Investigation on the mechanical interface stability of curved high aspect ratio x-ray gratings made by deep x-ray lithography

Abstract

We describe the rationale for selecting graphite as a substrate material suitable for manufacturing curved high-aspect ratio metallic x-ray gratings and experimentally validate that its properties satisfy requirements relevant for clinical phase-contrast and dark-field x-ray imaging. Selection criteria applied to two candidate materials graphite and polyimide were compliance to bending, mechanical tenacity of the attachment of the lamellar grating structure to the substrate, the substrate material’s x-ray robustness, and the compatibility with the x-ray LIGA process used to manufacture the grating structures. In contrast to other standard materials such as silicon wafers with titanium layer, graphite wafers could be bent to smaller radii and are natively electrically conductive. While polyimide wafers allowed for even smaller bending radii, we found their high risk of grating structure detachment to be a strong detractor. Minimum achievable bending radii were 55 and 70 mm for pure graphite wafers and graphite wafers with mounted grating structure, respectively. Electron microscopy of graphite surface and cross-sections revealed a fine porous structure conducting to a very stable metal–wafer interface. Grating structures with heights of more than 200 μm were bonded to graphite wafers and their integrity confirmed in flat as well as in bent state using microfocus x-ray imaging. We conclude that graphite is a very well-suited substrate material for manufacturing curved x-ray gratings.