ABSTRACT
Stain etching of silicon is a particular variant of electroless etching in which an oxidant is added to an acidic fluoride solution. Stain etching results in the formation of a porous silicon layer, which can be nanocrystalline and exhibit visible photoluminescence. In this review, the surface chemistry of silicon etching in acidic fluoride solutions is discussed as is pore formation and propagation. From these general discussions, we develop an understanding of the mechanism of the action of stain etchants and what we need to know for their rational development, including the following requirements: (1) an acidic fluoride solution, (2) sufficiently high fluoride concentration compared with the oxidant concentration, (3) the oxidant must be able to inject holes into the Si valance band with 0.7 VE (4) oxide formation needs to be slow or nonexistent,
(5) reduction of the oxidant must lead to soluble products, (6) film homogeneity is enhanced if the oxidant’s half-reaction does not evolve gas, and finally (7) the net etching reaction has to be sufficiently anisotropic to support pore nucleation and propagation. On this basis, four new formulations of stain etchants containing Mn O4-, Fe3+, V O 2+, and Ce4+ are described here. These new formulations can get around many of the problems associated with common nitrate/nitrite based stain etchants. The new formulations need no “activation,” exhibit short induction times, produce homogeneous films and are quite reproducible.
