Etching kinetics of silicon dioxide in aqueous fluoride Solutions: A surface complexation model

A molecular-level model of SiO₂ dissolution in aqueous fluoride solutions is presented. The model recognizes the fact that the SiO₂/water interface contains neutral hydroxylated surface groups (Si-OH) which can protonate to give positively charged sites (Si-OH⁺₂) or deprotonate to give negatively charged sites (Si-O^-), depending on the pH. The adsorption of the fluoride ion, which is formulated as a surface ligand exchange reaction (Si-OH + HF = Si-F + H₂O), results in the polarization of the underlying Si-O bonds. The subsequent detachment of the surface Si-F complex constitutes the effective dissolution event. It is shown that the effects of aqueous phase variables (e.g., pH) on the dissolution rate are correlated with the effects of these same variables on the surface concentration of adsorbed fluoride. The model is compared with published experimental results, and it is demonstrated that a unified theory is obtained when the role of surface complexation is incorporated into the dissolution mechanism. The frequently reported observation of a decline in etching rate at elevated NH₄F concentrations (and therefore high pH) is attributed to a competition between OH^- ions and F^- ions for adsorption sites.