Optical, structural and aging properties of Al/Sc-based multilayers for the extreme ultraviolet

Highlights: • Structural characterization and modeling of Al/Sc-based multilayers. • Discovery of a 1 to 3 nm thick interfacial layer and Al₃Sc phase identification. • Thickness dependence of crystallization for Al and Sc layers. • Reflectance measurements covering 5 orders of magnitude from 17 nm to 80 nm. • Good temporal stability of Al/Sc multilayers capped with a 3 nm thick SiC layer. This manuscript presents the structural characterization of Al/Sc-based periodic multilayer coatings for the extreme ultraviolet (EUV) spectral range. Based on transmission electron microscopy and electron diffraction as well as grazing-incidence and large-angle x-ray diffraction, a model for the layer structure and the interfacial effects of Al/Sc coatings is built. The onset of crystallization in nanoscale Al and Sc layers as a function of thickness is also revealed in these characterizations. The Al/Sc layer model is validated and further refined by fitting in-band and out-of-band EUV reflectance measurements across 5 orders of magnitude in an extended wavelength range from 17 to 80 nm. The same type of EUV reflectance measurements is used to test the Al/Sc aging properties and to demonstrate the spectral response of optimized two- and tri-material multilayers including Al/Sc, Al/Sc/SiC and Mo/Al/Sc.