Passivation of laser-drilled via holes for emitter-wrap-through-cells

A high speed via-hole drilling laser process for emitter-wrap through cell production is investigated concerning the removal of induced crystal damage by a subsequent alkaline etching step. The etching time necessary for damage removal is identified. Further the passivating properties at the via-hole surface of two passivation layers are compared. The passivating effect of a PECVD-SiN layer deposited on both sides of the wafers is confronted to thermal oxide passivation by lifetime measurement. A theoretical frame work using the analogy of via-holes and dislocations is applied to extract values for the surface recombination velocity at the via-hole wall. Both analysis indicate that the passivating effect of the PECVD layer depends on the via hole radius whereas the passivation quality of the SiO2-layer saturates for an etching time of equal or greater 240 s. For the highest etching times surface recombination velocities of down to 60 cm/s are found for SiN.