Near Infrared Photoelasticity of Polycrystalline Silicon and it's Relation to In-Plane Residual Stresses
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The goal of this research was to investigate an experimental infrared transmission technique and associated analysis tools that extract the in-plane residual stresses in thin single and poly-crystalline silicon sheet, and try to relate the residual stresses to physical parameters associated with silicon growth and cell processing. Previous research has suggested this concept, but many engineering and analytical details had not been addressed. In this research, a system has been designed and built. A fringe multiplier was incorporated into the system to increase the sensitivity. The error was analyzed and the resolution of the system was found to be 1.2~MPa. To convert the experimental results to residual stresses, the stress-optic coefficients of (001), (011) and (111) silicon were analyzed analytically and calibrated using a four-point bending fixture. Anisotropy in (001) and (011) silicon was found to be 33%, and the coefficient of EFG silicon is 1.7 times larger than that of (001) silicon. The polariscope together with other techniques was applied to silicon wafers after various processing steps in the manufacture of photovoltaic cells. The influence of the processing on residual stress was investigated and positive correlations between residual stresses, PL and efficiency were obtained.