A survey of the effect of cationic, anionic and non-ionic surfactants on the rate and morphological evolution of nickel electrodeposition from a Watts-type bath is presented. Particular attention is given to the prospect for void free filling of sub-micrometer trenches. Cationic species such as protonated, nitrogen-bearing polyethyimmine (PEI) and cetyl-trimethy-ammonium (CTA+) yield significant inhibition of nickel deposition. For a range of concentrations the single cationic surfactant systems exhibit hysteretic voltammetric curves that, when corrected for ohmic electrolyte losses, reveal an S-shaped negative differential resistance. Void-free bottom-up feature filling is observes when operating at potentials within the hysteretic regime whereby metal deposition begins preferentially in the mist densely patterned regions of the wafer followed by propagation of the growth front laterally across the remaining wafer surface. In contrast, at low overpotentials and concentrations sulfur-bearing additives such as thiourea (TU) exert a depolarizing effect on nickel deposition and negligible hysteresis. When PEI and TU are both present, the suppression provided by PEI is diminished and feature filling leads to uniform deposition on the wafer scale. Suitable combinations of PEI and TU enable near void-free filling of >230 nm wide trenches with sloping (~3.5 degrees) sidewalls. Initial conformal growth is followed by geometric leveling once the deposits on the sloping sidewalls meet.
Citation: Journal of the Electrochemical Society
Pub Type: Journals
electrodeposition, nickel, surfactants, trenches