High resolution phase zone plates for water window wavelengths

High-resolution X-ray microscopy has applications in the fields of biology, colloid physics, and solid state physics. For each of these applications it is essential that the micro zone plates used as objectives in the X-ray microscope combine high resolution with optimal diffraction efficiency, so as to minimize exposure time and the X-ray dose absorbed by the specimen. The outermost zone width constitutes an important property of the zone prate, since the first-order spatial resolution scales linearly with this parameter. The other fundamental property of a micro zone plate is its diffraction efficiency, which should be both high and uniform. The combination of small outermost zones and the zone height required for optimal diffraction efficiency leads to high aspect ratios of the nanostructures. We report here the technique to manufacture these micro zone plates using microscopic galvanoforms and electrodeposition. Copolymer galvanoforms for nickel micro zone plates were irradiated with high doses of X-ray radiation to increase the degree of cross-linking of the copolymer network; in this way, exact pattern replication in the galvanoform was obtained for zone aspect ratios of up to 8:1. Using these galvanoforms, nickel micro zone plates were produced for the soft X-ray wavelength 2.4 nn, with smallest zone widths of 30 nn and 40 nm, achieving first-order diffraction efficiencies of up to 15% and 20%, respectively.