ABSTRACT
The anodization of silicon at high potentials leads to the formation of porous silica. The process may be brought to a high level of control and understanding by using a dilute fluoride electrolyte of neutral pH and low buff er strength. In such conditions, the growth of a porous silica layer is triggered by sweeping the potential above a critical value. At fixed potential, the porous layer is continuously dissolved, and a constant anodic current is obtained, while the porous layer reaches a steady-state thickness, in the micrometre range. The obtained layers exhibit a porosity on the 10 nm scale. However, depending on the preparation conditions, macromorphologies may appear superimposed on the mesoporosity. These macromorphologies originate from an uneven dissolution of the mesoporous layer, a phenomenon which stems
from the presence of oxygen evolution occurring in parallel with silicon oxidation. A map of the diff erent macromorphologies as a function of the diff erent experimental parameters has been established. On the microscopic scale, the layer structure appears anisotropic. From a variety of experimental observations, including direct SEM observation after electrodepositing nickel metal into the porous structure, it appears that this anisotropy consists of a stratified structure, which is directly related to the oscillatory behaviour observed for silicon dissolution in fluoride electrolytes. It is inferred that the distinct morphologies of electrochemically formed porous silica, compared with porous alumina or porous titania, originate from this very peculiar feature of silicon electrochemistry.
