Double-sided zone plates
To obtain Fresnel zone plates with very high resolution and good diffraction efficiency requires the fabrication of nanostructures with extreme aspect ratios. Within a collaboration funded by the Nanoscopium beam line of Synchrotron SOLEIL, we have developed schemes for the stacking of two zone plates, in order to increase the effective structure height of Fresnel zone plates beyond todays nanofabrication limits. By patterning both sides of the same support membrane (see Figure 1&2), we can provide a pair of stacked zone plates as a monolithic device.
Figure 1: Fabrication process of a double-sided Fresnel zone plates. (a) The membranes are patterned with gold alignment markers. (b, c) The resist is patterned on the front and the back side using the alignment markers. (d) Coating with iridium using atomic layer deposition.
Using this approach were able to produce double-sided zone plates with 30 nm outermost zone width and 1200 nm effective zone height, corresponding to an aspect ratio of 40 . The devices showed an enhanced diffraction efficiency of 10% at 9 keV photon energy (see Figure 3). The shape of the focal spot was reconstructed from a ptychographic data set , revealing a spot size matching the expected value of 30 nm FWHM.
Figure 2: FIB cross section of a double-sided Fresnel zone plate with 30 nm outermost zone width. The structure heights two sides add up to a value of 1200 nm.
- I. Mohacsi, I. Vartiainen, M. Guizar-Sicairos, P. Karvinen, C. Kewish, A. Somogyi, V.A. Guzenko, E. Müller, E. Färm, M. Ritala, and C. David, Double-sided diffractive X-ray optics for hard X-ray microscopy Optics Express 23 (2015) p. 776
- J. Vila-Comamala, A. Diaz, M. Guizar-Sicairos, A. Mantion, C. M. Kewish, A. Menzel, O. Bunk and C. David , Characterization of high-resolution diffractive x-ray optics by ptychographic coherent diffractive imaging, Optics Express 19 (2011) p. 21333