Tunable hard x-ray nanofocusing with Fresnel zone plates fabricated using deep etching

Fresnel zone plates are used widely for x-ray nanofocusing, due to their ease of alignment and energy tunability. Their spatial resolution is limited in part by their outermost zone width dr_N, while their efficiency is limited in part by their thickness t_zp. We demonstrate the use of Fresnel zone plate optics for x-ray nanofocusing with dr_N = 16 nm outermost zone width and a thickness of about t_zp = 1.8 µm (or an aspect ratio of 110) with an absolute focusing efficiency of 4.7% at 12 keV, and 6.2% at 10 keV. Using partially coherent illumination at 12 keV, the zone plate delivered a FWHM focus of 46 × 60 nm at 12 keV, with the first-order coherent mode in a ptychographic reconstruction showing a probe size of 16 nm FWHM. These optics were fabricated using a combination of metal-assisted chemical etching and atomic layer deposition for the diffracting structures, and silicon wafer back-thinning to produce optics useful for real applications. This approach should enable new higher resolution views of thick materials, especially when energy tunability is required.