In-Plane Residual Stress and its Relationship to Dislocation Density in Polycrystalline (EFG) Silicon Sheet

Ribbon produced by the Edge-defined Film-fed Growth (EFG) technique now supplies about 4% of the polycrystalline silicon wafers used to manufacture solar cells for photovoltaic products. A critical gap exists in knowledge of thermal stress effects which lead to residual stress and dislocation generation in wafer material. This knowledge is needed to apply stress models to improve the electronic quality of wafers made from EFG crystalline silicon sheet, enable production of thinner wafers, and to raise productivity of EFG crystal growth furnaces by lifting limitations on growth rate. We present new data for EFG ribbon in this paper which identifies a key relationship between residual stress and dislocation density. In order to get this data, we measured and correlated point-by-point residual stress and dislocation density in the same wafer. The residual stress was obtained with a resolution of 150 mum using an infrared photoelastic techniques [1,2,3], and dislocation densities were mapped at corresponding locations in the sample using etch pit demarcation and an automated PVSCAN approach. We will discuss how the new data can aid in modeling of stress in the crystal growth process and help in predicting and understanding the effects of thermal stress on wafer properties during crystal growth