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.
This wafer contains tiny computers using carbon nanotubes, a material that could lead to smaller, more energy-efficient processors. Credit: Norbert von der Groeben
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.
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