Nickel zone plates for soft X-ray microscopy

Soft X-ray microscopes require high-spatial-resolution Fresnel zone plates with high and uniform diffraction efficiency as imaging optics. The combination of the zone height for optimal diffraction efficiency with the fact that the first-order spatial resolution of a zone plate scales linearly with its outermost zone width leads to high aspect ratios of the nanostructures. In the water window wavelengths region nickel is a well suited zone plate material, yielding high diffraction efficiency at moderate aspect ratios of the zones. Since it cannot be structured by reactive ion etching a tri-layer process making use of electrodeposition techniques was developed. All nickel zone plates presented in this work have demonstrated excellent first-order diffraction efficiencies at 2.4 nm wavelength. Zone plates with outermost zone widths of 40 nm, 30 nm and 25 nm were fabricated with the tri-level process yielding 21.6%, 18.0% and 14.7% first-order groove efficiency, respectively. These measured efficiencies correspond to maximum obtainable efficiencies determined by the galvanoform height of 100%, 95% and 85%.