Nanotube circuit could boost chip speeds
Technology 23 March 2006
By Stu Hutson
A single-molecule logic circuit has shown that using carbon nanotubes instead of silicon pathways could someday soup up integrated circuits to near-terahertz processing, up from today's low-gigahertz range.
Researchers at IBM's Thomas J. Watson Research Center used techniques similar to conventional chip-making technology to create field effect transistors along a carbon nanotube – one very large carbon molecule – that had been deposited onto a silicon wafer. Unlike shrunken conventional silicon circuits, the resulting logic circuit yielded virtually no electron flow impedance, meaning current flowed faster.
"This isn't about making the circuits smaller, it's about making them faster," said Joerg Appenzeller, one of the circuit's developers. "Nanotubes fit the characteristics we need to advance high-end processing."
The components of today's computer chips are made by doping tracts of a silicon substrate with metals of different electronic properties. While this technique was the breakthrough technology behind the integrated circuit, it becomes increasingly problematic in the race for smaller and smaller components.
Bad vibrations
Moore's Law, coined in 1965, predicted that with the speed of technological development, the number of transistors in a chip and therefore a chip's speed would double roughly every 18 months.
The problem is that as electrical paths shrink, electrical resistance increases proportionally. Also, the process of doping – adding impurities to the silicon to alter its electrical properties – means that the impurities left behind scatter electron flow, which becomes more of a problem at smaller scales.
The major reason behind the resistance, however, is an odd phenomenon known as plasmonic resonance, in which an electron's path is hindered when it becomes coupled with vibrations in the surrounding lattice structure. But because the carbon nanotube is a single molecule with electrons passing along the tube, this problem is averted and resistance minimised, even at tiny scales.
Additionally, smaller silicon pathways make it easier for electrons to "jump tract" and leech into other nearby components. But in a nanotube circuit, this would be highly improbable as electrons would be carried down the molecular tract of the nanotube, Appenzeller explains.
Proof of principle
These properties will allow the manufacture of smaller transistors with electrons flowing faster through their wires, making for faster processing. The newly created circuit only operated at 52 megahertz – a pittance compared with the gigahertz speeds of modern silicon processors, but optimisation of transistor speed was not the team's primary goal.
"We wanted to show the basic properties of the circuit," Appenzeller says, who adds they have "a long way to go" before thinking about processing speeds.
And development goes beyond simply optimising transistor sizes or design architecture. Current nanotube production methods are not yet able to produce tubes with the exact shapes or consistent sizes that would be needed to build even one chip, let alone mass manufacturing them.
"If we put a Manhattan Project type of effort into this, we could do it in five years," says Andrew Rinzler, the physicist at the University of Florida who made the IBM team's tubes. "But, in reality, it's going to take a little longer."
Journal reference: Science (DOI: 10.1126/science.1122797)