Title : Nanofabrication of multilayer fresnel zone plates used for hard x-ray microscope
Fresnel zone plat (FZP) is one of the high-resolution X-ray focusing optics. Challenges in fabricating high aspect ratio FZP limit the improvement of the efficiency and the resolution. Multilayer is an ideal technique to obtain FZPs with virtually unlimited high aspect ratio. Here we show the method to fabricate multilayer FZPs based on multilayer deposition with atomic layer deposition (ALD) and slicing with focused ion beam (FIB). The Al2O3/HfO2 multilayer films were deposited alternately on a glass fiber core with diameter 55 μm. Multilayer FZPs working in hard X-ray range with diameter 75 μm, outermost zone width 40 nm, multilayer thickness 10 um and zone height 7.5 um were prepared. And for the first time an Al2O3/HfO2 multilayer FZP could achieve an image with resolution of sub-100 nm in a transmission hard X-ray (8 keV) microscope. For the multilayer method, the precision of the core diameter is important to the quality of FZP due to the accumulative error which can lead to phase reverse. Therefore the influence of central core diameter deviation is theoretically analyzed. Calculation results indicate that for the FZP we designed and fabricated in this paper the tolerable range of the core diameter deviation is from -400 nm to 400 nm when the focal length is 18 mm. Besides that, FIB slicing is a vital and time-consuming process. We discussed the effects of curtain structure coming from FIB slicing on the images quality and the test result showed that the severe curtain structure will result in the failure of FZP. Based on that we optimized the process considering the efficiency and quality by combining quick slicing, local polishing and the way of fixing sample. The theoretical analysis and technique mean a lot for the fabrication of FZP in hard X-ray range.
Audience take away:
• Uniform multilayer films can be deposited on the surfaces of curved substrates by ALD.
• The multilayer FZP fabricated based on ALD and FIB can be used in other shorter wavelength X-ray.
• Changing the way of fixing sample can make the process, FIB slicing and mechanical arm transferring large sample, more efficient.