Because of the physics of electron motion in their structures, carbon nanotubes look like the perfect conductors for replacing copper-damascene interconnections on ICs. The tubes bring with them several minor problems, however. One is that, after you grow the tubes, they may not turn out to be conductors. Another issue is that the only reliable way to grow the tubes is in a forest: You must anchor them at the bottom and vertically grow them in a soup of available carbon atoms. That limitation is not helpful for IC interconnect, in which tiny bundles of tubes would have to follow routing channels from one contact to another. Yet another issue is that a single tube is of little use; you need a bundle of them to carry useful current.

At least the last problem is yielding to solutions—pun intended. Researcher James Jiam-Qiang Lu, associate professor of physics and electrical engineering at Rensselaer Polytechnic Institute (Troy, NY), led a small team investigating the bundling of the nanotubes. He presented the work at the International Interconnect Technology Conference, which took place last month in Burlingame, CA. The tubes normally grow in loose bunches, rather like hay. Lu found that, when his team immersed the bunch of nanotubes in isopropyl alcohol and then let it dry, capillary action pulled the tubes together into a much tighter bundle, typically 25 times denser than the original bunch. This density is still insufficient to compete with high-quality metallic copper for current density, but it is getting closer. And the problems of fabricating interconnect lines from copper are growing with each new process node.