Simulating the Influence of Supporting Electrolyte Concentration on Copper Electrodeposition in Microvias

Robust, void-free Cu electrodeposition in high-aspect ratio features relies on careful tuning of electrolyte additives, concentrations, and electrochemical parameters for a given feature dimension or wafer pattern. Typically, Cu electrodeposition in electronics manufacturing of microscale or larger features (i.e., microvias, through-holes, and high-density interconnects) employs a CuSO4-H2SO4 electrolyte containing millimolar levels of chloride and, at a minimum, micromolar levels of a polyether suppressor. Research and optimization efforts have largely focused on the relationship between electrolyte additives and growth morphology, with less attention given to the impact of supporting electrolyte. Accordingly, a computational study exploring the influence of supporting electrolyte on Cu electrodeposition in microvias is presented herein. The model builds upon prior experimental and computational research on localized Cu deposition by incorporating the full charge conservation equation with electroneutrality to describe potential variation in the presence of ionic gradients. In accord with experimental observations, simulations predict enhanced current localization to the microvia bottom as H2SO4 concentration is decreased. This phenomenon derives from enhanced electromigration within recessed features that accompanies the decrease of conductivity with local metal ion depletion. This beneficial aspect of low acid electrolytes is also impacted by feature density, CuSO4 concentration, and the extent of convective transport.