Abstract

Investigation of scattering in optical coatings has been considerably extended in the last several years, and numerous factors (e.g., substrate and residual roughness, correlation and cut-off frequency, autocorrelation length and isotropy degree) have been studied in detail. Previous results have shown that residual roughness owing to TiO₂/SiO₂ and Ta₂O₅/SiO₂ materials produced by ion-assisted deposition and ion plaitng is negligible compared to the roughness produced by a 0.5 nm substrate defect reproduction. In these conditions, measurement of scattering before coating is sufficient to permit an accurate prediction of angular and total scattering. In calculation, interface roughnesses are identical and are perfectly correlated to the substrate surface. We show that prediction of scattering remains valid over a large spectral range, but the deposition conditions of materials must be carefully defined. For example, one can show how the correlation between interfaces is modified by using an oblique incidence deposition. Moreover, by determining the isotropy degree of scattering, we can point out the consequences of local defects in the multilayers, especially for large scattering angles. For substrate roughnesses less than 0.5 nm, structural information on the coatings can be deduced from the residual scattering and is useful for the study of optimal conditions of deposition by such techniques as ion-assisted deposition and ion plating.

© 1990 Optical Society of America