Polymer-based X-ray masks patterned by direct laser writing

X-ray masks are indispensable tools in deep X-ray lithography (XRL). To date, hardly any fabrication technology can provide affordable and readily available masks with good structure quality. The bottleneck of adequate masks to a large extent limits the widespread use of XRL. In this article, an alternative XRL mask fabrication process is described to significantly improve availability and cost efficiency of XRL masks as key instruments in XRL processing: A 355 nm UV-laser is applied to expose SU-8 resist on an antireflective coating and a copper sacrificial substrate. The voids in this resist template are filled by a two-step electroplating process with sacrificial nickel and 3.6 μm thick gold absorbers. A second SU-8 coat embeds the absorbers, forming the 40 μm mask membrane. This configuration allows for XRL into resists of up to about 200 μm thickness at the SyLMAND beamline, Canada. The absorber structure accuracy is about 1 μm, at smallest tested lateral dimensions of 2 μm isolated features and 500 nm details. Upon release from the substrate, the membrane locally deforms by up to 1.79 μm. PMMA microstructures patterned with such a mask have smooth and vertical sidewalls. The SyLMAND chopper allows one to limit thermal deformations during exposure to the micrometer range: At a beam power of 0.42 W, typical thermal deformations are 0.5 μm-1.4 μm, depending on the layout, and position inaccuracies are about 3.3 μm.