Optimization of laser writer-based UV lithography with high magnification optics to pattern X-ray lithography mask templates

Abstract

Deep X-ray lithography is a preferred fabrication approach for those micro devices that depend on smooth and vertical sidewalls of comparatively deep structures rather than extreme lateral resolution. The structure quality obtained depends on, and is limited by, the quality of the X-ray mask applied. A critical component of the mask is its absorber patterns. They get fabricated by electroplating into voids of a polymer template. These templates must usually be at least 3 μm deep and exhibit smooth and vertical sidewalls with a lateral resolution of micrometers and possibly below. Primary patterning of the templates is very demanding. Best results are obtained when dedicated electron beam writers with acceleration voltages of 100 kV and above are applied. This, however, limits access to patterning infrastructure and substantially drives delivery timeline and cost, making mask absorber template patterning a bottleneck of the entire process sequence. We propose, evaluate and optimize an alternative absorber patterning approach based on direct laser writing. An ultraviolet laser with 355 nm wavelength and 250 mW beam power by Heidelberg Instruments is applied to expose 2.9 μm thick, chemically amplified, high contrast, negative tone resist mrx-5. Exposure parameters analyzed include the dose and focal settings. Experiments are carried out on bare silicon wafers as well as on chrome-gold and on titanium oxide plating bases. For all cases, results with and without an additional antireflective coating of 200 nm AZ BAR-Li are studied. Aspects of the resist template structure quality analyzed include the sidewall verticality and its smoothness and defects, resist adhesion to the substrate, minimum feature size and structure accuracy, as well as irregularities due to stitching of partial layouts. In an optimized process, a dose of 14 mW on oxidized titanium and BAR-Li was used. We were able to demonstrate 1.5 μm minimum feature size of isolated structures and structural details of about 1 μm. The sidewalls are vertical and exhibit a roughness of dozens of nanometers. When an antireflective coating is used, chamfers are observed at the resist bottom. The structure accuracy occasionally deviates from the original layout by 200–300 nm, particularly at stitching singularities or towards the end of resist walls. The described absorber template patterning process delivers a resolution that much extends beyond previous UV patterning approaches. The structure accuracy, however, is inferior to electron beam written samples. Given the cost and timeline benefit, results of the study will allow users to identify which primary patterning approach is best suited for their micro devices.