Electric field induced motion of metallic droplets: application to submicron contactor

H. Dallaporta (1) , M. Prestigiacomo (2) , F. Bedu (1) , D. Tonneau (1) , D. Chatain (1) , P. Sudraud (3)

1 CINaM - Centre Interdisciplinaire de Nanoscience de Marseille
2 CRMCN - Centre de recherche de la matière condensée et des nanosciences
3 ZAC Saint Charles - Orsay Physics

This article reports the monitoring of reversible displacement of a gallium droplet on a tungsten submicron wire deposited by focused ion beam from tungsten hexacarbonyl precursor. The authors demonstrate that by applying a voltage to the wire terminals, the internal electric field created along the wire produces the motion of the droplet. Since the matter involved in this displacement is conductive, the authors show that it is possible to build a submicron electrical switch. Contact can be switched on and off between two electrodes separated by a submicron gap, by electrical monitoring the position of the conductive droplet. In nanoscience, connections of nano-objects are needed to demonstrate the peculiar electrical properties of such objects. A lot of studies have been devoted to direct writing of high quality metallic wires by local decomposition of gaseous molecules induced by tightly focused ion or electron beams. 1–4 These techniques are particularly suitable to link the micron world optical lithography to the nanoworld. Cross beam systems coupling a focused ion beam FIB and a scanning electron microscope SEM equipped with a gas injection system have been widely used in microelectronic industry for integrated circuit rewiring at the submicron scale. The deposited materials are often contaminated by C and O coming from the decomposition of the ligands of the precursor molecules, usually organometallic or metal organic molecules. Furthermore, extra contamination of the deposited wires occurs by implantation of gallium atoms coming from the Ga + ion beam itself. Consequently, the metal wire resistivity is usually up to 100 times higher than the bulk one. Tungsten nanowires 200 nm wide by 80 nm thick have been fabricated by a focused ion beam induced deposition process 5,6 FIBID from the dissociation of WC0 6 molecules and studied in our cross beam system equipped with in situ electrical measurements. A current density above a threshold value 5,6 of 1.4 10 7 A / cm 2 induces a wire annealing by Joule effect which improves their electrical conductivity. The high current density induces the exudation of gallium out of the wire and its merging as several droplets Fig. 1. The exudation of gallium is closely related to the light 3% decrease of the wire resistance between 1.1 and 4 V Fig. 1. Droplet formation along the wire takes place when the voltage reaches about 2.5 V. This value increases with the initial value of the resistance of the wire to get the current density threshold required. Increasing the voltage up to 4 V drives all the droplets toward the lowest potential electrode, leading to the formation of a unique droplet of about 550 nm in diameter SEM image inserted in Fig. 1. For a voltage above 4 V, the resistance of the wire drops due to the nonreversible crys-tallization of the tungsten wire. 5,6 The value of the resistance after the first ramping can be ten times less than the value of the resistance of the as-deposited wire. The droplet contains about 90 at. % of gallium atoms and 10 at. % of oxygen atoms as observed by ex situ energy dispersive x-ray analysis. Most of the oxygen contained in the droplet is a consequence of the sample transfer through air from one SEM to the other. A gallium droplet, which has been formed at one extremity of a wire, as explained previously, 5,6 migrates toward the opposite terminal if the voltage is reversed. The droplet is always driven toward the lowest potential electrode when the voltage applied 2.5 V induces a current density of about 1.4 10 7 A / cm 2 through the tungsten wire. Droplet position change was followed in situ by SEM. Figure 2 displays three images grabbed from a SEM video, separated in time by 0.6 s between Figs. 2a and 2b and 0.4 s between Figs. 2b and 2c. We started with one drop-let nearby the left electrode and slowly increased the voltage. One can see that the left droplet diameter decreases slightly while the right one increases. It demonstrates that the gal-lium transfer occurs from the left electrode to the right one by changing the polarity of the voltage between the two electrodes. During the transfer, the W wire remains unchanged between the initial and final states. It should be stressed that instead of moving at once from one point of the wire to the other, the droplet diameter decreases while simultaneously a Electronic

Domaines: Science des matériaux [cond-mat.mtrl-sci]

H. Dallaporta, M. Prestigiacomo, F. Bedu, D. Tonneau, D. Chatain, et al.. Electric field induced motion of metallic droplets: application to submicron contactor. Journal of Vacuum Science and Technology, 2010, 28 (4), pp.L35-L38. ⟨10.1116/1.3462963⟩. ⟨hal-00523864⟩