A novel optical technique for the estimation of porosity in porous siliconthin films

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.