Optimization of the spectral performance of multilayer coatings on complex optics in plasma assisted deposition processes

Abstract

The performance of optical components is usually improved by optical coatings. Some of these optical components exhibit complex geometrical shapes and are therefore very difficult to coat in a homogeneous way. The spectral performance of the optical coatings on such substrates will vary as a function of its geometry making it very difficult to keep the spectral performance within customer specifications all over the substrate. Examples for optics with complex geometries are half sphere lenses, freeform surfaces, diffraction gratings, microlense arrays, large substrates etc. We developed a simulation tool that can calculate and optimize the spectral performance of a given multilayer stack on arbitrarily shaped optics as a function of the processing parameters of the coating plant. This tool will obviously reduce the risk and the development costs. The spectral performance of a multilayer stack is given in general by the coating design, that means by the individual layer thicknesses and the refractive indices of the different layer materials. On curved optics different coating materials exhibit different thickness and refractive index distributions. Consequently the optical layer stack will exhibit varying spectral performance at different positions on the substrate. Empirical models for thickness and refractive index distributions have been developed as a function of the most important processing parameters (e.g., deposition rate, deposition angle, ion impingement rate and temperature).