Step-wise reduction and reversible deposition of ruthenium in calcium chloride hexahydrate based eutectic solvent

The controlled deposition of a nanometre-thick layer of ruthenium as barrier material is highly desired in the micro-electronics industry but not without technical challenges. For the first time, fully reversible electrodeposition followed by subsequent stripping of ruthenium metal from calcium dichloride-based hexahydrate and ethylene glycol (EG) based eutectic solvent (CaCl 2 ∙6H 2 O:2EG) has been performed. The work explores the electrochemical characteristics of Ru 3+ on a glassy carbon disc electrode, where, for the first time, a voltammogram showing stepwise electron transfers of Ru 3+ was reported in ambient conditions. In the absence of additives and brighteners, ruthenium was found to deposit homogeneously onto the surface of glassy carbon as well as copper electrodes at constant applied potentials (−0.45 V vs 5 mM Ag/AgCl ref), leading to a mirror-like surface finish. The thickness of the deposition layer could be finely tuned with deposition time. Scanning electron microscopy/ energy dispersive X-ray analysis revealed a highly homogeneous and crack-free nanometre-thick layer of ruthenium. The results herein indicate that CaCl 2 .6H 2 O:2EG is a better eutectic solvent to deposit otherwise hard-to-plate metals. New insights from this work highlight the importance of establishing a high potential drop at the electrode-solution interface to facilitate the deposition of hard-to-deposit metals as well as the need for inert cations with high Lewis acidity to reduce the activity of surface water molecules. In the case of CaCl 2 ∙6H 2 O:2EG, we infer from the reversible and stepwise redox activity of ruthenium that water molecules are tightly bound to Ca 2+ ions at the Helmholtz layer, thereby reducing the activity of water.