Non-contact microcrack detection from as-cut wafer to finished solar

Decreasing wafer thickness is a clear trend in photovoltaic industry, leading to increasing problems with wafer breakage. At the same time, handling systems are sped up and forces due to higher acceleration are more and more increasing. These impacts are likely to cause microcracks in wafers or cells, and even the lowest stress leads to breakage during processing, thus decreasing yield. Nearly every processing step can generate defects in the silicon material; hence it is very important to be able to inspect the wafers or cells for microcracks after each handling/processing step. It is obvious that these tests shall occur without generating new defects in order to keep yield up - a non-contact method is essential. We use two new non-contacting optical systems to investigate the presence of cracks in wafers and solar cells. Both systems use infrared light: the GP NANO-D detects microcracks in wafers; the GP MICRO-D serves for detecting similar defects in processed cells. Together, the two tools enable crack detection after each processing step, from the as-cut wafer to the finished cell. To define the influence of microcracks on the mechanical stability of the sample we compare mechanical test results of samples with and without detected cracks, and find a strong dependence of breakage force on presence and position of the detected cracks.