Electroless deposition of Co(W) thin films

Thin films of cobalt that contain small amounts of tungsten [Co(W)] were deposited by the electroless process. Those films do not contain either phosphorus or boron which are included in most electroless cobalt films processes. The deposition bath for Co(W) thin films include Co ions, tungstate ions as a source for tungsten, di-methyl-amine-borane (DMAB) complex as a reducing agent, ammonium hydrate as a complexing agent, acetic acid for buffering and surfactants. Co(W) layers were deposited on two types of seed layers: (a) thin sputtered cobalt or copper films on 100 nm SiO₂/Si and (b) bare silicon wafers activated by an aqueous Pd/PdCl₂ solution. The deposited layer thickness range was 40-1000 nm with deposition rate at 90 °C and pH 9 of ∼7 nm/s for both Pd activated Si and sputtered Co seed, and ∼5 nm/s for the sputtered Cu seed. Lowering the temperature to 70 °C lowered the deposition rate to ∼0.7 nm/s for the Pd activated Si. The deposited layers were bright coloured, uniform, and with low defect density under visual inspection. The thin films composition was found to be Cobalt with 3-4 at.% tungsten for all types of seed layers. The Co(W) thin films specific resistivity was in the range of 60-90 μΩ cm. Finally we present the thin film morphology as it was characterized using atomic force microscopy and scanning electron microscopy.