Fabrication of hard x-ray zone plates with high aspect ratio using metal-assisted chemical etching

Abstract

Fresnel zone plates are widely used as nanofocusing optics for x-ray microscopy, where the spatial resolution is set by the width of the finest rings while the efficiency is set by their thickness. This leads to the requirement for high aspect ratio nanofabrication. Metal-assisted chemical etching and atomic layer deposition has already been used to produce high aspect ratio zone plate structures on unthinned silicon wafers. The authors demonstrate here a substantial improvement on the achieved aspect ratio up to a value of 500:1, by producing 16 nm wide platinum zones with thicknesses up to 8 μm. At the same time, the silicon substrate was thinned to 15 μm as required for a practically useful optic. First tests have shown 4.8% diffraction efficiency using 20 keV x rays. This x-ray focusing efficiency is higher than most Fresnel zone plates for this photon energy and near what has been achieved with multilayer-coated Kirkpatrick–Baez mirrors, and multilayer Laue lenses. Hard x-ray zone plates offer the advantages of easy alignment, energy tunability, and one can make many zone plates in a batch on a single silicon chip.