Etching mechanism of silicon nitride in HF-based solutions

A reaction mechanism for the etching of silicon nitride layers in aqueous h ydrofluoric acid solutions is proposed. The surface of Si3N4 consists of Si NH2 groups that are etched from the solid matrix via three possible routes. Depending on the pH, these SiNH2 groups are protonated (pK(a) = 1.4) to Si NH3+ At <pH 4, the rate-limiting step consists of an elimination of NH3 and a subsequent addition of F- or HF to the vacant surface site to form Si-F. At > pH 3, the elimination of NH; is assisted by HF,; followed by a transf er of one of the fluorides of HF2- to the vacant site. Ail subsequent react ion steps to remove the SiF unit are nucleophilic substitution reactions wi th low activation energies. The etch rates and mechanism of different types of silicon nitride films are compared with that of SiO2 etching. Therefore , etch selectivity between these two materials can be explained. The theory is also applicable for silicon hydrogen passivation. (C) 2001 The Electroc hemical Society.