High spatial-resolution reflectivity and fluorescence mapping of multilayers using a sub-micrometer focused synchrotron X-ray beam

The quality of X-ray multilayers (roughness, composition, layer registry) is usually derived from specular reflectivity and diffuse scattering measurements, these signals being usually averaged over a beam footprint a few millimeters wide. However, some imaging applications involving multilayer optics, e.g., Extreme Ultra-Violet Lithography (EUVL) or synchrotron phase holo-tomography, require multilayers that provide a uniform response at the $\mu$m-scale or below. Therefore, the détection of defects at this scale, either present in or replicated through the multilayer, is essential to the development of defect-free reflective surfaces. In this paper a method capable of detecting such defects has been investigated. White a x-y piezo-driven stage raster scans the sample under the beam, the reflected signal can be recorded. Results obtained for Ru/Al$_2$O$_3$ and Ru/B$_4$C designed for synchrotron optics and for Mo/Si multilayers for EUVL applications are presented. By choosing an incident energy slightly higher than the absorption edge of the heavy element of the multilayer structure, the fluorescence signals of the various components could be monitored simultaneously during reflectivity mapping