Copper Films Now Support Unimpeded Electron Flow

Researchers are increasingly focused on achieving ballistic transport, unimpeded movement of electrons, in nanoscale materials to improve device performance. Yongjin Cho from the Department of Physics, Pohang University of Science and Technology, Su Jae Kim from the Crystal Bank Research Institute, Pusan National University, and Min-Hyoung Jung from the Department of Energy Science, Sungkyunkwan University, working with colleagues at Copper Innovative Technology (CIT) Co., the Graduate School of Semiconductor Materials and Devices Engineering, Ulsan National Institute of Science and Technology, and including Young-Min Kim, Hu-Jong Lee, Seong-Gon Kim, Se-Young Jeong, and Gil-Ho Lee, report observations of ballistic transport in copper thin films. This is significant because, unlike materials such as carbon nanotubes and graphene which exhibit ballistic transport but lack scalability, metal films offer excellent potential for large-scale nanodevice fabrication. Their investigation of 90nm-thick copper films without grain boundaries revealed ballistic transport in channels smaller than 150nm at temperatures below 85 K, potentially paving the way for low-loss signal transmission and improved interconnects in future semiconductor technologies.

Copper films, meticulously engineered to be free of grain boundaries, have enabled unimpeded electron flow at the atomic level and promising to unlock improved signal transmission and interconnects for future semiconductor devices. Researchers have achieved ballistic transport in 90nm-thick copper films, a feat previously limited to nanoscale materials like carbon nanotubes and graphene.

This breakthrough overcomes a significant hurdle in materials science, as deposited metal films typically exhibit short electronic mean free paths that hinder the unimpeded flow of electrons necessary for ballistic transport. The work details the fabrication of 90nm-thick copper films, meticulously engineered to be free of grain boundaries, crystalline defects that disrupt electron movement.

By measuring a negative bend resistance in devices with channel widths below 150nm at temperatures below 85 K, the team demonstrated that electrons can travel through the copper without significant scattering. This discovery unlocks new possibilities for understanding the intrinsic quantum properties of copper, including its electronic band structure and Fermi surface topology.

The ability to observe ballistic transport in a scalable material like copper opens avenues for probing fundamental quantum phenomena, such as phase-coherent quantum interference and the quantum Hall effect, which were previously difficult to study in metallic thin films. The atomically flat single-crystalline copper thin films were grown using an atomic sputtering epitaxy technique, addressing challenges in conventional metal-film deposition.

Measurements revealed an electronic mean free path of approximately 150nm below 85 K, confirming the ballistic nature of the electron transport. This achievement not only facilitates investigations into the quantum behaviour of copper thin films but also holds promise for applications in quantum circuits, spintronic devices, and next-generation interconnect technology. The precise control over film growth and the observation of negative bend resistance, a hallmark of ballistic transport, represent a significant advancement in the field of nanoscale electronics.

Negative bend resistance confirms ballistic electron transport in grain boundary-free copper films

Bend resistance measurements reveal ballistic transport in 90nm-thick copper films fabricated without grain boundaries. Devices with channel widths of 250nm or less exhibited a deviation from expected behaviour, and the 150nm-wide device demonstrated negative bend resistance below 90 K, signifying that electrons traverse the copper film without significant scattering.

The observed negative bend resistance arises from electron accumulation at the measurement terminals, creating a negative electrical potential and reducing the overall resistance. For the 150nm channel width device, the electronic mean free path was estimated to be approximately 150nm below 85 K, a value comparable to, or even shorter than, the device dimensions.

This confirms that ballistic transport dominates when grain boundaries are eliminated, allowing electrons to travel without the interruptions that typically cause scattering. Analysis of polycrystalline copper films revealed a high density of grain boundaries, contrasting sharply with the single-crystalline films where these defects are drastically reduced.

Misorientation line maps show that the polycrystalline films contain trillions of different grain orientations, while the single-crystalline films exhibit only two orientations stacked along specific crystallographic planes. Electron backscatter diffraction (EBSD) mapping further confirms the perfect alignment of copper atoms in the single-crystalline films, with a negligible density of grain boundaries, less than 1nm/m². The high crystalline quality, evidenced by 2.07 Å atomic spacing, is maintained even after fabrication processes, ensuring the integrity of the ballistic transport pathway.

Single-crystalline copper thin film growth and nanoscale device fabrication by atomic sputtering epitaxy

Atomic sputtering epitaxy (ASE) served as the foundational technique for growing single-crystalline copper (SCCF) thin films, addressing challenges inherent in conventional metal deposition such as surface oxidation and defect formation. This method facilitates the creation of atomically flat films, crucial for minimising scattering events and enabling ballistic transport.

The resulting SCCF, approximately 90nm thick, was meticulously characterised using transmission electron microscopy (TEM) to confirm its crystalline structure and the consistent 2.07 Å atomic spacing. Focused ion beam (FIB) milling prepared cross-sectional TEM samples, allowing detailed examination of the film’s microstructure after device fabrication.

Hall bar-shaped devices with channel widths of 10μm, 1μm, 250nm, and 150nm were then defined using standard electron beam lithography and patterned with Argon ion etching. This precise fabrication process ensured the creation of nanoscale structures necessary to observe ballistic transport phenomena. A cross-geometry configuration was implemented for bend resistance (RB) measurements, injecting current from one side of the device and measuring the voltage difference across a perpendicular section.

This specific geometry was chosen because it is particularly sensitive to deviations from diffusive transport, allowing for the detection of ballistic behaviour. The measurement of bend resistance, defined as the bend voltage divided by the current, was central to identifying ballistic transport. By comparing the observed RB values with predictions based on the van der Pauw formula, applicable to diffusive transport regimes, researchers could discern whether electron movement was limited by scattering or occurred ballistically.

Ballistic transport observed in unexpectedly thick copper films offers scalable interconnect potential

Scientists have long sought to create materials where electrons flow without resistance, a phenomenon known as ballistic transport. Achieving this in practical, scalable devices has proven remarkably difficult, largely because the materials best suited to demonstrate it, pristine nanotubes or graphene, are notoriously hard to manufacture consistently at scale.

This new work, detailing ballistic transport in surprisingly thick copper films, represents a significant departure from that established paradigm. The observation of near-frictionless electron flow in 90-nanometer copper structures, achieved through meticulous fabrication to eliminate grain boundaries, suggests a pathway towards scalable, low-loss interconnects that could dramatically improve the efficiency of electronic devices.

What makes this particularly notable is the unexpected thickness of the copper film. Conventional wisdom held that ballistic transport in metals required extremely thin layers, approaching the dimensions of individual atoms. This research demonstrates that, with careful control of material quality, a relatively substantial 90-nanometer film can still support ballistic behaviour at cryogenic temperatures.

However, the requirement for very low temperatures, below 85 Kelvin, remains a substantial hurdle for widespread application. Future work will likely focus on manipulating the film’s electronic structure through alloying or strain engineering, seeking to preserve ballistic transport at more practical operating temperatures and exploring whether these principles can be extended to other metallic materials. The ultimate goal is not just to demonstrate a phenomenon, but to engineer a truly lossless pathway for electrons in future technologies.