N. Ookubo et al., MICROSCOPE OBSERVATION OF A SELF-STANDING FILM OF POROUS SILICON, Materials science & engineering. B, Solid-state materials for advanced technology, 20(3), 1993, pp. 324-327
A self-standing film (SSF) of porous silicon electrochemically isolate
d from the substrate has been observed under optical and electron micr
oscopes. The SSF consists of two layers: the top layer, on the side fr
om which anodization starts, cracks into cells exhibiting a fine porou
s structure: the bottom layer, on the side of isolation, shows a unifo
rm and coarse porous structure. Under the optical microscope the film
is found to expand upon wetting with methanol and to shrink back when
it is dried. The change in the surface area amounts to about 10%, whil
e any change in the thickness is undetectable. This expansion-shrinkag
e process can be repeated, but the film is occasionally cracked when i
t shrinks.