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Highly reflective multilayer (ML) coatings deposited on precisely polished mirror substrates have enabled imaging at EUV and x-ray wavelengths at near-normal angles of incidence. These ML films essentially represent synthetic Bragg crystals made of alternating layers of materials, where the constructive interference of light between the layers results in significant reflectivity at normal incidence. Stable interfaces and sufficient contrast in the refractive index between the material layers are the most fundamental requirements for these ML structures to function efficiently. The first attempt in 1940 by DuMond and Youtz to make copper-gold (Cu-Au) MLs resulted in the loss of reflective performance after a few days due to interdiffusion between the layers. However, Dinklage in 1967 and Spiller in 1972 were the first to make successful experimental demonstrations of ML films with stable reflective performance over time, operating at EUVâx-ray wavelengths, followed by T. Barbee and J. Underwood. In the following two decades, significant activity occurred in this direction by the groups at Bell Laboratories and at Lawrence Berkeley, Lawrence Livermore, and Sandia National Laboratories (LBNL, LLNL, and SNL, respectively) in the U.S., and NTT Laboratories in Japan. These early efforts were motivated by the need for ML mirrors for EUVâx-ray solar physics, EUVâx-ray lithography x-ray microscopy, and x-ray lasers for defense applications. These researchers established the vacuum-deposition techniques and general principles of making such ML structures into practical elements for EUV and x-ray instrumentation. The rapid advancement of laser-produced plasma (LPP) source EUV reflectometers and second- and third-generation synchrotron facilities that occurred at about the same time made possible the accurate and reproducible at-wavelength characterization of ML films, thus further accelerating the development of ML technology.
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