Electropolishing Silicon in Hydrofluoric Acid Solutions

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.