Carbon Nanotubes: A Viable Alternative to Silicon?

Peter Singer, Editor-in-Chief -- Semiconductor International, 6/1/2001

The day when integrated circuits are based on transistors made of carbon nanotubes instead of silicon got a little closer recently, thanks to the development of a new fabrication technology developed by scientists at IBM (Yorktown Heights, N.Y.). As reported in the April 27 issue of the journal Science, IBM researchers have built the world's first array of transistors out of carbon nanotubes — cylinders of carbon atoms that measure as little as 10 atoms across and are 500 times smaller than today's silicon-based transistors.

In an interview with Semiconductor International, Phaedon Avouris, lead researcher on the project and manager of IBM's Nanoscale Science Research Department, explained the advantages of the carbon nanotube. "It's extremely strong mechanically, at least 10 times more than steel. It has extreme thermal stability — in the absence of oxygen, it can be heated to over 2500°F without decomposition. It also has good thermal conductivity, better than diamond."

Avouris said the cylinders tend to look like chicken wire rolled into a cylinder. "Depending on the details of the structure — how that chicken wire is rolled — you end up with either metallic or semiconducting nanotubes. So you have your two basic elements that enter in the circuit — the wiring and the semiconductors in one type of material." The nanotubes can also be grown in relatively long lengths (often called "ropes"), measuring several millimeters.

The first individual transistors made out of nanotubes were produced in 1998, but this was a time-consuming process in which nanotubes had to be positioned one at a time or by random chance. Since then, scientists have been trying to develop alternative fabrication methods that could conceivably be used in mass production.

The problem was that all synthetic methods of nanotube production yield a mixture of metallic and semiconducting nanotubes. "You cannot just take what is produced by synthesis and try to make a transistor, because the metallic tubes in the transistor will short it out," Avouris explained. "That has been considered as a very big barrier in further advances of any kind of nanotube devices." The IBM team overcame this problem with "constructive destruction," a technique that allows the scientists to produce only semiconducting carbon nanotubes where desired and with the necessary electrical properties.

IBM's new process results in a dense array of semiconducting nanotube transistors. (Source: IBM)

The Figure shows how it works: 1) The scientists deposit ropes of "stuck-together" metallic and semiconducting nanotubes on a silicon-oxide wafer. 2) A lithographic mask is then projected onto the wafer to form electrodes (metal pads) over the nanotubes. These electrodes act as a switch to turn the semiconducting nanotubes on and off. 3) Using the silicon wafer itself as an electrode, the scientists "switch off" the semiconducting nanotubes, which essentially blocks any current from traveling through them. 4) The metal nanotubes are left unprotected and an appropriate voltage is applied to the wafer, destroying only the metallic nanotubes because the semiconducting nanotubes are now insulated. 5) The result is a dense array of unharmed, working semiconducting nanotube transistors that can be used to build logic circuits.

For additional information on emerging technologies, go to www.semiconductor.net/emerging

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