Nanofabrication of Fresnel Zone Plates for X-Ray Focusing

In this paper E-beam nanolithography and gold electroplating on very thin silicon nitride membranes, are combined for fabrication of amplitude soft X-ray Zone Plates (λ=40 Å) with resolution down to 50 nm and external diameters up to 200 µ m. In particular, challenges to the lithographic process introduced by advanced and non-standard requirements, such as large external diameters together with a very high resolution external zones pitch, and ring-to-space ratio optimization for higher order focusing (2nd order), will be studied in detail. The mechanism of image formation and control is studied by means of Monte Carlo electron scattering modelling, and a self consistent proximity effect routine is specifically developed for the case of Zone Plates. Some of the fabricated ZPs were tested at a dedicated synchrotron microscopy beam line working at 3.7 nm, showing 1st-order efficiencies up to 9.5% (against a theoretical 10%) and resolutions down to 50 nm for the second order.