Graded multilayer mirrors for the carbon window Schwarzschild objective

The paper deals with the recent progress in fabrication of the graded multilayer mirrors to be used in a 21X Schwarzschild objective operating at the wavelengths about 4.5 nm ("carbon window" region). The graded Co/C reflective multilayer coatings were fabricated using DC-magnetron sputtering. Mask-assisted deposition was used to create the required radial variation of the multilayer period. Accuracy of the multilayer's parameter measurements and quality of nm-scale layer deposition were improved significantly with application of a number of new methods and approaches. The soft X-ray measurements were conducted at the ALS 6.3.2 beamline to quantify the graded periods on concave and convex mirrors of the Schwarzschild objective. They demonstrated that the reflectivity curves were adjusted with the accuracy of about 0.008 nm (0.3%) over the entire mirror surfaces. The total throughput of the objective with full working aperture (N A ~ 0.2) is estimated to be as high as 0.25%.