Water-in-Salt Electrolytes for the Electrodeposition of Ruthenium for Interconnect Applications
Authors: Sides William D. , Huang Qiang 
Published: February 2020

As interconnect trench dimensions continue to scale down, the use of electrodeposited copper faces challenges due to its exponentially increasing resistance as trench widths approach and become shorter than the electron mean free path. This limitation has resulted in the desire to develop electroplating processes suitable for metallization with alternate elements with lower electron mean free paths. Namely, Co and Ru are particularly well-suited to interconnect applications, both of which being less prone to scaling issues than Cu[1]. Whereas much effort has been devoted to developing Co damascene processes[2], electrodeposition processes for Ru are much less developed due to its slow deposition kinetics, which generally entails deposition occurring in the presence of significant hydrogen formation[3]. Many studies of electrodeposited Ru have been performed using various non-aqueous solvents to widen the solvent electrochemical window and deposit Ru in the absence of hydrogen formation[4], however the use of non-aqueous solvents has significant obstacles, such as high impurity incorporation, atmospheric instability, toxicity, and expense, all of which must be overcome before industrial use becomes practical. A super-high concentration of LiTFSI salt has recently been implemented in Li-ion batteries to expand the electrochemical window of the electrolyte[5]. This effect is partially due to a lack of free water molecules, as they are nearly all within the solvation sheaths of the Li salt and are therefore less electrochemically active. A similar approach can be implemented to reduce the activity of water molecules during electrodeposition, in this case by using a super-high concentration of LiCl salt. So-called water-in-salt electrolytes enable reduced hydrogen evolution in high-overpotential electrodeposition while retaining compatibility with current industrial processes designed for aqueous electrolytes.
Water-in-salt electrolytes have been used to permit the electrodeposition of Ru at high cathodic potentials with limited hydrogen evolution. Electrochemical studies show that high concentrations of LiCl suppress hydrogen evolution, inducing a mass-transfer limit of hydrogen evolution at significantly reduced current densities. The significance of limiting hydrogen evolution during Ru deposition is compounded by the observation that fast hydrogen evolution acts to suppress the Ru plating current, a phenomenon that has also been observed in other metals, including Co and Mn. In addition to electrochemical studies, characterizations of Ru films deposited from water-in-salt electrolytes at various conditions will be presented. Focus will be on the crystallization behavior and corresponding resistivity change of films deposited from water-in-salt electrolytes, which has been previously observed to vary significantly from metal films deposited from aqueous electrolytes[6], and is crucially important for interconnect applications.

References

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