Page 417 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

14

MEMS-IDT Microsensors

14.1 INTRODUCTION

The combination of a microelectromechanical system (MEMS) device with an interdigital
transducer (IDT) (surface acoustic wave (SAW)) microsensor is a relatively new concept.
MEMS-IDT-based microsensors can offer some significant advantages over other MEMS
devices, including the benefits of excellent sensitivity, surface conforaiability, greater
robustness, and durability. These new MEMS-IDT combinations have fewer moving
mechanical parts that ultimately gives rise to these valuable benefits. Indeed, the superior
performance removes not only the need for associated electronic circuitry to balance (or
measure) the movement of moving structures but also leads to even smaller devices.
In this chapter, we give a detailed description of the fabrication, characterisation, and
testing of MEMS-IDT microsensors for inertial navigation systems. Inertial accelerometers
generally consist of a suspended seismic mass that is displaced from its rest position under
the influence of an external load. Thus, the position of the seismic mass depends on the
momentary acceleration applied to the device. This displacement is usually transduced
into either a change in resistance (in piezoresistive accelerometers) or a change in capac-
itance (in capacitive accelerometers). Each of these two basic electrical parameters has
its own relative advantages and disadvantages: piezoresistive accelerometers tend to have
a wide frequency range, low cost, and low precision, whereas capacitive accelerome-
ters have high sensitivity but a small dynamic and frequency range. The topic of silicon
microaccelerometers has already been discussed in Chapter 8 and further details on iner-
tial devices may be found in the following references: Esashi (1994), Roylance and Angell
(1979), Rudolf etal (1987), Suzuki et al. (1990), Seidel et al. (1990), and Matsumoto
and Esashi (1992).
Conventional accelerometers incorporate a large amount of electronics within the sensor
package. They also require an internal battery to drive the associated electronics. These
requirements result in sensors that are relatively bulky and require regular maintenance.
The size of the sensors is a major drawback in applications in which the sensors need to
be conformal to the structure and should not add excessive amounts of weight.
We now provide a detailed description of two worked examples of MEMS-IDT micro-
sensors, namely, an accelerometer and a miniature inertial navigation system. The inertial
navigation system uses both gyroscopes and accelerometers to measure the state of
motion of an object (i.e. its kinematics) by sensing the changes to that state caused
by the accelerations. The characteristic features required for many applications are a
high level of precision and wide dynamic and frequency ranges. We also show that it