He Yan,Yuan Zewei,Song Shuyuan,Gao Xingjun,Deng Wenjuan
Photocatalysis-assisted chemical mechanical polishing, in which the photocatalysis oxidation and silica abrasives polishing are combined, is a novel finishing technique for 4H–SiC wafer. This paper characterizes the effect of pressure and abrasives on the MRR and surface quality and discusses mechanical and chemical interaction based three slurries that resulted in maximum material removal rate (1.18 μm/h) with PCMP slurry. The polished silicon carbide wafer was examined with atomic force microscope, transmission electron microscope and X-ray photoelectron spectroscopy for surface quality and material removal mechanism. The results show that an atomically smooth and flat 4H–SiC (0001) surface (Ra about 0.247 nm) was obtained by PCMP. The interface of crystal and amorphous layer of 4H–SiC wafer was flat without introducing crystallographic subsurface damage, and the atoms and lattice fringes of the crystal layer are arranged in regular order. The existence of a thin silicon oxycarbide layer, which are various forms of functional groups such as Si–C, Si–C–O, Si–O, Si 4 C 4 O 4 , C–O, and C = O, at the interface. And, it also demonstrates that the amorphous layer is composed of oxide layer (4–6 nm) and distorted layer. The silicon carbide surface is mechanically activated, oxidized and mechanically removed in turn, which is the main method of material removal.
Chemical mechanical polishing (CMP) has revolutionized the processing of semiconductor material such as silicon carbide (SiC). However, the strong chemical bond between silicon atoms and carbon atoms gives SiC materials a very high mechanical hardness and chemical stability. Therefore, SiC wafers may suffer from a low material removal rate (MRR) and serious subsurface damage when traditional CMP methods are used. In this study, to improve the CMP properties of Si-face of the 4H-SiC wafers, the synergistic effect of UV and TiO 2 on the activation of persulfate (PS) was investigated for the first time. The response surface method was employed to study the effect of pH, PS concentration, and TiO 2 dosage on CMP in-depth, and to ultimately optimize the polishing process. The analysis demonstrated that an MRR of 608 nm/h can be obtained with the average surface roughness ( Sq ) of 0.521 nm when the pH is 6, in which the TiO 2 dosage is 0.015 wt% and the PS concentration is 3 wt%. Polarization curves , AC impedance spectra and X-ray photoelectron spectroscopy tests were performed to further analyze the chemical action of the slurry, which showed that the introduction of UV-TiO 2 strongly enhances the chemical action of slurry and results in the formation of a passivated layer on the SiC surface. Finally, based on the radical quenching experiments and the results mentioned above, a model that relates to the removal of 4H-SiC by UV-TiO 2 synergistic activation of PS is proposed.
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