Optimisation of Cr/Sc-based multilayer coatings for water window applications
Résumé
Development of efficient normal incidence optics for the soft x-ray range and, in particular for the water window (a region between absorption edges of carbon and oxygen, from 282 to 533 eV) remains quite a challenge. Proposed more than 20 years ago, a most promising system, the Cr/Sc multilayer, provides theoretical reflectivity about 60% at near-normal incidence around the Sc L2,3 edge at 397 eV. However, the measured peak reflectance of Cr/Sc multilayers achieved so far is much lower (around 20% in the previous publications) because of a number of reasons. Apart of the problem of stability while depositing several hundreds of sub-nanometer layers, the most significant reflectivity loss is caused by formation of a rough interfaces as a result of the material interdiffusion and/or thin film growth process.
At the last PXRNMS conference in 2018 we reported on our approach to design high- reflectance Cr/Sc-based multilayer mirrors using a process of nitridation of chromium during deposition and adding boron carbide (B4C) as a third material in the periodic structure. We have already demonstrated that with this type of coating a peak reflectance exceeds 20% in the vicinity of the Sc L-edge at near-normal incidence [1].
Since that time we have elaborated our method and produced several series of CrN/B4C/Sc multilayer mirrors deposited by magnetron sputtering. The multilayers have been characterized by grazing incidence x-ray reflectometry and at-wavelength measurements. The normal incidence reflectivity with a peak reflectance higher than 30% was measured at the Metrology beamline of Soleil synchrotron. We will present and discuss the main results of the study and problems that we have encountered. In fact, an accurate calculation of the CrN/B4C/Sc multilayer reflectivity is hardly possible due to a lack of reliable optical constants of CrN in general and in this spectral range in particular. So we had to use an experimental approach in order to find an optimal thickness ratio of materials in the multilayer structure which provides a maximum reflectance gain. It was determined with a series of multilayers of relatively small period number (N=50). Based on this finding, we have proceeded with 500-periods samples, the number closed to that corresponding to saturated values of the peak reflectance for this system.
This study was able to confirm our strategy of optimisation for this new multilayer system which proved to be potentially interesting for various water window applications such as, for instance, x-ray microscopy.