Experimental Studies of Surface Transport Aspects of Electrodeposition and Corrosion

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ABSTRACT CTS-9527497 The goal of this effort is to develop and apply a methodology to measure surface diffusion on electrodes at the solid-liquid interface. The approach uses voltammetry, differential capacitance, and surface second harmonic generation (SHG). Systems studied include lead on gold (111) and chlorine, lead, or copper on silver (111). Experiments on vicinal gold (111) use a purely voltammetric measurement to obtain the relative coverage on terraces and steps; the dependence of this partitioning on the deposition rate from dilute solutions gives an indirect measure of surface diffusion. Direct measurements of surface diffusion. Direct measurements of surface diffusion on silver (111) use surface SHG, a nonlinear optical probe of adsorption capable of submonolayer selection; these experiments are two types: measurement of the decay of a surface diffraction grating, and measurement of the decay of a one-dimensional concentration step. The first technique employs laser-induced thermal desorption (LITD) to create a diffraction grating and uses SHG to monitor its rate of decay, which is caused by surface diffusion. The second techniques involves creation of a step concentration profile, either by LITD or by masked vacuum evaporation, the decay of which is monitored by SHG. A Boltzmann-Montano analysis of the decay gives the coverage-dependent diffusivity that is needed to describe macroscopic electrodeposition processes. The SHG signal is related empirically to surface coverage by two electrochemical methods: stripping voltammetry and differential capacitance. Surface diffusion is one of the fundamental steps in electrodeposition and electrodissolution, and is rate-limiting in some industrial situations. Nevertheless, the phenomenon is little studied because it is experimentally inaccessible. This study aims to rectify this with the eventual goal of incorporating microscopic information on surface diffusion and nucleation into feature-scale (morphological) simula tion of sub-half-micron electrodeposition of noble-metal interconnects and electrical contacts. ***