Fracture Strength of Polycrystalline Silicon Wafers for the Photovoltaic Industry

The fracture strength of polycrystalline silicon wafers has been investigated by means of twist and four-point bending tests. Under a twisting configuration, which generates high tensile stresses within the middle of the wafers, a unimodal distribution in strength is obtained. The characteristic strength and Weibull modulus are 131.0 MPa, and 14.4, respectively. Under a four-point bending configuration, which generates high stresses both at the surface and at the edges, an additive bimodal distribution is obtained. The first mode of the distribution has a characteristic strength of 76.0 MPa and a Weibull modulus of 1.6; the second mode has a characteristic strength of 161.2 MPa and a Weibull modulus of 11.5. Fractographic observations confirm that the first mode (lower strength) corresponds to wafers which failed from large edge chips (sizes up to 90 mu m). Weibull analysis suggests that the second mode (higher strength) corresponds to wafers which failed from smaller surface chips (sizes up to 50 mu m). The results obtained point to large edge chips as the most dangerous defects degrading the fracture strength of the wafers. This is of great relevance for the photovoltaic industry, as fracture of silicon wafers limits both the performance and lifetime of the solar cells, and production yields.