The semiconductor industry faces a tremendous challenge in the development of a transistor device with sub-10 nm complex features. Self-limiting atomic layer etching (ALE) is essential for enabling the manufacturing of complex transistor structures. In this study, we demonstrated a thermally driven ALE process for tungsten (W) using sequential exposures of O2 and WF6. Based on the insight gained from the previous study on TiO2 thermal ALE, we proposed that etching of W could proceed in two sequential reaction steps at 300 °C: (1) oxidation of metallic tungsten using O2 or O3 to form WO3(s) and (2) formation and removal of volatile WO2F2(g) during the reaction between WO3(s) and WF6(g). The O2/WF6 etch process was experimentally studied using a quartz crystal microbalance (QCM). We find that both the O2 and WF6 ALE half reactions are self-limiting, with an estimated steady-state etch rate of ∼6.3 Å/cycle at 300 °C. We also find that etching of W proceeds readily at 300 °C, but not at temperatures lower than 275 °C. Thermodynamic modeling reveals that the observed temperature dependence is likely due to the limited volatility of WO2F2. The use of WF6 with O3 in place of O2 also allows W etching, where the stronger oxidant leads to a larger mass removal rate per cycle. However, we find O2 to be more controllable for precise metal removal per cycle. In addition, etched W films were examined with ex situ analytical tools. Using spectroscopic ellipsometry (SE) and scanning electron microscopy (SEM), we confirm etching of tungsten film on silicon substrates. Surface analysis by X-ray photoelectron spectroscopy (XPS) revealed a minimal fluorine content on the W film after partial etching and on the silicon surface after full etching. This suggests that W ALE does not significantly alter the chemical composition of W films. This work serves to increase the understanding of ALE reactions and expand the base of available ALE processes for advanced material processing.
Keywords: ALE; O2; W; W selectivity on SiO2 and Si−H; WF6; atomic layer etching.