphire substrate is essential for epitaxial growth of GaN, which is used for high brightness light-emitting diodes (LEDs), high-power and high-frequency devices. However, the material removal rate (MRR) of sapphire in conventional polishing is very low because of its high hardness and chemical inertness. We proposed application of plasma assisted polishing using a resin-bonded silica grinding stone for finishing of sapphire surface and investigated basic removal properties. The results of a ball-on-disc type test results showed that irradiation of water vapor containing atmospheric pressure Ar gas plasma promoted the MRR of sapphire by a factor of 7.4. Strong emission from hydroxyl radical was observed by an optical emission spectroscopy measurement of the plasma. XPS measurements revealed that the surface of both sapphire and silica were hydroxylated after the plasma irradiation. From these experimental and measurement results, we proposed the removal model in plasma assisted polishing of sapphire as follows. Firstly, irradiation of water vapor plasma hydroxylates the surfaces of sapphire and silica. Then, Al-O-Si bonding is formed by dehydration reaction between sapphire and silica surfaces. Finally, surface atom of sapphire is removed by the motion of silica abrasive. In this paper, we describe the preliminary experimental results and measurement results which support the proposed removal model in plasma assisted polishing of sapphire.

Abstract: Preliminary study on anodic-oxidation-assisted polishing (AOAP) of 4H-SiC (0001) using ceria polishing film was demonstrated. In the case of using deionized (DI) water as an electrolyte, rms roughness of 0.16 nm was obtained, which is almost the same as roughness of the surface finished by conventional chemical mechanical polishing (CMP). However, the polishing rate was very low and was 23 nm/h. In contrast, the polishing rate of 0.84 μm/h, which is equal to that of conventional CMP of single-crystal SiC or greater, was obtained when we used 1 wt% of phosphoric acid (H₃PO₄) as the electrolyte, although the surface roughness increased to rms roughness of 1 nm order. These experimental results indicate that the polishing rate greatly depends on the oxidation rate of anodic oxidation and the balance between the oxidation rate and the removal rate of oxide by abrasive greatly affects the roughness of the processed surface.

Abstract: 4H-SiC is considered as one of the most promising next-generation semiconductor power-device materials. An atomically flat 4H-SiC surface with a well-ordered step/terrace structure was essential for epitaxial growth or applications in electrical devices. Plasma assisted polishing (PAP), in which the irradiation of atmospheric-pressure water vapor plasma and ceria (CeO₂) abrasive polishing were combined, was successfully applied to the atomic-scale flattening of 4H-SiC. To clarify the atomic-scale flattening mechanism of 4H-SiC in PAP process, investigations of thermal oxidation of 4H-SiC were conducted. Cross-sectional transmission electron microscopy (XTEM) observations revealed that the interfaces between the thermal oxidized oxide layer and SiC were very flat regardless of the thickness of the oxide layer. Dipping in hydrofluoric acid for 10 min and CeO₂ abrasive polishing for 3 h were respectively conducted on a 4H-SiC surface which was thermally oxidized for 2 h. A flat surface was obtained after dipping in HF acid. However, no step/terrace structure, which corresponds to the inclination of the crystal plane, could be observed due to the residual of silicon oxycarbide. A well-ordered step/terrace structure was obtained on the surface polished by CeO₂ abrasive. The step height was about 0.25 nm, which corresponds to a one-bilayer structure of 4H-SiC. The different oxidation rates of Si atoms on the cubic face and Si atoms on the hexagonal face were considered the reason why two kinds of terraces with different width were generated.

Abstract: Single crystal sapphire is widely used as the material for precision equipments, due to its high hardness, chemical inertness and light transmission. However, it is difficult to obtain a scratch-free and damage-free sapphire surface with high-efficiency through traditional mechanical polishing or etching. We developed plasma assisted polishing (PAP) for the finishing of difficult-to-machine materials, such as silicon carbide, diamond, and sapphire. In this article, preliminary research results are showed about PAP applied to polishing of single crystal c-plane sapphire substrates. Combination of helium based atmospheric pressure water vapor plasma irradiation and silica abrasive polishing drastically increased removal rate of the sapphire c-plane. XPS measurements of the surfaces with and without irradiation of water vapor plasma revealed that alumina hydrate was formed by plasma irradiation at low temperature of less than 40°C. It is assumed that formation of alumina hydrate promoted the removal rate of sapphire.

Abstract: Plasma chemical vaporization machining (PCVM) is an ultraprecise figuring technique for optical components without introducing the subsurface damage. In our previous study, the material removal volume was controlled by changing the scanning speed of the worktable. However, because of inertia of the worktable, a discrepancy between the theoretical scanning speed and the actual scanning speed will occur if the spatial change rate of speed is rapid. Therefore, we proposed the application of the pulse width modulation (PWM) control and the amplitude modulation (AM) control of the applied RF power to control the material removal rate (MRR). Experimental results showed that the relationship between the MRR and the average RF power had high linearity, the control range of the PWM control mode was from 0.19 x 10^-2 mm³/min to 3.90 x 10^-2 mm³/min (from 5% to 100%), which was much wider than that of the AM control mode.

Abstract: Reaction-sintered silicon carbide (RS-SiC) is a promising material for optical components used in space, or molds for precision glass lens because of its excellent properties. For processing of RS-SiC, diamond tools are utilized because RS-SiC is difficult-to-machine material due to its high hardness. In that case, subsurface damage (SSD) and scratches are inevitably introduced on the processed surface, and they deteriorate the qualities of products. To resolve these issues, we proposed a complex machining technique named anodic oxidation assisted process (AOAP), in which localized anodic oxidation and removal of the oxidation layer by grinding or polishing were combined, for figuring or polishing of RS-SiC without introducing any scratches and SSD. The grinding or polishing tool used in AOAP has a lower hardness than that of RS-SiC, but higher than that of the oxidation products. It is possible to figure the objective shape and polish the surface by changing the conditions including the oxidation time, the composition of electrolyte, the configuration of the cathode electrode, applied voltage, and so on. In our previous study, we found that RS-SiC was oxidized efficiently by anodic oxidation with various electrolytes such as phosphoric acid, ultrapure water, and a mixture of hydrochloric acid and hydrogen peroxide. In this research, we investigated the preliminary processing characteristics of AOAP for RS-SiC. We ascertained that irradiating UV light with photon energy higher than the band gap of processed materials is very effective for increasing the oxidation rate of anodic oxidation. And we proposed a novel polishing process of RS-SiC, which combining oxidation only SiC area in RS-SiC by anodic oxidation with the electrolyte of ceria slurry, with polishing by ceria slurry which removes both oxidized layer and unoxidized layer in RS-SiC. The results of investigation for the oxidation rate and the polishing rate of SiC, Si and SiO₂ with ceria slurry implies that we can remove SiC grain and Si grain in RS-SiC at the same MRR by combing the anodic oxidation and polishing with ceria slurry at the same time, and obtain the smooth surface.

Abstract: Silicon carbide (SiC) is a promising semiconductor material for high-temperature, high-frequency, high-power, and energy-saving applications. However, because of the hardness and chemical stability of SiC, few conventional machining methods can handle this material efficiently. A plasma chemical vaporization machining (PCVM) technique is an atmospheric-pressure plasma etching process. We previously proposed a novel style of PCVM dicing using slit apertures for plasma confinement, which in principle can achieve both a high removal rate and small kerf loss, and demonstration experiments were performed using a silicon wafer as a sample. In this research, some basic experiments were performed using 4H-SiC wafer as a sample, and a maximum removal rate of approximately 10 μm/min and a narrowest groove width of 25 μm were achieved. We also found that argon can be used for plasma generation instead of expensive helium gas.

Abstract: 4H-SiC is difficult to be polishing due to its high hardness and chemical inertness. We proposed a novel polishing technique named plasma assisted polishing (PAP), in which oxidation by water plasma and polishing soft abrasive were combined. In order to increase the material removal rate of PAP and clarify the atomic-scale flattening mechanism, experimental studies on water vapor plasma oxidation and thermal oxidation of 4H-SiC (0001) were conducted. Experimental results indicated that the initial oxidation rate of water vapor plasma oxidation (185 nm/h) was much higher than that of thermal oxidation (29nm/h). In the case of water vapor plasma oxidation, the oxide/SiC interface was rough when the oxide layer was thin and it became flatter along with the increase of the thickness of the oxide layer. In contrast, the oxide/SiC interface was atomically flat regardless of the thickness of the oxide layer in the case of thermal oxidation. CeO₂ abrasive polishing was conducted on the oxidized SiC surfaces, well-ordered step/terrace structures were obtained in both cases. The step height was about 0.25 nm, which corresponds to a one-bilayer structure of 4H-SiC.

Abstract: Silicon carbide (SiC) is a promising semiconductor material for high-temperature, high-frequency, high-power, and energy-saving applications. However, because the hardness and chemical stability of SiC are high, few conventional machining methods can handle this material efficiently. We previously developed a plasma chemical vaporization machining (PCVM) technique, which is an atmospheric-pressure plasma etching process, and investigated its application to the processing of SiC substrates. In this paper, we propose a novel style of PCVM technique for dicing, using slit apertures to confine the plasma. From experiments by means of an apparatus with a one-slit aperture formed by two masks, it was found that the kerf loss was almost proportional to the slit width, and that the etching depth increased with RF power. Furthermore, from experiments on a SiC wafer, we obtained a 130-μm etching depth and 300-μm kerf loss for an 11-min processing time and 200-μm slit width.

Abstract: Plasma-assisted polishing (PAP) was successfully applied to single-crystal SiC to obtain an atomically flat surface without introducing any scratches. To clarify the flattening mechanism and increase the material removal rate (MRR) of PAP, investigation of the oxidation process in PAP is essential. In this study, we observed 4H-SiC (0001) surfaces processed by water vapor plasma oxidation using angle resolved X-ray photoelectron spectroscopy (ARXPS). Water vapor plasma oxidation was conducted for 1 min and 5 min. SiO2 and silicon oxycarbide were observed as the oxidation products. A decrease in the plasma irradiation time decreased the thickness of the oxide layer, particularly that of the silicon oxycarbide layer.