Page 379 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

13

IDT Microsensors

13.1 INTRODUCTION

Surface acoustic wave (SAW) devices possess several properties such as high reliability,
crystal stability, good reproducibility, and relatively small size that make them suitable
for many sensing applications. They can be used to sense many different properties, for
example, strain, stress, force, pressure, temperature, gas concentration, electric voltage,
and so forth. Readers are referred to a recent article by Hoummady et al. (1997) for a
review of their applications.
One attractive feature of some types of SAW sensor is that they can be read remotely.
The operating frequency of a SAW device typically ranges from 10 MHz to a few GHz,
which corresponds to the operating frequency range of radio and radar communication
systems, respectively. Thus, when an interdigital transducer (IDT) sensor is directly
connected to an antenna, the electromagnetic waves received by wireless transmission
can excite SAW in the piezoelectric material. The fundamentals of both SAW devices
and acoustic waves in solids were considered in Chapters 9 and 10, and it was evident
that passive, wireless (or remotely operable) SAW devices can be made. The latter is an
attractive proposition when low-power sensors are needed and are even more attractive
for use in remote, inaccessible locations, for example, when buried in concrete or in the
ground. Wireless SAW-based microsensors are described in detail in Section 13.3.
The sensing mechanism of SAW- IDT microsensors is based on a change in the prop-
erties of the SAW (e.g. amplitude, phase, frequency, or velocity) when the measurand
changes. Basic descriptions of the acoustic parameters that can be used in a generalised
measurement system have been given in Chapter 11.
In this chapter, we present a number of different applications of SAW microsensors
together with the equations that govern their behaviour. For example, in chemical sensors,
the SAW couple into a thin chemically sensitive coating and its properties perturb the
nature of the waves. Several different properties of the film coatings can affect the acoustic
waves, namely, mass, density, conductivity, electrical permittivity, strain, and viscoelas-
ticity. In general, the change in acoustic velocity u a can be related by the total differential
theorem to the change in any property or properties. The following equation applies for
changes in mass, electrical, mechanical, and environmental parameters (Hoummady et al.
1997).

(13.1)
c
"elec "mech