Structure and performance of Si/Mo multilayer mirrors for the extreme ultraviolet

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Abstract
We report the results of structural, chemical, and extreme ultraviolet (EUV) characterization of Si/Mo multilayers grown by sputtering and by UHV evaporation. This study includes mirrors designed for normal incidence with peak reflectivities Rpeak between 22 and 24 nm, and 45° mirrors having Rpeak between 16 and 19 nm. The deposition conditions were varied to produce multilayers with a wide range of interface morphologies. A variety of techniques were used to determine the structure and composition of the multilayers, including x‐ray diffraction, transmission electron microscopy, Rutherford backscattering spectroscopy, and Auger depth profiling. All of the mirrors have amorphous Si layers and polycrystalline Mo layers with thin amorphous alloy interlayers. We obtain good fits to the low‐angle x‐ray diffraction data only when these interlayers are taken into account. The best sputter‐deposited mirrors were made at the lowest Ar pressure studied, 3 mTorr. The best evaporated mirrors were produced at a substrate temperature of 200 °C. The EUV reflectivity as a function of wavelength was measured using synchrotron radiation. Both the multilayer structure and surface contamination significantly affect the EUV reflectivity, and must be considered to obtain good fits to the reflectivity curves. The best 45° mirror had a peak reflectivity of 53% at 18.6 nm for 100% S‐polarized light, and the best normal‐incidence mirror had a peak reflectivity of 33% at 23.6 nm.

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