Recent advances in development of Cr/Sc based reflective multilayer coatings for x-ray applications
Résumé
Many scientific and industrial applications in x-ray range require the use of reflective multilayer coatings with a good efficiency and stability and often of a particular spectral band width. Here we report on recent advances in optimization of Cr/Sc-based multilayer systems for various x-ray applications. We have investigated and developed several approaches to design periodic and aperiodic multilayer mirrors using as a base the Cr/Sc pair of materials. A method to characterize sub-nanometric layers based on grazing incidence X-ray reflectometry has been proposed and tested experimentally. We have elaborated a certain number of periodic and large-band aperiodic multilayer mirrors for plasma diagnostics at grazing incidence in the hard x-ray range between 2 to 10 keV [1].
The development of efficient normal incidence optics in the soft x-ray range remains quite a challenge. Proposed more than 20 years ago as a most promising system for the water- window range, the Cr/Sc multilayer could provide about 60% peak theoretical reflectance at near-normal incidence around the Sc L2,3 edge at 397 eV. However, the values of measured reflectance of Cr/Sc multilayers achieved so far are much lower than predicted 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 interface as a result of the material interdiffusion.
We studied an influence of boron carbide (B4C) introduced in the multilayer structure as a barrier layer or a third material and an effect of nitridation of chrome layers during deposition on optical and structural properties of the Cr/Sc reflective coatings with the aim to improve the performance of multilayer mirrors in the water-window range. Several sets of Cr/Sc-based multilayer mirrors were deposited by magnetron sputtering and characterized by using both soft and hard x-ray reflectivity techniques. Complementary analysis of the structural parameters was performed by using the x-ray reflectivity and standing waves enhanced x-ray fluorescence techniques later on [2]. The normal incidence reflectivity was measured at the metrology beamline of Soleil synchrotron [3]. We will demonstrate that combining both effects (the nitridation of Cr and the use of B4C barrier layers), we have been able to produce
a new multilayer system (CrNx∕B4C∕Sc) with the peak reflectance as high as 23% measured at near normal incidence of 85.7° around the photon energy of 397 eV.