Early stages in the electroplating of copper and silver on gold

Abstract

The early stages of copper deposition on gold were studied, employing the electrochemical quartz crystal microbalance (EQCM). It was observed that a thin layer of copper, of the order of 5 nm or less, causes significant stress, which exhibits itself as an increase in frequency during deposition. Part of the stress is relaxed on open circuit at room temperature. Experiments conducted with silver instead of copper, under otherwise similar conditions, gave no indication of internal stress. The difference in behavior was attributed to the difference in lattice parameters. That of silver is almost equal to gold, while that of copper is about 13% smaller. During copper deposition on copper, the rate of change of frequency with charge is a few percent below that calculated, implying that the apparent Faradaic efficiency is less than 100%. This is attributed to the two-step reduction of Cu 2+ and the diffusion of a fraction of the Cu + ions, formed as intermediates, into the bulk of the solution. A Pourbaix-type representation was developed to identify the regions of potential and concentration in which the Cu + ion is thermodynamically stable.