Fabrication of nickel diffractive phase elements for x-ray microscopy at 8 keV photon energy

Abstract

The ability to resolve small details using x-ray microscopy is critically dependent on the properties of the optical elements used in the microscope’s setup. Today, Fresnel zone plates (ZPs) are widely used at synchrotron radiation sources, due to their ability to concentrate x-ray beams to spots with diameters in the tens of nanometers range. Unfortunately, fabricating ZPs with high efficiencies and sharp foci proves to be extremely challenging technologically, especially in the range of hard x-rays. A widely recognized fabrication issue is the mechanical instability of narrow and tall features made up of polymeric resists. These features often collapse either during the development or the drying of the structures due to the action of capillary forces, or in the step of electroplating because of the stress applied to them by the growing metal. The authors demonstrate a fabrication strategy that solves such issues by forming high-aspect-ratio templates in materials as hard and mechanically stable as silicon. The strategy is implemented by a combination of high-resolution lithography techniques, electroplating and plasma etching. In the present work, Fresnel zone plates with 200 nm outermost zone widths were demonstrated by electroplating nickel up to 2.1 μm height using 3.5 μm deep template structures.