Mapping of a thick transparent layer

Thickness measurement of a layer is a frequent application of ellipsometry. To this end the observables Delta and Psi are simulated with the optical model of the sample. The model contains free parameters, i.e. layer thickness d, which is varied until the difference (mean square error) of measured and observed delta and psi is minimal. The optical model can be used to simulate delta and psi as a function of the thickness d. For a transparent layer (i.e. extinction k = 0, fig. 1) delta and psi are periodic. The period length depends on the angle of incidence and the wavelength. With a single pair of delta and psi the thickness can only be single correctly evaluated up to a multiple of the period length, which is typical 250 nm. In order to evaluate a unique thickness either an angle of incidence or a wavelength spectrum of delta or psi has to be fitted. The imaging ellipsometer EP3-SW can record a delta map of the sample surface at one wavelength instead of single point measurements of delta usually done by nonimaging ellipsometers.

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