A Novel Approach Of High Speed Scratching At Nanoscale Depths Of Cut By Diamond Tips With Submicron Radius

Solar cell could transform solar energy to electrical energy which can be used in our daily life conveniently. In the past years, solar photovoltaic industry developed with a high speed because solar energy is abundant and have a good application prospect. Due to its high energy conversion efficiency, silicon-based solar cell occupies large proportion in solar photovoltaic industry.Silicon wafer should undergo multi-wire sawing in solar cell fabrication process. Material removal is finished by numerous grits in simultaneously and damaged layer will generate in the subsurface. In order to improve the utilization of silicon and decrease the thickness of damaged layer, it is necessary to study the mechanisms of material removal and damaged layer formation. Single-point diamond tip scratching is the fundamental to investigate the mechanisms. In recent, there are there scratching methods:nanoindentation scratching, atomic force micrograph scratching and precision motion stage scratching. In the first two methods, the scratching speed are at μm/s level which is six magnitude lower than that of multi-wire sawing. Despite of the speed of last scratching method is at m/s level, its radius of curvature is several to tens of micron which is not consistent with that of grits. In conclusion, the above methods can not simulate the actual cutting process to properly provide experimental evidence of material removal and damaged layer formation.Taking care of the disadvantages of the above methods, we firstly fabricated diamond tips with submicron curvature radius. Nanoscratching experiments were operated using developed diamond tips in an ultra-precise silicon wafer grinder under different conditions. Successively, cross-section of the onset of debris and crack were prepared through focused ion beam etching technology to measure the width and depth using scanning electron micrograph. And then, Hertz contact theory, Oliver-Pharr mode and Jing X.N. mode were employed to calculate the critical messages, such as damaged layer size, cutting force and displacement of diamond tips, depending on the shape and curvature radii of diamond tips. Finally, cross-section samples of transmission micrograph were prepared using in situ focused ion beam etching to characterize its microstructure. There is perfect single crystal silicon under an amorphous layer at the onset of debris. This is different with the precious studies, in which it is damaged layer between amorphous layer and single crystal. This new phenomenon lay a foundation to develop novel ultra-precise machining method. At the onset of crack, there is an amorphous layer following by a damaged layer, this is consistent with traditional studies of scratching mechanisms and high speed grinding. High pressure was also not found in the subsurface through selected area electron diffraction and high resolution transmission electron micrograph.