Study of the temporal stability of the reflection coefficient in the vicinity of 58.4 nm of narrow-band Sc/Al mirrors with Si or ScN_x interlayers and a MoSi₂ protective cap layer

Observation of a sample of Si/Al/Sc mirror with a MoSi2 protective cap layer, which was stored in air for 40 months, showed that the peak reflectivity at a wavelength of 58.4 nm during this period decreased from 32 % to 20.5 %, and over the last 20 months of observation-from 21.5 % to 20.5 %, which indicates a significant decrease in the rate of decline and almost stabilization of the peak reflectivity at the level of 20 %. The spectral reflection peak width of this mirror at half maximum has not changed over the past 20 months and amounted to 6.3 nm. The given data of secondary ion mass spectrometry and grazing incidence X-ray reflectometry show that the decrease in reflectance of Si/Al/Sc multilayer mirrors with a MoSi2 protective cap layer is mainly due to an increase in the amount of impurities (oxygen and carbon) in the structure and on the surface of the mirrors and, to a lesser extent, due to the broadening of the interlayer boundaries. Comparison of the rate of decrease in reflectance at 58.4 nm of Sc/Al mirrors with interlayers of silicon and nitride scandium and a MoSi2 cap layer indicates that neither the material and thickness of the interlayer nor the thickness of the protective MoSi2 cap layer within 3-6 nm have a significant effect on this rate.