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For the finishing of some difficult-to-machine materials, such as silicon carbide, diamond, and so on, a novel polishing technique named plasma-assisted polishing (PAP) was proposed, which combined with the irradiation of atmospheric pressure water vapor plasma and polishing using soft abrasives. In this article, application of PAP to 4H-SiC (0001) substrate was conducted. We used helium-based water vapor plasma to modify the mechanical and chemical properties of the SiC surface. The results of X-ray photoelectron spectroscopy measurements indicate that the surface was efficiently oxidized after plasma irradiation, and the main product was SiO. CeO was used as the abrasive material in PAP, the hardness of which was near to that of the oxidized surface. The scanning white light interferometer images of the PAP processed surface showed us that scratches disappeared and surface roughness also decreased from 4.410 nm p-v, 0.621 nm root mean square (rms) to 1.889 nm p-v, 0.280 nm rms. From the atomic force microscopy images, step and terrace structure was observed on the surface after PAP, which means an atomically flat surface was obtained. The PAP processed surface was observed using cross-sectional transmission electron microscope, which indicated that almost no crystallographical defects were introduced. [ABSTRACT FROM AUTHOR]

• Electrochemical etching-enhanced CMP is proposed for efficient polishing of GaN. • Identical-site observation was conducted to study the etching behaviors of GaN. • A maximum etching rate of 1.462 μm/min has been achieved. • Sa roughness of a dia-lapped GaN surface has been reduced from 69.8 nm to 0.64 nm. An electrochemical etching-enhanced CMP process was proposed to realize the highly efficient and damage-free finishing of GaN. In this process, electrochemical etching was first used to remove the damaged layer induced by grinding or lapping, and CMP was then conducted to flatten the etched surface. The etching rate could reach 1.46 μm/min using NaOH solution as the electrolyte which was highly efficient. It was found that the etching rate and surface roughness can be balanced by adjusting the applied potential. CMP was carried out on an etched GaN sample and the surface roughness was reduced from 69.8 nm to 0.64 nm, and its surface quality was also confirmed to be desirable through photoluminescence. These results demonstrate that the proposed electrochemical etching-enhanced CMP process is highly effective and efficient to obtain a crack-free, damage-free and smooth GaN surface. [ABSTRACT FROM AUTHOR]

[Display omitted] • Atomic scale smoothing of GaN is realized by plasma-based atom-selective etching. • Temperature is a key factor in determining the PASE etching mode of GaN. • Sa roughness can be reduced from 135.8 nm to 0.527 nm by 2 min of PASE. • PASE demonstrates an extremely high MRR of 93.01 μm/min. Plasma-based atom-selective etching (PASE) of GaN is conducted to realize the highly efficient planarization of the GaN surface. Inductively coupled plasma with high temperature and high concentration of radicals is used as the source of PASE. The non-toxic carbon tetrafluoride is chosen over chlorine as the reaction gas, and the volatility of the etching products will be improved at high temperatures in the PASE of GaN. After 2 min of PASE, the GaN surface roughness is reduced from Sa 135.8 nm to Sa 0.527 nm. The material removal rate of PASE of GaN is measured to be 93.01 μm/min, thousands of times higher than that of the conventional chemical mechanical polishing method. The crystal structure of the GaN subsurface is well-ordered without any damage or defects. PASE is thus proven to be a non-destructive etching method. In this study, the effects of radio frequency power and reaction gas flow rate on PASE are also investigated. Surface temperature and concentration of radicals are found to be the critical factors in the PASE of GaN. [ABSTRACT FROM AUTHOR]