|
research:
introduction | integrated
packages | sensor
arrays | challenges
& limitations
MEMS & NEMS Sensors
Our work involves designing and developing micromechanical sensors
based on micro-machined cantilever beams. The unprecedented sensitivity
of microcantilever chemical, physical, and biological sensors,
as demonstrated by our group and many other laboratories around
the world, suggests that in the years to come, micro- and nano-
cantilever sensors will be an integral part of many sensor devices.
The microcantilever concept will serve as a general platform
for a myriad of extremely sensitive, highly selective, real-time
micro and nanosensors that can be mass-produced using conventional
techniques.
Cantilever detectors work in one of two modes, depending on what
is to be detected and under what environmental conditions:
- Resonant frequency shift due to mass adsorption
- Adsorption-induced bending of the microcantilever
We are most interested in the latter. In the adsorption-induced
bending mode, the deflection changes as a function of adsorbate
coverage due to free energy change. A relation between cantilever
bending and changes in surface stress is given by the Stoney formula.
In general the surface stress depends on both the surface free
energy and the surface strain, but for high aspect ratio cantilevers
one can easily neglect the contribution from surface strain effects
and equate the free energy change to surface stress variation.
Currently we are developing sensors for:
- Groundwater monitoring
- Explosive vapors
- Cancer and cardiac disease markers
| introduction
| integrated packages | sensor
arrays | challenges
& limitations
Integrated Packages
The trend in miniaturization of intelligent sensors couples very
well with the versatility of cantilever arrays. The cantilever
signal can be readout optically but designs using piezoresistive
and capacitive pickups are well suited to integration with on-chip
electronic circuitry. The electronic processing could further
incorporate analog intelligence to decouple non-selective responses
to environmental effects. Thus, the need to find coatings that
individually respond only to a particular chemical is obviated.
| introduction | integrated
packages | sensor arrays | challenges
& limitations
Sensor Arrays
Micromachining technologies currently available could be used
to make multitarget sensor arrays involving hundreds of cantilevers,
analog processing, and even local telemetry on a single chip.
The number of sensing elements in a sensor array can be used for
lower noise, much higher selectivity, and increased robustness.
Simplicity, low power consumption, potentially very low cost to
manufacture, inherent compatibility with array designs, and the
ability to operate in air or liquid make cantilever sensors very
attractive for a variety of applications.
| introduction
| integrated packages
| sensor arrays |
challenges & limitations
Challenges & Limitations
There are a number of challenges to overcome before MEMS and
NEMS sensors come into widespread use. The technology for designing
and simulating electronic chips is well advanced. Software to
integrate electronic, mechanical, and fluidic designs is still
in its infancy, yet huge investments presently being made will
soon accelerate the design of fully integrated devices. The effects
of environmental influences on coatings and cantilevers will need
to be fully characterized and incorporated into data libraries,
so that a prospective chip can be completely characterized before
hardware fabrication.
Additional coatings and attachment methods will need to be developed
and added to the libraries. The development of on-chip microfluidics
will also be needed to facilitate some applications. Material
stability is an issue that potentially limits long-term reliability
in harsh conditions and will need to be addressed. Just as similar
challenges have been met by the semiconductor industry, we believe
these will be overcome.
|
