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IBM Research Breakthrough Paves Way for Post-Silicon Future with Carbon Nanotube Electronics





  • Carbon nanotube transistors can operate at ten nanometers


  • Equivalent to 10,000 times thinner than a strand of human hair 


  • Less than half the size of today’s leading silicon technology

  • Could also mean wearables that attach directly to skin and internal organs




IBM Research has  announced a major engineering breakthrough that could accelerate carbon nanotubes replacing silicon transistors to power future computing technologies.

IBM scientists demonstrated a new way to shrink transistor contacts without reducing performance of carbon nanotube devices, opening a pathway to dramatically faster, smaller and more powerful computer chips beyond the capabilities of traditional semiconductors. 

A microscopic image showing the scaling of new CNT contact, showing that the contact size can shrink without reducing device performance.


IBM's breakthrough overcomes a major hurdle that silicon and any semiconductor transistor technologies face when scaling down. In any transistor, two things scale: the channel and its two contacts. As devices become smaller, increased contact resistance for carbon nanotubes has hindered performance gains until now. These results could overcome contact resistance challenges all the way to the 1.8 nanometer node – 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 made smaller year after year, but they are approaching a point of physical limitation. With Moore's Law running out of steam, shrinking the size of the transistor – including the channels and contacts – without compromising performance has been a vexing challenge troubling researchers for decades.


IBM has previously shown that carbon nanotube transistors can operate as excellent switches at channel dimensions of less than ten nanometers – the 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 power consumption.

Earlier this summer, IBM unveiled the first 7 nanometer node silicon test chip, pushing the limits of silicon technologies and ensuring further innovations for IBM Systems and the IT industry. By advancing 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.A Cross-sectional TEM image showing the fabricated nanotube transistor with an end-bonded contact and a contact length below 10 nm.



“These chip innovations are necessary to meet the emerging demands of cloud computing, Internet of Things and Big Data systems,” said Dario Gil, vice president of Science & Technology at IBM Research. “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.”
A New Contact for Carbon Nanotubes

Carbon nanotubes represent a new class of semiconductor materials that consist of single atomic sheets of carbon rolled up into a tube. The carbon nanotubes form the core of a transistor device whose superior electrical properties promise several generations of technology scaling beyond the physical limits of silicon.

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 shrink in size, electrical resistance increases within the contacts, which impedes performance.



A Boy And His Atom: The World's Smallest Movie

 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 and invented a metallurgical process akin to microscopic welding that chemically binds the metal atoms to the carbon atoms at the ends of nanotubes. This ‘end-bonded contact scheme’ allows the contacts to be shrunken down to below 10 nanometers 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.”


CARBON COMPUTING: A Q&A WITH AN IBM RESEARCH'S SHU-JEN HAN

How are silicon and carbon similar when it comes to transistors?
Let's start with carbon because it has so many different allotropes, from carbon nanotubes, graphene to diamonds. But diamonds, for example, are electrical insulators, not semiconductors – which are what we need for a transistor. Graphene is a two-dimensional sheet of pure carbon (yes, one-atom-thick) that can conduct current well, but it does not have a bandgap, therefore, transistors made with graphene cannot be switched off. Carbon nanotubes are a rolled-up form of graphene, which are somewhat similar to Silicon since they both have band gap and can be used as the center piece of the transistor – the channel.
Why are carbon nanotubes not in use like silicon?
Silicon has offered many advantages as a transistor material for the last half century. One biggest perhaps was that it forms a great gate dielectric – SiO2. For carbonnanotubes, many material issues have to be solved to obtain similar high-quality carbon nanotube wafers for device fabrication. We can’t switch to an entirely new material over night, but silicon is reaching its scaling limits.
How have you and your team solved this issue of contact resistance?
Carbon nanotubes conduct electricity much faster than silicon, and perhaps more importantly, they use less power than silicon. Plus, at just slightly over one nanometer in body thickness, they’re significantly thinner than today’s silicon, providing good electrostatic control. The challenge has, until now, been how to form high quality contacts between metal electrodes and carbon nanotubes.
In any transistor, two things scale: the channel and its two contacts. It's at the contacts where carbon nanotube resistance, like silicon, has hindered performance. Especially when channel continues to shrink and channel resistance becomes less and less important. Essentially, current just cannot flow into the channel effectively when you hit atomic dimensions.
Dr. Qing Cao and my other teammates at [the IBM Watson Research Center] developed a way, at the atomic level, to weld - or bond – the metal molybdenum to the carbon nanotubes' ends, forming carbide. Previously, we could only place a metal directly on top of the entire nanotube. The resistance was too great to use the transistor once we reached about 20 nm. But welding the metal at the nanotubes' ends, or end-bonded contacts, is a unique feature for carbon nanotubes due to its 1-D structure, and reduced the resistance down to 9 nm contacts. Key to the breakthrough was shrinking the size of the contacts without increasing electrical resistance, which impedes performance. Until now, decreasing the size of device contacts caused a commensurate drop in performance.
What is necessary to scale this technology? And what is your next step in this work?
We must scale our carbon nanotube transistor onto a wafer. The challenge is twofold: it includes how to orient and place these 1 dimensional structures from the solution onto the wafer as well as how to purify them (initial solution has about 1/3 metallic tubes which are not useful for transistors and need be removed).
We've developed a way for carbon nanotubes to self-assemble and bind to specialized molecules on a wafer. The next step is to push the density of these placed nanotubes (to 10 nm apart) and reproducibility across an entire wafer.
What future nanotechnology are you looking forward to?
I can see the potential of our carbon nanotube chips to replace silicon for conventional computing uses. Better transistors can offer higher speed while consume less power. Plus, carbon nanotubes are flexible and transparent. They could be used in futuristic “more than Moore” applications, such as flexible and stretchable electronics or sensors embedded in wearables that actually attach to skin – and are not just bracelets, watches, or eyewear.



About IBM Research
Now in its 70th year, IBM Research continues to define the future of information technology with more than 3,000 researchers in 12 labs located across six continents. Scientists from IBM Research have produced six Nobel Laureates, 10 U.S. National Medals of Technology, five U.S. National Medals of Science, six Turing Awards, 19 inductees in the National Academy of Sciences and 14 inductees into the U.S. National Inventors Hall of Fame – the most of any


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