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.