Selective etching of silicon in aqueous ammonia solution

Abstract

Abstract The current-voltage ( I-V ) characteristics of silicon in aqueous ammonia have been studied. As the bias is swept to the cathodic direction and kept cathodic for a few minutes, clear passivation peaks in the I-V curves can be observed for both p- and n-type silicon. A linear current-voltage relation is observed for the n-Si/NH 4 OH contact. A rectifying current-voltage relation with large leakage current is found for the p-Si/NH 4 OH contact. These observations indicate that there is no potential barrier for the n-Si/NH 4 OH contact and only a very small potential barrier exists for the p-Si/NH 4 OH contact. This small potential barrier allows easy transfer of holes between silicon and aqueous ammonia and promotes the spontaneous formation of the oxide film. A systematic method to etch either the n- or the p-type region selectively on a silicon substrate has been proposed based on observed I-V curves and verified experimentally. Very good etching selectivity has been achieved in aqueous ammonia with a suitable applied external bias. If the external bias is between −0.85 and −0.6 V versus SCE, only the p-type region is etched. Smooth and hillock-free etched surfaces are achieved in this bias region. For a bias smaller than −1.4 V versus SCE, the etching rate for the n-type region is lower than that for the p-type region. A bias between −1.3 and −0.9 V versus SCE should be applied to the sample if a higher etching rate of the n-type region than that of the p-type region is required.