Stress-Free Engineering
by David Pescovitz
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Materials
Science professor Robert O. Ritchie. Click for larger
image.
Courtesy
Robert O. Ritchie
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"The whole basis of my research is looking at why things break," says Professor of Materials Science Robert O. Ritchie.
The most recent structures he's studying can't be seen without a scanning electron microscope though. Ritchie and his colleagues are working to understand why tiny electromechanical machines, most invisible to the naked eye, are more prone to failure than engineers have expected.
Fabricated en masse from silicon, similar to the way integrated
circuits are made, microelectromechanical systems (MEMS) are integrated
into an increasing number of devices — from sensors that trigger
auto airbag deployment after a collision to inkjet printers to medical
drug delivery systems. Each gear, actuator, or motor that make of
a MEMS chip is often smaller in diameter than a human hair.
On-chip
resonant fatigue device used to evaluate the fatigue endurance
of silicon films used in microscale devices. Click for larger
image.
Courtesy
Robert O. Ritchie
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"MEMS may be small,
but they're structures just like airplanes, ships, or bridges,"
says Ritchie, who also heads the structural materials research efforts
at Lawrence Berkeley National Laboratory. "If you want to make a
structure out of something that will last, silicon is often right
at the bottom of the list because it's so extremely brittle."
That brittleness means that the micron-thick silicon films that
make up MEMS components should simply fail at the maximum stress
they're capable of withstanding, not unlike window glass. But several
years ago, investigators reported that the silicon films oddly suffer
what's known as fatigue failure. Essentially, the material doesnít
fail immediately but rather after millions and millions of cycles
at stresses which can be as low as half the stress that would break
the structure in a single cycle. The curiosity of Ritchie and his
former graduate student Christopher Muhlstein's was piqued.
"With silicon, such fatigue failure simply shouldn't happen," Ritchie says. "But it did."
This
electron microscopy image of a section of Ritchie's resonant
fatigue device (photo above left) shows cracks forming in
the oxide layer on top of the silicon film. Click for larger
image.
Courtesy
Robert O. Ritchie
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To get a better
sense of what actually was occurring in the microworld, Ritchie
and Muhlstein used a 300 micron square MEMS resonator, a structure
that electrostatically flaps back and forth 40,000 times a second.
The resonator serves as both a sample and a machine to test the
sample. By electronically measuring the change in the resonant frequency
of the device, the frequency at which certain electrical variables
become equal, they were able to show that the fatigue failure was
associated with nanometer-scale cracks that were growing in the
component until they were long enough to completely break the device.
Moreover, they were able to estimate the actual size of these tiny
cracks and their velocities.
To find out why these cracks were growing, Ritchie and Muhlstein turned to Eric Stach at the National Center for Electron Microscopy. With Stach's collaboration, the researchers used the Center's state-of-the-art 1-million volt electron microscope to show that the cracks were not growing in the 2000 nanometer thick silicon film itself, but rather the 100 nanometer thick oxide layer that formed on top of the silicon as a result of exposure to air, similar to rust forming on unprotected steel. Once the oxide layer forms, moisture in the air can cause cracking. And therein lay the answer to the mysterious fatigue failure.
"When you have a very thin film of silicon, the oxide layer where
the moisture-induced cracking can occur is a very large portion
of the whole structure," Ritchie explains. "So the crack can grow
long enough in the oxide to break the whole structure. With large
sections of bulk silicon, this would not occur."
Currently, Ritchie and his colleagues are collaborating with Berkeley Chemical Engineering professor Roya Maboudian to utilize her coatings to bond directly to the surface of the silicon and prevent the oxide layer from forming. The coatings, Ritchie says, are effective surface barriers to keep out air and moisture, thereby markedly reducing the possibility of fatigue failure in the silicon films.
"Metals are expected to fatigue and therefore one knows that you have to take care of it, either by design or choice of material," Ritchie says. "Fatigue of micron-scale silicon was unexpected. But it's essential to know about and understand it from the perspective of being able to design safe silicon micro-machines and structures."
The Ritchie Group
The Maboudian Group
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© 2002 UC Regents.
Updated 11/26/02.
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