Surface oxidation of Al masks for deep dry-etch of silica optical waveguides

The surface oxidation of Al metal masks in an oxygen plasma was studied for realizing deep dry-etch of silica optical waveguides. The oxidation efficiency of the plasma was found to depend on mainly substrate bias and plasma power. Net sputtering effect happened when ion bombarding potential exceeds certain critical value. However, suitable ion bombarding energy is of benefit to the oxidation process. There was a saturation thickness of the Al2O3 layer, beyond which the growth rate of Al2O3 films became very low. The saturation thickness increased with the plasma power. According to these growth characteristics, the oxide growth mechanisms were discussed, and suitable plasma conditions were chosen for the surface oxidation of Al metal masks. Under the chosen plasma conditions, a thick Al2O3 layer of ∼ 6.5 nm was generated in a short time of 2 min. Thus the surface of the Al metal mask could be periodically oxidized during the breaks of the silica etching process to enable much higher SiO2/mask etching selectivity of ∼ 100:1, in comparison with ∼ 15/1 obtained without the surface oxidization process. This greatly reduced the required Al mask thickness from over 500 to 100 nm for a deep silica etch of over 5 μm, and leaded to the achievement of high-quality silica waveguides with vertical and very smooth sidewalls.