IBM Scientists Develop Breakthrough Carbon Nanotube Transistor Technology

"Constructive Destruction" method overcomes major hurdle for building computer chips beyond silicon

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Yorktown Heights, N.Y - 27 Apr 2001: ... IBM scientists have developed a breakthrough transistor technology that could enable production of a new class of smaller, faster and lower power computer chips than currently possible with silicon.

As reported in the April 27 issue of the journal Science, IBM researchers have built the world's first arrayof transistors out of carbon nanotubes -- tiny cylinders of carbon atoms that measure as small as 10 atoms across and are 500 times smaller than today's silicon-based transistors. The breakthrough is a new batch process for forming large numbers of nanotube transistors. Until now, nanotubes had to be positioned one at a time or by random chance, which while fine for scientific experiments is impossibly slow and tedious for mass production.

This achievement is an important step in finding new materials and processes for improving computer chips after silicon-based chips cannot be made any smaller -- a problem chip makers are expected to face in about 10-20 years.

"This is a major step forward in our pursuit to build molecular-scale electronic devices," said Phaedon Avouris, lead researcher on the project and manager of IBM's Nanoscale Science Research Department. "Our studies prove that carbon nanotubes can compete with silicon in terms of performance, and since they may allow transistors to be made much smaller, they are promising candidates for a future nanoelectronic technology. This new process gives us a practical way of making nanotube transistors, which is essential for future mass production."

Using Carbon Nanotubes as Transistors in Chips
Depending on their size and shape, the electronic properties of carbon nanotubes can be metallic or semiconducting. The problem scientists had faced in using carbon nanotubes as transistors was that all synthetic methods of production yield a mixture of metallic and semiconducting nanotubes which "stick together" to form ropes or bundles. This compromises their usefulness because only semiconducting nanotubes can be used as transistors; and when they are stuck together, the metallic nanotubes overpower the semiconducting nanotubes.

Beyond manipulating them individually, a slow and tedious process, there has been no practical way to separate the metallic and semiconducting nanotubes -- a roadblock in using carbon nanotubes to build transistors. The IBM team overcame this problem with "constructive destruction", a technique that allows the scientiststo produce only semiconducting carbon nanotubes where desired and with the electrical properties required to build computer chips.

The basic premise of "constructive destruction" is that in order to construct a dense-array of semiconducting nanotubes, the metallic nanotubes must be destroyed. This is accomplished with an electric shockwave that destroys the metallic nanotubes, leaving only the semiconducting nanotubes needed to build transistors.
Here is how it works:
1. The scientists deposit ropes of "stuck together" metallic and semiconducting nanotubes on a silicon-oxide wafer,
2. Then a lithographic mask is 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, since the semiconducting nanotubes are now insulated,
5. The result: a dense array of unharmed, working semiconducting nanotube transistors that can be used to build logic circuits like those found in computer chips.Moore's Law states that the number of transistors that can be packed on a chip doubles every 18 months, but many scientists expect that within 10-20 years, silicon will reach its physical limits, halting the ability to pack more transistors on a chip. Transistors are a key building block of electronic systems -- they act as bridges that carry data from one place to another inside computer chips. The more transistors on a chip, the faster the processing speed, indicating why this advance by IBM scientists could have a profound impact on the future of chip performance.

Related Carbon Nanotube Work at IBM
In the same report, the IBM scientists show how electrical breakdown can be used to remove individual carbon shells of a multi-walled nanotube one-by-one, allowing the scientists to fabricate carbon nanotubes with the precise electrical properties desired. The report also shows how the scientists fabricate field-effect transistors from carbon nanotubes with any variable band-gap desired.

In parallel studies of carbon nanotubes, IBM researchers have been working to improve the electrical characteristics of individual nanotube transistors. The unpublished data from these studies show that if the carbon nanotubes are scaled up to the size of today's silicon-based transistors, the performance would be the same. This proves that the smaller carbon nanotube transistors should allow for Moore's Law to continue on its path when silicon cannot be made any smaller.

The report on this work is published in Science, Vol. 292, Issue 5517, April 27, 2001. The authors of the report "Engineering Carbon Nanotubes and Nanotube Circuits Using Electrical Breakdown" are Phaedon Avouris, of IBM's T.J. Watson Research Laboratory in Yorktown Heights, N.Y., Philip G. Collins, formerly of IBM, now with Covalent Materials in Emeryville, California, and Michael S. Arnold, an IBM intern from the University of Illinois.

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