Simulations point to nanotube future

Researchers at Rensselaer Polytechnic Institute have created models to determine whether it is worth pursuing carbon nanotubes as replacements for copper wiring in chip designs.

To better understand and more precisely measure the key characteristics of both copper nanowires and carbon nanotube bundles, the researchers used models at the quantum-mechanical level to work out how well copper nanowires will perform versus nanotubes.

After crunching numbers for months with the help of Rensselaer’s Computational Center for Nanotechnology Innovations, the research team concluded that the carbon nanotube bundles boasted a much smaller electrical resistance than the copper nanowires. This lower resistance suggests carbon nanotube bundles would therefore be better suited for interconnect applications.

“With this study, we have provided a road map for accurately comparing the performance of copper wire and carbon nanotube wire,” said Saroj Nayak, an associate professor in Rensselaer’s department of physics, applied physics and astronomy, who led the research team. “Given the data we collected, we believe that carbon nanotubes [at a width of] 45nm will outperform copper nanowire.”

The research results will be featured in the March issue of Journal of Physics: Condensed Matter.

Because of the nanoscale size of interconnects, they are subject to quantum phenomena that are not apparent and not visible at the macroscale, Nayak said. Empirical and semi-classical laws cannot account for such phenomena that take place on the atomic and subatomic level, and, as a result, models and simulations based on those models cannot be used to accurately predict the behaviour and performance of copper nanowire. Using quantum mechanics, which deals with physics at the atomic level, is more difficult but allows for a fuller, more accurate model.

“If you go to the nanoscale, objects do not behave as they do at the macroscale,” Nayak said. “Looking forward to the future of computers, it is essential that we solve problems with quantum mechanics to obtain the most complete, reliable data possible.”

Nayak said the simulations took into account the contact resistance of both types of interconnect. Over long distances, because electrons can move ballistically through nanotubes over distances in the micromillimetre region makes them significantly better than copper wiring, where the ballistic transport distance is less than 40nm, he noted. However, there are still many challenges to overcome before mass-produced carbon nanotube interconnects can be realised. There are still issues concerning the cost of efficiency of creating bulk carbon nanotubes, and growing nanotubes that are solely metallic rather than their current state being of partially metallic and partially semiconductor. Simply placing nanotubes where they are needed is something that eludes chipmakers.

More study will also be required, he said, to model and simulate the effects of imperfections in carbon nanotubes on the electrical resistance, contact resistance, capacitance, and other vital characteristics of a nanotube interconnect.

Image: Carbon nanotubes could improve on copper wiring once interconnect diameters shrink to 45nm

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