Pressure and depth dependence of sidewall roughness of polymer optical waveguides during reactive ion etching

Sidewall roughness (SWR) of fluorinated polyether waveguides fabricated using reactive ion etching in pure oxygen gas was directly measured using atomic force microscope (AFM). We confirmed that SWR is not the replicate of line edge roughness (LER) of the waveguides. We also confirmed the pressure dependence of SWR for shallow structures and discovered an additional etch depth dependence for deeper structures which counteracts the pressure dependence. Lower O₂ pressure etching produces SWR which increases with depth while higher O₂ pressure etching produces declining SWR with depth. The depth dependence at lower pressure is explained by the change in the arrival dynamics of etchant ions in a mechanism involving both shadowing and first order reemission effects.