Nanofabrication of 50 nm zone plates through e-beam lithography with local proximity effect correction for x-ray imaging

High resolution Fresnel zone plates for nanoscale three-dimensional imaging of materials by both soft and hard x-rays are increasingly needed by the broad applications in nanoscience and nanotechnology. When the outmost zone-width is shrinking down to 50 nm or even below, patterning the zone plates with high aspect ratio by electron beam lithography still remains a challenge because of the proximity effect. The uneven charge distribution in the exposed resist is still frequently observed even after standard proximity effect correction (PEC), because of the large variety in the line width. This work develops a new strategy, nicknamed as local proximity effect correction (LPEC), efficiently modifying the deposited energy over the whole zone plate on the top of proximity effect correction. By this way, 50 nm zone plates with the aspect ratio from 4:1 up to 15:1 and the duty cycle close to 0.5 have been fabricated. Their imaging capability in soft (1.3 keV) and hard (9 keV) x-ray, respectively, has been demonstrated in Shanghai Synchrotron Radiation Facility (SSRF) with the resolution of 50 nm. The local proximity effect correction developed in this work should also be generally significant for the generation of zone plates with high resolutions beyond 50 nm.