SUBJECT: M.S. Thesis Presentation
BY: Vanessa Pogue
TIME: Monday, March 28, 2016, 9:30 a.m.
PLACE: MARC Building, 114
TITLE: Measurement and Analysis of of Wire sawing Induced Residual Stress in Photovoltaic Silicon Wafers
COMMITTEE: Dr. Shreyes Melkote, Chair (ME)
Dr. Steven Danyluk (ME)
Dr. Ajeet Rohatgi (ECE)


Silicon (Si) wafers are used in over ninety percent of solar cells and are the most important material for solar cell production today. As Silicon is a very brittle material, breakage during processing is a significant issue leading to lower production yields and also contributes to a large proportion of the overall solar cell manufacturing cost. The manufacturing process of a 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.

Specifically, 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 in multicrystalline Silicon. 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.