Reflectance and transmittance of a slightly inhomogeneous thin film bounded by rough, unparallel interfaces

Inhomogeneity of a thin film, rough interfaces of the film with air and sub strate and non-parallelism of these interfaces severely affect reflectance and transmittance. Neglecting these film imperfections could give rise to e rroneous optical characterisation of the film from measured spectra. Each o ne of these imperfections, considered alone, has its own characteristic inf luence on the interference fringes of the reflectance and transmittance spe ctra. In this work, a model of an inhomogeneous thin film bounded by rough, unparallel interfaces is introduced for calculating reflectance and transm ittance at normal incidence. The effects on the spectra due to roughness as predicted by the proposed model are compared with those evaluated by an ap proach involving the effective-medium approximation (EMA). In this latter a pproach, a thin transition layer replaces the rough interface, but it prove s inadequate for reproducing the transmittance spectrum. Finally, the model is successfully applied to the characterisation of a LiF film thermally ev aporated on a glass substrate. (C) 2001 Elsevier Science B.V. All rights re served.