State of the art Fresnel Zone plates (FZPs) have always challenged the technological limits of micro- or nano-manufacturing, thereby attracting zone-plate aficionados and compelling them to push the boundaries of what is possible. Trends are moving towards higher resolution FZPs that operate at higher photon energies, to keep track with recent upgrades of synchrotron radiation facilities worldwide. There is a particularly strong need for FZPs that can focus X-rays to smaller than 20 nm spot size at energies of 20 keV and higher. Such FZPs would enable breakthrough nanoscale imaging and elemental analyses of multi-element samples, such as nano-composites, biological or geological samples, in situ observation of electrodes, and diagnosis of nanofabricated electronic devices. However, FZPs with such parameters are not yet available. Most attempts to achieve FZPs for high energy x-rays seek to incrementally improve on conventional, lithography-based approaches. Examples of this include efficiency-reducing zone doubling as well as difficult-to-achieve stacking of FZPs. Even with these techniques it is difficult to push beyond the mid-hard (5-10keV) energy range. .Therefore, we are revisiting the old idea of jelly-roll fabrication, and breathing new life into it through the use of atomic layer deposition (ALD) and wafer-level fabrication.
Recently Alcorix proposed and started work on a new fabrication method for multilayer (“jelly-roll”) FZPs that has the potential to achieve Fresnel zone widths down to 5 nm. The method uses repeated, sequential ALD of two materials (e.g. Al2O3 and Ta2O5) to form Fresnel zones around tall, cylindrical pillars of Si that have sidewalls smoothened to <1 nm roughness. Tilt control of the Si sidewalls assures that the finest zones (also the most numerous and most important for resolution) are oriented to fulfill the Bragg tilt condition for highest diffraction efficiency, as necessary for ultra-high aspect ratio FZPs. After growing the zones via ALD, the fabrication consists of wafer-level polishing, bonding to another Si (carrier) wafer, polishing the back side, and etching windows through the carrier wafer to expose the zone plate structures on a membrane. Over the next two years, Alcorix plans to sequentially produce prototype FZPs of 20, 15, 10 and finally 5 nm outermost zones. The fabrication process will improve over time since each prototype will benefit from the characterization of the previous generation. Each FZP generation will be characterized for layer thickness accuracy and placement, tilt control of outermost zones, resolution and efficiency for in-beam performance. This presentation will show proof of principle experiments and FZP output variations with diverse manufacturing parameters, discussing what is critical and what can be tolerated. An outlook towards future capabilities of the method will be given.