Measurement and analysis of wire sawing induced residual stress in photovoltaic silicon wafers

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Pogue, Vanessa Ann
Melkote, Shreyes N.
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The manufacturing process of a photovoltaic Si wafer comprises of first a high temperature heating process to produce a Si ingot from polycrystalline Silicon, which is then cut into bricks and subsequently sawn into wafers using a wire saw. These processes create residual stresses both from the thermal gradient induced by solidification and from either the rolling-indenting or scratching-indenting processes caused by the type of wire saw used. The objective of this research is to study silicon wafer residual stress as a result of the typical industry manufacturing processes and by doing so, better understand the mechanical properties that lead to increased fracture. This thesis aims to quantify the amount of residual stress generated by the solidification/thermal gradient produced during the casting of Si ingots separately from the residual stress generated by the wire sawing process. Samples from industry are used to compare the effects of the manufacturing processes on residual stress in multi-crystalline silicon (mc-Si) wafers including the effects of fixed abrasive diamond wire sawing (DWS) vs. loose abrasive (LAWS) slurry wire sawing used in the wafering process. Near-infrared birefringence polariscopy and polarized micro-Raman spectroscopy are used to study wafer residual stresses within grains and at grain boundaries in mc-Si as a function of etch-depth. While near-infrared birefringence polariscopy allows for the measurement of full-field maximum shear stress, micro-Raman spectroscopy provides decomposition of the stress tensor into both principal and shear in-plane stress components. Consequently, regions of high tensile stress, which are detrimental to the mechanical integrity of the wafer, can be easily identified. In addition to the mechanical characterization, the residual stress produced by the thermal gradient/solidification process for multi-crystalline Si wafers was also correlated to electrical performance of mc-Si wafers using photoluminescence.
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