Page 251 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

THERMAL SENSORS 231

measurement; for example, a thermal anemometer measures air flow. However, according
to our classification of measurand energy domain, this would be regarded as a mechanical
sensor and appear under Section 8.2.3. Consequently, the most important thermal sensor
is the temperature sensor.
Temperature is probably the single most important device parameter of all. Almost
every property of a material has significant temperature dependence. For example, in the
case of a mechanical microstructure, its physical dimensions - Young's modulus, shear
modulus, heat capacity, thermal conductivity, and so on - vary with operating tempera-
ture. The effect of temperature can sometimes be minimised by choosing materials with
a low temperature coefficient of operation (TCO). However, when forced to use standard
materials (e.g. silicon and silica), the structural design can often be modified (e.g. adding
a reference device) to compensate for these undesirable effects.
It is often necessary to use materials that are not based on complementary metal
oxide semiconductor (CMOS), such as magnetoresistive, chemoresistive, ferroelectric,
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pyroelectric; these compounds tend to possess strong temperature-dependencies . In fact,
the problem is particularly acute for chemical microsensors, as most chemical reactions
are strongly temperature-dependent.
Many nonthermal microsensors (and MEMS devices) have to operate either at a
constant temperature - an expensive and power-intensive option when requiring heaters
or coolers, - or in a mode in which the temperature is monitored and real-time signal
compensation is provided. Clearly, microdevices that possess an integrated tempera-
ture microsensor and microcontroller can automatically compensate for temperature and
thus offer a superior performance to those without. This is why temperature sensors
are a very important kind of sensors and are commonly found embedded in microsen-
sors, microactuators, MEMS, and even in precision microelectronic components, such as
analogue-to-digital converters.

8.2.1 Resistive Temperature Microsensors

Conventionally, the temperature of an object can be measured using a platinum resistor,
a thermistor, or a thermocouple. Resistive thermal sensors exploit the basic material
property that their bulk electrical resistivity p, and hence resistance R, varies with absolute
temperature T. In the case of metal chemoresistors, the behaviour is usually well described
by a second-order polynomial series, that is,

2
2
P(T} ^ p 0(l + a TT + ftrT ) and R(T) « R 0(l + a TT + frT ) (8.1)
where po/Ro are the resistivity or resistance at a standard temperature (e.g. 0 °C) and otj
and ß T are temperature coefficients. C*T is a sensitivity parameter and is commonly known
as the linear temperature coefficient of resistivity or resistance (TCR) and is defined by

1 dp
<*T=--~ (8.2)
podT

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The properties of common metals, semiconductors, and other materials are tabulated in Appendices F, G,
and H.