Nanowires have superior electrical and
mechanical properties and can be put to good use in pressure sensors
Miniaturized
pressure sensors are widely used in mechanical and biomedical applications, for
example, in gauging fuel pressure in cars or in monitoring blood pressure in
patients. Woo-Tae Park and co-workers at the A*STAR Institute of
Microelectronics1 have now developed a nanowire-based sensor that is so
sensitive it can detect even very low pressure changes.
Most
miniaturized pressure sensors harness the intrinsic properties of
piezoresistive materials. A structural change in such a material, induced for
example by an external force, results in a complementary change in its
electrical resistance. However, piezoresistive materials have two major
limitations. Firstly, these materials are not particularly sensitive, which
means that low pressures produce weak electronic signals. Secondly, these materials
can generate a lot of electrical noise, which can mask the true measurement
signal. An ideal transducer should have a high signal-to-noise ratio (SNR).
Park and his co-workers have now used nanowires to create a pressure sensor
with enhanced SNR properties.
Previous
research has shown that nanowires can exhibit high piezoresistive effects
because of their small size. To take advantage of this, Park and his co-workers
used state-of-the-art material processing techniques to suspend two silicon
nanowires between two electrodes on a silicon-on-insulator substrate. Each wire
was a few hundred nanometers long and approximately 10 nanometers wide. They
were covered in amorphous silicon which both protected them and acted as an
electrical connection, referred to as the gate. The researchers attached to
this a circular diaphragm: a two-layer membrane of silicon nitride and silicon
dioxide. Any stress in the diaphragm was therefore transferred to the nanowire
structure.
The
team characterized their sensor by passing a controlled stream of air across
it. Ammeters measured the current flowing through the device as a known
electrical potential was applied across the two electrodes. An additional
voltage, the gate bias, was also applied between one of the electrodes and the
gate. Park and his co-workers demonstrated that they could achieve a four-fold
increase in pressure sensitivity by reversing the direction of this gate bias.
The researchers observe that this may be a result of the bias voltage
controlling the confinement of the electrons within the nanowire channels — a
concept commonly employed in so-called field-effect transistors. An assessment
of the device noise characteristics also showed significant improvements with
the right choice of operating parameters.
Park
and his co-workers envisage that the device will provide a promising route for
applications requiring miniaturized pressure sensors that use little power.
The
A*STAR-affiliated researchers contributing to this research are from the Institute of Microelectronics
References
1.
Singh,
P., Miao, J., Park, W.-T. & Kwong, D.-L. Gate-bias controlled sensitivity
and signal-to-noise ratio enhancement in nanowire FET pressure sensor. Journal
of Micromechanics and Microengineering 21, 105007 (2011). | article
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