Development of an alkali-metal-free bath for electroless deposition of Co-W-P capping layers for copper interconnections

A bath containing alkali-metal-free chemicals was developed for electroless deposition of Co-W-P thin films on a copper substrate and an optimisation of bath compositions was made. Ammonium cobalt sulphate, ammonium tungstate and ammonium hypophosphite were used as the precursors of cobalt, tungsten and phosphorus, respectively. Dimethylamine borane and ammonium citrate were used as reducing and complexing agents, respectively. It was found that the cobalt content, film thickness and grain size increased with increase in cobalt ion concentration in the bath. Tungsten in the films increased from 1 to 6 at.% when its concentration in the bath was increased from 0.001 to 0.009 M. A variation of phosphorus content from 2 to 12 at.% was made by increasing its concentration from 0.01 to 0.05 M. It was found that the deposition rate decreased with increasing citrate ion concentration. Amorphous films were obtained when the combined amount of phosphorus and tungsten exceeded 12 at.% in the films. The crystalline film had small spherical crystallites with diameter less than 40 nm.