Pinhole Detection using RUV System

Ultrasonic Technologies developed a proprietary method to identify silicon wafers and solar cells with small, sub-millimeter “Pinhole” defects and cracks. “Pinhole” represents a seed point anomaly which dramatically reduces wafer and cell strength and ultimately leads to breakage and yield reduction. UST researched and developed an Activating Station add-on which is once integrated with our RUV system allows successful detection of “Pinhole” cracks.

Ultrasonic Technologies has a proven record detecting small to medium size (3 to 20 mm) cracks and defects in solar cells and wafers. It was recently indicated by our customers that other mechanical problem posses a high probability of wafer/cell breakage in production. A small, sub-millimeter in diameter “pinhole” represents a seed point defect which dramatically reduces the cell strength and ultimately leads to breakage and yield reduction. Illustrative example of this defect in production grade silicon cell is presented here. A 3.6 x 3.6 mm map of a silicon wafer using high-resolution Scanning Acoustic Microscope (SAM) with sub-millimeter pinhole. Note, that unopened cross cracks provide directions when the cell is broken.

To address this production problem, Ultrasonic Technologies developed a proprietary protocol to identify the wafers and cells with pinholes quoted here as an “Activating Station” (AS).

Software modification: considering that pinholes can be found with high repeatability in a high-volume crack testing, and potentially may be a long sequence of damaged cells, AS will use a new concept of “Critical Bandwidth” (BWcr) meaning that the cells showing BW value above certain threshold, BWcr will be rejected. The software automatically select BWcr based on measured good cells. Multiple SAM imaging confirmed that cells with BW > BWcr exhibit cracks. Following tests illustrate the concept. This rejection concept can operate in parallel with the standard RUV statistical algorithm.

Activating Station concept: prior to RUV testing the cell is passing through the AS with settings defined by the operator. After activation the cell is transferred to the RUV system and analyzed on BWcr as well as using a standard RUV statistical algorithm. Some heavily damaged cells are broken during the activation step and eliminated by the hardware. Our tests show that pinholes located at the central area of the cell are covered with 100% accuracy.

This delivers a turn-key quality control solution to solar cell and module manufacturers.

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Source: Ultrasonic Technologies

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