Characterization of thin films for X-ray and neutron waveguiding by X-ray reflectivity and atomic force microscopy

Abstract

X-ray and neutron guiding in thin-film waveguides are finding numerous applications, such as sub-micron beam production for X-ray microscopy, and applications in neutron interferometric devices and polarizers. Thin-film waveguides are composed of a three-layer stack where the central layer, displaying low absorption for X-rays/neutrons, act as a guiding film. The utilization of such systems with low brilliance X-rays and neutron sources, requires the thickness of the guiding film to be increased. The efficiency of the waveguides critically depends on the thickness of the guiding layer and its surface roughness. In this paper we address the problem of producing relatively thick and smooth guiding layers for a high efficiency, thin-film neutron waveguide. We have produced a Ni/Al/Ni tri-layer structure with a 500 nm thick Al layer optimized for neutron waveguiding. The characterization is performed by complementary X-ray reflectometry and atomic force microscopy. The surface roughness estimation by mean of the two methods is presented and discussed. We show that a combination of sputtering/evaporation processes is beneficial in reducing the roughness of the Al film.