Rational Design of Etchants for Electroless Porous Silicon Formation

摘要

From fundamental considerations of the surface chemistry of Si etching in fluoride solutions as well as pore formation and propagation, we develop a mechanistic understanding of what must be known for rational development of etchants: (I) an acidic fluoride solution because the presence of OIF promotes step flow etching, (2) sufficiently high fluoride concentration compared to the oxidant concentration, (3) the oxidant must be able to inject holes into the Si valance band at a sufficient rate, hence E° > ～0.7 V is required, (4) oxide formation needs to be slow or nonexistent so as to avoid the formation of a uniform oxide and associated electropolishing, (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.