CoSi Nanoflakes Could Replace Copper Interconnects in Chips

Copper is a highly coveted – and frequently stolen – metal, owing to its role in the expansion of data centres, electric vehicles and energy infrastructure.

It is also used to make interconnects: the wires that carry electricity between the semiconducting transistors serving as the fundamental components of electronic chips.

Remarkably, a cutting-edge chip may incorporate more than 100 kilometres (62 miles) of copper interconnects.

Yet copper could be reaching its limits.

Why copper interconnects struggle at small scales

As copper interconnects are reduced in size, the flow of electrons becomes restricted, since these basic particles are increasingly scattered by the surfaces of the wires.

Known as the resistivity size effect, this happens when surface area grows far larger in proportion to volume. The resulting microscopic mosh pit raises resistance and causes signal delays. Higher current densities can also harm interconnects, reducing their reliability.

Physicists have now shown the advantages of a seemingly unlikely replacement for copper: a nanoscale material combining cobalt and silicon.

Earlier work established that cobalt silicide (CoSi) semimetal materials possess a notable characteristic. As their dimensions shrink, their electrical resistivity falls, because quantum mechanics produces highly conductive surface routes that reduce electron scattering.

Writing in Nature Materials, the researchers produced single-crystal CoSi "nanoflakes" in several sizes and showed their practical potential to replace copper.

This may offer a way around an increasingly serious bottleneck, as interconnects account for a significant share of the volume and cost of integrated circuits. Although transistors are becoming quicker, their overall performance is constrained by impeded electrical flow through progressively smaller interconnects.

CoSi nanoflakes outperform copper interconnects

The team discovered that reducing the thickness of CoSi components from one micrometre (one millionth of a metre, or 0.00004 of an inch) to roughly 20 nanometres (20 billionths of a metre) caused CoSi's resistivity to decline by an order of magnitude.

At room temperature, therefore, a 20-nm-thick CoSi nanoflake is ten times less resistant to electrical flow than a copper interconnect with the same thickness.

CoSi nanoflakes also have a current-carrying capacity 100 times higher than copper interconnects, which "substantially surpasses that of conventional metals," according to the researchers.

These characteristics make CoSi a particularly promising copper alternative. The question, however, was whether it could genuinely take copper's place in interconnect applications.

To assess this, the researchers performed two tests demonstrating CoSi's practical usefulness.

Testing CoSi in radio-frequency and silicon chips

For the first test, they used a ground-signal-ground (GSG) arrangement to assess how effectively a CoSi interconnect could send signals without delay or energy loss.

This radio-frequency experiment showed that CoSi interconnects can transmit signals with minimal loss at frequencies up to 40 GHz – bands used, for instance, in spacecraft communications systems.

The second test, designed to reflect real-world use, examined the inherent compatibility of CoSi interconnects with CMOS silicon chip technology, the standard electronic-chip platform.

The researchers substituted a CoSi nanoflake for part of the conventional metal interconnects in a silicon ring oscillator. This device passes an electrical signal around a loop containing an odd number of logic gates.

The oscillator could consequently run at the same frequency with CoSi, producing "a negligible change in total resistance of the interconnect wire," the researchers note.

Taken together, the "radio-frequency and on-chip testing unequivocally demonstrate CoSi to be an interconnect material with highly scalable potential," they add.

The material can also withstand harsh conditions. Further measurements indicated that CoSi remains stable under high current densities and can endure temperatures of 450 degrees Celsius (840 degrees Fahrenheit) for 200 hours.

Overall, the material demonstrated strong potential as a replacement for copper.

"These studies show CoSi to be a superior alternative to copper interconnects, surpassing them in terms of both conductivity and reliability at the nanoscale," the researchers conclude.

The research was published in Nature Materials.

This article was fact-checked by Peter Dockrill and edited by Peter Dockrill. While we take pride in our process, we are only human. If you notice an error, please let us know.