High reflectance Cr/V multilayer with B(4)C barrier layer for water window wavelength region.

To develop the high reflectance mirror for the short wavelength range of the water window region (λ=2.42-2.73  nm), Cr/V multilayers with B₄C barrier layers are studied. The grazing incidence x-ray reflectometry results show that the multilayer interface widths are significantly reduced down to 0.21-0.31 nm, after the introduction of 0.1 nm B₄C barrier layers at both interfaces. The [B₄C/Cr/B₄C/V] multilayer with a large number of bilayers of N=300 maintains the same small interface widths while the surface roughness is only 0.2 nm. According to the transmission electron microscope measurements, the layer structure improvement with barrier layers can be attributed to the suppression of the crystallization of vanadium inside the structure. Using the interface engineered multilayer, a maximum soft x-ray reflectance of 24.3% is achieved at λ=2.441  nm, under the grazing incidence of 42°.