Ultrathin porous silicon layers have been stain-etched upon multicrystalline silicon (multi-Si) substrates. We studied optical and structural properties of porous silicon by photoluminescence, photo-luminescence excitation, reflection, atomic force microscopy and scanning tunnel microscopy methods. It was observed that the thickness of porous silicon did not exceed 20 nm. The photoluminescence method has shown that photoluminescence spectra of porous silicon of different grains have shown that they differ insignificantly (similar to 10%) in intensity. It was found that por-Si layers with optimal antireflection characteristics was obtained during etching time 7 min. In the paper the comparison of the reflection characteristics of investigated samples por-Si with industrial antireflection coating is presented. (C) 2005 Springer Science + Business Media, Inc.