Depolarization of Cu electrodeposition in the presence of Ag: A cyclic-voltammetry study

Abstract

Cyclic voltammetry profiles of Cu-Ag acidic sulfate deposition baths at Cu, Cu-Ag, Au or Ru substrates have been systematically studied, revealing the existence of an early Cu reduction peak before the bulk Cu deposition peak. The Cu deposit thickness produced by this early Cu reduction peak is estimated to decrease with scan rate and increase with the [Ag+] concentration in the electrolyte. The early Cu reduction peak generates Cu deposits thicker than 3 nm for the Cu-Ag electrolyte with 2 mM Ag(I). Electrochemical quartz crystal microbalance (EQCM) and CV reveal that the early Cu reduction peak is associated with a Cu(II)/Cu(0) reaction at 65% Coulombic efficiency, slightly smaller than that of the main Cu(II)/Cu(0) reduction peak (∼78%). Directly observing the linear regions in the polarization curves produced by CV, the early Cu reduction peak is estimated to show a Tafel slope ∼30 mV/dec. We hypothesize that the early Cu reduction peak is caused by the catalytic effect of Ag in the substrate or in the initial deposit on the intermediate reduction step of Cu(II)/Cu(I), leading to a shift of the rate determining step of Cu(II)/Cu(0) reduction.