Scratch Fracture of Polycrystalline Silicon Wafers

Abstract

Fracture of silicon wafers is responsible for lower than desirable manufacturing yields in the photovoltaic industry. This study investigates the fracture response of polycrystalline silicon wafers under sliding contacts at different length scales, by means of macro and microscratch tests which simulate cutting processes. The dominant fracture modes were found to be partial cone cracking (macro) and radial cracking (micro). Statistical analysis of the critical loads for crack initiation showed that polycrystalline wafers are weaker than their single-crystal counterparts, that is, they crack at lower applied loads under comparable conditions. Moreover, the Weibull modulus of polycrystalline silicon was found to be the average of the relevant single-crystal directions. Subsequent microscopic observations and flexure tests reveal that the lower resistance of polycrystalline silicon to scratch fracture is due mainly to the presence of relatively large polishing defects, and not to the weakness of its grain boundaries. Alternatives are proposed to minimize damage during ingot cutting, with a view to minimizing wafer breakages during wafer handling and machining.