Theoretical studies were conducted for thickness measurements using transparent substrates on the external and internal reflection configurations. For three-phase systems consisting of ambient, film, and substrate, the refractive index of the substrate could be optimized to obtain the high sensitivity of an ellipsometric quantity Δ to the film thickness and the small susceptibility of Δ to errors in the incident angle. It was shown that the combination of an ordinary glass substrate and an additional dielectric layer with an appropriate layer thickness works as a synthetic high-index single substrate (SHIS). The optical effect of the combination was approximately described by use of the effective refractive index of SHIS. A method to select the refractive index of the additional layer was also given.
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