Kazuya Yamamura

Competition between surface modification and abrasive polishing: a method of controlling the surface atomic structure of 4H-SiC (0001)

The surface atomic step-terrace structure of 4H-SiC greatly affects its performance in power device applications. On the basis of the crystal structure of 4H-SiC, we propose the generation mechanism of the a-b-a*-b* type, a-b type and a-a type step-terrace structures. We demonstrate that the step-terrace structure of SiC can be controlled by adjusting the balance between chemical modification and physical removal in CeO2 slurry polishing. When chemical modification plays the main role in the polishing of SiC, the a-b-a*-b* type step-terrace structure can be generated. When the roles of physical removal and chemical modification have similar importance, the a-b-a*-b* type step-terrace structure changes to the a-b type. When physical removal is dominant, the uniform a-a type step-terrace structure can be generated.

Atomic-scale planarization of 4H-SiC (0001) by combination of thermal oxidation and abrasive polishing

Thermal oxidation (TO) and abrasive polishing were combined for atomic-scale planarization of 4H-SiC. It was found that the oxide/SiC interface was atomically flat regardless of the thickness of the oxide. The specimen prepared by TO was dipped in HF solution to remove the oxide. However, owing to the residual silicon oxycarbide (Si-C-O), the step/terrace structure of 4H-SiC could not be observed. Nanoindentation tests revealed that the hardness of Si-C-O was much lower than that of SiC. A thermally oxidized SiC surface was polished using CeO2 abrasives, which resulted in an atomically flat surface with a well-ordered two-bilayer step/terrace structure.

Comparison of thermal oxidation and plasma oxidation of 4H-SiC (0001) for surface flattening

The thermal oxidation and water vapor plasma oxidation of 4H-SiC (0001) were investigated. The initial oxidation rate of helium-based atmospheric-pressure plasma oxidation was six times higher than that of thermal oxidation. The oxide-SiC interface generated by plasma oxidation became flatter with increasing thickness of the oxide, whereas the interface generated by thermal oxidation was atomically flat regardless of the oxide thickness. Many pits were generated on the thermally oxidized surface, whereas few pits were observed on the surface oxidized by plasma. After the oxide layer generated plasma oxidation was removed, an atomically flat and pit-free SiC surface was obtained.

Efficient processing of reaction-sintered silicon carbide by anodically oxidation-assisted polishing

Abstract. Reaction-sintered silicon carbide (RS-SiC) is a promising optical material for the space telescope systems. Anodically oxidation-assisted polishing is a method to machine RS-SiC. The electrolyte used in this study is a mixture of hydrogen peroxide (H2O2) and hydrochloric acid (HCl), and the oxidation potential has two modes: constant potential and high-frequency-square-wave potential. Oxide morphologies are compared by scanning electron microscope/energy dispersive x-ray spectroscopy and scanning white-light interferometer. The results indicate that anodic oxidation under constant potential can not only obtain a relatively smooth surface but also be propitious to obtain high material removal rate. The oxidation depth in anodic oxidation under constant potential is calculated by comparing surface morphologies before and after hydrofluoric acid etching. The theoretical oxidation rate is 5.3u2009u2009nm/s based on the linear Deal–Grove model. Polishing of the oxidized RS-SiC is conducted to validate the machinability of the oxide layer. The obtained surface roughness root-mean-square is around 4.5 nm. Thus, anodically oxidation-assisted polishing can be considered as an efficient method, which can fill the performance gap between the rough figuring and fine finishing of RS-SiC. It can improve the machining quality of RS-SiC parts and promote the application of RS-SiC products.

Atomic-scale and pit-free flattening of GaN by combination of plasma pretreatment and time-controlled chemical mechanical polishing

Chemical mechanical polishing (CMP) combined with atmospheric-pressure plasma pretreatment was applied to a GaN (0001) substrate. The irradiation of a CF4-containing plasma was proven to be very useful for modifying the surface of GaN. When CMP was conducted on a plasma-irradiated surface, a modified layer of GaF3 acted as a protective layer on GaN by preventing the formation of etch pits. Within a short duration (8u2009min) of CMP using a commercially available CeO2 slurry, an atomically flat surface with a root mean square (rms) roughness of 0.11u2009nm was obtained. Moreover, etch pits, which are inevitably introduced in conventional CMP, could not be observed at the dislocation sites on the polished GaN surface. It was revealed that CMP combined with the plasma pretreatment was very effective for obtaining a pit-free and atomically flat GaN surface.

Investigation of Removal Mechanism of Sapphire in Plasma Assisted Polishing

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.

Open-air type plasma chemical vaporization machining by applying pulse-width modulation control

Photolithography techniques have been used to enable the low-cost and high-speed transfer of a pattern onto a silicon wafer. However, owing to the high integration of semiconductors, extreme ultraviolet will be increasingly used as the exposure light source and all optics must be reflective to focus light because the wavelength of the light will be so short that it cannot pass through a lens. The form accuracy of reflective optics affects the accuracy of transfer, and a flatness of less than 32 nm on a 6 inch photomask substrate is required according to the International Technology Roadmap for Semiconductors roadmap. Plasma chemical vaporization machining is an ultraprecise figuring technique that enables a form accuracy of nanometre order to be obtained. In our previous study, the removal volume was controlled by changing the scanning speed of the worktable. However, a discrepancy between the theoretical scanning speed and the actual scanning speed occurred owing to the inertia of the worktable when the change in speed was rapid. As an attempt to resolve this issue, we controlled the removal volume by controlling the electric power applied during plasma generation while maintaining a constant scanning speed. The methods that we adapted to control the applied electric power were amplitude-modulation (AM) control and pulse-width modulation (PWM) control. In this work, we evaluate the controllability of the material removal rate in the AM and PWM control modes.

Preliminary study on atmospheric-pressure plasma-based chemical dry figuring and finishing of reaction-sintered silicon carbide

Abstract. Reaction-sintered silicon carbide (RS-SiC) is a research focus in the field of optical manufacturing. Atmospheric-pressure plasma-based chemical dry figuring and finishing, which consist of plasma chemical vaporization machining (PCVM) and plasma-assisted polishing (PAP), were applied to improve material removal rate (MRR) in rapid figuring and ameliorate surface quality in fine finishing. Through observing the processed RS-SiC sample in PCVM by scanning white-light interferometer (SWLI), the calculated peak-MRR and volume-MRR were 0.533u2009u2009μm/min and 2.78×10−3u2009u2009mm3/min, respectively. The comparisons of surface roughness and morphology of the RS-SiC samples before and after PCVM were obtained by the scanning electron microscope and atomic force microscope. It could be found that the processed RS-SiC surface was deteriorated with surface roughness rms 382.116 nm. The evaluations of surface quality of the processed RS-SiC sample in PAP corresponding to different collocations of autorotation speed and revolution speed were obtained by SWLI measurement. The optimal surface roughness rms of the processed RS-SiC sample in PAP was 2.186 nm. There were no subsurface damages, scratches, or residual stresses on the processed sample in PAP. The results indicate that parameters in PAP should be strictly selected, and the optimal parameters can simultaneously obtain high MRR and smooth surface.

Experimental Studies on Water Vapor Plasma Oxidation and Thermal Oxidation of 4H-SiC (0001) for Clarification of the Atomic-Scale Flattening Mechanism in Plasma Assisted Polishing

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.

Material Removal Rate Control in Open-Air Type Plasma Chemical Vaporization Machining by Pulse Width Modulation of Applied Power

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.