Design of a submillimeter crack-detection tool for Si photovoltaic wafers using vicinal illumination and dark-field scattering

Microcracks in silicon solar cells reduce the mechanical strength of the wafer and cause breakage during manufacturing, transportation, and field operation. Therefore, there is a need to trace where microcracks initiate in the manufacturing line. As wafers become thinner, the critical crack length required for fracture significantly decreases for the same loading conditions. Currently, very few industry-standard tools can reliably detect submillimeter cracks, which will become more critical for thinner wafers. In this work, we demonstrate a technique to detect submillimeter cracks located at the edges of various multicrystalline silicon wafers and solar cells. The proposed technique, which is based on near-infrared dark-field imaging with vicinal laser illumination from the wafer edge, has several advantages over state-of-the-art optical transmission imaging and dark-field scattering techniques. Moreover, we adapt this technique to achieve the high-throughput requirement of inline metrology; hence, it can be used to detect submillimeter cracks in a manufacturing line. With a high-frame-rate line-scan camera, this proposed crack technique is designed to theoretically achieve a scanning throughput of less than 1 s per wafer.