Characterization of different porous silicon structures by spectroscopic ellipsometry

The results of multiparameter fitting of spectroscopic ellipsometric (SE) spectra on porous silicon layers (PSL) were connected with the processing parameters (oxidation, etching time, porosity, argon implantation dose). Two optically different types of silicon forms, a bulk-type silicon (c-Si) and polycrystalline-like silicon with enhanced absorption in the grain boundaries (p-Si) needed to be mixed with voids in the appropriate ratio, and the PSL had to be divided in depth in several different sections in order to obtain the best fit, The sectioning reflects the effect of upper and lower interfaces or inhomogeneity in depth. The effective porosity and the sublayer thicknesses are determined with high precision. In the case of argon implantation, we used the dielectric function of c-Si and implanted amorphous silicon (a-Si) mixed with voids. The regression analysis of SE spectra clearly shows the effect of different doses of implantation. The sectioning reflects that to the full range of argon ions the porous silicon became amorphous and denser. The overall thickness of the originally porous layer also significantly reduced (from 670 nm to 320 nm) due to the argon implantation.