Atomic layer etching of Al₂O₃ with NF₃ plasma fluorination and trimethylaluminum ligand exchange

In this study, a cyclic isotropic plasma atomic layer etching (ALE) process was developed for aluminum oxide that involves fluorination with NF₃ plasma and ligand exchange with trimethylaluminum (TMA). The isotropic plasma ALE consists of two steps: fluorination and removal. During the fluorination step, the Al₂O₃ surface was fluorinated to AlOF_x with NF₃ plasma at 100 °C. The formation of the AlOF_x layer was confirmed by x-ray photoelectron spectroscopy analysis, and the atomic fraction of fluorine on the surface was saturated at 25% after 50 s of plasma fluorination. The depths of the fluorinated layers were in the range of 0.79–1.14 nm at different plasma powers. In the removal step, the fluorinated layer was removed by a ligand exchange reaction with TMA at an elevated temperature range of 250–480 °C. The etch per cycle (EPC) was 0.20–0.30 nm/cycle and saturated after 30 s in the temperature range of 290–330 °C. No etching was observed below 250 °C, and the EPC increased in the temperature range of 250–300 °C during the removal step with the ligand exchange reaction and reached the maximum at 300 °C. Then, the EPC was significantly reduced at high temperatures, possibly due to TMA decomposition. The fluorine atomic fraction on the surface was reduced to 14% after the removal. In conclusion, Al₂O₃ was successfully etched at the atomic scale by the cyclic plasma ALE process. The average surface roughness of Al₂O₃ was reduced from 8.6 to 5.3 Å after 20 cycles of etching.