Ill. - Scientists using molds derived from carbon nanotubes
have approached the ultimate resolution - defined by
molecular scale dimensions - in a widely used polymer
nanoimprinting technique. By accurately replicating
features with nanometer dimensions, the technique could
play future roles in fabricating structures in fields
as diverse as microelectronics, nanofluidics and biotechnology.
Polymer nanoimprint lithography
works by pressing a mold with embossed relief structures
against a thin polymer film. Little is known, however,
of the basic physics and chemistry that operate between
the two surfaces at the molecular level, let alone
how these interactions relate to resolution.
"A better understanding
of the basic physics and resolution limits of nanoimprint
lithography will allow us to develop design criteria
for better polymer materials for molds and films that
would improve the performance," said John Rogers,
a professor of materials science and engineering at
the University of Illinois at Urbana-Champaign and
a researcher at the Beckman Institute for Advanced
Science and Technology.
In a paper published in the
December issue of the journal Nano Letters, Rogers
and colleagues at Illinois and Dow Corning Corp.
explored the fundamental resolution limits of polymer
nanoimprint lithography. The work involved a broad
interdisciplinary collaboration between experts in
several fields, including nanoimprint lithography,
carbon nanotubes, nanoscale imaging techniques for
polymers, and polymer chemistry.
The researchers began by growing
single-walled carbon nanotubes on a silicon wafer.
Then they prepared a mold of the nanotubes by pouring
a thermal-setting polymer over the wafer.
After curing the mold, they
gently pressed it against a thin layer of photocurable
polyurethane. Passing light through the transparent
mold caused the material to cross-link and harden.
The researchers then used atomic force microscopy
to measure the heights of the resulting relief structures
and transmission electron microscopy to determine
"Our approach allowed
us to reach a critical size regime never explored
before," Rogers said. "From a detailed analysis
of the microscope images, we were able to demonstrate
reliable patterning at the 2 nanometer scale, and
even some capability down to 1 nanometer.
These dimensions are comparable to the sizes of individual
To obtain features with a resolution
of 2 nanometers, both the average distance between
polymer cross-links (approximately 1
nanometer) and the lengths of individual chemical
bonds (approximately 0.2 nanometers) become important
in the molding process.
"We normally wouldn't
be concerned with the molecular structure of the polymer,"
Rogers said, "but at these dimensions we have
feature sizes that are only a few times larger than
the length of individual bonds in the polymer. In
addition, we have a countable number of polymer bond
lengths that are available to replicate the relief
By varying the density of cross-links
in the polymer, the researchers also established a
connection between resolution limit and molecular
structure of the polymer. "The ultimate resolution
is correlated to the ability of the prepolymer to
conform to the surface and the ability of the cross-linked
polymer to retain the molded shape,"
The ability to mold nano-scale
features can benefit many fields, from semiconductor
device manufacturing to emerging areas of biotechnology.
For example, polymer nanoimprint lithography could
help the electronics industry achieve the resolution
requirements needed for next-generation devices. By
structuring materials with dimensions smaller than
the wavelength of light, the technique also could
create photonic devices whose optical properties are
defined by the geometry of the relief structures embossed
In other applications, polymer
molds with molecular scale channels could prove useful
in nanofluidics, where the tiny tunnels would transport
fluids or separate materials based on size, Rogers
And, by allowing for the nanoimprinting of individual
macromolecules, the technique might open new paths
to molecular recognition, drug discovery and catalysis.
The work was funded by Dow
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James E. Kloeppel, Physical
217 244-1073; email@example.com