Combined AFM and Brewster-angle analysis of gradually etched ultrathin SiO2 - Comparison with SRPES results

Two surface sensitive techniques are employed to assess both structural and optical properties of an inhomogeneous Si(1 1 1)/silicon oxide multilayer system. Upon gradual etch-back of the native silicon oxide layer, structural changes of the respective interfaces were determined by contact-mode atomic force microscopy (CM-AFM); optical data were obtained by Brewster-angle analysis (BAA) at a single wavelength. It is shown, that the sensitivity of BAA leads to the identification of an additional strained sub-surface layer that was investigated by subsequent etching experiments and following optical analysis. Inclusion of this layer and its interfaces into a multilayer model allowed precise numerical evaluation of the respective oxide thicknesses in the range between 12 Å and 2 Å. These values, obtained by combination of BAA and AFM, are in excellent agreement with results obtained by synchrotron radiation photoelectron spectroscopy (SRPES). It is furthermore shown, that the thickness resolution limit of BAA (at constant nanotopographic roughness) is well below 1 Å. A limitation of BAA single-wavelength analysis is reached when the roughness variation, in terms of an effective layer and its thickness, exceeds the oxide thickness variation.