Microsphere Photolithography for the Fabrication of Metasurfaces

Metasurfaces are two-dimensional materials that possess artificially engineered nano/micro-scale structures, providing precise control over the amplitude, phase, and polarity of transmitted and reflected electric fields. While standard semiconductor fabrication techniques offer high-resolution capabilities down to a few nanometers and great flexibility in patterning, they are prohibitively expensive for large-scale production of metasurfaces. However, it is not necessary to have extremely fine feature sizes with arbitrary pattern geometries for most metasurface applications in the visible-near infrared range. Microsphere Photolithography (MPL) combines both top-down and bottom-up fabrication techniques by utilizing self-assembled microspheres as optical elements for photolithography. It presents an appealing alternative for producing nano/microstructures over large areas with a better balance between flexibility and cost. The fabrication process parameters of MPL associated with the exposure and development process are investigated and modeled for feature geometry prediction. A DMD-based direct-write (DW) MPL system is developed to improve patterning flexibility. Both feature geometry, as well as the overall metasurface topology is dynamically controlled from the spatial or angular modulated illumination with fast turnaround. A reusable microsphere mask is employed to reduce the cost of MPL. Lower pressure is required for this mask-based MPL compared to nanoimprint lithography. A tradeoff is observed between the yield and mask wear in terms of pressure. Additionally, the application of MPL fabricated functional metasurface is explored as frequency filters, anti-condensation surface as well as refractive index sensors. Overall, MPL demonstrates promise as a cost-effective alternative fabrication technique for large-scale metasurfaces.