Study Of Surface And Subsurface Damage Of Single Crystal SiC Wafer In Ultraprecission Machining

Single crystal SiC as the third generation wide band gap semiconductor material has the advantages of wide band gap, high thermal conductivity, high breakdown field, high saturated electron drift velocity and high bond energy that is widely used as the substate material of high frequency and high power electronic devices and optoelectronic devices. The surface equality of the ultra-Preeision machined substrates greatly afectets the performance of the thin films grown on it. A cross-sectional cleavage detection method was proposed base on cross-sectional microscopy method. The surface and subsurface damage characteristics and material removal mechanism were studied, and abrasive machining processes were investigated.Compared with the traditional cross-sectional sample preparation method, cross-sectional cleavage sample preparation method has a similar detection result but its sample preparation process is simpler. Impact factors are reduced as lapping and polishing did not performed on the cross-section. The detection results reflect the actual state of subsurface more directly.The indentation cracks formation and propagation on single crystal SiC wafer were investigated by Vickers indentation test, and the indentation formation process was analysed. The material deformation includes plastic deformation, crack propagation and micro crushing during loading and unloading of the indentor. Plastic indentation, lateral cracks and radial/median cracks were induced in single crystal SiC indentation. The brittle-ductile transition was found with the decreasing of the load of indentor.Base on the fracture mechanism of single crystal SiC, surface and subsurface damage characteristics and material removal mechanism were studied through observation of the surface and cross-section of slicing, lapping, grinding, polishing SiC wafer. The surface and subsurface damage induced during rough machined includes chipping layer and subsurface cracks layer. The subsurface cracks layer mainly includes lateral cracks and radial/median cracks.Different machining processes were used in the single crystal SiC wafer machining to study the ultra smooth surface formation process. In lapping, when the particle size was changed to 1.5μm, the surface roughness Ra was reduced to 24.0nm and the maximum subsurface crack was 1.2μm. Grinding has a higher efficiency than lapping, and plastic removal can be achieved by changing the process parameters. The maximum undeformed chip thickness was influenced by abrasive size, feedrate and wheel speed.With the decrease of the maximum undeformed chip thickness, the grinding surface roughness decreased and a trend of a brittle-ductile transition was occurred. While the maximum undeformed chip thickness is larger than the critical grit depth of cut, grinding was conducted within brittle fracture regime. On the contrary, the material removal mode was mainly plastic flow when the maximum undeformed chip thickness was less than the critical grit depth of cut. When grinding with 325# diamond wheel, plow scratches and chipping pits were found on the ground surface. The surface roughness Ra was 17.7nm and maximum subsurface crack depth was 5.8μm. Plastic removal was achieved while using 8000# diamond wheel. Plastic scratches were found on the surface. A smooth surface of roughness Ra 2.5nm without any subsurface cracks was obtained. In magnetorheological finishing, atomic scale removal was possible with diamond abrasive size of 0.5μm. The material removal was totally in plastic. A super smooth surface eventually obtained with a roughness of Ra 0.4nm without any subsurface crack.