Effects of edge defects induced by multi-wire sawing on the wafer strength

The focus of the photovoltaic industry is a continuous reduction of the cost of solar energy. Lowering the wafer thickness during the processing by means of multi-wire slurry saw technology is one of the key issue to reduce these costs. However, reducing the wafer thickness without increasing the wafer strength leads to a larger breakage rate during the subsequent fabrication steps. Hence, recent studies have been carried out to enhance the sawing process by minimizing the sub-surface defects. Nevertheless, little efforts have been made to determine at which stage in the wafering process, the most dangerous defects are created for solar cells processing. Are they made during the shaping of the silicon bricks from cast ingots, or during the slicing operation into wafers, or else? State of the art consists in polishing the bricks prior to wafering by multi-wire slurry saw. The goal of this paper is to bring some insights on the importance of the edge defects on the wafer strength. Results using various methods such as roughness measurement, wafer strength measurement with the 4-lines bending tests and finite elements calculations are presented. The main conclusion of this study is that the defects made during the shaping of the bricks prior to wafering can be of high importance with respect to wafer strength.