DESIGN AND FABRICATION OF FRESNEL ZONE PLATES WITH LARGE NUMBERS OF ZONES

The advent of high-brightness x-ray sources in the 10-40 keV region op ens new possibilities of experiments with microbeams. Techniques to fo rm these focused beams may be based on glancing mirrors, phase element s, or diffractive optics, in particular Fresnel zone plates (FZPs). Be cause of the long focal length and large acceptance, FZPs designed to work in the hard x-ray region tend to have quite large diameters and l arge numbers of zones. For instance, the zone plate described in this article has a 1860 mu m diam, a focal length of f=3 m (for 8 keV), and 1860 zones. On a standard pattern generator, circular shapes are alwa ys approximated as simpler structures. The tolerance requirement for s hape and positions of zones depends on the number of zones, and it is necessary to guarantee that the circular structures are approximated t o the required degree of accuracy while keeping the size of the data s tructure to a reasonable size for processing by the exposure system. F or instance, if polygons are used to approximate circular zones, a for mula for the minimum acceptable number of polygon sides can be derived . An x-ray mask for a Fresnel phase zone plate (FPZP) with 1860 zones was designed directly in Cambridge source pattern data format and fabr icated using the Leica Cambridge e-beam tool installed in the CXrL. Th e zone plates presented in this article were designed for hard x rays, and multilevel x-ray lithography was employed as a fabrication techni que to form absorber thickness sufficient to provide the necessary pha se shift. Minimum gold features of 0.25 mu m with thicknesses of 1.6 a nd 3 mu m, were formed to be used with 8 and 20 keV photons, respectiv ely. Finally, in order to estimate the quality of the zone plates duri ng fabrication, a scanning electron microscope based moire method was used. (C) 1997 American Vacuum Society.