Last week, researchers at Osaka University said they have bulk-produced carpets of micron-long chains of crystalline globules and silicon dioxide stems that they believe could vie with nanotubes to produce next-generation semiconductor circuits.

Both teams of nanotechnology researchers have made marked strides in the quest to develop processes for the electronics industry that may lead to the mass-production of nanoelectronic circuits within the decade.

Predictions vary as to how long it will take the two technologies to reach the stage of mass production. U.S. researchers say that time may come in as little as two or as many as 15 years, while Japanese scientists foresee a 10-year development time frame.

The IBM scientists' creation of a logic circuit within a single-molecule carbon nanotube lays the foundation for that technology to succeed silicon in electronic circuits in 10 to 15 years. Up until now, transistors have been created in carbon nanotubes, but functioning logic circuits have not.

"The ability to build a whole circuit on a single molecule has always been the holy grail of computing," said Phaedon Avouris, lead scientist on the project and manager of nano-scale science at IBM Research (Yorktown Heights, N.Y.). "This gives us hope that by the time silicon reaches its limits in another decade, we will be ready to bring nanotubes out."

Tiny technology, big strides
Scientists believe that the recent advances could spur more research in nanotechnology, ultimately leading to the study of production-scale techniques for building such molecular circuits.

At IBM, the current focus is on the molecular-scale carbon tubes known as nanotubes. Nanotubes, the IBM team says, offer the potential to change computing, enabling research- ers to build switches measuring just 5 nanometers across, which is about 100 times smaller than today's silicon switches.

Using such nanoscale circuits, scientists say that chip makers could produce higher-density electronic devices that consume less power and dissipate less heat. Theoretically, the inherent speed of such circuitry also presents the opportunity for chip makers to create devices that operate at terahertz clock speeds, assuming that all other bottlenecks are eliminated.

IBM's Avouris will announce today that his team reached two significant milestones in nanotube technology: They created an n-type nanotube transistor, which had never been done before; and they integrated p-type and n-type transistors to create a NOT gate on a single molecule.

To form an n-type transistor, Avouris said that researchers covered a p-type nanotube transistor with photoresist, placed it in high vacuum (about 10--6 Torr) and heated it to 400C.

To create a functioning NOT circuit, researchers created two p-type nanotube transistors in series, covered the transistors with photoresist, then used e-beam lithography to expose a window above one of them. By opening the window, the researchers were able to expose that transistor to a potassium dopant, which changed it to an n-type transistor. By having p- and n-type transistors in series, they were then able to create a NOT gate, or voltage inverter. Avouris said that researchers were particularly hopeful because the gain of the inverter is greater than one, which means that the output of the logic gate can be used as the input to drive other logic gates or circuits along the length of the nanotube.

"If you put the circuits along the length of the nanotube, you can save on wiring and make your devices much smaller," Avouris said.

Not production-ready
IBM researchers stressed that their efforts were aimed at proving out the concepts, and were not geared toward the creation of devices in production volume.

"The procedures we're using are too time-consuming and therefore not appropriate for mass production," Avouris said.

IBM plans to continue to study the physics of nanotubes for no more than two more years, then hopes to move into the next phase, which would involve development of methods to mass-produce such circuits. In contrast to today's effort, which includes just a few PhD scientists, that endeavor would involve dozens of scientists and engineers.

Future efforts would also concentrate on scaling down the size of the current nanotube systems. While gate widths of the existing nanotube circuits measure about 250 nm, researchers believe they could reduce those sizes by a factor of 50 or more.

"We believe we can make transistors as small as 5 nm without adversely affecting their performance," Avouris said.

The company's researchers think that carbon nanotube technology offers the best alternative to silicon on the horizon today. "There are physical limits to how small you can go with silicon, and we expect to reach those limits in about 10 to 15 years," Avouris said. "Right now, among the different molecular alternatives that have been proposed, this is by far the most promising."