Superconformal Copper Electroplating on an Ultrathin Cobalt Seed in an Acidic Copper Sulfate Electrolyte

Abstract

Cu electroplating on an ultrathin Co seed has been developed for superconformal filling of advanced interconnects, in an acidic CuSO4 electrolyte containing plating additives, i.e., halide, suppressor, accelerator, and leveler. A suppressor-halide adlayer is found to play a bifunctional role in both suppressing Cu growth and inhibiting Co dissolution. Corrosion inhibition is attributed to adsorption of hydrophobic suppressor molecules on a halide-terminated Co surface that blocks water from interacting with Co, thereby retarding the formation of Co(OH)+, a corrosion immediate with which hydronium from the electrolyte would react to form soluble Co2+. With enhanced suppression, Co loss is mainly confined to the removal of native Co oxides in acid. Correspondingly, galvanic Cu deposition forms a monolayer shortly after immersion at open-circuit potential, becoming self-limited with growth of a second layer over the next 20 s as dynamic surface processes make more underlying Co available for the displacement reaction. Growth of the first Cu layer is controlled by the receding of native oxides in an exponential-decay manner. Native Co oxides, if not removed, promote Cu protrusions in electroplating. The proposed process produces void-free fill on a 22 nm wide feature with a Co liner about 20 Å thick after fill.