Simulating Cu electrodeposition in high aspect ratio features: Effect of control mode and uncompensated resistance in S-NDR systems

Abstract

Void-free Cu electrodeposition in high aspect ratio features requires, at a minimum, an additive package containing micromolar halide and polyether that combine to form a co-adsorbed adlayer that inhibits metal deposition on the electrode interface. Successful feature filling relies on preferential growth proceeding from the most recessed surfaces where sustained breakdown of the polyether-halide suppressor layer occurs. Localization is the result of positive feedback between inhibitor breakdown and metal deposition subject to transport limitations of the suppressor precursor(s). This gives rise to a S-shaped negative differential resistance (S-NDR) that, convolved with uncompensated ohmic resistance, results in electrode bifurcation into active and passive zones. The interplay between the additive derived S-NDR behavior, uncompensated cell resistance, and potentiostatic regulation is explored in comparison to galvanostatic feature filling. Uncompensated resistance arises from the working electrode contact and electrolyte between the working and reference electrodes. For a CuSO4 – H2SO4 electrolyte containing 80 µmol/L Cl− and 40 µmol/L polyether, simulations of potentiostatic deposition with minimal uncompensated resistance reveal a narrow window between fully passive and voided feature filling; bottom-up filling terminates prematurely even under the most favorable conditions. In contrast, optimized galvanostatic operation enables void-free feature filling with termination dictated by coulometry. Increasing the uncompensated resistance along with application of accordingly more negative applied potentials produces filling dynamics that blend the positive attributes of galvanostatic and potentiostatic deposition to enable complete, void-free feature filling with spontaneous passivation near the feature opening. Importantly, these beneficial filling effects are also evident for trench arrays having variable widths or heights.