The smoothing effect of si layers in multilayer Be/Al mirrors for the 17- to 31-nm range

The smoothing effect of thin silicon films used as buffer layers in multilayer Be/Al mirrors optimized for wavelengths longer than 17.1 nm has been investigated. The objects of investigation have been near-normal incidence (an angle of incidence of 88$^\circ$) and grazing incidence (33.5$^\circ$) mirrors. This effect has been observed in multilayer Be/Si/Al mirrors with periods of up to at least 29 nm. For normal incidence mirrors optimized for a wavelength of 17.14 nm, a record-high value of the peak reflection coefficient, 62.5%, has been achieved at spectral selectivity $\lambda/\Delta\lambda$ = 59. The temporal stability of the mirrors has been studied.