Additives for Superconformal Gold Feature Filling

An overview of the effect of additives on Au electrodeposition from Na₃Au(SO₃)₂ based electrolytes is presented with an emphasis on filling of fully metallized recessed surface features such as trenches and vias. The impact of heavy metals additives Tl⁺, Pb²⁺, and Bi³⁺ is reviewed and accompanied by a brief survey of the effects of Sb³⁺, Te⁴⁺, SeCN^-, 3-mercapto-1-propanesulfonic acid (MPS) and polyethyleneimine (PEI) additions. The addition of Tl⁺, Pb²⁺, Bi³⁺ or Sb³⁺ accelerates the kinetics of Au(SO₃)₂ ^3- reduction to Au, as manifest in hysteretic voltammetry and rising chronoamperometric transients, and yields bright specular deposits. Gold deposition with Pb²⁺ addition exhibits superconformal filling in sub-micrometer trenches while Bi³⁺ addition induces a more extreme bottom-up filling, but Tl⁺ and Sb³⁺ additions yield essentially conformal deposition for the conditions examined. Modest acceleration and hysteresis observed with Te⁴⁺ addition reflect roughening due to limited nucleation, 3D growth and delayed coalescence, rather than catalysis, and are associated with conformal feature filling. Unlike the other additives, SeCN^-, MPS and PEI inhibit the deposition kinetics. Breakdown of suppression during deposition in micrometer size trenches is biased toward recessed surfaces where the flux of suppressor is constrained, yielding localized deposition and superconformal filling.