<title>Multilayer reflection-type zone plates and blazed gratings for the normal incidence soft x-ray region</title>

Abstract

Here we report on the fabrication and analysis of two different types of laterally structured multilayers, which act as highly efficient x-ray optical elements for focusing or spectral dispersion of soft x-rays at (near) normal incidence. The microfocusing of soft x-rays at (near) normal incidence with plane reflection optics can be obtained by multilayer Bragg-Fresnel zone plates. A circular condensor zone plate structure (2.9 mm diameter, 417 zones, inner zone radius 50 micrometers , outer zone width 0.9 micrometers ) has been recorded by holographic lithography followed by ion beam etching into a Mo-Si multilayer mirror of 24 periods and a doublelayer thickness of 7.2 nm. The focusing properties analyzed by soft x-ray reflectometry at (lambda) equals 13.8 nm show a focal spot size smaller than 40 micrometers measured 160 mm behind the multilayer zone plate. Multilayer blazed gratings offer the opportunity for highly resolved spectral dispersion of soft x-rays due to the possible combination of high efficiency and high diffraction orders with almost the whole intensity diffracted in one diffraction order. The sawtooth profile of a blazed grating structure (1221 l/mm, blaze angle 1.5 degree(s)) has been ruled into a 200 nm thick Au-film which has been deposited onto a plane glass substrate. A Mo/Si multilayer of 15 periods and a doublelayer thickness of 7.2 nm has been deposited onto the grating substrate. In order to smooth the rough Au-surface and to prevent interdiffusion of the Au-film with the upper Mo-Si multilayer a carbon film has been evaporated onto the Au-grating surface before the deposition of the soft x-ray coating. This procedure results in a significant increase of diffraction efficiency. The multilayer grating has been matched working on blaze in the third diffraction order, where an absolute diffraction efficiency of 3.4% at (lambda) equals 14 nm has been measured, while only 1.1% has been achieved for a similar grating without a carbon interlayer.