Monday, October 29, 2012
Researchers Bring Commercial Fabrication of Carbon Nanotubes Closer
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IBM scientists have demonstrated a new approach to carbon
nanotechnology that opens up the path for commercial fabrication of
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.
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
"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
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
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
To overcome these barriers, IBM researchers developed a 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
This achievement was published today in the peer-reviewed journal