Ten years from now, your PCs and servers could well be running on carbon nanotube-based technology. NEC has developed a method of positioning tiny tubes of carbon in a way that it has reported will make circuits run faster and consume less power than the fastest and most powerful silicon chips.
NEC said the process was an important step toward NEC's goal of developing chips that run at 15GHz to 20GHz while consuming about the same power as today's Intel Pentium 4 processors. The company says it made a breakthrough in the way it makes carbon nanotubes, bringing them closer to being used in transistors in LSI (large-scale integrated circuit) chips.
"Before we developed this process, we could not control the position or the diameter of carbon nanotubes simultaneously. Now we can," said Yukinori Ochiai, principal researcher at NEC's nanotechnology group at the company's fundamental and environmental research laboratories at Tsukuba, north of Tokyo. "We think, frankly, it's a major step forward and a breakthrough. Nobody else can do both position and diameter," he said.
Ever since their discovery by NEC researcher Sumio Iijima in 1991, carbon nanotubes have held promise as the building blocks for transistors and circuit wiring that are much more efficient than those with conventional materials. Carbon nanotubes are made when carbon atoms form hollow, open-ended cylinders that have diameters between 0.4nm and 1.8nm. Carbon nanotubes vary in length up to several hundred nanometres long, depending on how they are made.
Electrons can flow through carbon nanotubes 10 times faster than they can in circuits made using silicon, and carbon nanotubes can carry 100 times the current and dissipate 20 times the heat of circuits made with silicon. Carbon nanotubes in transistors can also amplify about 20 times more current than conventional silicon-based transistors, Ochiai said.
For example, Pentium 4 chips contain about 54 million transistors and these transistors are embedded in lines on the chip's silicon wafer that are 90nm across. Next year, Intel will introduce chips built in 65nm lines. The transistors have gates, the switches that turn off and on, measuring 35nm across, according to Intel.
According to the International Technology Roadmap for Semiconductors, an organisation that helps standardise how companies advance chip technologies, processors will be built in lines as small as 22nm by approximately 2016, and chips will contain billions of transistors.
It is at this bleeding edge that carbon nanotubes will be useful, Ochiai said. As circuits get smaller, silicon's comparative lack of efficiency means chips have to use more power and generate more heat. In addition, ever stronger yet thinner insulators are needed to stop electrons straying or leaking between the gate and other parts of the transistor. Placed in the circuits, as well as carrying more current faster, carbon nanotubes can also act as great insulators. Carbon nanotubes will solve many of the technical issues faced by transistors built on silicon alone, Ochiai said. "With carbon nanotubes, the chips will run faster, but with the same power as today's chips," he said.
And the downside? Carbon nanotubes' big drawback is that their size makes them difficult to manipulate, and unlike semiconductor technology, carbon nanotubes have not benefited from more than 50 years of technical development.
However, researchers have not been able to reliably grow carbon nanotubes simultaneously into set sizes, positions and directions in a way that they fit into transistor circuits. NEC's new process solves the issues of size and position, because the company can grow carbon nanotubes to a standard diameter on a chip, and control their position to an accuracy of 5nm, Ochiai said. The process that NEC is using will also enable the company to solve the remaining issue -- direction -- Ochiai said.
The company uses an electron beam to etch patterns into a film in the positions where it wants to anchor the tips of carbon nanotubes. Carbon atoms vaporised in ethanol gas are sucked over the anchor points, then condense into carbon nanotubes at those points, a process Ochiai said is like crystallising salt from sea water. Ochiai's team are near to being able to control carbon nanotube direction using electric fields that wash over the carbon nanotubes as they are formed. "The next step isn't so difficult. I guess that we are two to three months from being able to do that," he said.
Beyond this, NEC believes that it can follow the history of semiconductor transistors and chips and cram more transistors into less space. If the company succeeds with its nanotube-direction strategy, it will be able to consider making transistors in groups of 10 then hundreds.
"It'll take a couple of years," Ochiai said. NEC still has work to do on a process for making and mass producing the structures using carbon nanotubes to form transistors to go beyond this, he said.
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