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Electrochemical atomic layer etching (e-ALE) is a unique approach for etching metals one atomic layer at a time. If practiced under optimal conditions, e-ALE ensures minimal evolution of surface roughness due to the atomic layer-by-layer etching characteristics. During e-ALE of copper (Cu), the crucial first step is the formation of a cuprous sulfide (Cu₂S) monolayer via the surface-limited sulfidization reaction. In this paper, we investigate the surface coverage of this sulfide layer as a function of the sulfidization potential, and show that the equilibrium coverage attained can be modeled using the Frumkin adsorption isotherm. At a potential of –0.74 V vs SHE, sulfidization provides near-complete monolayer coverage of Cu by Cu₂S, which then facilitates e-ALE in a layer-by-layer etching mode thereby maintaining a smooth post-etch surface. Operation at potentials negative with respect to –0.74 V provides sub-monolayer coverage, which manifests in roughness amplification during etching. This work provides a thermodynamics-guided foundation for the selection of operating conditions during Cu e-ALE. 

A novel process for the electrochemical atomic layer etching (e-ALE) of ruthenium (Ru) is described. In this process, the surface Ru is electrochemically oxidized to form a monolayer of ruthenium (III) hydroxide—Ru(OH)₃. The Ru(OH)₃monolayer is then selectively etched in an electrolyte containing chloride (Cl^–) species. This etching process is selective towards Ru(OH)₃and does not attack the underlying Ru metal. Adsorbed Cl^–on the Ru electrode is then cathodically desorbed before the sequence of Ru oxidation and Ru(OH)₃etching is repeated. This e-ALE sequence is shown to etch Ru at approximately 0.5 monolayer per cycle while practically avoiding any surface roughness amplification. The proposed Ru e-ALE process uses a single electrolyte which eliminates the need for electrode transfer or electrolyte switching between process steps. In this report, we employ electrochemical, microscopic and spectroscopic techniques to gain insights into the various characteristics of the Ru e-ALE process.