Ion Beam Milled Fresnel Zone Plate

In this article I discuss some interesting developments in the area of FIB technology, and its use for nano-fabrication. In this case the production of of X-ray Fresnel Zone Plates with some surprising new characteristics. These results were published recently in the journal ’Optics Express’ and so maybe weren’t very visible via the normal Nano-fabrication, Lithography or FIB technology channels. (and yes I am a co-author).

Ion beam lithography for Fresnel zone plates in X-ray microscopy  - Optics Express, Vol. 21 Issue 10, pp.11747-11756 (2013)

This paper is interesting for 2 main reasons. The first is how FIB technology can benefit, by moving from an analytical SEM platform (FIB-SEM), to an Electron Beam Lithography architecture more suited to ultra-fine nano-patterning. The second reason is that this new precision has been successfully employed in advancing our understanding of X-ray optics, in this case, the structure of Fresnel Zone Plates and its effects.

What’s new for FIB heads? Well this example shows a high precision, fully automated, large pattern FIB milling operation, being used to directly fabricate a working Fresnel Zone Plate lens. This was done over a extended period (16 hrs) and within a tiny total cumulative positional error budget (total of 20nm’s) – far beyond conventional instrument capabilities. 

Also FZP lens functionality relies absolutely on the total pattern precision across the full write field (100x100microns). In this case this had to be accurate down to <5 nanometers (beyond the deflection precision and stability of a conventional microscope). The additional requirements of using a small Gallium beam size with a ‘low tails’ profile, combined with constant beam ‘circularity’ under high deflection, and current stability over long periods are also of particular note. (These were only achievable by removing the suppressor element from the source assembly and using a novel control method for maintaining beam current over long periods). 

And for those in the X-ray optics field, this result is disruptive and interesting as it shows that FIB machining offers many advantages over conventional EBL FZP fabrication. It goes on to prove the advantages of FIB machined ring profiles for the preferential activation of higher X-ray focal orders. These higher focal orders are shown to deliver higher X-ray imaging resolutions for rings of the same equivalent size. This resolution enhancement effect of higher orders was shown to match the theoretical predictions.

You can read the paper here

or Email me at Lloyd.peto@nanoscopeservices.com if you’re interested to learn more.