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Three main machining processes of large-diameter 6H-SiC wafers were introduced in this paper. These processes include cutting, lapping and polishing. Lapping causes great residual stresses and deep damage layer which can be reduced gradually with subsequent polishing processes. Surfaces prepared by mechanical polishing (MP) appeared a large number of scratches with depth of 5~8 nm. These scratches can be effectively removed by chemo-mechanical polishing (CMP). After CMP, extremely smooth and low damage layer surface with roughness Ra=0.3 nm was obtained. Atomic force microscopy (AFM) and optical microscopy were used to observe the surface morphology of samples and a high resolution X-ray diffractometer (HRXRD) was used for the crystal lattice perfection of the subsurface region. Changes of surface residual stresses during machining processes were investigated by HRXRD.