3rd October 2015

A breakthrough in replacing silicon with carbon nanotubes

IBM has announced a breakthrough that will accelerate the replacement of silicon transistors with carbon nanotubes. Their new method could work down to 1.8 nanometre node sizes.

IBM this week announced a major engineering breakthrough that could accelerate carbon nanotubes replacing silicon transistors to power future computing technologies. Researchers at the company have demonstrated a new way to shrink transistor contacts, without reducing performance of carbon nanotube devices – paving the way to dramatically faster, smaller and more powerful chips beyond the capabilities of traditional semiconductors. The details were published yesterday in the journal Science.

IBM has overcome a major hurdle that silicon and other transistor technologies face when scaling down. In any transistor, two things scale: the channel and its two contacts. As devices become ever smaller, increased contact resistance for carbon nanotubes has hindered performance gains, until now. These results could overcome contact resistance challenges all the way to 1.8 nanometre nodes – four technology generations away.

Carbon nanotube chips could greatly improve the capabilities of high performance computers, enabling Big Data to be analysed faster, increasing the power and battery life of mobile devices and the Internet of Things, and allowing cloud data centres to deliver services more efficiently and economically.

Silicon transistors – tiny switches that carry information on a chip – have been shrunk year after year since the mid-20th century, but are now approaching the limits of miniaturisation. With Moore's Law running out of steam, shrinking the size of the transistor, including the channels and contacts – without compromising its performance – has been a major challenge in recent years.

IBM has previously shown that carbon nanotube transistors can operate as excellent switches at channel dimensions of less than ten nanometres – equivalent to 10,000 times thinner than a strand of human hair and less than half the size of today's leading silicon technology. IBM's new contact approach overcomes the other major hurdle in incorporating carbon nanotubes into semiconductor devices, which could result in smaller chips with greater performance and lower consumption of power.

Earlier this summer, IBM unveiled the first 7 nanometre node silicon test chip, pushing the limits of silicon technologies. By advancing the research of carbon nanotubes to replace traditional silicon devices, IBM is paving the way for a post-silicon future and delivering on its $3 billion chip R&D investment announced in July 2014.

"These chip innovations are necessary to meet the emerging demands of cloud computing, Internet of Things and Big Data systems," said vice president of Science & Technology at IBM Research, Dario Gil. "As silicon technology nears its physical limits, new materials, devices and circuit architectures must be ready to deliver the advanced technologies that will be required by the Cognitive Computing era. This breakthrough shows that computer chips made of carbon nanotubes will be able to power systems of the future sooner than the industry expected."







IBM's new carbon nanotube bonding technique, showing the fabricated nanotube transistor with end-bonded contact and a contact length below 10 nm, with potential to scale to 1.8 nm.

Carbon nanotubes represent a new class of semiconductor materials, consisting of single atomic sheets of carbon rolled into a tube. They form the core of a transistor device whose superior electrical properties could allow Moore's Law to continue for at least several more generations.

Electrons in carbon transistors can move more easily than in silicon-based devices, and the ultra-thin body of carbon nanotubes provide additional advantages at the atomic scale. Inside a chip, contacts are the valves that control the flow of electrons from metal into the channels of a semiconductor. As transistors begin to shrink in size, electrical resistance increases within the contacts, which impedes performance. Until now, decreasing the size of the contacts on a device caused a commensurate drop in performance – a challenge facing both silicon and carbon nanotube transistor technologies.

IBM researchers had to forego traditional contact schemes by inventing a metallurgical process akin to "microscopic welding", which chemically binds the metal atoms to the carbon atoms at the ends of nanotubes. This end-bonded contact scheme allows the contacts to be shrunk below 10 nanometres without deteriorating performance of the carbon nanotube devices.

“For any advanced transistor technology, the increase in contact resistance due to the decrease in the size of transistors becomes a major performance bottleneck,” Gil added. “Our novel approach is to make the contact from the end of the carbon nanotube, which we show does not degrade device performance. This brings us a step closer to the goal of a carbon nanotube technology within the decade.”



A set of end-contacted nanotube transistors. Credit: IBM Research

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