Low-stress coatings for sputtered-sliced Fresnel zone plates and multilayer Laue lenses

摘要

The application of thin film coating processes for the fabrication of diffractive X-ray optical elements like sputtered-sliced zone plates or multilayer Laue lenses (MLL) is a very promising approach for X-ray focusing down to spot sizes of ＜ 10 nm. However, for practical useful focal length in the order of several millimeters or a few centimeters, multilayer thicknesses of several 10 μm up to a few 100 μm are necessary in order to have large enough numerical apertures of the lenses. Currently one of the main challenges is to coat low-stress multilayers with large total thicknesses in the order of 100 μm. Usually sputter deposition results in thin films with significant compressive stress. With new material combinations such as Mo/MoSi_2/Si/MoSi_2 and W/WSi_2/Si/WSi_2 the overall stress can be reduced to almost zero if the individual thicknesses are properly adapted. In the case of these four-layer-systems only the period thickness d_p follows the zone plate law. In case of Mo/MoSi_2/Si/MoSi_2, stress-free multilayers are obtained with d_(Mo) = 0.5~*d_p, d_(MoSi20 = 0.16~*d_p and d_(Si) = 0.34~*d_p.