FOR RELEASE: July 22, 1997
Contact: Larry Bernard
Office: (607) 255-3651
Internet: [email protected]
Compuserve: Larry Bernard 72650,565
http://www.news.cornell.edu
ITHACA, N.Y. -- The world's smallest guitar -- carved out of crystalline
silicon and no larger than a single cell -- has been made at Cornell
University to demonstrate a new technology that could have a variety of
uses in fiber optics, displays, sensors and electronics.
The "nanoguitar" -- made for fun to illustrate the technology -- is just
one of several structures that Cornell researchers believe are the world's
smallest silicon mechanical devices. Researchers made these devices at the
Cornell Nanofabrication Facility, bringing microelectromechanical devices,
or MEMS, to a new, even smaller scale -- the nano-sized world.
"We have a new technology for building the smallest mechanical devices,"
said Harold G. Craighead, Cornell professor of applied and engineering
physics who has the directed the work performed by his doctoral student,
Dustin W. Carr.
The guitar has six strings, each string about 50 nanometers wide, the width
of about 100 atoms. If plucked -- by an atomic force microscope, for
example -- the strings would resonate, but at inaudible frequencies. The
entire structure is about 10 micrometers long, about the size of a single
cell.
A scanning electron microscope photo of the guitar won the
award for best scanning electron micrograph at the 41st Electron, Ion and
Photon Beam Technology and Nanofabrication Conference in Dana Point,
Calif., in May. Carr and Craighead presented their research at the
conference and submitted a paper to the Journal of Vacuum Science and
Technology.
While the guitar resulted in an award-winning electron micrograph, it is
the other structures and devices that will be of real utility, Craighead
says. Applications that require small-scale mechanical probes, high-speed
response or measurement of very small forces can benefit from this
technology. For example, mechanical force probes can be made much smaller
than a single cell, and forces associated with single biological molecules
could be measured.
An efficient and relatively non-invasive method of measuring the small
motion of the mechanical structures is performed by using the interference
of laser light beams. The Cornell researchers have made a Fabry-Perot
interferometer using this technology. These interferometers use parallel
mirrors, one of which moves relative to the other. The motion is detected
by variations in the reflected light. The devices currently under study in
Craighead's laboratories are moved by electrical forces. These
electrically driven devices can be used to modulate the intensity of the
reflected light.
"This could be of interest for light displays," said Craighead, former
director of the Cornell Nanofabrication Facility, a national resource.
"You could have arrays of these things because they're so small, with each
one independently driveable. We have tremendous flexibility in what we can
build."
In the near term, such nanostructures also can be used to modulate lasers
for fiber optic communications. These researchers already have
demonstrated the ability to make large amplitude modulation of light
signals at high speeds. "We can make reflected light pulses at a rate of
12 million per second," Craighead said. Such a rate is faster than the bit
rate of most ethernet connections.
The structures also are extremely sensitive to very small forces. "We make
these structures move and measure the motion by placing single electrons on
the surface of the devices," Carr said.
The key to the technology is electron-beam (or E-beam) lithography. E-beam
lithography is a technique for creating extremely fine patterns (much
smaller than can be seen by the naked eye) required by the modern
electronics industry for integrated circuits. Derived from the early
scanning-electron microscopes, the technique consists of scanning a beam of
electrons across a surface covered with a thin film, called a resist. The
electrons produce a chemical change in this resist, which allows the
surface to be patterned.
Most microelectromechanical devices are made by photolithography and
chemical etching and have minimum feature sizes of slightly less than 1
micrometer. To build devices with dimensions of nanometers rather than
micrometers requires a new fabrication approach.
A nanometer is one-billionth of a meter. For comparison, the diameter of a
human hair is about 200 micrometers, or 200,000 nanometers -- positively
huge compared to these newest structures, where a wire is about 50 to 100
nanometers in diameter.
"I know we can go smaller than this. The question is how small we can go
and still have dependable and measurable mechanical properties. That is
one of the things we would like to know," Craighead said. "We are nearing
the technological limit where it gets harder to get smaller than this."
Using high-voltage electron beam lithography at the Cornell Nanofabrication
Facility, one of only two similar machines in this country, Carr and
Craighead sculpted their structures out of single crystal silicon on oxide
substrates. A resist is used to pattern the top silicon layer. The oxide
that is underneath this layer can be selectively removed using a wet
chemical etch. The result: free-standing structures in silicon crystal.
"The nano-mechanical devices will also allow further exploration into
important physical questions regarding motion and mechanical energy
dissipation," the researchers wrote in their report.
The Cornell Nanofabrication Facility (CNF) at Cornell University, is
partially supported by the National Science Foundation. It is part of the
National Nanofabrication Users Network, a partnership of nanotechnology
centers across the nation. In addition, this research was funded partially
by the U.S. Department of Defense and the Department of Education.
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