In the field of nanoelectronic devices, the rapid development of miniaturization and high integration density poses severe challenges to the atomic-scale stability of nanoscale components under extreme operating conditions. As device dimensions shrink to the sub-10 nm scale, electromigration becomes a core bottleneck limiting reliability, where the directional diffusion of atoms driven by high-density current gradually damages metal interconnects, ultimately leading to device failure. For a long time, copper and gold, as mainstream interconnect materials, exhibit significant resistivity increases and low diffusion activation energies at the sub-10 nm scale, making them highly prone to failure. Traditional theory holds that the electron wind force is the primary driver of atomic migration in metal interconnects, with its direction always aligned with the electron flow, which has served as the cornerstone for formulating reliability strategies in the electronics industry.

Due to the extremely high technical barriers in the preparation, testing, and visualization of materials with complex electronic structures at the atomic scale, researchers know very little about the electromigration mechanisms of refractory transition metal interconnects. Although in-situ transmission electron microscopy has been capable of capturing the migration process in traditional metals, its application has mostly been limited to larger-scale structures ranging from tens to hundreds of nanometers. This lack of experimental data severely hampers the reliability design of nanoscale components for extreme operating conditions, making it difficult to predict and control the structural evolution of next-generation interconnect materials at the atomic level.

To address the above issues, a team from Zhejiang University, the Henan Academy of Sciences, and other institutions conducted systematic research using ZEPTOOLS' in-situ TEM measurement system. Through in-situ nanofabrication and electrical pulsing techniques, they discovered for the first time in sub-10 nm refractory metals such as tungsten and molybdenum an "upwind" electromigration phenomenon where atoms migrate opposite to the direction of electron flow, challenging the traditional theoretical understanding that the electron wind force dominates atomic migration.

Title: Upwind electromigration of sub-10-nm metallic nano-interconnects
Journal: Nature Communications
URL: doi.org/10.1038/s41467-

Through precise experiments, the research team discovered that in refractory metal nano-interconnects such as tungsten and molybdenum, surface atoms exhibit an "upwind" migration phenomenon under electrical pulsing that is completely different from conventional understanding. Traditional theory has long held that the electron wind force drives atoms to move in the direction of electron flow, but in these transition metals with complex electronic structures, atoms show a tendency to migrate in the opposite direction. This finding not only directly challenges the established paradigm of metal electromigration that has prevailed for half a century but also provides a critical theoretical basis for the reliability design of high-performance nanodevices under extreme operating conditions in the future.

To overcome the extreme technical difficulties in fabricating, testing, and real-time imaging of nano-interconnects at the atomic scale, the researchers developed an integrated in-situ nanofabrication and electrical pulsing testing method. In this process, using the ZEPTOOLS in-situ probe holder, the experimenters were able to precisely manipulate a nanoscale tungsten tip inside the electron microscope for in-situ nanowelding, thereby constructing single-crystal or bicrystal nano-interconnect models free of lattice defects and with controllable crystallographic orientation. This experimental approach, based on the precision hardware of the in-situ probe holder, ensured that the surface-diffusion-driven dynamic evolution of electromigration could be captured in real time at atomic resolution without the interference of surface contamination, providing highly reliable data support for revealing the intrinsic mechanism of electromigration.

Through first-principles calculations and density functional theory simulations, the research team thoroughly analyzed the physical origin of the driving force reversal. The study shows that the effective charge of metal atoms is influenced by both the direct force and the electron wind force. In materials such as tungsten and molybdenum, due to their unique electronic band structures, the electron-phonon coupling strength is weak and the electron scattering time is short, resulting in a significant reduction of the electron wind force. The calculations reveal that the ratio of the direct force to the electron wind force on the surface atoms of tungsten and molybdenum is much higher than that in traditional interconnect materials such as copper and gold, allowing the direct force to dominate in the competition and thus drive atoms to migrate opposite to the direction of electron flow. The elucidation of this mechanism marks a transition in the evaluation system of electromigration driving forces from a single electron wind model to a multi-force competition model.

The study further explored the application prospects of electromigration phenomena in failure prevention and precision manufacturing of nano-interconnects. Experiments observed that by changing the polarity of the electrical pulse or modulating the surface step kinetics, atomic accumulation and depletion can be artificially guided, and even self-repair of anodic damage can be achieved under specific conditions. This atomic-scale surface morphology evolution regulated by electrical pulses not only helps establish a more accurate framework for predicting the lifetime of nanodevices but also provides new process concepts for atomic-scale manufacturing technology. This research achievement not only deepens the understanding of metal transport behavior but also points the scientific direction for developing next-generation electromigration-resistant novel alloys and multilayer interconnect structures.

ZEPTOOLS, as a Chinese high-end precision instrument company, is a leading supplier of in-situ electron microscopy characterization solutions. The PicoFemto series of in-situ transmission electron microscopy characterization solutions have successively provided technical support for major research achievements of users both domestically and internationally. The figure below shows the ZEPTOOLS in-situ TEM sample holder product used in this research achievement: