Complete understanding of ellipsometric data needs the use of a model of either the sample structure and the materials to be characterized within the layers, including the delicate step of choosing a good dispersion function to model these materials. Particular problems appear when modelling materials with large absorption bands where classical Lorentzian oscillators cannot fit experimental data. This is for example the case of materials which present transitions with large inhomogeneous broadening in the visible and infrared spectral range. Oscillators in the collection of nominally identical atoms may then have slightly different resonance frequency because, for example, of local surroundings local crystal structures, because of defects, dislocations or lattice impurities. We report on the use of a dispersion function to model materials with such non Lorentzian bands. This dispersion function is the correlation of a Lorentzian oscillator by a Gaussian distribution, also known as the Voigt function. For the same area Gaussian have lower peaks, larger FWHM but have a faster extinction and so a smaller background. The use of Voigt profiles allows to introduce alternate variations between Lorentzian (strongly homogeneous limit) and Gaussian (strongly inhomogeneous limit). We propose the computation of this function using the complex scaled error function also known as the Faddeeva function. It is shown that the use of such oscillators greatly helps in the fitting procedure by reducing the number of fitting parameters. This is ilustrated by different examples concerning microelectronics materials in thin layers such as H-doped Silicon nitride (SiN), Polybenzoxazole (PBO) polymer dielectrics (PBO) or Fluorimated silicate glass FSG which belongs to the low k dielelectrics family. We also propose an alternate formulation of this oscillator when the broadening is not symmetric with frequency.

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1. Molecular fractal surfaces analysis with spectroscopic ellipsometry