Anisotropic atomic layer etching of W using fluorine radicals/oxygen ion beam

Atomic layer etching (ALE) has advantages such as precise thickness control, high etch selectivity, and no-increase in surface roughness which can be applied to sub 10 nm semiconductor device fabrication. In this study, anisotropic ALE of tungsten (W), which is used as an interconnect layer and gate material of semiconductor devices, was investigated by sequentially exposing to F radicals by NF₃ plasma to form a WF_y layer and following exposure to an oxygen ion beam to remove the WF_y layer by forming volatile WO_xF_y at room temperature. A wide ALE window of F radical adsorption time of (≥ 10 s/cycle) and O_x ⁺ ion desorption time of (10 ≤ t ≤ 50 s/cycle at + 44–51 eV of O_x ⁺ ion energy) could be identified, and at the ALE conditions, a precise etch rate of ~2.6 Å/cycle was obtained while increasing the W etch depth linearly with increasing the number of etch cycles. At the optimized W ALE conditions, the W surface roughness after the W ALE was similar to the as-received W and the etch selectivity over SiO₂ was close to infinite. However, after the W ALE, ~ 10% F diffused into W was observed on the etched W surface, and which could be removed by a following process.