Comparative Study of Pd/B4C X-ray Multilayer Mirrors Fabricated by Magnetron Sputtering with Kr and Ar Gas

Ultrathin Pd/B4C multilayers are suitable X-ray mirrors working at the photon energy region of 7-20 keV. To further improve the layer structure, Pd/B4C multilayers with a d-spacing of 2.5 nm were fabricated by magnetron sputtering using the heavy noble gas Kr and compared with the conventional ones fabricated by Ar. Although the Kr-sputtering process can work at a lower pressure, the interface width-especially the interface roughness-is a little larger than that made by Ar. A stronger polycrystallization and a lower content of sputter gas atoms were found in the Kr-made sample, which can be explained by the joint effect from less recoiled particles and lower sputtering pressure. A good reflectance of 68% of the Kr made multilayer was measured at 10 keV, which is only slightly lower than that of the Ar made sample (71%).