Thermal Atomic Layer Etching of Copper via Sequential Chlorination and Volatility-Controlled Hydration

Abstract

Thermal atomic layer etching (ALE) of metals is fundamentally constrained by the low volatility of metal halides, which has limited the scalability, selectivity, and temperature flexibility of dry etching processes. Here, we introduce a hydration-activated volatilization strategy that establishes coordinated water molecules as an active chemical handle to overcome this long-standing limitation in thermal metal ALE. Using copper (Cu) as a model system, we demonstrate a two-step thermal ALE process in which sequential chlorination with sulfuryl chloride (SO₂Cl₂) forms surface CuCl₂, followed by a controlled hydration step with H₂O vapor that converts the non-volatile CuCl₂ into highly volatile CuCl₂•2H₂O. This hydration-induced phase transformation dramatically enhances volatility at low temperatures, enabling intrinsically thermal ALE by directly coupling hydration to volatility control, without reliance on organic ligands or plasma assistance. The process exhibits clear self-limiting behavior in both the chlorination and hydration half-reactions, with temperature-dependent etch rates ranging from 0.04 to 1.10 nm per cycle over 75-175 ℃. In situ quartz crystal microbalance measurements directly confirm cyclic mass gain and removal associated with surface modification and volatilization, while X-ray photoelectron spectroscopy and Raman spectroscopy verify the formation and removal of Cu-Cl-based surface species. Surface morphology evolution during etching is systematically examined using atomic force microscopy and scanning electron microscopy. Thermodynamic analysis further supports the energetic favorability of the SO₂Cl₂-driven chlorination pathway. By decoupling halide formation from volatilization through hydration-enabled volatility control, this work establishes a new design paradigm for thermal metal ALE that extends beyond conventional oxidation-based routes. This strategy provides a general framework for atomic-scale recess engineering of Cu and potentially other advanced interconnect materials, offering new opportunities for scalable three-dimensional integration in future electronic devices.