Enhanced etching rate of silicon in fluoride containing solutions at pH 6.4

The etching rate of silicon in fluoride-containing solutions was found to show a remarkable pH dependence, having a sharp peak at about pH 6.4. A very similar pH dependence was also observed for the anodic current of n-type silicon electrodes in the fluoride-containing solutions. The chemical reactions of silicon occurring in the fluoride-containing solutions were attributed to the oxidative breaking of the surface Si-Si bonds by HF2- ions on the sites where the fluorine atom is bonded. Under acidic conditions, the surface is very stable because it is terminated with hydrogen atoms. In solutions at pHs higher than 7, the reaction rate becomes low owing to the very low concentrations of HF2- ions in these solutions. These factors are concluded to be the reasons for the appearance of peaks at pH about 6.4 for the etching rate and the anodic current.