The mechanisms of SiO2 etching by CFm- (m = 3-1) and NFn- (n = 2,1) are investigated by applying quantum chemical ab initio methods on local molecular surface models. From the calculated energetic data of intermediate hypercoordinated centers formed by adsorption of the negatively charged ions, the reaction behavior at the surface is estimated. The CF,- and NF,- ions act as blockers for Si centers, provide chemisorbed F- ions, or enable the Si-O bond cleavage. At higher temperatures (energy supply of about 37 kcal/mol), the blocking ions desorb and Si-O bonds break.