By Roland Piquepaille
Even if researchers are routinely building all kinds of nanodevices in their labs, the current production process of nanowires or nanosensors is similar to the car manufacturing process before Henry Ford. These nanostructures are almost handmade. Now, researchers at University of California Davis (UC Davis) have adapted a technology developed for Hewlett-Packard Laboratories. And they came with two new ways to massively produce nanowires of precise length. Their 'nanobridges' and 'nanocolonnades' are totally compatible with existing microelectronics fabrication processes. This opens the way for to a wide range of industrial-strength applications, such as bio-chemical sensing, nanoelectronics, nanophotonics, memory and logic devices for future computing. Read more...First, let's look at how most nanostructures are produced today.
Nanotechnology, the ability to create and work with structures and materials on an atomic scale, holds the promise of extreme miniaturization for electronics, chemical sensors and medical devices. But while researchers have created tiny silicon wires and connected them together one at a time, these methods cannot easily be scaled up.
"It takes weeks to make one or two, and you end up with different sizes and characteristics," said M. Saif Islam, assistant professor of electrical and computer engineering, who joined UC Davis from Hewlett-Packard Laboratories in 2004.
Saif Islam and his Integrated Nanodevices and Systems Research (inano) group decided to adapt a technology developed for Hewlett-Packard.
While working at the Quantum Science Research group of Hewlett-Packard Laboratories, Islam and colleagues came up with a new approach. Silicon wafers used for building microcircuits are usually polished at one specific angle to the atomic planes of silicon. Instead, the group used a wafer that was polished at a different angle, changing the orientation of silicon atomic planes to the surface. Using a chemical vapor deposition technique, they could then grow identical, perpendicular columns of silicon.
The researchers have used this method to grow "nanobridges" across a gap between two vertical silicon electrodes. The nanobridges are strong, chemically stable and show better electrical properties than previous approaches, Islam said. They could be used for nanosized transistors, chemical sensors or lasers.
The research work about these 'nanobridges' has been published by Applied Physics A, in a special issue on nanotechnology. Here is a link to the abstract of this paper named "A novel interconnection technique for manufacturing nanowire devices" (Volume 80, Number 6, Pages 1133-1140, March 2005).
This paper reviews a novel bridging technique that connects a large number of highly directional metal-catalyzed nanowires between pre-fabricated electrodes and extends the technique to an electrically isolated structure that allows conduction through the nanowires to be measured.
Two opposing vertical and electrically isolated semiconductor surfaces are fabricated using coarse optical lithography, along with wet and dry etching. Lateral nanowires are then grown from one surface by metal-catalyst-assisted chemical vapor deposition; nanowires connect to the other vertical surface during growth, forming mechanically robust nanobridges.
By forming the structure on a silicon-on-insulator substrate, electrical isolation is achieved. Electrical measurements indicate that dopant added during nanowire growth is electrically active and of the same magnitude as in planar epitaxial layers.
Meanwhile, the research about 'nanocolonnades' was presented at the spring meeting of the Materials Research Society in San Francisco on April 1, 2005 under the name "Nano-Colonnades: A Novel Technique for Integration of Nanowire Devices." Here is a link to the program of the symposium where this technology was described.
Finally, I was unable to find any pictures of these 'nanobridges' or 'nanocolonnades.' If anyone knows about such images, please send me a pointer and I'll update this entry. Thank you in advance.
Sources: UC Davis News, April 7, 2005; and various websites
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