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 a deviation of the resonance frequency response curve measured on a wafer with peripheral or bulk millimeter-length crack and 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 identically processed silicon wafers or cells with the same geometry lead to “false positive” events reducing the 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 the RUV technique was applied to a set of production-grade Cz-Si wafers and finished solar cells. Cracked solar cells rejected by the RUV 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. High accuracy and selectivity of the RUV method to identify mm-size cracks in wafers and cells was confirmed.