A reaction mechanism for the etching of silicon nitride layers in aqueous hydrofluoric acid solutions is proposed. The surface of Si3N4 consists of SiNH2 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 SiNH3+ At pH 3, the elimination of NH; is assisted by HF,; followed by a transfer of one of the fluorides of HF2- to the vacant site. Ail subsequent reaction steps to remove the SiF unit are nucleophilic substitution reactions with 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 Electrochemical Society.