Significant softening of copper nanowires during electromigration studied by picosecond ultrasound spectroscopy

High-frequency vibrations related with copper nanowires on a silicon substrate are studied using picosecond ultrasound spectroscopy. The reflectivity change in the probe light pulse is monitored after irradiation of the specimen with the ultrafast light pulse, showing high (similar to 75 GHz) and low (<similar to 15 GHz) frequency vibrations, which are identified to be thickness resonance of nanowires and collective-mode resonances on the substrate, respectively. The nanowires are subjected to the current-loading test to induce electromigration. The thickness resonance frequency significantly decreases as the progress of the electromigration while the other frequencies and the resistance remain nearly unchanged. The micromechanics analysis and the vacancy diffusion theory indicate the growth of thin defects at grain boundaries for softening the nanowires.