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Growing Nanotubes on Silicon a Base for Nanoelectronics

Photonics.com
Aug 2006
WEST LAFAYETTE, Ind., Aug. 2, 2006 — Engineers have developed a technique to grow individual carbon nanotubes vertically on top of a silicon wafer, a step toward using nanotubes to make advanced electronics, wireless devices and sensors by stacking circuits and components in layers.

FisherSands.jpgThe technique might help develop a method for creating "vertically oriented" nanoelectronic devices, the electronic equivalent of a skyscraper, said Timothy S. Fisher, an associate professor of mechanical engineering at Purdue University who is leading the work with Timothy D. Sands, the Basil S. Turner Professor of Engineering.

"Verticality gives you the ability to fit more things into the same area, so you can add more and more layers while keeping the footprint the same size or smaller," Fisher said. "But before we can even think about using nanotubes in electronics, we have to learn how to put them where we want them." Purduefisher-nano.jpg
This diagram shows the porous anodic alumina template created by Purdue engineers to grow carbon nanotubes vertically out of tiny cavities on top of a silicon wafer. The technique might form a foundation for creating vertically oriented nanoelectronics that can pack more devices and circuits into a computer chip while keeping the same footprint as conventional chips. (Image: Birck Nanotechnology Center, Purdue University)
The engineers first created a thin film containing two layers of aluminum sandwiching one ultrathin layer of iron using electron-beam evaporation, a standard process employed in the semiconductor industry. The engineers then used anodization, a process that causes metals to oxidize -- like rusting -- to selectively create tiny cylindrical cavities and turn the film into a porous anodic alumina template less than 1/100th the width of a human hair. During the process, an electric field was used to form a precisely aligned array of nanoscopic holes, turning aluminum into porous alumina, the oxidized form of aluminum also known as aluminum oxide.

A mixture of hydrogen and methane gas was then flowed into the template's holes, and microwave energy was applied to break down the methane, which contains carbon. The iron layer acted as a catalyst that prompted the carbon nanotubes to assemble from carbon originating in the methane, and the tubes then grew vertically out of the cavities.

"You get a single nanotube in each pore, and that's important because we can start to think about controlling how and where to put nanotubes to vertically integrate them for future electronic devices and sensing technologies," Sands said.Purduenanotubes.jpg
This picture, taken with a transmission electron microscope, shows carbon nanotubes that have been grown out of tiny cavities in a porous anodic alumina template created by Purdue engineers. A layer of iron in the template acts as a catalyst to grow the nanotubes from methane gas flowed into the template. (Photo: Birck Nanotechnology Center, Purdue University)
The team's findings are detailed in a research paper that appeared July 11 in the journal Nanotechnology. The paper was written by graduate students Matthew R. Maschmann and Aaron D. Franklin; postdoctoral research associate Placidus Amama; Dmitri Zakharov, a staff scientist at Purdue's Birck Nanotechnology Center; Eric Stach, an associate professor of materials engineering; Sands and Fisher. The research is based at Birck Nanotechnology Center.

"The pores in the template and the nanotubes that grow in the pores really self-assemble once you set the process in motion," said Stach, who used two types of electron microscopes to take images of the nanotubes emerging vertically from the cavities.

The cavities form within seconds, and the nanotubes take several minutes to finish growing. The holes vary in width from 30-50 nanometers. A nanometer, or billionth of a meter, is about as long as 10 hydrogen atoms strung together.

Carbon nanotubes, which were discovered in the early 1990s, might enable industry to create new types of transistors and more powerful, energy-efficient computers, as well as ultrathin "nanowires" for electronic circuits. Reaching that potential promise, however, won't be possible unless carbon nanotubes can be integrated with other parts of circuitry and devices, Sands said.

The experiments at Purdue yielded both single- and double-walled nanotubes, meaning they are made of either one or two single sheets of carbon atoms, yielding tubes about one nanometer in diameter.

Researchers will continue their efforts to understand which conditions are needed to produce single-wall tubes versus the double-wall variety and to learn how to produce more of one or the other. Other researchers previously have made the templates, but the Purdue researchers are the first to add a layer of iron, which was Maschmann's idea, Fisher said.

"He was told by many people, including me, that it probably wouldn't work," Fisher said. "We were surprised to see that the nanotubes grow from the sidewall of the hole and then extend vertically."

Early applications are most likely in wireless computer networks and radar technology. Long-term uses are possible in new types of transistors, other electronic devices and circuits, the researchers said.

The work is funded by NASA through the Purdue-based Institute for Nanoelectronics and Computing. For more information, visit: http://news.uns.purdue.edu



GLOSSARY
wafer
A cross-sectional slice cut from an ingot of either single-crystal, fused, polycrystalline or amorphous material that has refined surfaces either lapped or polished. Wafers are used either as substrates for electronic device manufacturing or as optics. Typically, they are made of silicon, quartz, gallium arsenide or indium phosphide.
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