Dealloying Multiphase AgCu Thin Films in Supercritical CO2

Abstract

Multiphase AgCu thin films were dealloyed using a mixture of hexafluoroacetylacetone (hfacH) and H2O2 dissolved in supercritical CO2. AgCu alloys exhibit eutectic phase behavior, allowing the composition of the two phases to be fixed while varying the average size of the phase domains from 250 to 1000 nm by increasing the annealing temperature. Selective removal of Cu from both phases was observed, and higher concentrations of the etching solution increased the etching rate between 45 and 75 °C, where the reaction exhibited an apparent activation energy of ∼38 kJ/mol. The morphology after dealloying was quantified using the fast Fourier transform power spectrum obtained from electron microscopy images. For phase domains smaller than ∼500 nm, Ag atoms released in the open regions that had been occupied by the Cu-rich phase diffused separately or as clusters to the Ag-rich phase, forming a nanostructured morphology that mimicked the starting microstructure. For larger domains, stable Ag clusters (50 nm diameter) formed in the open regions, because the diffusion limit was reached, yielding an estimate of 4 × 10–12 cm2/s for the surface diffusivity in supercritical CO2.