High-fidelity patterning of AlN and ScAlN thin films with wet chemical etching

We report on the anisotropic wet etching of sputtered AlN and Sc_0.2Al_0.8N thin films. With tetramethyl ammonium hydroxide at 80 °C, the etch rates along the c-axis were 330 and 30 nm/s for AlN and Sc_0.2Al_0.8N, respectively. Although the etching was anisotropic, significant lateral etching below the mask occurred, perpendicular to the c-axis. With a 1 µm Sc_0.2Al_0.8N film, it could be up to 1800 nm. We studied the lateral etching with molybdenum, SiO₂, SiN_x and TiO₂ masks, and found the leading cause for the lateral etching to be modification of the AlN or Sc_0.2Al_0.8N surface caused by ion bombardment and surface oxidation by ambient air. The lateral etching was reduced by optimizing the mask deposition and with thermal annealing. With Sc_0.2Al_0.8N, the lateral etching was reduced down to 35–220 nm depending on the mask, while with AlN, it was reduced to negligible. These results can be used for developing optimised mask deposition processes for better etch characteristics and for microfabrication of AlN and Sc_xAl_1‑xN thin-film structures.