Page 324 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

304 INTRODUCTION TO SAW DEVICES

direct response to physical and chemical parameters, including surface mass, stress, strain,
liquid density, viscosity, permittivity, and conductivity (Grate et al. 1993a). Furthermore,
the anisotropic nature of piezoelectric crystals allows for various angles of cut, each cut
having different properties. Applications such as a SAW-based microaccelerometer, for
example, utilise a quartz crystal having a stable temperature (ST) cut because the reso-
nance frequency is almost independent of temperature (Bechmann et al. 1962). Again,
depending on the orientation of the crystal cut, various SAW sensors having different
acoustic modes may be constructed, which have a mode ideally suited toward a partic-
ular application. Other advantageous attributes include very low internal loss, uniform
material density, and elastic constants (Bechmann et al. 1962). Owing to these proper-
ties, many different sensors can be designed and optimised to meet the needs of specific
sensing applications, leading to their increasing role as chemical and physical sensors
(Grate et al. 1993a,b). Since the early 1960s, research and development in the acoustic
sensor field has increased significantly and has shown increased diversity.
The principal means of detection of a change of physical property follows from the
transduction mechanism of a SAW device, which involves the conversion of signals
from the physical (acoustic wave) domain to the electrical domain. Small perturbations
affecting the acoustic wave manifest themselves as large-scale changes when converted to
the electromagnetic (EM) domain because of the enormous difference in their velocities
(Varadan and Varadan 1997). This can be understood from the following calculations:
The SAW wavelength A is given by the ratio v/f 0 . The velocity v of a SAW wave on a
–1
piezoelectric substrate depends on the material and is typically 2 3490 ms , whereas the
synchronous frequency f 0 is set by the AC voltage applied to the interdigital transducer
9
(IDT) and is typically 1 GHz. Thus, the SAW wavelength A is 3490/10 or about 3.5 urn.
The EM wavelength X c is given by the ratio c/f o, where c is the velocity of light, that is,
3 x 10 8 m/s; in this case, X c is 0.3 m. The ratio of the two wavelengths is /A C , which takes
–5
a value of 1.1 x 10 here. The sensing action of such transducers involves any influences
that will alter the acoustic wave velocity v and, consequently, the associated properties
of the wave, such as frequency and time to travel between the sensor and the detector.
The slower acoustic velocity enables the use of simple, low-cost IC circuitry to transduce
the sensing signals that have a high level of precision as demonstrated in later chapters.
The attributes of an ideal sensor should include the following (d' Amico and Verona
1989):

1. High sensitivity
2. Fast and linear response
3. Fully reversible behavior
4. High reliability
5. Selectivity
6. Compact
7. High signal-to-noise ratio
8. Insensitive to surrounding environmental conditions

2
Value for lithium niobate.