High-Aspect Ratio Nanofabrication for Hard X-Ray Zone Plates

Hard x-ray nanoimaging enables structural investigations of newmaterials for many applications. For high-resolution experiments, zone plate x-ray optics are commonly chosen. Two methods of zone plate nanofabrication are presented in this thesis. Zone plates are circular diffraction gratings with radially decreasing grating period. Their optical resolution depends on the width of the smallest zone, which nowadays can be around 10 nanometers. However, the efficiency of a zone plate depends on its thickness and its material. For hard x-rays, the optimal zone plate thickness is in the order of micrometers. Therefore, high aspect ratio nanofabrication processes are needed. Two such methods are investigated in this study. First, an existing tungsten nanofabrication process based on reactive ion etching (RIE) was extended to 22:1 aspect ratio structures at 30 nm line width. The core improvement was a resist curing step that enhanced pattern transfer during RIE. Such a zone plate with 200 micrometer diameter and 2.2% efficiency was used in the commissioning experiment of NanoMAX, the nanoimaging beamline at the Swedish synchrotron facility MAX IV. Transmission imaging with 40 nm resolution, as well as the fluorescence imaging modality were demonstrated. Second, metal-assisted chemical etching (MACE) of silicon using gold catalyst patterns was investigated. MACE dependence on gold pattern geometry, etching solution composition, temperature, and substrate doping is described. The process is characterized in terms of etching rate, directionality, and nanostructure surface roughness. Finally, the Ronchi test is presented as a way to quickly judge the performance of x-ray optics in terms of present aberrations and x-ray sources in terms of coherence.