Source: National Science Foundation
Self-assembling Devices At The Nanoscale: A New Hybrid Technique Could Lead
To Mass-produced Chips With Molecular-scale Structure
Scientists at the University of Wisconsin's Materials Research Science and
Engineering Center (MRSEC) on Nanostructures, Materials, and Interfaces have
demonstrated a technique that could one day allow electronic devices to
assemble themselves automatically--giving semiconductor manufacturers a way
to mass-produce "nanochips" that have circuit elements only a few molecules
across, roughly ten times smaller than the features in current-generation
"In terms of storage alone, that could mean a computer with 4,000 gigabytes
of memory," says center director Juan de Pablo, a member of the Wisconsin
team, which is publishing its results in the July 24 issue of the journal
Nature. The Wisconsin MRSEC is one of 27 materials research centers
established by the National Science Foundation. Indeed, adds team leader
Paul Nealey, "we work closely with the Semiconductor Research Corporation,"
an industry consortium that includes such firms as IBM, Motorola, Intel,
AMD, and Shipley.
Basically, the two researchers explain, the chip-makers are worried about
what happens next. In today's fabrication plants, solid-state circuit
elements are etched onto the surface of a wafer of silicon via lithography:
a process that's somewhat like exposing photographic film and then
developing it. That approach has gotten the manufacturers down to features
on a scale of 100-150 nanometers, which is typical of current-generation
chips like the Pentium 4. "But the cost of the factories is increasing at an
exponential rate," says de Pablo, "and it's not clear if they can
extrapolate their current technology much below 50 nanometers."
Some experimental techniques can go smaller. For example, a tightly focused
electron beam can inscribe nanoscale circuit elements on the silicon surface
line by line, almost as if an artist were drawing them with a pen. The
problem is that drawing a single chip takes something like a week. With
lithography, which can imprint a pattern on the entire wafer surface at
once, the big fabrication plants can mass-produce thousands of chips in an
Another technique lately getting attention works from the bottom up, using
materials that will spontaneously assemble themselves into periodic
structures at the molecular scale. "Achieving dimensions of tens of
nanometers is inexpensive and routine," says Nealey--especially with certain
"block copolymers," which are compounds composed of two or more long polymer
chains connected at the ends, like so: .AAAAAAA·BBBBBBB·CCCCC.
Unfortunately, he says, left to their own devices, these materials tend to
organize themselves into roundish clumps and broad, swirling
patterns--lovely to look at, perhaps, but nothing like the precisely ordered
structures needed for technological applications.
Faced with that conundrum, Nealey and others have spent the past five years
or so combining lithography and self-assembly into a hybrid technique known
as "templated," or "directed" self-assembly. In the current work, the
Wisconsin group began by using lithographic techniques to chemically alter
the surface of a standard silicon wafer. However, because they employed
extreme ultraviolet light, which has a much shorter wavelength than the
light used in conventional lithography, and applied some clever optical
manipulations as they projected the light, they were able to lay down a
pattern of straight, parallel, chemically activated stripes only 20 to 30
Next, Nealey and his colleagues washed the patterned silicon surface with a
solution containing the block copolymer. In this case, it was a compound
containing just two types of component polymers: one that had a chemical
attraction to the stripes, and another that preferred to stay out in the
open air. By manipulating the length of the two polymers, and other such
factors, the researchers achieved a very precise balance between attraction
and repulsion. And as a result, the co-polymer organized itself right on top
of the nanoscale stripes, with no evidence of swirling or other undirected
Of course, de Pablo cautions, it's a long way from parallel lines of plastic
to fully operational electronic devices. "All that we've done in this work,"
he says, "is to create the pattern, show that the polymer follows the
pattern, and show that the final result is completely free of defects."
Nonetheless, Nealey points out, this work is a proof of principle: "We've
shown that this kind of hybrid technology can integrate self-assembling
materials, such as block copolymers, into existing manufacturing processes,
such as lithography, and deliver molecular level control."
One obvious next step is to grow the block copolymer in nanoscale vertical
columns, says Nealey. Such columns eventually could be engineered to hold
one bit of information each, leading to ultra-high density magnetic storage
media--probably "the most immediate and striking application" of the hybrid
technology, he says. A more speculative application would be to create
nanoscale integrated circuits. After all, says Nealey, many polymers can be
made to conduct electricity if they contain the right kind of metal ions. So
in principle, one could dope different parts of the polymer pattern with
different ions, and make devices such as diodes and transistors.
The tricky part would be designing the circuits themselves. "The industry
has been using the same integrated circuit designs for years," says Nealey,
"just shrinking them as the chips get smaller. But here we can't do that;
with our technique we can only make very simple shapes like lines and
circles. On the other hand, if we can make these simple designs very
inexpensively, then the question for the chip designers becomes, 'What can
we do with this?'"
"That's a huge unknown," says Nealey. "But it will be a big research area in
Editor's Note: The original news release can be found here.
Do Wah Ditty
"You're free. And freedom is beautiful. And, you know, it'll take time to
restore chaos and order - order out of chaos. But we will."
- Washington, D.C., April 13, 2003
- President Bushisms