Progress in the fabrication of high-aspect-ratio zone plates by soft x-ray lithography

Abstract

Fabrication of Fresnel zone plates for the hard x-ray spectral region combines the challenge of high lateral resolution (~100 nm) with a large thickness requirement for the phase-shifting material (0.5-3 µm). For achieving a high resolution, the initial mask was fabricated by e-beam lithography and gold electroforming. To prevent the collapse of the structures between the developing and electroforming processes, drying was completely eliminated. Fabrication errors, such as nonuniform gold electroplating and collapse of structures, were systematically analyzed and largely eliminated. We optimized the exposure and developing processes for 950k and 2200k polymethylmethacrylate of different thicknesses and various adhesion promoters. We discuss the effects of these fabrication steps on the zone plate's resolution and aspect ratio. Fresnel zone plates with 110 nm outermost zone width, 150 µm diameter, and 1.3 µm gold thickness were fabricated. Preliminary evaluation of the FZPs was done by scanning electron microscopy and atomic force microscopy. The FZP focusing performance was characterized at the Advanced Photon Source at Argonne National Laboratory.

Near-field stacking of zone plates for hard x-ray range

“…Current fabrication techniques only allow manufacture of aspect ratios of around 10:1 for a small outermost zone width. 7,8 Therefore, highest resolution can be obtained only at a significant loss of efficiency and an increase in the signal/background ratio.…”

Fabrication of Fresnel zone plates with high aspect ratio by soft X-ray lithography

“…This means that the aspect ratio of FZPs has to be very high to get both high spatial resolution and acceptable efficiency. Using three-step replication processes, FZPs which has an outermost zone width of 110 nm and aspect ratio of 11, has been fabricated in Advanced Photon Source using X-ray proximity printing and gold (Au) electroplating (Divan et al 2002). A method, which allows fabricating Au structure with high aspect ratio, has been developed at National Synchrotron Radiation Laboratory (NSRL).…”

Fabrication of X-ray imaging zone plates by e-beam and X-ray lithography

“…Through e-beam lithography, trilayer resist processes and gold electroplating techniques, the zone plates with outermost zone width of sub-30 nm and aspect ratio more than 12 have been fabricated (Feng et al 2007;Spector et al 1997). Using X-ray lithography and gold electroplating techniques, which allows to replicating sub-100 nm and high aspect ratio nanostructures, the zone plates with outermost zone width of 110 nm and aspect ratio more than 11 has been fabricated (Divan et al 2002).…”