Influence of the Number of Bilayers on the Optical Performances of X-UV Multilayer Interferential Mirrors

The influence of the number of bilayers on the optical performances of actual X-UV multilayer interferential mirrors (MIMs) has been studied in order to emphasize the experimental restrictions in the designing of "thick" mirrors used for the development of etched multilayer gratings. Several sets of samples (W/C, Mo/Si) with increasing number of bilayers have been manufactured in the very same conditions by means of a sputtering technique. X-ray diffraction characterization at Cu-K_α radiation (λ = 1.54018 Å) exhibits technical constraints in the achievement of multilayer structures with large number of bilayers. We obtain a gradual loss of reflectivity for deposition times greater than 1h 30 min to 2h without significant drift of the MIM's geometrical parameters (period and division parameter). In the same time, absolute reflectivity measurements at Cu-L_α radiation (λ = 1.333 nm) emphasize satisfying optical and spectroscopic performances of W/C thick samples ( 150 bilayers).