Electrochemical Filling of Nanoporous Gold with Silver via Surface-Limited Redox Replacement

Abstract

Nanoporous (np) metals feature a unique network of interconnected pores and ligaments structured at a length scale of a few to hundreds of nanometers. As a result, np metals have an inherently higher surface area-to-volume ratio resulting in lower melting temperature compared to their bulk metal counterparts. Most applications of np metals have been associated with electrocatalysis. However, in recent years, a growing interest in the use of np metals in microelectronics have been pursued. Among these applications is the development of np-Cu-based chip-to-substrate interconnections that is achieved by Cu-on-Cu bonding. np-Cu is an ideal candidate for solder alternatives because of its relatively lower melting and sintering temperatures. Furthermore, the incorporation of a thin film of Sn in the pores and/or the surface of the np-Cu ligaments could further lower its sintering temperature while maintaining low resistivity. In order to fabricate these composites, there must be strict control over the uniform and flat deposition of the Sn thin film on np-Cu.Here we present a proof-of-concept study on the development of an all-electrochemical metal-filling of np structures. Conventional methods like electron beam induced deposition and overpotential deposition (OPD) have been developed for the filling of porous structure. However, these approaches showed only partial filling that are also accompanied by the growth of three-dimensional protrusions forming outside of the pores. In order to overcome these complications, we developed an approach based on electrochemical atomic layer deposition by surface-limited redox replacement (SLRR). This method allows for a layer-by-layer deposition of metals that has been shown to form relatively flat and uniform deposits. In this study, the model porous structure is np-Au, which is prepared by the selective de-alloying of pre-deposited Au-Cu alloys. The np-Au structure was then filled with Ag via SLRR (in a one-cell configuration) that is realized by successive cycles of the adsorption of a Pb monolayer, serving as a sacrificial metal, that is immediately replaced by Ag via electroless galvanic displacement.A comparative study suggests that the SLRR approach results in a more uniform Ag-filling compared to a stepwise potentiostatic OPD approach that was also done independently. Morphological characterization via surface and cross-sectional scanning electron microscopy (SEM) reveals progressive filling of np-Au with Ag. Additionally, electrochemical BET characterization via cyclic voltammetry of Pb-underpotential deposition (Pb-UPD) was also used to monitor the filling process by measuring the electrochemical surface area of the Ag-filled np-Au composite. The findings from this study will be discussed in detail. Reference:Castillo, Y. Xie, and N. Dimitrov “Filling in nanoporous gold with silver via bulk deposition and surface-limited redox replacement approaches” Electrochimica Acta, 2021, 380, 138196.