Biomedical Engineering Reference
In-Depth Information
mode, the dependence of resonant frequency of the microcantilever on the
mass of the microcantilever is exploited. h e heat mode, takes advantage
of the bimetallic or bimorph ef ect that leads to a bending of a biomaterial
microcantilever with change in temperature. h e performance of a micro-
cantilever sensor relies on real-time measurements and the resolution of
measurements of cantilever mechanical parameters during sensing opera-
tion. Microcantilever transduction schemes are broadly classii ed as opti-
cal and electrical [8]. Microcantilever sensors with an optical transduction
scheme are expected to of er the highest sensitivity. However, owing to
the practical limitations pertaining to the i eld deployment of such opti-
cal transduction based sensors, the integrated electrical transduction
mechanism inside the mechanical element are usually preferred. h ere
are dif erent varieties of electrical transduction schemes reported such as
piezoresistive, piezoelectric, capacitive, electron tunnelling technique and
embedded MOSFET technique [15].
Most commonly used microcantilever structural materials are single
crystalline silicon, polycrystalline silicon, silicon nitride, silicon dioxide
and mechanically stable polymers like SU-8, TOPAS and Parylene [13-15].
As per the working principle of microcantilever based surface stress sen-
sor, the sensitivity is determined by the stif ness of the cantilever structure
and hence by the Young's modulus of the material. h e need for highly sen-
sitive and inexpensively fabricated microcantilever sensors motivated the
researchers to explore polymer based microcantilever technologies. Among
the polymers reported for microfabrication, SU-8 which is an epoxy based
polymer developed by IBM, is the most commonly used polymer struc-
tural material in MEMS [16-18]. Since 1999 [19], the use of SU-8 poly-
mer which is also considered as a high aspect ratio negative photoresist for
MEMS applications has been exponentially growing during the last couple
of years [20-22]. SU-8 has the ability in forming patterns with wide range
of thickness varying from few hundred of nanometres to a few millimetres
with high aspect ratios. SU-8 seemed to be a good candidate for structural
material for microcantilever sensors with its inherent advantages such as
low Young's modulus, inexpensive and less complex fabrication process, a
well understood UV and e-beam resist with low consumption of chemicals
and gases and low temperature for fabrication processes making it cost
ef ective and compatibility for integrating sensor with microl uidics.
h e performance of SU-8 based polymer nanomechanical sensors
could be enhanced by incorporating ultra-sensitive electrical transduction
schemes that are compatible with SU-8 processing [23-32]. Design and
development of SU-8 microcantilever sensors with four novel electrical
transduction schemes namely (1) polymer nanocomposite piezoresistive
