Ruthenium Layers Electrodeposited from Non-Aqueous Electrolytes as Interdiffusion Barriers for Electronic Interconnects
Authors: Bernasconi Roberto , Lissandrello Federico , Magagnin Luca 
Published: December 2024

Metallic ruthenium is one of the most promising candidates for tantalum and titanium substitution in diffusion barrier layers for microelectronic applications [1]. Ru barriers are characterized by high thermal stability, limited resistivity and great adherence. Moreover, the electrodeposition of copper for the Damascene process can be in many cases carried out directly on the barrier, without the need of a seed layer. Ru peculiar properties allow the controlled deposition of very thin layers, with thicknesses in the few nm range, by means of a wide variety of different techniques.

One of such techniques is electrodeposition, which makes possible the low-cost deposition of Ru layers on conductive substrates. Ru plating typically takes place from aqueous sulfamate based electrolytes, containing the metal in the form of a complex known as μ-nitridobisaquatetrachlororuthenate [Ru2(μ-N)(H2O)2Cl8]3-. These baths are routinely used at the industrial level and provide good coatings at acceptable cathodic efficiencies. However, ruthenium tends to oxidize in aqueous environment and the resulting deposits can contain impurities or oxides. These impurities, acceptable for aesthetic and for wear protection applications, can adversely affect the barrier properties of the thin Ru layers required by microelectronic applications.

In an attempt to deposit high purity layers, the use of non-aqueous electrolytes for the deposition of ruthenium coatings has been proposed. The substitution of water with alternative solvents, in particular, can potentially relieve the problem of ruthenium oxidation. In the context of non-aqueous electrolytes, ruthenium has been deposited from water and air stable ionic liquids [2, 3], from deep eutectic solvents [4] and from alcoholic solutions [5]. The latter case is of particular interest due to very high purity of the metallic layers obtained [5].

Starting from these premises, the present work aims at investigating the barrier properties against copper diffusion of ruthenium layers deposited from a non-aqueous electrolyte based on ethylene glycol. This electrolyte contains the metal in its divalent state. Nanometric Ru layers are deposited on Si substrates and subsequently coated with copper (electrodeposited from a commercial solution). The resulting multilayers are then subjected to high temperature annealing and characterized from the morphological, phase composition and electrical point of view. The observed results are compared with the performances of equivalent Ru layers plated from standard aqueous solutions based on the μ-nitridobisaquatetrachlororuthenate complex.

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[3] O. Mann, W. Freyland, O. Raz and Y. Ein-Eli, Chem. Phys. Lett. 460, 178 (2008)

[4] R. Bernasconi, A. Lucotti, L. Nobili and L. Magagnin, J. Electrochem. Soc. 165(13), D620-D627 (2018)

[5] F. Lissandrello, R. Bernasconi, C. L. Bianchi, G. Griffini and L. Magagnin, Electrochim. Acta 468, 143186 (2023)