Physical chemistry of wet chemical anisotropic etching of silicon

In this paper we explain a view to understand the anisotropy of the etching of silicon in certain wet chemical agents (such as KOH). The starting point is the assumption that the [Left angle bracket]111[Right Angle Bracket] face of silicon is a flat face, the etch rate of which is then governed by a nucleation barrier. We review the atomic surface structure of the [Left angle bracket]001[Right Angle Bracket], [Left angle bracket]110[Right Angle Bracket] and [Left angle bracket]111[Right Angle Bracket] face, with the result that the [Left angle bracket]111[Right Angle Bracket] face is the only flat face. We discuss the consequence of the assumption of steps on the orientation dependence of the etch rate, and we show that the observed experimental facts perfectly fit to this idea. We also review recent in situ STM images of slightly misaligned [Left angle bracket]111[Right Angle Bracket] n-silicon that show moving steps, and an increasing number of etchpits as a function of the voltage difference between silicon and an NaOH solution, reaching a maximum close to the passivation voltage. This observation is discussed in terms of the driving force for etching and the step free energy. Finally, the order of magnitude of the step free energy is discussed.