Tri-layer contact photolithography process for high-resolution lift-off

A novel UV contact lithography process is presented to realize diffraction-limited dimensions in the patterning and lift-off of structures. The process involves a tri-layer stack comprising a bottom layer of lift-off resist (LOR), followed by a back anti-reflection coating (BARC), capped by a layer of I-line optimised photo resist (PR). This stack enables diffraction-limited exposure of PR in a contact mask aligner. The BARC layer decouples the development of the PR and LOR layers into independent process steps that may be optimised separately. It also strengthens structurally the lithographic stack, reducing the likelihood of PR collapse, particularly for small features in close proximity. Moreover, the process may be applied to transparent (e.g., SiO2) or opaque (e.g., Si) substrates without change. We demonstrate the process by realising Au grating couplers aligned to underlying Au plasmonic waveguides on a multilayer substrate supporting Bloch long-range surface plasmons. The grating couplers consist of 16 unit cells arranged in a period of 980 nm, each unit cell comprising a 170 nm thick Au ridge and a duty cycle of similar to 50%. Physical and optical measurements are provided to validate the process.