High-precision deposition of a laterally graded X-ray multilayer with high reflectance on a strongly curved toroidal...

Multilayer coated toroidal mirrors have been widely used in X-ray optical systems to focus/collimate the X-ray beams. The variation of grazing incidence angles at different positions in the tangential direction of the toroidal mirror necessitates a lateral gradient of the multilayer d-spacing. Additionally, a small radius of curvature (RoC) in the sagittal direction and the compact size of the mirror make the deposition more difficult. Here, a toroidal mirror with a 10×10mm² effective area and an 18 mm sagittal RoC was coated with a W/Si multilayer with high precision. The multilayer d-spacing was changed from 3.48 to 4.18 nm along the tangential direction and kept constant along the sagittal direction, using both varied motion speeds of the substrate and a mask. The experimental deviation of the d-spacing with the designed value was less than ±1%. The interface quality of the multilayer at different positions of such a strongly curved mirror remains the same, and the measured X-ray reflectivities are all between 65% and 68% at 8 keV.