Improving multilayers for maximum reflectivity

Multilayer mirrors are the key optical elements in both EUV lithography and Soft X-ray metrology. In order to improve the reflectivity of multilayer optics, metrology is needed to resolve all physical processes at the interfaces that contribute to reflectivity losses. All imperfections, like intermixing, compound formation or roughness lower the reflectivity. We demonstrate that by resolving and understanding the process at the interfaces, we can optimize the multilayer reflectivity using diffusion barriers and ion polishing, to achieve record reflectivities, up to 71.2% at 13.5nm, and gains of tens of % in soft X-ray multilayers.