Free-standing silicon-nitride zoneplates for neutral-helium microscopy

The ground-state helium-4 beam employed by the microscopy technique discussed in this article interacts exclusively with the atoms in the topmost sample-monolayer. Its low-energy (tens of meV) and chemical inertness make the beam an almost ‘ideal’ imaging probe in the sense that a sample surface can be imaged without alteration by the probe. The microscopy technique therefore has promising applications in the imaging of fragile samples and metrology. In this article we present a fabrication process for the diffraction-based focusing element (Fresnel Zoneplate) in such a setup. Zoneplates made previously for this purpose have suffered from low transmission, a problem we have solved with our new process. In addition, we have measured the first-order diffraction efficiency of three zoneplates fabricated with this process. The efficiency of a zoneplate with 388 μm diameter was close to 70% of the theoretically predicted value. We believe the reduction stems mainly from misalignment between the writefields used to pattern the zoneplate. The fabricated zoneplates of 190 μm diameter, which we patterned using a single writefield, have close to the theoretically predicted transmission characteristics; a result that has not been achieved before for neutral atom Fresnel lenses.