A novel process for the electrochemical atomic layer etching (e-ALE) of copper (Cu) is presented. In this process, Cu first undergoes surface-limited sulfidization to form a monolayer of copper sulfide (Cu 2 S). The Cu 2 S layer is then selectively etched in hydrochloric acid without etching the underlying Cu. The steps of surface-limited sulfidization of Cu and selective etching of the resulting Cu 2 S are repeated sequentially to achieve a net etch rate of close to one Cu monolayer etched per e-ALE cycle. Surface-limited etching is shown to minimize roughness amplification thereby preserving the near-atomic flatness of the original Cu electrode. Atomic layer etching (ALE) processes are critically important for the precise tailoring of materials and structures in nano-electronics. 1For atomically precise etching of metals, plasma-based approaches are available which generate nonvolatile etch products thereby contaminating the metal surface. Hess and co-workers have developed a two-step process that etches copper (Cu) films with chlorine and hydrogen plasmas at low temperature (below 20• C). This process generates a volatile etch product that minimizes surface contamination. 2In most plasma-based approaches, the metal etching rate is higher than 1 nm per etch cycle. Such high etch rates do not provide the requisite atomic-scale control over etching required in ALE. While plasma-assisted ALE processes for oxides are mature, 1 ALE of metals is still in its infancy and numerous development efforts are currently underway. 3In this communication, we report on an electrochemical approach for the layer-by-layer etching of Cu with atom-scale control over the etching rate. The two-step approach consists of surface-limited electrochemical sulfidization of Cu followed by selective etching of the resulting copper sulfide (Cu 2 S) monolayer. Surface-limited sulfidization has been used previously for fabricating semiconductors. 4 Feasibility of the electrochemical ALE of Cu is demonstrated and process performance parameters (etch rate, surface roughness) are characterized. ExperimentalCyclic voltammetry and chronoamperometry.-Surface-limited sulfidization of Cu was studied using cyclic voltammetry (CV) performed using a three-electrode cell consisting of a sputter-deposited Cu substrate as the working electrode, a platinum (Pt) wire as the counter electrode, and a saturated Ag/AgCl reference electrode (Fisher Scientific). The electrolyte contained 0.1 M potassium hydroxide (KOH, Fisher Chemical) and 0.5 mM sodium sulfide (Na 2 S, SigmaAldrich) and was prepared using de-aerated DI water. A VersaSTAT 3 potentiostat was used for all electroanalytical measurements. All potentials reported below are referenced to the standard hydrogen electrode (SHE). The Cu substrate was rinsed first with ethanol and then with DI water before drying with N 2 . The cleaned substrate was immersed in 2 M sulfuric acid (H 2 SO 4 , Fisher Scientific) for 1 min to remove surface Cu oxides. A potential of -1.2 V vs. SHE was applied for 100 s to further ...

Electrochemical atomic layer deposition (e-ALD) technique offers a simple and effective "wet chemistry" approach enabling highprecision monolayer-by-monolayer deposition of metal films. The process of e-ALD of Au involves lead underpotential deposition (Pb UPD ) followed by its redox replacement by a Au monolayer. The time evolution of the deposit mass during "one-pot" Au e-ALD is known to exhibit a unique three-stage response that is presently not well understood. In this work, we probe this response using voltammetry, electrochemical quartz crystal microgravimetry (e-QCM), and chronoamperometry to unravel the underlying mechanistic events during Au e-ALD. The presence of adsorbed Au +3 -ligand complex(es) (Au-L) on the Au surface is established. In stage I of e-ALD, this Au-L adsorbed layer is reduced to Au while a Pb UPD adlayer is formed. In stage II, the Pb UPD adlayer undergoes spontaneous surface-limited redox replacement by nobler Au under open-circuit conditions. Finally, in stage III, readsorption of the Au-L occurs on the newly deposited Au. Quantitative analysis of the deposit mass transients obtained under a variety of conditions provides an estimate of the Au-L mass and its molecular weight.

The tubular flow-through electrode (FTE) is a facile electroanalytical tool for investigating electrochemical reaction kinetics; however, its suitability for this purpose requires careful design and operation under conditions that guarantee uniform current distribution. In this perspective article, we provide a scaling analysis of the transport and reaction processes within a tubular FTE leading to quantitative guidelines for the FTE design and operation. Two dimensionless parameters (classical Wagner number WaI and modified Wagner number incorporating the effect of mass transport WaII) are utilized to compare the magnitudes of the ohmic, activation and transport resistances of the tubular FTE, and to quantify its current distribution uniformity. With the aid of analytical modeling of the current distribution, optimal ranges for these two dimensionless parameters are defined so as to ascertain uniform current distribution. Application of these guidelines to a graphite tubular FTE is shown to enable the precise determination of the charge-transfer kinetics of the ferri/ferrocyanide redox reaction.

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