High Purity Tungsten Nanostructures via Focused Electron Beam Induced Deposition with Carrier Gas Assisted Supersonic Jet Delivery of Organometallic Precursors

A substantially enhanced purity of tungsten nanostructures, with up to 95% of metal content relative to carbon contaminants with no postprocessing, is achieved by using a supersonic inert carrier gas jet in the continuum flow regime to deliver the organometallic precursor for focused electron beam induced deposition (FEBID). Through impact-enhanced desorption of residual organic ligands at the deposition site, high kinetic energy of the gas jet assists in completing the secondary electron induced dissociation of precursor molecules, resulting in both increased metal purity and enhanced deposit growth rate. The inert gas jet at low Knudsen numbers also serves as a carrier gas to increase precursor flux to the substrate. Operating in the continuum flow regime reduces jet spreading and allows the use of smaller diameter nozzles to increase localization of precursor delivery to the deposition site. This drastic increase in localized precursor flux to the substrate is shown to compensate for the diminished sticking coefficient resulting from an increased impingement velocity of the continuum-flow gas jet relative to a molecular-flow gas jet. Increasing the jet temperature increases the delivered kinetic energy and allows tuning the balance between enhanced ligand contaminant desorption to increase deposit purity and the reduced sticking coefficient, resulting in the net increase of pure metal growth rate. Relevant physical mechanisms are discussed with support of experimental observations and multiscale numerical simulations of the gas jet, comparing the conventional molecular gas flow versus continuum gas flow. Collectively, our results suggest a promising path forward for delivery of organometallic precursors in gas phase that results in high purity metal deposition without sacrificing the high growth rate.