| In galvanic displacement, metal ions from solution are reduced and deposited onto a substrate, which is oxidized and dissolved to complete the oxidation-reduction reaction. This method can deposit metals selectively onto oxidizable substrates, but applications of galvanic displacement have been limited by the ability to control of film properties, such as morphology and adhesion to the substrate.; Cu was deposited onto Si rotating-disk electrodes (RDEs) to quantify the influence of mass transfer and reaction kinetics on Cu deposition rates. The rate-limiting process was determined by the relative concentrations of CuSO4 and HF, the Si etchant. Deposition rates were first order in CuSO4 concentration and independent of HF concentration when HF was in large excess ([HF]/[CuSO4] > 20). Deposition rates increased with increasing HF concentration for [HF]/[CuSO4] <20, demonstrating that Si dissolution limited deposition rates under these conditions. Mixed-potential theory was used to accurately predict deposition rates and open-circuit potential (OCP) values from the driving forces for the oxidation and reduction half-reactions.; Atomic-force and optical microscopies were used to probe the influence of the rate-limiting step on the structure of Cu films. Rough, discontinuous films resulted when Si dissolution limited deposition rates, while smooth films formed when Cu2+ reduction was the lone rate-limiting step. For large excesses of HF ([HF] = 3.0 M, [CuSO4] = 0.001-0.02 M), internal stress caused films to buckle and partially detach from the Si when film thicknesses exceeded 150 nm, leading to a decrease in deposition rates.; The effects of stabilizing Cu cations in solution were probed by using NH4F as the Si etchant. Deposition rates decreased with increasing mass transfer rates in NH4F solutions, in contrast to results in HF. A deposition model was developed which revealed that NH3 stabilized Cu+ intermediates, and these species diffused into the bulk solution before subsequent reduction and deposition as Cu0. Film properties, particularly adhesion to the substrate, improved when ascorbic acid was added to these solutions. AFM images of the underlying Si revealed enhanced local dissolution of Si, which promotes adhesion by increasing the surface area of Si in contact with the Cu film. |
