Wire-sawing processes: parametrical study and modeling
Reducing the wafer breakage rate without changing the wafer thickness and sawing thinner wafers while maintaining constant breakage rate are two possibilities to decrease the costs of solar cells. They are similar in the sense that both require stronger wafers. To achieve this goal, it is important to gain insight into the wire-sawing process, its underlying defect creation mechanisms and the impact of sawing parameters on wafer strength. Consequently, a series of bricks were sawn with different slurry densities, wire tensions and feed rates, and the results were analyzed in terms of the wafer strength measured by bending tests. Roughness and wafer thickness were also measured. It is found that the strongest wafers were obtained by using a low abrasive volume fraction in the slurry, a low wire tension and a slow feed rate. From the analyses, we provide a qualitative interpretation of the effects of the processes at work in slurry-based wafering that explains, for instance, the wafer thickness and roughness variations. Based on physical arguments about the interaction between the wire, the silicon carbide particles and the silicon wafer, a semi-empirical model relating defect creation to the sawing parameters is developed. With this model, the wafer strength distribution can be predicted, thus simplifying optimization of the sawing process. (C) 2014 Elsevier B.V. All rights reserved.