An achromatic four-mirror compensator for spectral ellipsometers

Abstract

Measurement and calculation results are presented that confirm that design four-mirror compensators can be designed for the spectral range of 200–2000 nm that is widely used in modern spectral ellipsometers. Measurements and calculations according to standard ellipsometric programs have been carried out on a broadband LED spectral ellipsometer with switching of orthogonal polarization states. Mirrors with the structure of glass substrate/Al₂O₃ layer (20–30 nm thick)/Al layer (150 nm thick)/upper Al₂O₃ layer (with specified thickness d) have been prepared by vacuum-evaporation method. It is shown that the phase-shift spectra of a four-mirror compensator, two mirrors of which have a native oxide 5.5 nm thick and the two others of which have an oxide layer 36 nm thick, measured on the ellipsometer, are flattened in comparison with similar spectra of a compensator, all four mirrors of which have a native oxide, especially in the short-wavelength spectral region. The results of calculating the phase-shift spectra of the four-mirror compensator with six variable parameters (angles of incidence of radiation on the mirrors and thicknesses of oxide layers on four mirrors) are presented. High-quality achromatization in a wide spectral range can be achieved for certain sets of parameters.