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Screening System Detects Wafer Defects

Scientists at the U.S. Department of Energy's National Renewable Energy Laboratory (NREL) have developed an instrument that puts pressure on wafers to find which ones are too fragile to make it through the manufacturing process. The device then kicks out weak wafers before they go through their costly enhancement. NREL's Silicon Photovoltaic Wafer Screening System (SPWSS), a cube-shaped furnace, can be retrofitted into an assembly line.

The screening system, developed by NREL scientist Bhushan Sopori with colleagues Prakash Basnyat and Peter Rupnowski, exposes a silicon wafer to thermal stress in the form of carefully calibrated high temperatures. As each wafer passes through a narrow — 15-millimeter — high-intensity illumination zone, different strips of the wafer are exposed to the heat.

The micro-cracks or breaks that occasionally develop from the thermal stress mirror the stress that will happen to weak wafers as they go through the assembly process. The difference is that the thermal testing happens first, before the expensive coatings and layers are added to the wafers.

The tool tests a pre-selection of wafers for high fracture strength, improving the yield of silicon solar cells by preventing breakage during cell fabrication. The manual version of SPWSS screens 1,200 wafers an hour.

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Also: Learn about inverted three-junction tandem thermophotovoltaic modules.