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

Fresnel zone plates are widely used 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, while their efficiency is limited in part by their thickness. We demonstrate the use of Fresnel zone plate optics for x-ray nanofocusing with 16 nm outermost zone width and a thickness of about 1.8 μm (or an aspect ratio of 110) that are able to deliver a 14 nm FWHM focus at 12 keV with 4.7% efficiency, and 6.2% efficiency at 10 keV. 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.