Plasma atomic layer etching of molybdenum with surface fluorination

This work developed a plasma atomic layer etching (ALE) process for molybdenum (Mo) with surface fluorination and ion bombardment. The Mo surface was fluorinated with CHF₃ or C₄F₈ plasmas, and the fluorinated surface was etched by Ar plasma. The fluorocarbon layers were generated with two fluorocarbon sources of CHF₃ or C₄F₈ plasmas on the Mo surfaces. C₄F₈ plasma generates more fluorine-rich fluorocarbons than CHF₃ plasma. The etch per cycle (EPC) of Mo was determined to be 0.8 nm/cycle for CHF₃ plasma and 2.8 nm/cycle for C₄F₈ plasma in the ALE window region. The dependence of the EPC of Mo on the ion energy in the etching step was investigated, and an ALE window was observed in the energy range of 100–225 V. The EPC of Mo increases with increasing Ar plasma time and saturates at 150 s for the CHF₃ plasma and 420 s for the C₄F₈ plasma. Fluorine residue after ALE was as low as 4%, which is lower than radical etching and RIE. The surface roughness was measured at 0.37 nm with the ALE process, which is lower than radical etching and RIE processes.