Physicists have demonstrated that cobalt silicide (CoSi) semimetal nanoflakes defy conventional physics by decreasing in electrical resistivity as they get thinner. Reported in Nature Materials, single-crystal CoSi scaling down from one micrometer to 20 nanometers drops resistance by an order of magnitude, offering a high-performance alternative to traditional copper chip interconnects.
The Physics Breaking Point of Copper Interconnects
Modern microprocessors are densely packed highways of metal. A state-of-the-art chip can contain over 100 kilometers, or 62 miles, of copper interconnects. These microscopic wires transfer electricity between the semiconducting transistors that form the basic building blocks of electronic chips. But copper is hitting a brutal physical wall.
As copper interconnects continue to shrink to match tighter transistor budgets, they choke electron flow. Fundamental particles are increasingly scattered by the surfaces of the wires. This phenomenon is known as the resistivity size effect. It occurs when surface area becomes much larger relative to volume, turning the microscopic interior into a mosh pit of resistance and signal delays. Rising current densities also damage these interconnects, cutting down their long-term reliability.
Why Cobalt Silicide Scales Downward Defensively
Enter a counterintuitive replacement: a nanoscale material made from cobalt and silicon. As detailed by Chen and colleagues in Nature Materials, cobalt silicide (CoSi) semimetal materials display an intriguing property. As they get smaller, their electrical resistivity actually decreases because quantum mechanics creates highly conductive surface paths with reduced electron scattering.
The research team created single-crystal CoSi nanoflakes of varying sizes to test their practical potential to supplant copper. Interconnects currently make up a substantial volume and cost of integrated circuits. Even as transistors become faster, overall performance remains bottlenecked by stymied electrical flow through ever-tinier metal lines.
By scaling down the thickness of CoSi components from one micrometer down to roughly 20 nanometers, the team observed a massive shift. The resistivity of CoSi dropped by an order of magnitude. A 20-nanometer-thick CoSi nanoflake proved to be ten times less resistive to electrical flow than a copper interconnect of equal thickness at room temperature.
Surpassing Conventional Metal Thresholds
Beyond resisting less, CoSi packs a serious punch when handling electrical load. The current-carrying capacity of CoSi nanoflakes is 100 times greater than that of conventional copper interconnects.
To evaluate if CoSi can truly fill copper’s shoes in real-world environments, researchers subjected the material to rigorous functional evaluations.
- Radio-Frequency Transmission: Using a ground-signal-ground configuration, the team measured how well a CoSi-composed interconnect could transmit signals without lag or energy loss. The test indicated that CoSi interconnects can transmit signals with little loss up to 40 GHz, matching frequency bands utilized in specialized communications systems like spacecraft communications systems.
- CMOS Compatibility: The second test probed the inherent compatibility of CoSi interconnects with CMOS silicon chip technology, the conventional platform for electronic chips. Researchers successfully replaced part of the traditional metal interconnects with a CoSi nanoflake inside a silicon ring oscillator, a device designed to transfer electrical signals between an odd number of logic gates in a loop.
The Road Ahead for Nanoscale Architecture
With data centers, electric vehicles, and advanced computing infrastructure demanding ever-greater efficiency, finding a viable successor to copper is paramount. Copper has served as the backbone of microprocessors since it supplanted aluminum back in 1997. Now, cobalt silicide is presenting a mathematically backed path forward, proving that at the nanoscale, getting thinner can mean getting better.
