Understanding the pH dependence of silicon etching: the importance of dissolved oxygen in buffered HF etchants

Dissolved O2 is found to have a profound effect on the morphology of Si(1 1 1) surfaces etched in buffered hydrofluoric acid (BHF). Atomically smooth surfaces were exposed to BHF containing varying amounts of dissolved O2 and the resulting changes in etch morphology were measured with scanning tunneling microscopy. These morphological changes were correlated with spectral changes in the infrared absorption spectrum. Surfaces etched in BHF containing dissolved O2 are much rougher than those etched in O2-free BHF. This effect is explained by kinetic competition between O2-induced and etchant-induced oxidation reactions which are followed by rapid etching of the oxidized species. Dissolved O2 alters the net site-specific reactivity of low pH BHF and leads to the production of rougher surfaces. The previously observed pH dependence of Si(1 1 1) etch morphology is also explained by this kinetic competition.