High-efficiency gold fresnel zone plates for multi-keV X-rays

We report a direct e-beam writing process of Fresnel zone plates (FZPs) using thick layers of PMMA resist as electroplating molds. We used 100-kV electron beam lithography to directly expose thick PMMA layers, which were later used as plating molds without intermediate etching steps. High-quality 500-nm- and 1-μm-thick Au FZPs with outermost zone widths down to 50-nm and 70-nm, respectively, and with diameters up to 600 μm were fabricated. In this paper we present the details of the optimized fabrication process, such as development times, developer, dose tables, and line shrinkages required to obtain the desired zone widths and gaps between the zones. The diffraction efficiencies of the fabricated FZPs were measured for a wide range of x-ray energies (2.8-13.2 keV) showing excellent values up to 65-75% of the theoretical maximum, reflecting the good quality of the FZPs. Spatially resolved diffraction efficiency measurements indicate the uniformity of the zone plates and lack of defects.