Effect of ion-beam polishing on the interface quality in a Ti/C multilayer mirror for `water window'

The effect of Ar+ ion-beam polishing after each layer deposition on e-beam evaporated Ti/C multilayer (87 periods) with the nominal period of 2.8 nm for imaging in the soft X-ray `water window' region was studied by the hard X-ray specular reflectivity and interface diffuse scattering at grazing incidence. The results were evaluated using Fresnel optical algorithm and a semikinematical modification of the distorted-wave Born approximation to extract and compare the basic interface parameters. In addition to an extremely low interface roughness of 0.1 nm, the main effect of polishing is a decrease of both the lateral and vertical interface correlation lengths by about one order of magnitude which may be ascribed to an interfacial reaction at the underlying interface induced by penetrating Ar+ ions. Consequently, a lower diffusely scattered intensity around the multilayer Bragg points in the reciprocal space occurs and if it falls within the field of view, an increased contrast for imaging with such multilayer mirror is expected. In addition to an enhanced specular reflectivity, this is the second benefit of ion-beam polishing which renders the multilayer prepared especially suited for imaging biological objects in the `water window' region.