Optical characterization of interface roughness of thin films on transparent substrates

Spectral diffuse reflectance and transmittance measurements have been performed on single films of tin oxide on Corning glass substrates. The measurements have been performed on a spectrophotometer equipped with an integrating sphere in the wavelength range 0.3 - 1.0 micrometers . The front surfaces of the films have been characterized by means of spectroscopic light scattering and optical profiler measurements after the deposition of a thin aluminum film. These measurements have given different results, which can be explained by the different bandwidths of the methods. The front surfaces have thus been found to have a short correlation length, indicating roughness with short spatial wavelengths. The front surface data obtained from the light scattering measurements have been used in a model to characterized both film interfaces. The calculated diffuse spectra agreed with experimental spectra both for reflectance and transmittance. The difference between the highest and the lowest scattering levels was more than an order of magnitude. The model calculations made it possible to establish an upper limit for the substrate-film interface.