chips draw near
Technology Research News
Carbon nanotubes are all the rage in research
circles because they are versatile and very small. But before these rolled-up
sheets of carbon atoms can take the rest of the world by storm researchers
have to make millions of them at a time in orderly arrays and connect
them to form useful devices.
Carbon nanotubes usually form in a jumble, like a pile of cooked spaghetti.
The challenge is to get them to form in the right places, and grow straight
and in the right direction.
Researchers at Stanford University have coaxed massive arrays of single-walled
carbon nanotubes to form on specific sites on the 4-inch silicon wafers
used to make computer
chips. Growing nanotubes in an orderly fashion on silicon wafers makes
it possible for manufacturers to use existing chipmaking technology to
connect the nanotubes into circuits.
The technique could eventually be used to produce massive transistor arrays
for computer processors, memory chips and chemical and biological sensors,
said Hongjie Dai, an assistant professor of chemistry at Stanford University.
"Full wafer growth enables scalability of nanotube devices," he said.
To make the nanotubes grow on a wafer, the researchers used photolithography,
an etching method that employs light and chemicals, to put an array of
10 to 100 million microscopic dots on the silicon wafer. The dots, which
are as small as one micron in diameter, served as catalysts to initiate
the growth of carbon nanotubes. A micron is one thousandth of a millimeter;
a red blood cell measures 5 microns in diameter. The researchers then
exposed the wafer to a hot vapor containing carbon. The carbon atoms condensed
to form nanotubes and began their growth from the catalyst dots.
The researchers used electric fields to orient the nanotubes as they grew,
said Dai. The nanotubes acted like microscopic magnets that were oriented
with electric field.
One, two or three single-walled nanotubes grew from each dot, according
to Dai. The nanotubes ranged from 1 to 3 nanometers in diameter and grew
as long as 10,000 nanometers, which is about twice as long as a red blood
cell. A nanometer is one millionth of a millimeter and one-thousandth
of a micron.
The chemistry that yields the nanotubes is independent of the size of
the catalyst islands, and the technique could theoretically produce a
nanotube for every 100 square nanometers of surface area on a wafer, or
about 800 million on a 4-inch wafer, said Dai.
The researchers' next step is using standard chip-making techniques to
place metal contacts over the nanotubes to connect them into circuits,
said Dai. The patterned nanotube growth technique could find using practical
applications in 1 to 5 years, said Dai.
Dai's research colleagues were Nathan Franklin, Yiming Li, Robert Chen
and Ali Javey of Stanford University. They published their research in
the December 31, 2001 issue of the journal Applied Physics Letters. The
research was funded by the Defense Advanced Research Projects Agency (DARPA).
Timeline: 1-5 years
TRN Categories: Materials Science and Engineering; Nanotechnology;
Story Type: News
Related Elements: Technical paper, "Patterned growth of
single-walled carbon nanotubes on full 4-inch wafers," Applied Physics
Letters, December 31, 2001; "Electric-field-directed growth of aligned
single-walled carbon nanotubes," Applied Physics Letters, November 5,
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