For the development of thin, doped microcrystalline silicon (pc-Si) layers, it is necessary to have an accurate tool to determine the thickness and material properties of layers around 20 nm. Here, we report on the interpretation of UV-VIS-NIR spectroscopy (reflection/ transmission) measurements using the O'Leary, Johnson, Lim (OJL) model in which we add extra information to compensate for the loss of density information due to the lack of fringes. Moreover, using this method we extract information that can be correlated to the crystalline ratio of pc-Si:H thin layers. We correlate thicknesses and material properties obtained from the optical method to the results obtained from various other techniques: Raman spectroscopy, Rutherford back scattering (RBS) and cross.-sectional transmission electron microscopy (X-TEM). By analyzing the data of thin mu c-Si:H layers (similar to 20 nm) as well as of thicker layers (similar to 100 nm) and comparing the results to thicknesses measured with X-TEM, we conclude that as long as the density of thin layers is identical to the thicker layers, with the optical method a good approximation of thickness of microcrystalline silicon layers is possible at a layer thickness down to 20 ran. (c) 2004 Elsevier B.V. All rights reserved.