Electrochemical study of the electroless deposition of Co(W, B) alloys

This paper presents the results of a study of the electroless deposition process and the film properties of cobalt-tungsten-boron [Co(W, B)] thin layers on thin copper films. Two solutions based on cobalt citric complexes and dimethylaminoborane reducing agent were developed. Tungsten was introduced into the solutions by using either sodium tungstate or tungsten acid. The deposited thin-film composition was characterized as a function of the bath formulation by X-ray photoemission spectroscopy. Electroless deposition was studied through electrochemical approach via linear sweep voltammetry analysis of the following two partial reactions: a. metal reduction and b. the oxidation of the reducing agent at the Co(W, B) plated electrodes. In situ mixed potentials of the process for different electrolytes were measured. The deposition rate of cobalt alloys was gravimetrically measured as well. Comparison between direct experimental values of the mixed potential and deposition rate with those derived theoretically from the current-potential curves for partial reactions shows the electrochemical nature of the deposition reaction with current yield value of about 0.7-0.8 and 0.98 for solution with sodium tungstate and alkali metals free bath, respectively. The optimal conditions (i.e., temperature, pH) and composition of aqueous solution for the deposition of Co(W, B) films were defined. Those films can be applied as both barrier and capping layers for ultra-large-scale-integration metallization for integrated circuits and microsystem-technology applications.