Northwestern Univ. researchers have
developed a new technique for rapidly prototyping nanoscale devices and
structures that is so inexpensive the “print head” can be thrown away
when done.
Hard-tip, soft-spring
lithography (HSL) rolls into one method the best of scanning-probe lithography—high
resolution—and the best of polymer pen lithography—low cost and easy
implementation.
HSL could be used in
the areas of electronics (electronic circuits), medical diagnostics (gene chips
and arrays of biomolecules), and pharmaceuticals (arrays for screening drug
candidates).
To demonstrate the
method’s capabilities, the researchers duplicated the pyramid on the U.S. one-dollar
bill and the surrounding words approximately 19,000 times at 855 million dots
per square inch. Each image consists of 6,982 dots. (They reproduced a bitmap
representation of the pyramid, including the “Eye of Providence.”)
This exercise highlights the sub-50-nm resolution and the scalability of the
method.
“Hard-tip,
soft-spring lithography is to scanning-probe lithography what the disposable
razor is to the razor industry,” said Chad A. Mirkin, the paper’s senior
author. “This is a major step forward in the realization of desktop
fabrication that will allow researchers in academia and industry to create and
study nanostructure prototypes on the fly.”
Mirkin is the George B.
Rathmann Professor of Chemistry in the Weinberg College of Arts and Sciences
and professor of medicine, chemical and biological engineering, biomedical
engineering and materials science and engineering and director of
Northwestern’s International Institute for Nanotechnology.
Micro- and
nanolithographic techniques are used to create patterns and build surface
architectures of materials on a small scale.
Scanning probe
lithography, with its high resolution and registration accuracy, currently is a
popular method for building nanostructures. The method is, however, difficult
to scale up and produce multiple copies of a device or structure at low cost.
Scanning probe
lithographies typically rely on the use of cantilevers as the printing device
components. Cantilevers are microscopic levers with tips, typically used to
deposit materials on surfaces in a printing experiment. They are fragile,
expensive, cumbersome and difficult to implement in an array-based experiment.
“Scaling
cantilever-based architectures at low cost is not trivial and often leads to
devices that are difficult to operate and limited with respect to the scope of
application,” Mirkin said.
Hard-tip, soft-spring
lithography uses a soft polymer backing that supports sharp silicon tips as its
“print head.” The spring polymer backing allows all of the tips to
come in contact with the surface in a uniform manner and eliminates the need to
use cantilevers. Essentially, hard tips are floating on soft polymeric springs,
allowing either materials or energy to be delivered to a surface.
HSL offers a method
that quickly and inexpensively produces patterns of high quality and with high
resolution and density. The prototype arrays containing 4,750 tips can be
fabricated for the cost of a single cantilever-based tip and made in mass,
Mirkin said.
Mirkin and his team
demonstrated an array of 4,750 ultra-sharp silicon tips aligned over an area of
one square centimeter, with larger arrays possible. Patterns of features with
sub-50-nanometer resolution can be made with feature size controlled by tip
contact time with the substrate.
They produced patterns
“writing” with molecules and showed that as the tips push against the
substrate the flexible backing compresses, indicating the tips are in contact
with the surface and writing is occurring. (The silicon tips do not deform
under pressure.)
“Eventually we
should be able to build arrays with millions of pens, where each pen is
independently actuated,” Mirkin said.
The researchers also
demonstrated the ability to use hard-tip, soft-spring lithography to transfer
mechanical and electrical energy to a surface.