(Invited) Electroless Deposition of Ruthenium Using Sodium Borohydride Reducing Agent: A Mechanistic Study Using Electrochemical Quartz Crystal Microbalance

Further miniaturization of interconnects and devices to keep up with Moore’s law in the 21st century requires materials innovation.  Copper, as the interconnect metal has been the workhorse of the semiconductor industry for the past two decades. As wire dimensions shrink below 20 nm in future technology nodes, the ability of copper to meet the conductivity and reliability benchmarks is strained1.  One metal that becomes attractive at smaller dimensions (<10 nm) is ruthenium, due to its high conductivity and electromigration resistance2.  Traditional routes for metallization of ruthenium includes physical vapor (PVD), chemical vapor (CVD) and atomic layer deposition (ALD).  PVD suffers from its line-of-sight, non-conformal deposition process that is not suitable for narrow, high aspect ratio geometries.  CVD and ALD processes are slower and more expensive than PVD.  Here we present an electroless deposition process (ELD) for ruthenium that overcomes some of the obstacles associated with the aforementioned processes.  ELD is inherently selective which yields conformal coating in challenging geometries.  Furthermore, ELD has lower cost of ownership due to relatively inexpensive precursors and significantly higher deposition rate than ALD and CVD.  Here, sodium borohydride is used as the reducing agent for electroless deposition of Ru, and the effects of temperature, precursor and ligand concentrations on ruthenium growth rate are characterized.  A linear increase in deposition rate is observed with increasing temperature and ruthenium concentration.  A critical borohydride concentration is observed at ~2 g/L, where the maximum deposition rate is obtained. The mechanism of the ruthenium deposition process (reduction half-reaction) is further investigated via electrochemical quartz crystal microbalance (EQCM) analysis to probe the role of pH and complexation.  Ru deposition on Cu initiates at a potential of -1.05 V vs. Ag/AgCl reference electrode, and further analysis indicates that a high pH of the electrolyte is needed to suppress any side reaction(s) and to form kinetically active ruthenium-hydroxy complexed species.  Finally, some challenges to integration and different applications for an electroless deposited Ru are highlighted.    

Figure 1: (a) Linear potential scan of the background electrolyte and the electrolyte containing Ru precursor. Increase in current density is observed at potential <-1.05 V vs. Ag/AgCl.  Coupled quartz crystal microbalance analysis (b) confirms Ru deposition at potentials more cathodic than -1.05 V vs. Ag/AgCl.

References:

1.         International Technology Roadmap for Semiconductors, www.itrs.net

2.         K. Sankaran, S. Clima, M. Mees, C. Adelmann, Z. Tokei and G. Pourtois. Proceedings of the IEEE International Interconnect Technology Conference, 193 (2014)