Scattering from multilayer coatings: a linear systems model

It is finally being recognized that residual surface roughness over the entire range of relevant spattalfrequencies must be specified and controlled In many precision optical systems. This includes the 'mid" spatial frequency surface errors that span the gap between the traditional "figure" and "finish" errors. This is particularly true for enhanced reflectance multilayers if both high reflectance and high spatial resolution are desired. If we assume that the interfaces making up a multilayer coating are uncorrelated at high spatial frequencies (microroughness) and perfectly correlated at low and mid spatial frequencies, then the multilayer can be thought of as a surface power spectral density (PSD) filter function. Multilayer coatings thus behave as a low-pass spatial frequency filter acting upon the substrate PSD, with the exact location and shape of this cut-off being material and process dependent. This concept allows us to apply conventional linear systems techniques to the evaluation of image quality, and to the derivation of optical fabrication tolerances, for applications utilizing multilayer coatings.