Bottom-up fabrication shrinks silicon circuit connections.

by Philip Ball

Minimizing the size of interconnects - the electrical links between different circuit components on a silicon microchip - is a challenge for conventional fabrication techniques. Now researchers may have found a solution in the shape of carbon nanotubes.

At present, interconnects are generally thin copper wires. At high current densities their metal atoms migrate, disrupting flow. Their electrical conductivity is also degraded at very small scales by electron scattering at surfaces and grain boundaries.

Jun Li and colleagues at NASA's Ames Research Center in Moffett Field, California, have grown vertical nanotubes at selected positions on a silicon chip, and then surrounded them with insulating silicon dioxide so that they cut a conducting channel through the film1.

Making these wires typically involves etching a hole - called a via - through the insulating film and then depositing the metal inside it. As interconnects become narrower, it is difficult to keep their sides straight - etching may make the via wider at the top than at the bottom. Filling very narrow channels completely with metal is also difficult.

Li's team essentially reverse the procedure: they grow free-standing, straight-sided interconnects first, and then fill in the gaps around them.

The researchers first cover a silicon wafer with a thin film of chromium; on this they lay islands of a nickel catalyst to act as a template. They then deposit vapour from a carbon-rich gas in an electrical field perpendicular to the surface, which ensures that the tubes grow vertically. The nanotubes are up to 10 millionths of a millimetre (10 nanometres) long and 30-200 nanometres wide.

Next they fill the space between the nanotubes with silica, again by vapour deposition. The silica coats the nanotubes completely, so the researchers then shave the top layers off to expose the nanotubes' ends.

For a compact bundle of nanotubes 250 by 500 nanometres across, the resistance is low - sufficient to furnish a good interconnect.

But a single nanotube has a resistance more than ten times greater than that of a bundle. The researchers suspect that this may be partly due to poor electrical contacts to the tips; it might also result from the nanotubes' structure. The deposition technique produces bamboo-like or nested-cone nanotubes, rather than those with continuous walls.

References

# Li, J. et al. Bottom-up approach for carbon nanotube interconnects.
Applied Physics Letters, 82, 2491 - 2493, (2003). |Article|