Tungsten atomic layer deposition on cobalt nanoparticles.

Tungsten (W) atomic layer deposition (ALD) was performed on cobalt (Co) nanoparticles using WF₆ and Si₂H₆ as reactants. A variety of techniques were then applied to analyze both the Co nanoparticles and flat Co substrates after W ALD. Analysis of the W ALD-coated Co nanoparticles is complicated because a CoO layer may exist on the Co nanoparticles and a WO₃ layer may be present on the W ALD coating. LECO measurements quantified the oxygen weight percent in the W ALD-coated Co nanoparticles. The oxygen weight percent decreased with increasing number of W ALD AB cycles. To determine the location of this oxygen, x-ray reflectivity (XRR) investigations measured the WO₃ film thickness on flat W ALD films. The XRR measurements yielded a WO₃ film thickness on flat W ALD films of ∼20 Å. X-ray photoelectron spectroscopy (XPS) studies also quantified the relative oxygen abundance at the W/Co interface for W ALD on flat Co films. The XPS measurements revealed that nearly all the oxygen was in the WO₃ layer on the W ALD film. Only an immeasurably small amount of oxygen was bonded as CoO at the W/Co interface. To determine the thickness of W ALD film on the Co nanoparticle, surface profilometry of W ALD on flat Co substrates measured a W ALD growth rate of 3.9 Å per AB cycle. A geometric model was then constructed to incorporate the information from all the measurements on Co nanoparticles and flat Co substrates. Excellent agreement between the geometrical model and the oxygen weight percent versus the number of W ALD cycles was obtained when the CoO thickness was negligible and the WO₃ thickness on the W ALD layer on the Co nanoparticles was 28.5 Å. This agreement indicates that the details of ALD on nanoparticles can be unraveled by a concert of techniques even when interfacial layers can form due to the high reactivity of nanoparticles. The W ALD-coated Co nanoparticles may be useful in fabricating WC–Co hardmetals with enhanced mechanical properties. [ABSTRACT FROM AUTHOR]