Enlarging focal depth using epsilon-near-zero metamaterial for plasmonic lithography

Plasmonic lithography can utilize evanescent waves to produce subdiffraction patterns. However, the high loss and shallow depth of patterns severely obstruct its application in practice. In this work, a large focal depth is achieved for deep subwavelength lithography. It is accomplished by employing radially polarized light to excite surface plasmons on a concentric annular grating and combining designed epsilon-near-zero metamaterial to select a high spatial frequency mode, which can shape an evanescent Bessel beam in a photoresist (PR). Moreover, the intensity distribution of the subdiffraction beam can be further enhanced and uniformized by adding reflective layers. It is shown that a needle-like beam with a focal depth of over 500 nm (1.23λ) is formed in the PR layer, and the full width at half maximum of the beam is widened from only 80 nm (0.2λ) to 94 nm (0.23λ). The analyses indicate that this design is applicable for direct writing lithography to produce super-resolution patterns with small feature size, high aspect ratio, and strong field intensity.