Resonance ultrasonic vibrations (RUV) for crack detection in silicon wafers for solar cells

The photovoltaic industry provides a pathway to allow renewable energy to meet world wide consumer energy needs. Past and present research and development on silicon based solar cells have helped make them the dominant player in the photovoltaic industry accounting for over 75% in 2005 as accounted by the US Department of Energy. One of the current technological problems is to identify and eliminate sources of mechanical defects such as thermo-elastic stress and cracks leading to the loss of wafer integrity and ultimately breakage of as-grown and processed Si wafers and cells.The RUV method, developed at the University of South Florida, enables fast and accurate crack detection with simple criteria for wafer rejection from solar cell production lines. The RUV system relies on variation of modal vibration characteristics due to physical variations in the wafers caused by cracks. Ultrasonic vibrations are introduced into the wafer using a vacuum coupled transducer and received by an acoustic probe mounted along the periphery of the wafer. Cracks are detected by monitoring shifts in the resonance peak\u27s frequency, bandwidth, and amplitude. In Cz-Si wafers it has been shown that increased crack length leads to a decrease in peak frequency and an increase in peak bandwidth and decreasing peak amplitude. Minimum crack length sensitivity is related to the uniformity of the RUV parameters from wafer to wafer within a batch as well as system characteristics. Typically the RUV system is capable of detecting sub-millimeter length cracks. The use of auto loading and unloading allows the RUV system to achieve mass production level speeds of approximately two seconds per wafer. The RUV system has been successful in detecting cracks in single crystalline and multi-crystalline silicon wafers. Further development of the RUV system would solidify its place in manufacturing plants for non-destructive crack detection in PV cells.The contributing work of the author toward the further development of the RUV crack detection method will be examined in this thesis