Growth of free-standing tungsten nanowires via voltage-energization nanotip-manipulation

In situ transmission electron microscopy demonstrated that the contact of a tungsten (W) nanotip with a W plate and subsequent tensile manipulation under electric voltage energization resulted in the growth of free-standing high-aspect-ratio W nanowires. The structural dynamics of the growth process was directly observed, and simultaneously the mechanical response was investigated on the basis of material-mechanics measurements. W nanocontacts between nanotip and plate elongated at a tensile speed of 1–9 nm/s under direct-current voltages of 0.9–2.5V. An approximately 10 nm diameter region at a positively biased electrode surface adjacent to the contact became a growth point, and atoms in the region aggregated to form and elongate a nanowire. The maximum width and length of the grown nanowires reached 20 nm and 189 nm, respectively. Saw-edge shape variations were observed in strain-stress relations derived during the growth process, exhibiting that the growth speed of the nanowires depended on stress acting on the nanowires. The present nanowire growth occurred based on the expansion of the wire end due to the atom aggregation promoted by voltage energization with the reduction of growth-suppressing stresses by tensile manipulation. When pulse voltage energization and tensile manipulation were performed alternatively, nanowires growth continued. In this condition, the maximum wire width and length increased to 38 nm and 294 nm, respectively. The free-standing high-aspect-ratio nanowires of the high-melting-point metal derived by the present growth method is expected to apply them to nanometer scale interconnections in next-generation high density three-dimensional packaging.