Multilayer coatings for hard x rays

Multilayer interference mirrors are enabling components in x-ray optical systems. These mirrors consist of periodic or non-periodic structures of alternating thin film layers of two or more materials deposited on an optical substrate. Today’s applications and experimental facilities (including the National Ignition Facility) require novel, multilayer-based hard x-ray multilayer optics that are beyond the current state of the art. The realization of hard x-ray multilayers is a challenging task, as it requires layers with thickness equivalent to a few atomic monolayers with stable layer interfaces. Interfacial effects (roughness, interdiffusion) directly control the magnitude of the reflectance, and in non-periodic multilayers, they can also drastically alter the shape of the spectral response. Furthermore, in non-periodic multilayers, the thickness, morphology, and composition of each interface may depend on the constituent layer thicknesses.

In a recent Optics Express publication, which follows an earlier result published in 2016, LLNL researchers detail a comprehensive study and optimization of molybdenum–silicon non-periodic multilayers operating at 17.4 kiloelectronvolts of photon energy. Molybdenum–silicon multilayers for much lower soft x-ray photon energies were developed at LLNL 30 years ago, but the multilayer structures were periodic (layer thicknesses were constant across the structures rather than varying), and the layers were much thicker than those in the current study. The researchers found that these differences resulted in dramatic changes in the physics of the layers and their interfaces, and in unique challenges in the design and calibration of the non-periodic molybdenum–silicon multilayer films. It was determined that the reflectance of these structures is maximized when boron carbide “barrier” layers are inserted at the layer interfaces, to limit inter-diffusion. Surprisingly, maximum reflectance is achieved with barriers on the molybdenum-on-silicon interfaces only, rather than on both molybdenum-on-silicon and silicon-on-molybdenum interfaces.

Funding was provided by the Laboratory Directed Research and Development Program (17-SI-001).

[C. Burcklen, T. Pardini, J. Alameda, J. Robinson, Y. Platonov, C. Walton, and R. Soufli, Depth-graded Mo/Si multilayer coatings for hard x-rays, Optics Express 27 (5), 7291–7306 (2019), doi: 10.1364/OE.27.007291.]