Inhomogeneity in films: limitation of the accuracy of optical monitoring of thin films

Abstract

With present-day refinements, thin film multilayers can be designed theoretically to meet virtually any reasonable filtering requirements. Often, when the optical properties are specified over very wide spectral regions the thicknesses of the various layers are not related in any simple way. The manufacture of such multilayers presents many difficulties. The tolerances on layer thickness and refractive indices in some designs are often very narrow. We have developed an optical method for the accurate control of layer thickness that involves the measurement of transmittance over a wide spectral region (400-1000 mm). This measurement is performed continuously during deposition by a rapid scanning monochromator. The accuracy of such a system depends on a precise knowledge of the indices of refraction that are produced during the multilayer deposition. In addition the structure of many optical thin films used for hard coatings departs considerably from the simple method that is traditionally used in optical coating designs. In the method we have developed to compensate for such discrepancies, optical inhomogeneity is included by assuming a linear refractive-index profile, determined by analyzing experimental results. These results are in agreement with other studies of structure.