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IBM researcher Hongsik Park observes different
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carbon nanotubes
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New York: IBM scientists have demonstrated a new approach to carbon
nanotechnology that opens up the path for commercial fabrication of
dramatically smaller, faster and more powerful computer chips. For the first
time, more than ten thousand working transistors made of nano-sized tubes of
carbon have been precisely placed and tested in a single chip using standard
semiconductor processes. These carbon devices are poised to replace and
outperform silicon technology allowing further miniaturization of computing
components and leading the way for future microelectronics.
Aided by rapid innovation over four decades, silicon
microprocessor technology has continually shrunk in size and improved in
performance, thereby driving the information technology revolution. Silicon
transistors, tiny switches that carry information on a chip, have been made
smaller year after year, but they are approaching a point of physical
limitation. Their increasingly small dimensions, now reaching the nanoscale,
will prohibit any gains in performance due to the nature of silicon and the
laws of physics. Within a few more generations, classical scaling and shrinkage
will no longer yield the sizable benefits of lower power, lower cost and higher
speed processors that the industry has become accustomed to.
Carbon nanotubes represent a new class of semiconductor
materials whose electrical properties are more attractive than silicon,
particularly for building nanoscale transistor devices that are a few tens of
atoms across. Electrons in carbon transistors can move easier than in
silicon-based devices allowing for quicker transport of data. The nanotubes are
also ideally shaped for transistors at the atomic scale, an advantage over
silicon. These qualities are among the reasons to replace the traditional
silicon transistor with carbon – and coupled with new chip design architectures
– will allow computing innovation on a miniature scale for the future.
The approach developed at IBM labs paves the way for circuit
fabrication with large numbers of carbon nanotube transistors at predetermined
substrate positions. The ability to isolate semiconducting nanotubes and place
a high density of carbon devices on a wafer is crucial to assess their
suitability for a technology – eventually more than one billion transistors
will be needed for future integration into commercial chips. Until now,
scientists have been able to place at most a few hundred carbon nanotube
devices at a time, not nearly enough to address key issues for commercial
applications.
"Carbon nanotubes, borne out of chemistry, have largely
been laboratory curiosities as far as microelectronic applications are
concerned. We are attempting the first steps towards a technology by
fabricating carbon nanotube transistors within a conventional wafer fabrication
infrastructure," said Supratik Guha, Director of Physical Sciences at IBM
Research. "The motivation to work on carbon nanotube transistors is that
at extremely small nanoscale dimensions, they outperform transistors made from
any other material. However, there are challenges to address such as ultra high
purity of the carbon nanotubes and deliberate placement at the nanoscale. We
have been making significant strides in both."
Originally studied for the physics that arises from their
atomic dimensions and shapes, carbon nanotubes are being explored by scientists
worldwide in applications that span integrated circuits, energy storage and
conversion, biomedical sensing and DNA sequencing.
This achievement was published today in the peer-reviewed journal
Nature Nanotechnology.
The Road to Carbon
Carbon, a readily available basic element from which
crystals as hard as diamonds and as soft as the "lead" in a pencil
are made, has wide-ranging IT applications.
Carbon nanotubes are single atomic sheets of carbon rolled
up into a tube. The carbon nanotube forms the core of a transistor device that
will work in a fashion similar to the current silicon transistor, but will be
better performing. They could be used to replace the transistors in chips that
power our data-crunching servers, high performing computers and ultra fast
smart phones.
Earlier this year, IBM researchers demonstrated carbon
nanotube transistors can operate as excellent switches at molecular dimensions
of less than ten nanometers – the equivalent to 10,000 times thinner than a
strand of human hair and less than half the size of the leading silicon
technology. Comprehensive modeling of the electronic circuits suggests that
about a five to ten times improvement in performance compared to silicon
circuits is possible.
There are practical challenges for carbon nanotubes to
become a commercial technology notably, as mentioned earlier, due to the purity
and placement of the devices. Carbon nanotubes naturally come as a mix of
metallic and semiconducting species and need to be placed perfectly on the
wafer surface to make electronic circuits. For device operation, only the
semiconducting kind of tubes is useful which requires essentially complete
removal of the metallic ones to prevent errors in circuits. Also, for large
scale integration to happen, it is critical to be able to control the alignment
and the location of carbon nanotube devices on a substrate.
To overcome these barriers, IBM researchers developed a
novel method based on ion-exchange chemistry that allows precise and controlled
placement of aligned carbon nanotubes on a substrate at a high density – two
orders of magnitude greater than previous experiments, enabling the controlled
placement of individual nanotubes with a density of about a billion per square
centimeter.
The process starts with carbon nanotubes mixed with a
surfactant, a kind of soap that makes them soluble in water. A substrate is
comprised of two oxides with trenches made of chemically-modified hafnium oxide
(HfO2) and the rest of silicon oxide (SiO2). The substrate gets immersed in the
carbon nanotube solution and the nanotubes attach via a chemical bond to the
HfO2 regions while the rest of the surface remains clean.
By combining chemistry, processing and engineering
expertise, IBM researchers are able to fabricate more than ten thousand
transistors on a single chip.
Furthermore, rapid testing of thousands of devices is possible
using high volume characterization tools due to compatibility to standard
commercial processes.
As this new placement technique can be readily implemented,
involving common chemicals and existing semiconductor fabrication, it will
allow the industry to work with carbon nanotubes at a greater scale and deliver
further innovation for carbon electronics.