Despite his initial successes, Lu said the density results obtained are not ideal and carbon nanotubes would have to be further compacted before they can outperform copper as a conductor.

A close-up photo, taken using a scanning electron microscope, reveals there are still large empty spaces between densified nanotubes. The research team is exploring various methods to achieve ever-higher density and higher quality of carbon nanotube bundles, he said.

Lu is confident that these densified carbon nanotubes, with their high conductivity, ability to carry high current density, and resistance to electromigration, will be key to the development of 3-D computer chips.

Chips used today can only shrink so much smaller, as their flat surface must have enough room to accommodate scores of different components. But the semiconductor industry and academia are looking at ways to layer chip components into a vertical stack, which could dramatically shrink the size of the overall chip.

Densified carbon nanotubes, with their ends trimmed and polished, can be the basic building blocks for interconnects that would link the stacked layers of a 3-D computer chip, Lu said.

“Carbon nanotubes are one of the most promising materials for interconnects in 3-D integration,” he says. Other potential applications of the densified nanotubes are high surface area electrodes for supercapacitors, fuel cell electrodes for hydrogen storage, heat dissipation materials for thermal conductors, and other situations that require high electrical, thermal, or mechanical performance.

Lu’s group closely collaborates with a research group led by Pulickel Ajayan, the Henry Burlage Professor of Materials Science and Engineering at Rensselaer, who provided Lu with the grown nanotubes.

Along with Liu, who leads the carbon nanotube densification process investigation, the research team includes research associates Navdeep Bajwa, Lijie Ci, Swastik Kar, and Sang Hwui Lee.

The team has filed a disclosure and is moving forward toward a patent for their densification process.

Lu’s research is supported by Defense Advanced Research Projects Agency (DARPA), the Microelectronics Advanced Research Corporation (MARCO), and the New York State Office of Science, Technology and Academic Research (NYSTAR) through the Interconnect Focus Center (IFC). The IFC is a multi-university research center focusing on the discovery and invention of new solutions that will enable the U.S. semiconductor industry to transcend known limits on interconnects that would otherwise decelerate or halt the rate of progress toward future terascale system integration.

For more information on Lu’s research into 3-D integration, go to: http://www.rpi.edu/research/magazine/spring05/chips.html.