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.