Low damage reactive ion etching for photovoltaic applications

Abstract

New concepts in silicon solar cell design require dry processing technologies. For this reason two reactive ion etching (RIE) processes have been developed: one for surface cleaning and one for the removal of phosphorous glass (PSG). However, damage is induced in silicon during reactive ion etching which deteriorates solar cell performance. Damage caused by SF6 RIE cleaning has been investigated by means of secondary ion mass spectroscopy, positron annihilation, and minority charge carrier lifetime measurements. Particles contained in the etch gas can be detected up to a depth of 50–80 nm in the silicon sample. A two layer model of vacancy distribution has been established: A layer of high vacancy concentration (1019 cm−3) up to a depth of 20 nm is followed by a second layer that extends over a depth of 1 μm with a vacancy concentration of 1016 cm−3. Effective minority charge carrier lifetimes decrease to about 10% of the lifetime of the wet etched control during RIE. If a heavily damaged layer of 20 nm is being removed by anodic oxidation, lifetimes return to the initial value. Under certain etching conditions it is possible to anneal plasma induced damage at 400 °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 etch gas, and high rf power or self-induced dc bias result in a significant decrease 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.