Thermal curing of crystallographic defects on a slim-cut silicon foil

SLIM-Cut method [1] addresses one of the most important challenges of crystalline-Si for photovoltaics: kerf-free watering of substrates as thin as 50 microns. The SLIM-Cut technology is fully based on mechanical stress and it is compatible with low-cost fabrication methods: a stress field is applied to a silicon wafer so that a crack propagates in the silicon substrate parallel to the surface at a given depth. The top silicon layer is separated from the parent substrate and processed into a solar cell. Nevertheless, the quality of the material must be assessed to ensure that this innovative watering technique does not jeopardize the potential energy conversion efficiency of the final solar cell. In order to execute this assessment, defect etches (Wright etchant) were performed on the original wafer and on the lifted-off sample to deduce the potential increase on the defect density. Among several possibilities, such as initiate the crack or changing lift-off conditions, we will focus on using an annealing step to decrease the defect density. Thus, we performed a curing step with LPCVD in H2 atmosphere. Furthermore, experimental work indicated, and in agreement with literature [5], <;111> orientation performs better relatively to the lift-off as it requires less energy to propagate a crack. Based on this consideration, <;111> wafers are investigated. Experimental analysis shows a reduction of the defect density after curing. This is an encouraging observation that tends to prove the preservation, or at least the recovery, of the crystal quality after the SLIM-Cut process.