Etching of silicon in fluoride solutions

The development and status of what is commonly called the Gerischer mechanism of silicon etching in fluoride solutions is reviewed. The two most widely used and studied wet etchants of silicon are F− and OH−. Their mechanisms of atom removal share many things in common; in particular, chemical passivation by a hydrogen-terminated surface plays an important role in both. Crucially, however, their initiation steps are different, and this leads to important differences in the structures of the materials produced by the etchants. The initiation of etching by F− is electrochemical in nature, responding to the electronic structure of the Si, and is, therefore, a self-limiting reaction that can produce nanocrystalline porous silicon. Hydroxide etching destroys porous silicon because its initiation step is a catalytic chemical reaction and not a self-limiting process. A number of unanswered questions regarding the dynamics of fluoride etching are highlighted.