Towards high diffraction efficiency zone plates for X-ray microscopy

Modern soft and hard X-ray microscopes use Fresnel zone plates as imaging or focusing optical elements. For short exposure times and to minimize the radiation load on the specimens, the diffraction efficiency of the zone plate objectives has to be maximized. Since the diffraction efficiency strongly depends on the height of the diffracting zone structures, the achievable aspect ratio of the nanostructures which can be fabricated by existing nanostructuring processes limits the zone plate efficiency. To increase the achievable aspect ratios especially for hard and multi-keV X-ray optics we develop a process to superimpose zone plates on top of each other. With this so-called stack process for zone plate fabrication the final aspect ratio is only limited by the number of single zone plate layers that can be stacked with sufficient accuracy. Several nanostructuring process steps have to be developed and/or improved to establish the stack process. For the first time, we present a polished zone plate on a subsequently etched support membrane. In addition, we present first results of stacked zone plates with two layers of nickel zones superimposed on top of each other.