Page 260 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

240 MICROSENSORS

400 T

50 75 100
7TQ

Figure 8.12 Effect of temperature on the phase shift from an integrated wireless SAW sensor.
After Bao et al. (1994)

This temperature microtransducer has great practical value in that the sensing part is
passive, that is, it requires no power supply; second, it is wireless and therefore can be
embedded in rotating or moving parts, such as car tyres, turbine blades, helicopter rotors,
and so on; and finally, it is very inexpensive to make. SAW-IDT microsensors have many
more important applications than simply acting as a temperature sensor. Consequently, the
microsensors are subsequently covered in further detail; Chapter 12 provides fabrication
details and Chapter 13 summarises the possible applications of IDT microsensors.

8.3 RADIATION SENSORS

Radiation sensors can be classified according to the type and energy of the measurand,
as illustrated in Figure 8.13, in which the energy E R in electron volts (eV) of the elec-
tromagnetic radiation is simply related to its frequency / in Hertz, and wavelength A. in
meters by

E R (in eV) = - = - (8.17)
e Xe
8
where h is Planck's constant, c is the velocity of light in a vacuum (i.e. 3.0 x 10 m/s),
-19
and e is the charge on an electron (i.e. 1.60 x 10 C).
Radiation can come in the form of particles, such as neutrons, protons, and alpha and
beta particles, emitted from the decay of nuclear material. However, the detection of
nuclear particles and high-energy electromagnetic radiation (i.e. gamma rays and X rays)
generally requires sophisticated instrumentation that cannot be readily integrated into a
miniature device. The exception to this general observation is perhaps the detection of
low-energy X rays or electrons through a solid-state photoelectric detector, the principle
for which is covered in Section 8.3.2.
The most common types of radiation microsensor detect electromagnetic radiation with
energies or wavelengths from the ultraviolet-to-near-infrared (UV-NIR) region, which
includes visible, through the NIR and thermal-infrared region and into the microwave
and radio regions. The most important regions are the visible light region and the NIR