Integrating a
complex, single-crystal material with "giant" piezoelectric
properties onto silicon, University of Wisconsin-Madison engineers and
physicists can fabricate low-voltage, near-nanoscale electromechanical devices
that could lead to improvements in high-resolution 3D imaging, signal
processing, communications, energy harvesting, sensing, and actuators for
nanopositioning devices, among others.
Led by Chang-Beom
Eom, a UW-Madison professor of materials science and
engineering and physics, the multi-institutional team published its results in Science.
Piezoelectric
materials use mechanical motion to generate an electrical signal, such as the
light that flashes in some children's shoe heels when they stomp their feet.
Conversely, piezoelectrics also can use an electrical signal to generate
mechanical motion-for example, piezoelectric materials are used to generate
high-frequency acoustic waves for ultrasound imaging.
Eom studies the
advanced piezoelectric material lead magnesium niobate-lead titanate, or PMN-PT.
Such materials exhibit a "giant" piezoelectric response that can
deliver much greater mechanical displacement with the same amount of electric
field as traditional piezoelectric materials. They also can act as both
actuators and sensors. For example, they use electricity to deliver an
ultrasound wave that penetrates deeply into the body and returns data capable
of displaying a high-quality 3D image.
Currently, a
major limitation of these advanced materials is that to incorporate them into
very small-scale devices, researchers start with a bulk material and grind, cut
and polish it to the size they desire. It's an imprecise, error-prone process
that's intrinsically ill-suited for nanoelectromechanical systems (NEMS) or
microelectromechanical systems (MEMS).
Until now, the
complexity of PMN-PT has thwarted researchers' efforts to develop simple,
reproducable microscale fabrication techniques.
Applying
microscale fabrication techniques such as those used in computer electronics,
Eom's team has overcome that barrier. He and his colleagues worked from the
ground up to integrate PMN-PT seamlessly onto silicon. Because of potential
chemical reactions among the components, they layered materials and carefully
planned the locations of individual atoms.
"You have
to lay down the right element first," says Eom.
Onto a silicon
"platform," his team adds a very thin layer of strontium titanate,
which acts as a template and mimics the structure of silicon. Next comes a
layer of strontium ruthenate, an electrode Eom developed some years ago, and
finally, the single-crystal piezoelectric material PMN-PT.
The researchers
have characterized the material's piezoelectric response, which correlates with
theoretical predictions.
"The
properties of the single crystal we integrated on silicon are as good as the
bulk single crystal," says Eom.
His team calls
devices fabricated from this giant piezoelectric material "hyper-active
MEMS" for their potential to offer researchers a high level of active
control. Using the material, his team also developed a process for fabricating
piezoelectric MEMS.
Applied in
signal processing, communications, medical imaging, and nanopositioning
actuators, hyperactive MEMS devices could reduce power consumption and increase
actuator speed and sensor sensitivity. Additionally, through a process called
energy harvesting, hyperactive MEMS devices could convert energy from sources
such as mechanical vibrations into electricity that powers other small
devices-for example, for wireless communication.
SOURCE
