A giant linear Fresnel zone plate as a hard x-ray condenser

Abstract

We present the design and test of a large aperture linear Fresnel zone plate (FZP) as a condensing lens for hard X-rays. The FZP is made of Si and is defined by electron beam lithography and chemical wet etching of oriented silicon substrates. The central zone is 75 μm wide, the outermost lines have a width of 0.35 μm, and the structures have a height of 10 μm. These parameters result in an optimal focusing distance of 9 m, a phase shift of π, and a diffraction efficiency of about 30% at 7.3 keV. We have tested the FZP at the Anomalous Scattering Beamline (ID01) of the ESRF. The FZP was inserted in the vacuum chamber in the Optics Hutch at 38.5 m from the source and 9 m from the sample. We have studied the dependence of beam size and gain as a function of energy in the range from 7 to 8.5 keV. We found the optimum energy range as 7.2-7.4 keV, where the focused beam width was 100 μm and the experimental gain was 9.2, for an expected theoretical value of 9.6.