Bundling carbon nanotubes for microchip interconnects

Physicists at the Rensselaer Polytechnic Institute in New York, US, have devised a way of compacting carbon nanotubes into dense bundles that could eventually replace copper interconnects in microchips.

James Jian-Qiang Lu and his colleague Zhengchun Liu have shown that immersing vertically grown nanotube bundles in an organic solvent and allowing them to dry results in capillary action pulling the nanotubes together into a denser bundle. Molecular bonds then ensure that the nanotubes retain their tightly packed form.

Carbon nanotube bundle - before compacting (© James Lu/Rensselaer Polytechnic Institute) Carbon nanotube bundle - after compacting (© James Lu/Rensselaer Polytechnic Institute)

Processing the nanotubes in this way boosts the density of the original bundles by up to 25 times, and the higher the density, the better the conductivity.

Factors that influence the density include nanotube height, diameter, spacing and synthesis method. And size is crucial. If the bundles are too short, they cannot be compacted; too tall and they collapse.

There is some way to go before the process can find practical application, but Lu is optimistic: “We have to find ways to further process the nanotube bundles, such as metal/nanotube contact, placing it to the right locations,… It is very challenging, but we have some ideas, and are working on the implementation of densified carbon nanotubes as the basic building blocks for interconnects.”

The researchers are currently exploring various methods of further increasing the bundle density, and the quality of the nanotubes used. The eventual aim is to develop 3-D microchips in which the constituent devices are layered into vertical stacks, thereby dramatically shrinking the size of the chips.

Azad Naeemi, an electrical engineer at Georgia Tech, comments: “Growth of densely packed carbon nanotube bundles is a grand challenge in fabricating nanotube interconnects. The 25 times increase in the density of nanotubes achieved by the Rensselaer researchers is very promising. More such out-of-the-box approaches are essential in circumventing the challenges facing nanotube interconnects.”

Figure: Carbon nanotube bundles before and after densification (&copu; James Lu/Rensselaer Polytechnic Institute).

Article first published in Nanomaterials News.