Two-Stage Electromigration Mechanism in Cu–Cu Direct Bonding Lines Revealed by In Situ Four-Dimensional Scanning Transmission Electron Microscopy

Cu-Cu direct bonding is increasingly adopted for three-dimensional integrated circuits, yet the competitive contributions of microstructural factors to electromigration (EM) remain poorly understood. Here, we investigated the EM behavior of void-free and voided Cu-Cu direct bonding lines using in situ four-dimensional scanning transmission electron microscopy (4D-STEM) at a current density of 107 A/cm2. Serial 4D-STEM crystallographic mapping revealed a distinctive two-stage EM mechanism in void-free samples: an initial stage governed by grain boundary diffusion, where the fraction of high-angle grain boundaries (HAGBs) increases at the expense of low-angle grain boundaries (LAGBs), followed by an accelerated stage dominated by surface diffusion after nanovoid nucleation at triple junctions. Notably, this preferential consumption of LAGBs and the resulting increase in relative HAGB fraction occurred consistently in both void-free and voided samples, independent of the dominant diffusion pathway. Atomic-resolution imaging captured the dynamic reconstruction of void surfaces toward low-energy {111} facets, further facilitating mass transport toward catastrophic failure. Concurrent electron energy loss spectroscopy (EELS) thickness mapping quantified directional volume reduction at the cathode. The void-free sample withstood current stressing approximately 4.8 times longer than the voided sample, demonstrating that pre-existing voids bypass the grain-boundary diffusion-controlled stage and immediately activate faster surface diffusion. These findings establish grain-boundary character distribution and triple-junction configuration as critical microstructural design parameters for enhancing the EM reliability in Cu-Cu bonding interconnects.