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Molecular scale resolution
achieved in polymer nanoimprinting technique
James E. Kloeppel, Physical Sciences Editor
217 244-1073; email@example.com
photo to enlarge
by Kwame Ross
Rogers, a professor of materials science and engineering at
Illinois, and his team used molds derived from carbon nanotubes
have approached the ultimate resolution – defined by
molecular scale dimensions – in a widely used polymer
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 their widths.
“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 macromolecules.”
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,” Rogers
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 said. 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 Corning.