Page 323 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

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Introduction to SAW Devices

9.1 INTRODUCTION

Physical and chemical sensors have and will continue to play an ever-increasing role in the
measurement of both physical (pressure, temperature, acceleration, strain etc.) and chem-
ical (ion or gas concentrations, chemical potential, etc.) properties. Sensors are important
for many processes, and it is impossible to imagine a world without their impact on some
part of our daily lives.
Sensors based on surface acoustic waves (SAWs) form an important part of the sensor
family, and in recent years, these have seen diverse applications ranging from gas and
vapour detection to strain measurement (Campbell 1998). A new generation of SAW-based
actuators modeled on microactuators based on microelectromechanical systems (MEMS)
have also been recently announced (Campbell 1998). The advantages of using SAW-based
devices in microsensors are that SAW devices are amenable to wireless interrogation, and
by the application of a suitable modification in the form of an onboard antenna, they
can be converted into sensors for use in remote and inaccessible locations. As the sensor
frequency increases, its physical size scales down, resulting in smaller devices permitting
a greater variety of applications. Consequently, we have dedicated several chapters in this
book to the topic of SAW devices and their sensing applications.

9.2 SAW DEVICE DEVELOPMENT AND HISTORY

Pierre and Jacques Curie discovered the piezoelectric nature of quartz in 1880, when
they observed that some crystals 1 they were studying would electrically polarise when
deformed by an applied force (Smith 1976). For 30 years, this phenomenon remained a
scientific curiosity. In 1910, Voigt published a book on the properties and theory of piezo-
electricity. The first practical applications using piezoelectric devices began during the
First World War of 1914 to 1918 (Grate et al. 1993a). In 1917, motivated by submarine
problems, P.1 Langevin of France and A. M. Nicholson of United States worked indepen-
dently on using piezoelectric devices as echo-detectors for detecting compressional waves
in seawater. Although too late for the war effort, these devices proved useful in shipping
as detectors in depth-sounding equipment (Smith 1976).
Acoustic sensors offer a rugged and relatively inexpensive means for the develop-
ment of wide-ranging sensing applications. A valuable feature of acoustic sensors is their

1
Crystalline materials and lattice structures are described in Chapter 3.