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In the study, Saif and graduate students
Jagannathan Rajagopalan and Jong H. Han explored aluminum films and
gold films. The aluminum films were 200 nanometers thick, 50-60
microns wide and 300-360 microns long. The gold films were 200
nanometers thick, 12-20 microns wide and 185 microns long. The average
grain size in the aluminum films was 65 nanometers; in the gold films,
50 nanometers.
"We found that the type of metal doesn’t matter,
said Saif, who also is a Willett Faculty Scholar and a researcher at
the university’s Micro and Nanotechnology Laboratory. "What matters is
the size of the grains in the metal’s crystalline microstructure, and
a distribution in the size."
Grain sizes are typically one-third to one-half the
thickness of a metal film. Raising the temperature by about 50 degrees
Celsius causes the grains to grow larger.
If the grains are uniformly too small, the metal
will be brittle and break while being bent. If the grains are
uniformly too large, the metal will bend, but then stay in that
position. To return to the initial shape, what’s needed is a balance
between brittleness and malleability.
That balance can be achieved through a combination
of small and large grains, the researchers report.
Variations in the microstructure lead to plastic
deformation in the larger grains and elastic accommodations in the
smaller grains, Saif said. The bigger grains bend, but push and pull
on the smaller grains, which become elastically deformed like a
spring.
If the metal is then left alone, the smaller grains
will release this energy and force the bigger grains back to their
original shapes over time. This local release of energy can be speeded
up by applying heat.
Controlling the crystalline microstructure of thin
films also could reduce energy loss in oscillators and resonators used
in electronic circuits, Saif said. Oscillators and resonators are
found in products ranging from air bag sensors and camcorders to
digital projectors and global positioning systems.
"If the grains that constitute the metal films in
these devices are between 50 and 100 nanometers, they can be very
lossy," Saif said. "However, if we decrease the grain size, we can
reduce much of the energy loss." |
