Wednesday, November 01, 2006

From the Labs: Nanotechnology

New publications, experiments, and breakthroughs in nanotechnology--and what they mean.

By Kevin Bullis

This nanowire-based CMOS circuit (the nanowires are too small to see) could lead to smaller, more powerful computers. (Courtesy of Dunwei Wang)

Nanowire Computing
A practical method for nanowire-based CMOS circuits

Source: "Complementary Symmetry Silicon Nanowire Logic: Power-Efficient Inverters with Gain"
Dunwei Wang et al.
Small 2(10): 1153-1158

Results: Caltech researchers have made silicon-nanowire-based logic circuits similar to the complementary metal-oxide semiconductor circuits used in computer chips. Such circuits combine two kinds of transistors that respond in opposite ways to electronic signals--a useful arrangement for energy-efficient chips. Because the new method can produce both types of transistors on a single surface, it could be suitable for mass production.

Why it matters: Because of their small size and excellent electronic properties, silicon nanowires could enable ultrasensitive handheld sensors for detecting cancer or identifying biological hazards. What's more, the nanowires could lead to more powerful, energy-efficient computer chips. But previous prototypes of nanowire-based circuits were made using techniques that don't lend themselves to batch processing. The new methods could make nano­wire circuits practical to manufacture.

Methods: To make p- and n-type transistors, the two types needed in CMOS circuits, researchers first created a checkerboard pattern of the p- and n-type silicon: they doped adjacent squares with different dopants, using photolithography-produced masks. Then, using a method they'd previously developed, the researchers selectively etched away silicon to form orderly arrays of nanowires. Finally, they connected these nanowires using e-beam lithography to form transistors and a fundamental type of logic circuit called an inverter.

Next steps: For mass production, the researchers will replace the e-beam lithography with the faster method of photolithography. They also need to demonstrate that an experimental process for making batches of nanowire arrays, called nano imprinting, will work in large-scale manufacturing.