Anomalous evolution of topology in nanoporous copper under thermal and electrolyte treatment

Abstract

Densification of nanoporous (NP) Cu under low temperatures and in the absence of applied pressure has recently attracted interest thanks to possible applications of NP Cu in performance interconnect or thermal interface materials in the electronics industry. In this work, the evolution of NP Cu structure under electrolyte exposure or heat treatment is examined with a view of uncovering the underlying kinetic processes and devising processing routes towards complete densification. It is demonstrated that electrolyte exposure or heat treatment of the same NP Cu structure yield vastly different final structures. Under electrolyte exposure at room temperature, NP Cu evolves self-similarly, i.e. both ligaments and junctions increase in size with similar scalings, which are consistent with surface diffusion limited models. When NP Cu is heat treated in forming gas, only junctions increase in size at the expense of decreasing ligaments. At temperatures as low as 30% of the bulk Cu melting temperature, heat treatment can result in densification of NP Cu to relative densities greater than 90%. In this case, the scaling of junction size with heat treatment time is consistent with bulk diffusion processes even if T/TM∼0.3.