Silicon PV Wafers: Mechanical Strength and Correlations with Defects and Stress

Abstract

Nanoindentation was used to measure the mechanical properties of 200mm diameter (100) CZ Si wafers subjected to the initiation and propagation of micro-crack defects. Silicon amorphization and phase changes were observed and accompanied by a monotonic decrease in hardness and elastic modulus, as the nanoindent tip approached the micro-crack shank or point. Identification and profiling of these localized phase transitions was obtained in the vicinity of the micro-cracks using electron back-scattered diffraction (EBSD) and Raman spectroscopy. It was found that the amorphous Si regions extend for about 10 µm at the edges and ahead of a moving crack tip. Wafers from ingots grown at faster growth rates with enhanced thermal gradients and associated point defect/impurity produce large localized stresses in the wafer core, which are capable of changing the path of propagating cracks. FTIR and Raman spectroscopy analysis were used to quantify local stresses due to radial oxygen precipitate variations. The resulting stress modified crack deviates considerably from energetically favorable [110]/(111) directions, following a radial path suggesting a ductile fracture failure mode.