Fracture Analysis and Distribution of Surface Cracks in Multicrystalline Silicon Wafers

Abstract

Solar silicon wafers are mainly produced through multiwire sawing. The sawing process induces micro cracks on the wafer surface, which are responsible for brittle fracture. Hence, it is important to scrutinize the crack geometries most commonly generated in silicon wafer sawing or handling process and link the surface crack to the fracture of wafers. The fracture of a large number of multicrystalline silicon wafers has been investigated by means of 4point bending and twisting tests and a failure probability function is presented. By neglecting the material property variation and assuming that one surface crack is dominating the wafer breakage, 3D finite element models with various crack sizes (depth, length, and orientation) have been analyzed to identify the distribution of surface crack geometries by fitting the failure probability from the experiments. With respect to the 63% probability, the existing surface cracks in the wafers studied appear to have depth and length ratios less than 0.042 and 0.19, respectively. Furthermore, it has been shown that the surface cracks with depth in the range from 10 to 20 خ¼m, length up to 10 mm and angles in the range of 30 deg–60 deg, can be considered as the most common crack geometries in wafers we tested. Finally, it has been found that the mechanical strength of the wafers tested parallel to the sawing direction is approximately 15 MPa smaller than those tested perpendicular to the sawing direction.