Scientists at the California Institute of Technology
(Caltech) have uncovered the physical mechanism by which arrays of nanoscale
(billionths-of-a-meter) pillars can be grown on polymer films with very high
precision, in potentially limitless patterns.
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Upper: Schematic showing typical experimental setup. Lower: AFM image of 260 nm high nanopillars spaced 3.4 microns apart which formed in a polymer film. Credit: Upper: Dietzel and Troian/Caltech; PRL. Lower: Chou and Zhuang, J. Vac. Sci. Technol. B 17, 3197 (1999).
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This nanofluidic process—developed by Sandra Troian,
professor of applied physics, aeronautics, and mechanical engineering at
Caltech, and described in a recent article in the journal Physical Review
Letters—could someday replace conventional lithographic patterning techniques
now used to build three-dimensional nano- and microscale structures for use in
optical, photonic, and biofluidic devices.
The fabrication of high-resolution, large-area nanoarrays
relies heavily on conventional photolithographic patterning techniques, which
involve treatments using ultraviolet light and harsh chemicals that alternately
dissolve and etch silicon wafers and other materials. Photolithography is used
to fabricate integrated circuits and microelectromechanical devices, for
example.
However, the repeated cycles of dissolution and etching
cause a significant amount of surface roughness in the nanostructures,
ultimately limiting their performance.
"This process is also inherently two-dimensional, and
thus three-dimensional structures must be patterned layer by layer," says
Troian.
In an effort to reduce cost, processing time, and roughness,
researchers have been exploring alternative techniques whereby molten films can
be patterned and solidified in situ, and in a single step.
About a decade ago, groups in Germany,
China, and the United States
encountered a bizarre phenomenon while using techniques involving thermal
gradients. When molten polymer nanofilms were inserted within a slender gap
separating two silicon wafers that were held at different temperatures, arrays
of nanoscale pillars spontaneously developed.
These protrusions grew until they reached the top wafer; the
resulting pillars were typically several hundred nanometers high and several
microns apart.
These pillars sometimes merged, forming patterns that looked
like bicycle chains when viewed from above; in other films, the pillars grew in
evenly spaced, honeycomb-like arrays. Once the system was brought back down to
room temperature, the structures solidified in place to produce self-organized
features.
In 2002, researchers in Germany who had observed this
phenomenon hypothesized that the pillars arise from infinitesimal—but very
real—pressure fluctuations along the surface of an otherwise quiescent flat
film. They proposed that the differences in surface pressure were caused by
equally tiny variations in the way individual packets (or quanta) of
vibrational energy, known as phonons, reflect from the film interfaces.
"In their model, the difference in acoustic impedance
between the air and polymer is believed to generate an imbalance in phonon flux
that causes a radiation pressure that destabilizes the film, allowing pillar
formation," says Troian. "Their mechanism is the acoustic analogue of
the Casimir force, which is quite familiar to physicists working at the
nanoscale."
But Troian, who was familiar with thermal effects at small
scales—and knew that the propagation of these phonons is actually unlikely in
amorphous polymer melts, which lack internal periodic structure—immediately
recognized that another mechanism might be lurking in this system.
To determine the actual cause of nanopillar formation, she
and Caltech postdoctoral scholar Mathias Dietzel developed a fluid-dynamical
model of the same type of thin, molten nanofilm in a thermal gradient.
Their model, Troian says, "exhibited a self-organizing
instability that was able to reproduce the strange formations," and showed
that nanopillars, in fact, form not via pressure fluctuations but through a
simple physical process known as thermocapillary flow.
In capillary flow—or capillary action—the attractive force,
or cohesion, between molecules of the same liquid (say, water) produces surface
tension, the compressive force that is responsible for holding together a
droplet of water. Since surface tension tends to minimize the surface area of a
liquid, it often acts as a stabilizing mechanism against deformation caused by
other forces. Differences in temperature along a liquid interface, however,
generate differences in surface tension. In most liquids, cooler regions will
have a higher surface tension than warmer ones—and this imbalance can cause the
liquid to flow from warmer- to cooler-temperature regions, a process known as
thermocapillary flow.
Previously, Troian has used such forces for microfluidic
applications, to move droplets from one point to another.
"You can see this effect very nicely if you move an ice
cube in a figure eight beneath a metal sheet coated with a liquid like
glycerol," she says. "The liquid wells up above the cube as it traces
out the figure. You can draw your name in this way, and, presto! You have got
yourself a new form of thermocapillary lithography!"
In their Physical Review Letters paper, Troian and Dietzel
showed how this effect can theoretically dominate all other forces at nanoscale
dimensions, and also showed that the phenomenon is not peculiar to polymer
films.
In the thermal-gradient experiments, they say, the tips of the
tiny protrusions in the polymer film experience a slightly colder temperature
than the surrounding liquid, because of their proximity to the cooler wafer.
"The surface tension at an evolving tip is just a
little bit greater, and this sets up a very strong force oriented parallel to
the air/polymer interface, which bootstraps the fluid toward the cooler wafer.
The closer the tip gets to the wafer, the colder it becomes, leading to a
self-reinforcing instability," Troian explains.
Ultimately, she says, "you can end up with very long
columnar structures. The only limit to the height of the column, or nanopillar,
is the separation distance of the wafers."
In computer models, the researchers were able to use
targeted variations in the temperature of the cooler substrate to control
precisely the pattern replicated in the nanofilm. In one such model, they
created a three-dimensional "nanorelief" of the Caltech logo.
Troian and her colleagues are now beginning experiments in
the laboratory in which they hope to fabricate a diverse array of nanoscale
optical and photonic elements. "We are shooting for nanostructures with
specularly smooth surfaces—as smooth as you could ever make them—and 3-D shapes
that are not easily attainable using conventional lithography," Troian
says.
"This is an example of how basic understanding of the
principles of physics and mechanics can lead to unexpected discoveries which
may have far-reaching, practical implications," says Ares Rosakis, chair
of the Division of Engineering and Applied Science (EAS) and Theodore von
Kármán Professor of Aeronautics and Mechanical Engineering at Caltech.
"This is the real strength of the EAS division."
The work in the paper, "Formation of Nanopillar Arrays
in Ultrathin Viscous Films: The Critical Role of Thermocapillary
Stresses," was funded by the Engineering Directorate of the National
Science Foundation.
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