Low-temperature plasma atomic layer etching of molybdenum via sequential oxidation and chlorination

In this study, an atomic layer etching (ALE) process for molybdenum was developed in two steps: plasma oxidation and plasma chlorination. In the plasma oxidation step, molybdenum was oxidized with oxygen plasma to form molybdenum oxide. As the plasma oxidation time increased, the atomic ratio of O-to-Mo, determined by x-ray photoelectron spectroscopy, increased, and then saturated to a value of 2.3. The oxidation depth of molybdenum was found to increase with increasing oxidation temperature—from 3.0 nm at 40 °C to 22.0 nm at 300 °C. It also increased with increasing RF (radio frequency) power—from 2.0 nm at 5 W to 5.5 nm at 25 W. In the plasma chlorination step, it is believed that molybdenum oxide was removed from the surface by forming molybdenum oxychloride (MoOCl₂, MoOCl₄, and MoO₂Cl₂) in chlorine plasma in the temperature range of 40–300 °C. The etch per cycle (EPC) continuously increased at temperatures above 100 °C; however, at temperatures below 40 °C, it was saturated. The RF power increased the EPC from 2.2 to 5.8 nm/cycle in the range of 5–25 W. It was found that the removal depth matched the oxidation depth at each RF power in ALE at 40 °C. The atomic composition of molybdenum after ALE was almost identical to that before ALE. This study demonstrates that the ALE of molybdenum at 40 °C can be realized by sequential plasma oxidation and chlorination.