Carbon nanotubes in a race against time to replace silicon

Nascent carbon nanotube chip technology may save the computer industry from silicon’s presumed looming demise. Or it may not.

carbon nanotube geoff hutchison flickr
Credit: Goeff Hutchison

The interplay of size and time may make carbon nanotubes the answer to the computer industry's prayers as it grapples with pressure to make silicon chips ever-smaller. Or the same factors may turn CNTs into a technological dead end.

Size refers to the dimensions of carbon nanotubes (CNTs) vs. the shrinking geometry of the components on today's silicon chips. A CNT is basically a tube whose wall is 1 carbon atom thick. The tube itself is 1 nanometer (nm, or one billionths of a meter, or one-thousandths of a micron) in diameter, although it can be tens of microns long. Although made of carbon, single-wall CNTs are excellent conductors thanks to quantum conductance, which allows electrons to propagate along the length of the tubes.

Time refers to the progression of Moore's Law, an observation by Intel co-founder Gordon Moore that the number of components on a chip can be expected to double every two years, without an increase in price. According to that, about more eight years from now silicon technology, which has reached 14nm geometry, will reach the atomic level. At that time, presumably the industry will no longer be able to uphold Moore's Law by making silicon components continually smaller.

Will CNTs, with their 1nm geometry, be ready by then?

The main work on CNT technology for chip design is happening at Stanford University and IBM Research. Workers at both facilities are optimistic -- but guardedly so.

"We feel that CNTs have a chance to possibly replace silicon transistors sometime in the future -- if critical problems are solved," says Supratik Guha, director of physical sciences at IBM Research.

"I am hopeful that CNTs will be used one day," says Max Shulaker, a Stanford graduate student who serves as its spokesman for CNT research.

Tiny roadblocks with huge impacts

The problem of laying down chip circuits with CNTs that match the size of silicon components hasn't been solved, Guha notes. Since individual CNTs don't carry enough current for a functional transistor, five or six parallel tubes are required for one connection. The tubes must be laid 6nm or 7nm apart to minimize interference, but greater separation would waste space.

"Currently we are able to space them 100nm apart, so an order of magnitude improvement is needed," Guha says. "This is where we need some new thinking. With the current separation there is no advantage over silicon."

A silicon circuit that's more than 30nm wide isn't an issue, as metal traces in 14nm devices are about 50nm wide, notes semiconductor industry analyst Linley Gwennap, head of The Linley Group. "There is nothing in a 14nm process that actually measures 14nm," he says, adding that the fin, or main body, of an Intel transistor at the 14nm level measures 10nm.

IBM's Guha says he's also concerned about the purity of the CNT fibers. Circuit construction requires single-walled CNTs, while tubes with two or more walls have different electrical characteristics and their presence constitute impurities.

"We need to be 99.999% pure" -- in other words, requiring single-wall nanotubes -- "and now we are at 99.9%," says Guha. "We are getting there, and I am confident we will fix the problem."

Beyond that, Stanford's Shulaker says the main obstacle to the commercialization of CNTs is the need to improve their contact resistance, or in other words, their connectivity with other conductors used in the system, like silicon and copper. The connection points are tiny and therefore create electrical resistance that requires additional voltage to overcome and operate the system, he explains. The issue is also present with silicon, but silicon designers have been working on solutions for decades, he adds.

carbon news stamford Norbert von der Groeben

This wafer contains tiny computers using carbon nanotubes, a material that could lead to smaller, more energy-efficient processors.

Shulaker also sees the need for better "doping" of the CNTs that are to be used as transistors. Doping is the intentional introduction of certain impurities to control the item's electrical properties so it can function as a transistor.

"It took years to refine doping with silicon," says Shulaker. "With CNTs we are at the stage where silicon was when it started."

The problem with potential silicon replacement technologies like CNT "is that you can do pretty cool things in the lab with them. But putting billions of them on a chip and trying to crank out millions of chips per month is a different problem. CNTs looks promising in the lab, but they must solve the problem of building them in a production environment," says Gwennap.

On a tight deadline

But solving CNT's various problems must happen within a specific time frame or the technology may as well be dropped as far as semiconductor progress is concerned.

With chip technology now at the 14nm level, in two years it will reach the 10nm level, and in four years the 7nm level, and then maybe the 5nm level in six years, Guha explains. But 5nm is about the width of 20 silicon atoms, so shrinking dimensions lower than 5nm may be difficult, barring the discovery of some way to manipulate individual atoms.

"We have another maybe three generations of technology left -- maybe four if you are really optimistic. After that improvements in silicon will cease," predicts Guha.

CNTs, of course, are at the 1nm level. But for the industry to adopt the technology, its problems must be resolved in time for planners to add it to their production road maps; before they make chips, they have to build factories.

Consequently, "we need to demonstrate the practicality of CNT technology in the next two to three years, or the window of opportunity will close and the technology will not be there when needed," says Guha.

If the problems can be solved, "We could see commercial products in six or seven years, at the earliest," Guha says. "Or development could drag on for a decade, or the technology may never become economical."

"At this point it looks like standard [silicon] transistors are solid enough to last to at least 7nm and perhaps 5nm," agrees Gwennap. "CNTs could come into production in six to eight years, maybe, which is pretty far out, but it's on a list of things people are looking at to replace standard transistors."

Not everyone believes it's possible to get there in time. "I don't see CNTs in under seven years, and even 10 years is farfetched," says David Kanter, senior editor at The Linley Group's Microprocessor Report. "What will be in production two to four years from now has already been selected, and anyone who claims to see further than 10 years ahead is not credible, to use the G-rated way of saying it," he adds.

nanotubes IBM

IBM scientists have measured the distribution of electrical charges in tubes of carbon that measure less than 2 nanometers in diameter, 50,000 times thinner than a strand of human hair. This technique, which relies on the interactions between electrons and phonons, provides a detailed understanding of the electrical behavior of carbon nanotubes.

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