Researchers create first ever integrated silicon circuit with nanotube transistors - DARPA
[Only registered users see links. ]
Researchers create first ever integrated silicon circuit with nanotube
By Sarah Yang, Media Relations | 05 January 2004
BERKELEY - The discovery of carbon nanotubes heralded a new era of
scientific discovery that included the promise of ultra-sensitive bomb
detectors and super-fast computer memory chips. But finding a way to
incorporate nanomaterials into a working nanoelectronic system has been a
frustratingly elusive achievement - until now.
In an important milestone in the fields of nanosciences and nanoengineering,
researchers at the University of California, Berkeley and Stanford
University are announcing that they have created the first working,
integrated silicon circuit that successfully incorporates carbon nanotubes
in its design.
"Until our work, no group has publicly reported success in directly
integrating nanotubes onto silicon circuits," said Jeffrey Bokor, UC
Berkeley professor of electrical engineering and computer sciences and
principal investigator of the project. "It is a critical first step in
building the most advanced nanoelectronic products, in which we would want
to put carbon nanotubes on top of a powerful silicon integrated circuit so
that they can interface with an underlying information processing system."
Researchers say the development brings them a significant step closer to
using carbon nanotubes for memory chips that can hold orders of magnitude
more data than current silicon chips - 10,000 times greater, according to
some estimates - or for sensors sensitive enough to detect traces of
explosives or biochemical agents at the molecular level.
UC Berkeley engineers teamed up with chemists at Stanford to develop an
integrated circuit that can dramatically speed the analysis of thousands of
synthesized carbon nanotubes. The description of this work appears in the
January 2004 issue of Nano Letters, a publication of the American Chemical
"These results represent a dream come true," said Hongjie Dai, associate
professor of chemistry at Stanford and co-principal investigator of the
project. "This achievement opens up a vast number of possible applications
A carbon nanotube, which looks like rolled chicken wire when examined at the
atomic level, is tens of thousands of times thinner than a human hair, yet
remarkably strong. It has attractive electrical properties, which several
research groups - including the one led by Dai at Stanford - have harnessed
to create high performance transistors.
The road to creating the first nano-silicon hybrid circuit began as a
solution to a practical research problem: How to refine the process of
growing nanotubes so that they are created with predictable qualities.
Depending on the molecular structure specific to each carbon nanotube, it
can either be metallic and capable of conducting electricity, or act like
semiconductors, with conductivity that can be turned on and off. But the
current synthesis process results in an unpredictable proportion of metallic
and semiconducting nanotubes, leaving researchers uncertain as to how much
of each type they'll get in any one batch.
Analyzing whether a batch yielded metallic or semiconducting nanotubes
involved a labor-intensive processing of manually checking the electrical
conductivity of each carbon nanotube.
To resolve this problem, the researchers set out to build a device that
would automate the process of decoding thousands of carbon nanotubes on a
silicon chip. Working with UC Berkeley's Microfabrication Laboratory, they
created a chip with silicon metal oxide semiconductor (MOS) circuitry. The
chip, dubbed the random access nanotube test chip, or RANT, contains a
network of silicon wires and switches that form a circuit.
Researchers then proceeded to grow carbon nanotubes directly onto "islands"
on the circuit platform that contained the necessary catalyst for nanotube
synthesis. The extreme heat required to grow nanotubes would typically melt
the circuitry of traditional semiconductors, but the researchers got around
that problem by interconnecting the silicon transistors with molybdenum, a
refractory metal that can withstand very high temperatures.
"We first envisioned a patterned growth of carbon nanotubes on silicon
wafers five years ago, but it wasn't clear at that time whether that
approach would work as an integrated nanotube-silicon hybrid circuit," said
Dai. "It was the combined expertise in chemistry, materials science and
electrical engineering that made this a reality."
The resulting chip contained thousands of carbon nanotubes connected to the
circuit on a 1-square-centimeter silicon chip. By turning certain switches
on and off, researchers were able to isolate the path that leads to an
individual nanotube. Not only could researchers pinpoint which nanotube was
responding to electrical current passing through the system, they could tell
whether the conductivity could be turned on or off. If they were able to
change the conductivity of the nanotube, they knew that it was a
semiconductor and not metallic.
"The circuit is interconnected in such a way that only 22 control signals
are needed in testing more than 2,000 nanotubes," said Yu-Chih Tseng, a UC
Berkeley graduate student in electrical engineering and computer sciences
and lead author of the paper. "The key is that this can all be done by a
machine and computer.
We succeeded in making a tool for nanotechnology researchers, and in the
process, we demonstrated the broader proof of principle that nanotubes can
be successfully integrated in a complex circuit."
Research such as this is an important component of the UC Berkeley-based
Center for Information Technology Research in the Interest of Society, or
CITRIS. The center includes a major emphasis on nanosciences and
nanoengineering, and is funding the construction of a new nanofabrication
laboratory on the UC Berkeley campus that would significantly enhance
researchers' ability to conduct such fundamental and innovative work, said
Ruzena Bajcsy, director of CITRIS.
Bokor cautions that the integrated circuit they have built is not a likely
candidate for commercialization just yet. For one, the molybdenum they used
to protect the circuit from heat damage is not a typical material used in
the semiconductor industry because it is a high-resistance metal.
Nevertheless, the achievement opens the door for other promising research on
nanotechnology devices, including those made of silicon nanowires and
organic polymers, researchers said.
"Carbon nanotubes have fascinated many scientists and those interested in
science ever since they were discovered," said Ali Javey, a graduate student
in chemistry at Stanford and co-author of the paper. "This work takes us an
important step forward by proving the compatibility of the nanotube
synthesis process with modified silicon technology and leading the way to
future nanotube-based commercial applications."
The research was conducted as part of the Materials, Structures and Devices
Center, one of five multi-university focus centers funded by the
Microelectronics Advanced Research Corporation (MARCO), a subsidiary of the
Semiconductor Research Corporation. The project was also supported by the
Defense Advanced Research Projects Agency (DARPA).
Other co-authors of the study include Peiqi Xuan and Ryan Malloy at UC
Berkeley, and Qian Wang at Stanford.
Do Wah Ditty
"Our country puts $1 billion a year up to help feed the hungry. And we're by
far the most generous nation in the world when it comes to that, and I'm
proud to report that. This isn't a contest of who's the most generous. I'm
just telling you as an aside. We're generous. We shouldn't be bragging about
it. But we are. We're very generous."
- Washinton D.C., July 16, 2003