Impact of plasma conditioning-state evolution on Cl2/Ar plasma-assisted Si atomic layer etching

Plasma-assisted atomic layer etching (ALE) requires tightly controlled plasma conditions to achieve reproducible self-limiting material removal; however, the temporal stability of conditioned plasma environments and their influence on etching behavior remain insufficiently understood. In this work, the influence of conditioning state on plasma-assisted Si ALE using Cl2/Ar plasma chemistry is systematically investigated through in situ optical emission spectroscopy correlated with etch characterization. Time-resolved plasma diagnostics reveal strong idle-time-dependent evolution of the plasma environment, where oxygen-related emission initially dominates after extended idle periods before gradually transitioning into a stabilized chlorine-active regime associated with effective etching-related plasma–surface interactions. Corresponding cross-sectional scanning electron microscopy demonstrates a clear correlation between plasma conditioning state and etching characteristics, including negligible etching during early conditioning stages, rough and nonuniform etching during transitional stages, and smooth self-limiting etching under fully stabilized conditions with an etch per cycle of ∼13.5 Å/cycle. In contrast, continuous Cl2 plasma reactive ion etching exhibits comparatively stable plasma characteristics and etching behavior regardless of prior conditioning history under the present conditions. These results demonstrate that plasma-assisted ALE exhibits substantially greater sensitivity to conditioning history and plasma-environment evolution than conventional continuous plasma etching due to its self-limiting surface-reaction nature. The findings further highlight the critical importance of controlling conditioning-dependent plasma dynamics for achieving stable and reproducible atomic-scale etching processes.