Two-Stage Electromigration Mechanism in Cu-Cu Direct Bonding Lines Revealed by In Situ Four-Dimensional Scanning
Eun-Byeol Park1, Min-Hyoung Jung1, Su Jae Kim2
1Department of Energy Science, Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea.
ACS Nano
|July 13, 2026
Summary
Copper-copper direct bonding reliability in integrated circuits is enhanced by controlling grain boundaries and triple junctions. Understanding electromigration mechanisms, like grain boundary and surface diffusion, is key to preventing failures.
Area of Science:
- Materials Science
- Electrical Engineering
- Nanotechnology
Background:
- Copper-copper direct bonding is crucial for advanced 3D integrated circuits.
- Electromigration (EM) in these bonds is a critical reliability concern.
- Microstructural influences on EM in Cu-Cu bonds are not fully understood.
Purpose of the Study:
- Investigate electromigration behavior in void-free and voided Cu-Cu direct bonding lines.
- Elucidate the distinct stages and mechanisms governing EM.
- Identify key microstructural factors for enhancing EM reliability.
Main Methods:
- In situ four-dimensional scanning transmission electron microscopy (4D-STEM) at 10^7 A/cm^2.
- Serial 4D-STEM crystallographic mapping.
- Atomic-resolution imaging and electron energy loss spectroscopy (EELS) thickness mapping.
Main Results:
- A two-stage EM mechanism was observed in void-free samples: initial grain boundary diffusion followed by surface diffusion after void nucleation.
- Low-angle grain boundaries (LAGBs) were preferentially consumed, increasing high-angle grain boundary (HAGB) fraction.
- Void-free samples lasted 4.8 times longer than voided samples, highlighting the impact of pre-existing voids on EM pathways.
- Void surfaces dynamically reconstructed to low-energy {111} facets, accelerating mass transport.
Conclusions:
- Grain boundary character distribution and triple junction configuration are critical for EM reliability in Cu-Cu interconnects.
- Pre-existing voids accelerate EM by bypassing initial diffusion stages.
- Microstructural engineering is essential for robust Cu-Cu direct bonding in integrated circuits.
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