The mechanism of Si etching in fluoride solutions

Several mechanisms for (photo)electrochemical etching of Si in aqueous fluoride solutions have been proposed. These models are reviewed and quantitative aspects of the kinetics and energetics of these mechanisms are evaluated. In particular, the roles of HF, F−, HF2−, OH− and H2O in etching both in the dark and under illumination are considered. The presence of a hole in a bulk band (as opposed to a surface state/resonance associated with the Si–H bond or the Si–Si backbond) initiates (photo)electrochemical etching. The sticking coefficient of F−(aq) ions on H-terminated Si increases by 11 orders of magnitude in the presence of this hole. The sticking coefficients of HF(aq) and HF2− (aq) in the course of the rate determining step are also calculated. The possible involvement of abstraction reactions within the overall mechanism is discussed. A modified reaction mechanism, which is consistent with the kinetic and energetic parameters discussed here, is presented.