Effects of surface and sub-surface damage to wafer strength

Polycrystalline silicon (polysilicon) currently serves as the primary material for solar cells in the photovoltaic industry. The material is cast into large ingots, which are cut with a band saw into smaller ingots (i.e. bricks). The bricks are then sliced into wafers with a wire saw. Damage is introduced within the wafers during these processes. If sufficient, the damage can cause brittle failure during the sawing processes, or in handling during further processing downstream. In this study, the surface quality of eight polycrystalline silicon bricks cast at BP Solar and exhibiting a characteristic quality was investigated using contact profilometry. The bricks were then returned to BP Solar where they were sliced into wafers. Wafers from each of the eight bricks were collected and the surface topography of the wafer surface and wafer edges was examined using white light interferometry. In addition, mechanical properties of the wafers were determined using biaxial flexure and a novel edge flexure test. Fractography revealed that wafers failed from either surface flaws introduced during wire saw sectioning or edge flaws introduced during band saw sectioning. In edge flexure, the wafer strength ranged from 135 to 158 MPa. Fractography revealed that flaws from wafers tested in edge flexure ranged from 10 to 50 mum in depth. An inverse approach based on Linear Elastic Fracture Mechanics estimates the critical crack length to range from approximately 10 to 60 mum for typical edge strengths. The strongest correlations are between the peak to valley height (Ry) of transverse brick profiles and the wafer edge strength and the Weibull modulus (m) and effective stress concentration factors (K¯t) taken from longitudinal profiles. The wafer strength decreased beyond a R y of 15 mum and the Weibull modulus decreases with an increase in K¯t. Results from the investigation distinguished that brick quality should be quantified in terms of peak to valley roughness measured transverse to the brick.