Acceleration Mechanism of Triethanolamine in Electroless Bath for Pure Cobalt Deposition

Abstract

The utilization of hydrazine as a reducing agent in the electroless cobalt bath is indispensable for the application of pure cobalt film fabrication in ultra-large-scale integration (ULSI). However, the deposition rate using the ordinary coating bath is relatively low. After conducting numerous experiments in this study, we have made a groundbreaking discovery: the deposition rate of electroless plating pure cobalt bath can be significantly increased from less than 1.0 μ·h−1 to more than 5.0 μ·h−1 by adding triethanolamine (TEA) as an accelerator. This remarkable finding greatly enhances the practical value of the electroless cobalt plating bath and enables its application in microholes filling of cobalt interconnect lines below 10 nm. According to the results of linear sweep voltammetry and mixed potential theory, the addition of TEA essentially forms [Co(C6H5O7)(TEA)2]2 2− complex with more feasible reductivity than that of the [Co(C6H5O7)(H2O)]2 2− in the ordinary plating bath and the cobalt center is favorable to be released under the electroless reduction attack to the unstably strong steric hindrance of TEA, which significantly increases the reduction current of Co(II) ions. After the further optimization of pH, temperature and annealing process, not only does resistivity decrease to 12.1 μΩ·cm, but also the surface morphology and crystallinity improve significantly. Therefore, an applicable electroless cobalt plating protocol with a moderate deposition rate is obtained.