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 decrease. 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 strongly influences the optical properties in this spectral range. Using an optical model that contains the plasma frequency of the free carriers, it is possible to interpret the spectra. It is shown that the method is suitable for the determination of the epitaxial layer thickness, the doping concentration of the substrate and the doping profile within the interface. Calculated and measured spectra of typical epitaxial layers are shown that illustrate the method. A comparison with data from spreading resistance measurements shows the advantage of ellipsometry as a non-destructive method for highly doped substrates.