Experimental X-ray methodologies with the combined capabilities for chemical identification on sub-micrometer scale and large penetration depth are currently being developed and routinely used as powerful tools for material characterization. They rely on optical elements able to provide submicrometer-sized x-ray beams; Fresnel Zone Plates (FZP) are the most widely used optics for this task because of their performances in spatial resolution, efficiency and focal distance. Yet, presently, manufacturing of FZPs relies on complex multi-step fabrication procedures.
We demonstrate that the focused ion beam (FIB) mask-less lithography is a reliable and fast technique for the fabrication of 100 nm resolution micro-FZPs. A prototype of a FZP having 100-nm resolution and 38 zones made by direct-writing on a nickel substrate by using a dual beam FIB/SEM is shown here. A careful optimization of ion current and overlapping allowed the fabrication of the FZP in only one hour. Our prototype of FZP has an aspect ratio of 3, yet measurements of side-wall angles of the etched trenches indicate that an aspect ratio of 6 can be obtained. This limit is set by the redeposition process ongoing during the ion milling that leads to trenches with a V-shaped profile.
Finally, whereas the conventional manufacturing methods can produce only step-like profiles limiting the FZP efficiency to values as low as 55%, the trapezoidal shape of the FIB fabricated trenches shown here should yield an increase in the FZP efficiency since, as theoretical consideration show, FZPs with parabolic profile of the zones would have the highest efficiency. Eventually such profile could be even obtained by a fine tuning of the ion beam parameters.
This makes FIB lithography an attractive technique for fabrication of FZPs to be used in the high energy region of X-ray spectrum and with highly brilliant X-ray sources.
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