Green etching of indium tin oxide metasurfaces

Metasurfaces have become integral elements of the modern nanophotonics toolkit, finding applications in many areas of photonics. With the recent rise in the use of transparent conducting oxides (TCOs), there is a desire to fabricate metasurface structures directly into materials such as indium tin oxide (ITO) to utilize their tunability and non-linear effects. Current ITO dry etching techniques use harsh chemically active gases such as Cl-2 or environmentally damaging gases such as CH4. As a result, etching processes often require metal masks and aggressive metal etchants that can damage deposited ITO thin films. Here, we present a straightforward reactive ion etch (RIE) using green and environmentally friendly source gases, namely an Ar and H-2 gas mix, together with a soft resist mask. By variation of the process parameters, an optimal etch condition is realised, producing uniform features for nanoscale structures at a steady rate of 15 nm/min for small features (< 50 nm). As a proof of concept, guided-mode resonance (GMR) gratings are fabricated in ITO thin films, with the optical results showing a comparably high reflection and Q-factor to common silicon nitride GMR gratings. Hence, this work provides an environmentally friendly alternative for dry etching ITO, in addition to highlighting the potential of ITO as a photonic material for future applications.