High-reflectivity Cr ∕ Sc multilayer condenser for compact soft x-ray microscopy

The condenser is a critical component in compact water-window x-ray microscopes as it influences the exposure time via its efficiency and the resolution via its numerical aperture. Normal-incidence multilayer mirrors can reach large geometrical collection efficiencies and match the numerical aperture of the zone plate but require advanced processing for high total reflectivity. In the present article we demonstrate large-diameter normal-incidence spherical Cr ∕ Sc multilayer condensers with high and uniform reflectivity. Dc-magnetron sputtering was used to deposit 300 bilayers of Cr ∕ Sc with a predetermined d-spacing matching the λ = 3.374 nm operating wavelength on spherical substrates. The mirrors show a uniform reflectivity of ∼ 3 % over the full 58 mm diameter condenser area. With these mirrors an improvement in exposure time by a factor of 10 was achieved, thereby improving the performance of the compact x-ray microscope significantly.