Saw Damage Gettering for Improved Multicrystalline Silicon

In this work we describe a technique, namely Saw Damage Gettering (SDG),which improves the lifetime of highly contaminated multi-crystal silicon wafers. The wafers to which it is applied are from the top and bottom of ingots, the so called “red zones” of ingots. Such wafers are currently discarded before processing due to high impurity levels which result in low efficiency cells. SDG entails an extra annealing step before the saw damage is removed by etching. The effect of the anneal is to dissolve metallic precipitates present in the material after casting. On subsequent cooling precipitates are preferentially nucleated in the high defect density regions associated with saw damage at the surfaces of the wafer so leaving the concentration of impurities in the bulk of the material significantly reduced. The anneal conditions, typically ∼850oC for ∼20 minutes, are chosen so that the majority of metal precipitates dissolve. The cooling step, taking ∼ 30 minutes, is such that the impurities have sufficient opportunity to diffuse from the center of the wafer to the damaged regions at the edges. This processing is found to improve carrier lifetimes by up to a factor of 4. This enhancement may be sufficient to enable red-zone wafers to be processed in the same manner as other, higher quality, mc-Si wafers without sacrificing cell efficiency. SDG is expected to be cost effective and easily incorporated into current cell processing procedures.