RUV in Solar Si Wafers
The Resonance Ultrasonic Vibrations (RUV)technique was developed for in-line non-destructive crack detection in full-size silicon wafers and solar cells. The RUV methodology relies on deviation of the resonance frequency response curve measured on a wafer with peripheral or bulk millimeter-length crack and on identical non-cracked wafers. Three RUV frequency curve crack detection criteria were identified: (1) shift of the peak position; (2) increase of the bandwidth, and (3) reduction of the amplitude. It was observed that statistical variations of the RUV parameters on a similarly processed silicon wafers/cells with the same geometry lead to “false positive” events reducing accuracy of the RUV method. We proposed a simple statistical approach using three independent RUV crack detection criteria to resolve this issue and demonstrated its validity experimentally. Crack detection using RUV technique was applied to a set of production-grade Cz-Si wafers and finished solar cells from the Isofoton’s production line. Cracked solar cells rejected by the RUV method using the statistical approach were imaged with Scanning Acoustic Microscopy (SAM)and room-temperature photoluminescence (PL) mapping. A comparison of three independent techniques for crack detection, RUV, SAM and PL, was performed on selected samples. A high accuracy and selectivity of the RUV method to identify mm-size cracks in wafers and cells was confirmed.