LIPSS in thin films as a large-scale mask for deep etching of sub-micron photonic structures

Abstract

Reactive ion etching (RIE) processes are often involved in the fabrication of high aspect ratio structures. While a material with high etch selectivity to substrate is preferred, it can be compensated for by increasing the thickness of the mask. However, fabricating a mask that is both thick and maintains high lateral resolution is a difficult technological task. Laser-induced periodic surface structures (LIPSS) offer a promising route to surpass the diffraction limit in conventional photolithography. Here, we demonstrate the formation of sub-micron LIPSS using femtosecond laser processing as a rapid, wafer-scale alternative for fabricating hard masks suitable for RIE. We identify optimal conditions for LIPSS formation in chrome thin films by varying the spatial pulse overlap of a 1030 nm wavelength Yb:KGW laser and demonstrate how LIPSS formation regime transitions from short-range to long-range order in the fluence interval from 147 mJ/cm² to 245 mJ/cm². Under optimized conditions, we produce nearly ideal patterns that we subsequently employ as a mask for diffraction grating fabrication. We transfer these periodic structures into silicon using RIE, resulting in 880 nm period linear diffraction gratings. Measurements show that the structures formed using the highest laser fluence have < 1 % reflectance in Vis-nIR. Whereas LIPSS made with 220 mJ/cm² fluence indicated 1.3 % relative diffraction efficiency in reflection at 635 nm wavelength. Scalability of high-quality diffraction grating origination technology under 245 mJ/cm² fluence was evidenced by wafer-scale patterning and feasibility for replication in PDMS, carrying 8.2 % diffraction efficiency in transmission at 405 nm wavelength.