Residual stresses in multi-crystalline silicon photovoltaic wafers due to casting and wire sawing

Abstract

A significant portion of the total manufacturing cost of crystalline silicon solar cells is attributed to the manufacturing and material costs of the silicon wafer. In addition to its high cost, silicon is very brittle, therefore wafers are prone to fracture during handling and processing. In this paper we investigate the manufacturing-induced residual stresses in photovoltaic silicon wafers due to casting and wire-sawing processes which affect the mechanical integrity of the wafers. Specifically, the paper addresses measurement of residual stresses in multi-crystalline silicon (mc-Si) wafers by photoelasticity and polarized micro-Raman spectroscopy methods, as well as the effects of diamond wire sawing and loose abrasive slurry sawing on the residual stresses within the grains and at the grain boundaries. The micro-Raman method probes the residual stresses in the near-surface of the wafer while the photoelasticity technique probes the through-thickness residual stress in the wafers. The results show that diamond wire sawing and loose abrasive slurry wire sawing produce compressive residual stresses in the as-cut mc-Si wafer surface. Loose abrasive slurry wire sawing produces larger compressive stresses in the as-cut surface layers compared to diamond wire sawing. Beneath the saw damage layer in the sawn wafers, low residual tensile stresses are present from the casting process.