Fabrication of high-resolution Fresnel zone plates by a single layer resist process

In this work, fabrication of a high-resolution Fresnel zone plate (ZP) by a single layer electron beam (EB) resist process combined with a gold electroplating process is described. The ZP designed has a diameter of 80 μm and is composed of 250 rings (499 zones) of which the width of the outermost ring is nominally 40 nm. In order to fabricate a 150-nm-thick gold layer which acts as a phase-shifter for X-rays, a 200-nm-thick polymethylmethacrylate (PMMA) is used as a single layer resist. A 50 kV EB lithography system with a probe diameter of 3 nm is employed for high-resolution patterning. To prevent resist patterns with an aspect ratio of 5 from collapsing, we reinforce the ZP patterns by combining neighboring rings with ribs. It is demonstrated that a modified ZP pattern with a 40-nm-wide outermost ring which will not collapse can be fabricated whereas the original ZP pattern collapses at a high aspect ratio. Although control of the width of the gold phase shifter should be improved, the outermost ring with the nominal width of 40 nm can be electroplated.