Mechanism of Bismuth Stimulated Bottom-up Gold Feature Filling

The mechanism underlying Bi ³⁺-stimulated bottom-up Au filling and self-passivation in trenches and vias in slightly alkaline Na ₃ Au(SO ₃)₂ + Na ₂ SO ₃ electrolytes is explored. The impacts of electrolyte components Na ₃ Au(SO ₃)₂, Na ₂ SO ₃ and Bi ³⁺ and potential-dependent kinetic factors on the rate of Au electrodeposition are quantified experimentally. Derived parameters are applied within the surfactant conservation Curvature Enhanced Accelerator Coverage model to simulate the filling of high aspect ratio trenches. It is observed that Bi adsorption accelerates the Au deposition rate with a non-linear dependence occurring around a critical coverage. Further, the impact of electrolyte composition is such that gradients of $Au{\left(S{O}_3\right)}_2^{3-}$ and $S{O}_3^{2-}$ derived from reduction of $Au{\left(S{O}_3\right)}_2^{3-}$ during deposition accentuate deposition farther from the feature opening. These factors and surface area reduction at the bottoms of filling features localize active deposition to feature bottoms. Ultimately, weakening of the concentration gradients and associated kinetics as bottom-up feature filling progresses reduces the Bi coverage on the growth front below the critical value and bottom-up growth terminates. Good agreement is observed with key experimental features including the incubation period of conformal deposition, transition to bottom-up growth, subsequent bottom-up filling and finally self-passivation as the growth front nears the field.