Recent advance in development of Cr/Sc-based multilayer coatings for water window applications

Development of multilayer mirrors for the water window (a region between absorption edges of carbon and oxygen, from 282 to 533 eV) remains quite a challenge. Proposed 25 years ago, the Cr/Sc multilayer provides theoretical reflectivity about 60% at near-normal incidence near the Sc L2,3 edge. However, the maximum measured peak reflectance achieved so far just slightly exceeds 20%. We report on our approach to design highly reflective Cr/Sc-based multilayer coatings using a process of nitridation of chromium during deposition and adding boron carbide as a third material in the multilayer structure. We will report on our strategy of optimisation of the CrN/B4C/Sc multilayer system and discuss the main findings and results. The peak reflectance as high as 32% at 397 eV was measured with this type of coating which proves to be potentially interesting for various water window applications such as x-ray microscopy.