Characterisation of epitaxial layers on silicon by spectroscopic ellipsometry

The characterisation of epitaxial layers on silicon substrates is becoming more and more important, especially as the thicknesses of the layers decrea se. Spectroscopic ellipsometry in the mid- and far-infrared range is a non- destructive method suitable for the determination of the optical properties of the sample. The absorption of free carriers of the doped material stron gly influences the optical properties in this spectral range. Using an opti cal model that contains the plasma frequency of the free carriers, it is po ssible to interpret the spectra. It is shown that the method is suitable fo r the determination of the epitaxial layer thickness, the doping concentrat ion of the substrate and the doping profile within the interface. Calculate d and measured spectra of typical epitaxial layers are shown that illustrat e the method. A comparison with data from spreading resistance measurements shows the advantage of ellipsometry as a non-destructive method for highly doped substrates. (C) 2000 Elsevier Science S.A. All rights reserved.