Copper interconnects carry current in today's integrated circuits, but in the nanometer-size future, the metal just won't do the job. At this month's IEEE 2010 International Electron Devices Meeting, in San Francisco, European researchers plan to announce that they're one step closer to a replacement. Nanotubes made of carbon, if grown in dense bundles, can transport large quantities of charge through tiny channels reliably. As part of the European ViaCarbon project, a team led by Jean Dijon, the head of nanotube research at the French government research organization CEA LITEN in Grenoble, says they've grown the densest bundles yet, packing 2.5 trillion carbon nanotubes per square centimeter. The density of their interconnects is within an order of magnitude of what's needed for replacing copper. In the future, such bundles have the potential to exceed copper's current-carrying capabilities by a factor of 100.
Copper needs a replacement particularly in the narrow pegs, called vias, that connect the silicon surface to the chips' wiring and connect one layer of wiring to another. According to the 2009 International Technology Roadmap for Semiconductors, engineers predict that as the features on chips shrink, not only will copper vias be more difficult to manufacture and suffer from more resistance, but by 2015 they may not work at all.
Vias start as holes etched into a layer of dielectric. Depositing copper ions from an electrolyte fills the holes, but at the nanometer scale, the metal could cling to the walls, leaving gaps. Also, the vias' walls must be lined with a metal nitride barrier to keep copper from seeping into the surrounding dielectric. The liner wastes space and increases resistance.
Engineers are also concerned that copper vias will prove unreliable at future nanometer-scale dimensions. Skinnier copper means that current density and resistivity both increase. Together, those factors cause the interconnects to heat up and break. "For each new generation, as dimensions decrease, current densities increase and will soon approach the copper limit," says Murielle Fayolle-Lecocq, a microelectronics engineer who worked on the new nanotube vias at France's CEA-LETI.
Carbon nanotubes have the perfect shape for fitting into future vias' voids: "Naturally, they're tall and thin," says the leader of the ViaCarbon project, John Robertson, who is an electrical engineering professor at the University of Cambridge, in England.