Porous silicon (por-Si) can be produced when silicon single crystals are su
bmerged in fluoride solutions and irradiated with laser light. The shape of
the por-Si film is determined by the laser beam intensity profile. When la
ser light is reflected from a Gaussian-shaped film, a divergent beam, which
exhibits ring patterns, is observed. The rings are formed by a combination
of optical interference and Fresnel diffraction. The size of the pattern i
s determined by the shape and depth of the film interfaces as well as the i
ndex of refraction of the film. The index of the film is determined by the
porosity and the index of the fluid that fills the pores. We explore the ap
plication of measurements of the reflected beam patterns to the determinati
on of porosity for per-Si thin films. We report the first direct estimation
of the porosity of photochemically produced porous silicon. Porosities of
70%-95% are found for n-type Si(111) etched in 48% HF with 633-nm illuminat
ion. Having demonstrated the success of this technique, we discuss improvem
ents and extensions that can be made. PACS: 78.66.Db; 68.55.-a; 78.55.Mb; 8
1.05.Rm; 81.65.Cf.