Electroless Deposited Cobalt-Tungsten-Boron Capping Barrier Metal on Damascene Copper Interconnection

The metal capping barrier deposited by the electroless cobalt tungsten boron (CoWB) alloy plating method for ultralarge scale integration applications was investigated. The CoWB film was formed directly on copper without a palladium catalyst, using dimethyl amin borane (DMAB) as a reducing agent, and it was deposited selectively on 0.25 p.m wide copper interconnects separated with 0.25 mm spacing SiO2. The CoWB thin films were effective barriers against copper diffusion even at CoWB thicknesses as low as 50 nm. Compared with the CoWB film, cobalt tungsten phosphorus films deposited directly on copper using DMAB as a deposition initiator was not effective as a copper diffusion barrier. The plating films contained mainly cobalt with a significant amount of tungsten (up to 20 atom %) and a small amount of boron. Additionally, we propose a newly developed alkaline metal free electroless CoWB plating solution using tetramethyl ammonium hydroxide as a pH adjuster.