(Invited) Electroless Rhodium Deposition for Metallization of Advanced Interconnects
Authors: Philipsen Harold , Viviani Prisca 
Published: July 2021

Over the past decades, the semiconductor industry managed to continuously increase performance at an impressive, steady rate by scaling all parts of the integrated devices to nanometer-size dimensions, thereby enormously increasing the number of transistors on the chip as well as decreasing the dimensions of the metallization [1]. Presently, Cu constitutes the largest material fraction as conductor in damascene trenches which can be deposited by electrochemical deposition (ECD) or electroless deposition (ELD) [2 – 6]. For next-generation interconnects, the material properties (electromigration) of Cu render it unreliable for producing ever more performant chips. Additionally, the necessity of a diffusion barrier layer for Cu strongly increases line resistance and negatively impacts device performance upon further down-scaling. Recently, various alternative metals, such as ruthenium, rhodium, and molybdenum, have been studied and start to be implemented to tackle performance losses due to the small dimensions of the metal structures [7 – 9]. Consequently, both wet-chemical deposition as well as controlled etching of these metals has become of interest [10 – 12].
In our contribution, we present results of the development of an electroless deposition bath for rhodium, using ruthenium (II) hexammine as reducing agent. An electrochemical characterization of the anodic and cathodic reactions involved was performed in hydrochloric acid and ammonium chloride solutions. The stability of Rh3+ and [Ru(NH3)6]2+ was monitored using UV-vis spectroscopy. It was found that under oxygen-free conditions the hexammine complex can be kept stable in the ammonium-containing solution.
Initially, deposition experiments were performed on PVD-Pt blanket thin films, for which a non-uniform coverage was observed. Therefore, to enhance the nucleation of the ELD process, we first deposited a thin Rh layer by electrochemical deposition, after which Rh was deposited using our ELD bath. Transmission electron microscopy and elemental mapping was used to characterize the morphology and chemical composition, respectively. Upon thermal treatment (forming gas, 420 °C, 20 min) crystal grain growth was observed.

Acknowledgements

The authors would like to thank the partners of IMEC's Industrial Affiliation Program (IIAP) on Advanced Interconnects.

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