Highly reflective Ru/Sr multilayer mirrors for wavelengths 9–12 nm

Abstract

The results of investigations of Ru/Sr multilayer coatings optimized for the spectral range of 9–12 nm are presented in this Letter. Such mirrors are promising optical elements for solar astronomy and for the development of beyond extreme ultraviolet (BEUV) lithography. A near-normal incidence reflectivity of up to 62.3% (λ = 11.4 nm) right after the synthesis is measured. The reflection coefficient decreases to 56.8% after five days of storage in air with a subsequent stabilization of its value. At a wavelength of λ = 9.34 nm, the reflection coefficient is 48.6% after two months of storage in air. To date, to the best of our knowledge, this is the highest reflectivity measured in this spectral range. The possibility of further increasing the reflectivity is discussed.