Direct measurement of the electron wind force in cobalt nanowires

Copper back-end-of-line interconnects have been the industry standard for decades, but copper's susceptibility to electromigration failure has motivated a search for alternatives. While cobalt's resistivity is higher than copper's in bulk form, it scales more slowly with decreasing interconnect size, making cobalt a promising alternative for highly scaled interconnects. To better understand the electromigration behavior of cobalt, we map both temperature- and the current-induced strain in cobalt nanowires using a scanning transmission electron microscope equipped with an electron energy loss spectrometer. We consistently see cobalt move away from the anode, and we find cobalt's effective ionic charge Z*=+2±1 near 300 °C. This result will inform the design of highly scaled cobalt interconnects.