X-ray scattering study of interface roughness correlation in Mo/Si and Ti/C multilayers for X-UV optics

The X-ray reflectivity and interface diffuse scattering at grazing incidence were measured on two couples of multilayers, namely on Mo/Si multilayers (50 periods) prepared by e-beam evaporation and sputtering techniques and on e-beam evaporated Ti/C multilayers (87 periods) prepared with and without Ar+ ion-beam polishing after each layer deposition. The results were evaluated using Fresnel's optical algorithm and a semikinematical modification of the distorted-wave Born approximation to extract and compare the basic interface parameters within each couple. For both Mo/Si multilayers, a frequency-dependent vertical correlation function of the interface roughness corresponding to a model of kinetic roughening was applicable, the vertical correlation length (at a given frequency) being more than an order of magnitude shorter for the sputtered sample. This effect may be explained by high lateral mobility of sputtered adatoms when the Ar plasma pressure is below the thermalization threshold. The main effect of polishing the Ti/C multilayer is a decrease of both the lateral and vertical correlation lengths by about an order of magnitude which may be ascribed to an interfacial reaction at the underlying interface induced by penetrating Ar+ ions. Different lateral and vertical correlations of the interface profiles evidenced within each couple of multilayers affect the distribution of the interface diffuse scattering intensity in the reciprocal space which has some implications for X-UV optics applications. (C) 1998 Elsevier Science B.V. All rights reserved.