Silicon is electropolished in hydrofluoric acid solutions if a critical current density is exceeded. Below the critical c.d., silicon dissolution is largely divalent, and a thick solid layer forms. This film is unstable and reacts slowly with the electrolyte to form tetravalent silicon and hydrogen gas. In the electro‐polishing region, silicon dissolution is mainly tetravalent with the formation of a very thin high resistance type of film.Experimental results on the effect of HF concentration, viscosity, and temperature indicate that electropolishing begins when the HF concentration at the silicon becomes limited by the rate of “mass transfer” of HF from the solution bulk to the surface.

The anodic dissolution rate of silicon is investigated as a function of the electrode potential, the doping of the crystal and of the hydrofluoric acid concentration. It is shown that at lower anodic electrode potentials two holes and at higher electrode potentials four holes are consumed. For the rate determining step which depends exponentially on the electrode potential in concentrated HF-solutions, however, only one hole is required. A dissolution mechanism is proposed partly based on the experimental results obtained by Turner.

Si wafers with (100) or (111) oriented surfaces were treated in hydrofluoric acid (40% HF, 1 min) and then water rinsed for different times from 10 s to more than 50 h. Oxygen uptake and oxide formation were investigated by x‐ray photoelectron spectroscopy and high‐resolution electron energy‐loss spectroscopy. The initial state after the HF dip is characterized by a coverage with Si–hydride and small amounts of oxygen and fluorine. The interaction with the liquid phase of water was investigated up to the monolayer range. It shows distinct features: The first step is a rapid exchange of Si–F with H2 O to form Si–OH groups followed by a slow nucleophilic attack of OH? on surface Si–H to produce Si–OH. Growth law is logarithmic and extends to 3–5 h of water contact. The surface Si–OH act as nuclei for the attack of water on the polarized Si–SiOH backbonds. Interior Si–H and Si–OH groups develop. Further attack of OH? on interior Si–H yields Si–OH. Condensation of Si–OH forms Si–O–Si bridges and SiO2?x nuclei appear. Strain and altered surface topography lead to a changed rate of the logarithmic oxide growth. The oxide formation is accompanied by a slight corrosive attack of H2 O, leading to roughening of the surface.