Low damage reactive ion etching for photovoltaic applications

New concepts in silicon solar cell design require dry processing technologi es. For this reason two reactive ion etching (RIE) processes have been deve loped: one for surface cleaning and one for the removal of phosphorous glas s (PSG). However, damage is induced in silicon during reactive ion etching which deteriorates solar cell performance. Damage caused by SF6 RIE cleanin g has been investigated by means of secondary ion mass spectroscopy, positr on annihilation, and minority charge carrier lifetime measurements. Particl es contained in the etch gas can be detected up to a depth of 50-80 nm in t he silicon sample. A two layer model of vacancy distribution has been estab lished: A layer of high vacancy concentration (10(19) cm(-3)) up to a depth of 20 nm is followed by a second layer that extends over a depth of 1 mu m with a vacancy concentration of 10(16) cm(-3) Effective minority charge ca rrier lifetimes decrease to about 10% of the lifetime of the wet etched con trol during RIE. If a heavily damaged layer of 20 nm is being removed by an odic oxidation, lifetimes return to the initial value. Under certain etchin g conditions it is possible to anneal plasma induced damage at 400 degrees C. The influence of RIE induced damage on solar cells is quantified by open circuit voltage analysis: Long process times, addition of oxygen to the et ch gas, and high rf power or self-induced de bias result in a significant d ecrease in open circuit voltage. Nearly damage free RIE processes have been developed for surface cleaning as well as PSG removal. Dry processed solar cells thus show the same performance as wet etched cells. (C) 1999 America n Vacuum Society. [S0734-2101(99)00803-9].