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