Numerical study of the plasmonic slab lens for improving direct-write nano lithography.

Plasmonic direct-write lithography (PDWL) provides a potential tool for the fabrication and manufacturing at the nano scale due to its high-resolution and low-cost. However, the shallow exposure depth hinders its practical application. Here, we incorporate the plasmonic slab lenses (PSLs) into PDWL to amplify and compensate evanescent waves, leading to improved light intensity, depth, resolution and better tolerance to the air gap beyond the near field optical lithography. Two typical plasmonic probes with different nanostructure and localized plasmonic resonance mechanisms are designed and fabricated as representatives, the local intensity enhancement of which mainly depend on the oscillations of transverse and longitudinal electric field components, respectively. Optimizations considering the PSL structure, material and the illuminating wavelength are performed to amplify different field components and figure out the best lithography configuration. Simulation results indicate that Ag-Ag cavity PSL and 355 nm illumination is the best combination for the lithography with bowknot aperture probe, while the semi-ring probe exhibits better performance under the condition of Ag-Al cavity PSL and 405 nm illumination. The semi-ring probe in combination with a plasmonic cavity, for instance, is demonstrated to enhance the light intensity by 4 times at the bottom layer of the photoresist compared to that without PSL and realize a lithography resolution of 23 nm. Our scheme is believed to boost the application of PDWL as a high-resolution and low-cost nanofabrication technology, and it may even serve as an alternative for the high-cost scanning method, such as focused ion beam and electron beam lithography.