In situ monitoring of electromigration in a single nano wire by picosecond ultrasonics

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Recent nano-scale fabrication has realized the progress of integrated circuits, where current density increases with the downscaling of each electronic component, leading to failure due to electromigration. Electromigration is the transport of materials via momentum exchange from high-density conducting electrons to metal atoms. Defects of atoms result in the disconnection of the circuits and the accumulation of atoms causes the short circuit. Therefore, electromigration due to highly dense current flow has been studied for long years. However, it is difficult to non-destructively evaluate the structural and physical property changes due to the electromigration in an in-situ and microscopic measurement. In this study, we develop a real-time monitoring method of structural and mechanical property changes of a single nanowire using picosecond ultrasonics. Picosecond ultrasonics is ultrafast pump-probe laser ultrasonics using femtosecond pulse laser to excite and detect sub-THz ultrasound. We can focus the laser light within 1-micrometer diameter, enabling us to evaluate the mechanical property changes in a single nanowire from the ultrasound propagation. By applying direct current and observing ultrasound, we discuss the relationship among mechanical, electrical, and structural changes due to electromigration.