Modeling of Etching Reactions: Ab Initio Calculations of the Reactions of CFm+ (m = 1−3) and NFn+ (n = 1, 2) with Local Models of SiO2 Surface Structures

Abstract

Using ab initio calculations, reactions of CFm+ (m = 1−3) and NFn+ (n = 1, 2) with ≡SiOH groups and ≡SiOSi≡ bridges of SiO2 structures are investigated. The local surface structures are modeled by cutouts of the surface. Dangling bonds are saturated by hydrogen atoms. Reaction and activation enthalpies are estimated by applying the MP2/6-31G**//3-21G(*) procedure. According to the calculations, all the considered ions are able to form C−O and N−O bonds to ≡SiOH groups and ≡SiOSi≡ bridges (59−92 kcal/mol). NF+ and CF2+ have the strongest bonds with the O atoms. The existence of O−F, Si−C, and Si−F bonds could not be demonstrated. Starting from these structures the Si−O bond breaking is investigated. The direct dissociation of the Si−O bond neighboring the attacked O atom requires an energy of 30−45 kcal/mol. More favorable reactions go over cyclic transition structures with activation enthalpies in the range between 10 and 21 kcal/mol. The reaction enthalpies show that under low-temperature conditions the ...