Thermal atomic layer etching of metallic tungsten via oxidation and etch reaction mechanism using O2 or O3 for oxidation and WCl6 as the chlorinating etchant

Abstract

Atomic layer etching (ALE), offering highly controlled removal of thin film materials, is considered as an enabling process technology for future development of transistor devices. The authors previously reported a thermal tungsten (W) ALE process using WF6 and O2 for temperatures ≥275 °C, and they recently discovered the opportunity for low-temperature W etching using WCl6 as the etchant instead of WF6. This article demonstrates a two-step, thermal W ALE process viable for temperatures ≥200 °C, consisting of an oxidation half-reaction with O2 or O3 and an etch half-reaction using WCl6 as the chlorinating etchant. In situ quartz crystal microbalance (QCM) analysis reveals that W ALE using O2 and WCl6 is self-limiting and proceeds at an etch rate of ∼7.3–8.2 Å/cycle for temperatures between 205 and 235 °C. QCM analysis further reveals a surface dependence in the etch rate of the O2/WCl6 process, where the etch rate is the largest during the first cycle and decreases to a smaller value in later cycles. In addition, the authors show that O3 is a more effective oxidant than O2 for W ALE at lower temperatures; saturation is achieved with a much shorter exposure. Etching of W films on silicon substrates was confirmed using ex situ techniques. Overall, this study increases the understanding of surface reactions in thermal ALE and expands the range of etchants and coreactants that are useful for thermal etching of metallic thin films.