New method for the geometrical and optical characterization of dielectric thin films with surface roughness

A new method for the geometrical and optical characterization of non-uniform thin isotropic dielectric films deposited onto a transparent substrate, has been developed. The algorithm is based on using the two envelopes of the optical reflection spectrum taken at normal incidence. The method allows determining the average thickness and the refractive index of the films with accuracies better than 1%. The formulation is not only valid in the case of wedge-shaped thin films, but also for films showing a significant surface roughness. In the latter, the amplitude of the surface roughness can be estimated. Amorphous As,,S,, thin films have been deposited by spin coating onto glass substrates, from a solution of the bulk material in n-propylamine. Indications of the surface roughness in these films were found from total reflectance measurements (specular + diffuse), by using a integrating sphere, and also from mechanical measurements, by using a profilometer. The latter technique provided a value for the average surface roughness of 20 +/- 4 nm, which is in excellent agreement with that optically determined value of 17.4 +/- 0.4 nm.