Wet etching of linear Fresnel zone plates for hard X-rays

We describe the fabrication and testing of a novel type of hard X-ray optics. The diffractive elements are generated by electron beam lithography and chemical wet etching of 〈110〉 oriented silicon substrates. Structures with widths down to 100 nm and very high aspect ratios were obtained using this method. To reduce the destructive effects of capillary forces during the drying of the diffracting structures, freeze drying and critical point drying were applied. The diffraction efficiency of our devices was measured using synchrotron radiation with photon energies between 8 and 29 keV. By tilting the lenses with respect to the X-ray beam, the effective path through the diffracting structures can be varied to optimize the diffraction efficiency over a wide range of photon energies. In this way, the effective aspect ratio can be increased to values exceeding 100, which cannot be obtained with untilted optics. The obtained efficiencies exceed values reported previously for this energy range.