Refractive index and inhomogeneity of thin films

Abstract

The fact that the optical characteristics of thin-film materials are generally different from those of the same materials in bulk form is well known. The differences depend very much on the conditions in which the deposition has been carried out. A good understanding of these differences, their causes, and the influence of deposition parameters is vital if we are to be able to improve coating quality. We have developed two complementary methods with the objective of deriving information on the index of refraction and its variation throughout the thickness of the film. Perceptible optical inhomogeneity is normally present and appreciable inhomogeneity is frequently present in thin films. Such inhomogeneity is usually associated with layer microstructure. The first is a postdeposition technique that makes use of measurements in air of the transmittance and reflectance of the layer under study over a wide wavelength region. The second, in contrast, makes use of in situ measurements, that is measurements made under vacuum and during the actual deposition of the layer. We shall show with the help of several examples that the two methods lead to results that are consistent and demonstrate the existence in deposited materials of an inherent variation of the index of refraction normal to the surface. The thermal sensitivity of the layer properties and their tendency to adsorb atmospheric moisture must be taken into account before the residual differences between the two techniques can be explained.