IEEE Spectrum April, 2008 - 9

crusher, the autonomous vehicle funded by the u.s. Defense Department and built by carnegie mellon
university, lived up to its name during urban and desert field trials at Fort Bliss, texas. crusher not only drives
itself, but unlike other unmanned vehicles it also sketches out real-time maps of the terrain by using laser
range finders, radar, and stereoscopic optical cameras. watch crusher crush things (and maneuver around
things that don't crush well) at http://spectrum.ieee.org/video?id=241 photo: sarah adee

Carbon-Nanotube
Wiring Gets Real

with a need to replace copper chip
interconnects in sight, a potential successor
is finally proving itself

ould carbon nanotubes have a shot at
replacing the copper
wires that connect millions
of transistors on today's
silicon chips? Chip makers
replaced aluminum interconnects with better conducting
copper ones about seven years
ago, but now copper's days
are numbered too. Higherperformance chips with moretightly packed transistors,
expected as soon as 2012, will
need interconnects less than
40 nanometers wide, at which
point copper's resistance will
slow signaling down too much.
Late last month, at the
Materials Research Society's spring meeting in

San Francisco, a team of
engineers from Stanford and
Toshiba reported that they
have used carbon nanotubes
to wire logic-circuit components on a conventional
silicon CMOS chip. They
claim to have shown that
nanotubes can shuttle data
at speeds of a little faster
than 1 gigahertz, close to
the range of state-of-the-art
microprocessors, which
run at speeds of 2 to 3 GHz.
In principle, nanotubes can
handle a current density 1000
times as great as that of copper
or silver. Accordingly, many
chip makers, including Intel,
have been trying to figure out
whether nanotubes can be

practically combined into an
integrated circuit and, if so,
how their properties hold up.
Stanford electricalengineering professor
H.S. Philip Wong and his
collaborators at Toshiba
fabricated common test
circuits, called ring oscillators,
on a silicon chip. Each
oscillator was missing one
wire that would complete
the circuit. Then researchers
laid down nanotubes on top
of the circuits to make that
last connection. Of the 19 ring
oscillators, 16 worked at over
800 megahertz, and the best
worked at 1.02 GHz. "This is
the first time that a nanotube
as a wire is operating in
a conventional chip-type
environment," Wong says.
Alexander Tselev, a
chemist studying carbonnanotube interconnects at
Duke University, in Durham,
N.C., hails this as "a step
from basic science to real
application." Still, a number
of big challenges remain,
particularly devising a reliable
method to make nanotubes
with consistent properties
and finding a good way to
arrange tubes in a pattern.
Today's manufacturing
processes result in batches
containing nanotubes of
different sizes and electrical
properties, some that conduct
electricity and others that
are semiconductors. Indeed,
it is the inconsistencies
in resistance and in the
length of the nanotubes
that result in the different
tube teSter: carbon
nanotubes in this 256-oscillator
circuit operated at about 1 gHz.
photo: GaeL CLose

Na * iEEE SpEctrum * april 2008

operating speeds of the ring
oscillators, Wong says.
There is also no known
way to precisely place
nanotubes on a surface.
The researchers use a
standard method called dye
electrophoresis. It involves
depositing a nanotube
solution on a surface and
applying electric fields to
attract the nanotubes to the
required spots. The method
is unpredictable. Wong and
his colleagues started out by
fabricating 256 oscillators;
carbon nanotubes completed
the wiring in only 19.
There are many other
problems to be solved.
Interconnects would need
to be made from bundles of
carbon nanotubes, because
they conduct current much
better than single nanotubes
do. But bundles of tubes
would be hard to lay down
horizontally. Unlike integrated
circuits, which have layers of
semiconductor, insulator, and
other materials "nicely stacked
one on top of the other," says
Vladimir Stojanovic, an
electrical-engineering and
computer-science professor at
MIT, "tubes are hard to handle,
because they don't stack up
very well." Manufacturers will
have to find either a way to
place prefabricated nanotubes
in the right spots or a way to
grow nanotubes where they're
needed at temperatures that
match silicon-fabrication
temperatures, Stojanovic says.
Clearly, carbon-nanotube
researchers have a lot left to
do. With copper needing a
replacement as soon as 2012,
they might have to speed
things up. -Prachi Patel-Predd
www.spectrum.ieee.org

http://spectrum.ieee.org/video?id=241 http://www.spectrum.ieee.org

Table of Contents for the Digital Edition of IEEE Spectrum April, 2008