Directional Heating and Cooling for Controlled Spalling

The fabrication of thin solar cells by kerfless wafering techniques offers a high potential for the reduction of photovoltaic costs. We present an experimental setup for the exfoliation of thin crystalline silicon foils from a silicon substrate induced by the difference in thermal expansion coefficient of the silicon and an aluminum stressor layer at moderate temperatures. A moving temperature gradient across the substrate controls the crack propagation parallel to the silicon surface. We measure and simulate the spatial temperature distribution during thermal treatment and find that the direction of crack propagation is controlled by the temperature distribution. We detach foils with an area of 19.6 cm² with thickness values ranging from 50 to 80 μm within one layer. The foils have a smooth surface with some irregularities near the edge.