Fabrication of soft x-ray zone plates using carbon, titanium as sacrificial layer

• A novel process for the fabrication of Fresnel zone plate (FZP) in poly methyl methacrylate (PMMA).
• Fabrication of mechanically stable FZP using ultra thin titanium film on carbon as sacrificial layer.
• Negative tone of PMMA utilized to provide mechanical strength to the structure.
• FZPs of 800 μm diameter, 25 mm focal length for a capillary discharge based argon x-ray laser operating at 46.9 nm.
• FZP had 165 zones with an outer most zone width of 1.34 μm, and a numerical aperture of 17 mrad.

In this paper, we report a novel process for the fabrication of Fresnel zone plate (FZP) in poly methyl methacrylate (PMMA) using ultra-thin titanium film on carbon as sacrificial layer by electron-beam lithography at 30 keV with proximity correction. The process involves fabrication of free standing structure with high mechanical stability. A study was undertaken to understand the interaction of an electron beam in the resist stack, leading to the optimization of the process for high aspect ratio structures in a single step of exposure and development. The negative tone of PMMA was also utilized to provide mechanical strength to the structure. FZPs of 800 μm diameter were designed and fabricated for a capillary discharge based argon x-ray laser operating at 46.9 nm, to have a focal length of 25 mm. Each FZP had 165 zones with an outer most zone width of 1.34 μm, and a numerical aperture of 17 mrad.