Biomedical Engineering Reference
In-Depth Information
therefore display no piezoelectric effect (unless strain is induced). Other
materials that lack a center of symmetry, such as quartz, lead zirconate
titanate (PZT), and zinc oxide (ZnO), are commonly used as piezoelectric
materials in MEMS sensors. The latter two can be deposited in thin-film
form on silicon or other material substrates.
Microactuator Technology
A variety of basic principals are used to implement MEMS actuators, includ-
ing electrostatic, piezoelectric, magnetic, magnetostrictive, bimetallic, and
shape memory alloy. Each of these has its respective advantages and dis-
advantages, and therefore careful consideration of the specific application
requirements must be part of the selection process. We shall review a few
of the more popular methods for MEMS microactuators here. Readers are
referred to [9-12, 37, 38] for a more comprehensive review.
Electrostatic actuation is based on the mutual attraction of two oppositely
charged plates. The force F generated between plates under the application
of a voltage potential V is given by
F = 1/2 (e
o
e
r
A ) ( V/d )
2
where e
o
is the permittivity of free space, e
r
is the relative permittivity of a
dielectric, A is the area of the plates, V is the applied voltage potential on the
plates, and d is the separation between the plates [41]. There are some inher-
ent advantages of electrostatic microactuators that make them attractive and
popular for MEMS devices, including being easy to fabricate and integrate
with electronics, having very low power consumption during operation, and
being able to exhibit very high mechanical bandwidths. Some of the disad-
vantages of electrostatic actuators are that the force is non-linear with dis-
placement and applied voltage, the resultant force is relatively small, and the
operating voltages can be relatively high.
Another popular method of implementing microactuators for MEMS is
based on the bimetallic effect. The bimetallic effect uses the differing ther-
mal expansion coefficients of two different materials to realize a thermal-
based microactuator. When these two materials are made into a composite
structure and heated, a thermally induced stress is generated in the struc-
ture if it is sufficiently compliant. The thermal strain is given by
α = (a
ilma
- a
ilmb
) (T
ele
- T
amb
)
where a
ilma
and a
ilmb
are the thermal expansion coefficients of the top and bot-
tom films, respectively, and T
ele
and T
amb
are the temperature of the bimetallic
element and ambient, respectively [9, 12]. Some of the attributes of a bimetal-
lic microactuator for MEMS are high power consumption for heating, low
mechanical bandwidth, relatively complex design and fabrication, relatively
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