Page 250 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

230 MICROSENSORS

The second set of devices is named according to the nature of the electrical signal.
Therefore, a capacitive sensor could be called a potentiometric sensor when a change in
voltage is recorded or a coulombic sensor when a change in electric charge is recorded.
In practice, sensors tend to be classified according to both the primary measurand (or
actuand) and the basic principle involved, for example, a capacitive pressure sensor.
Using this nomenclature, it is possible to describe reasonably clearly the type of device
in question.
Many books that have been published on the topic of sensors 2 often focus on one prin-
ciple, such as thermal, pressure, chemical, and so on. Appendix K lists a number of general
books on sensors, but interested readers are referred to two books in particular. First, an
introductory text by Hauptmann (1991), which gives an excellent overview of sensors for
readers unfamiliar with the field, and second, a more advanced eight-volume book series
by Gopel published by Wiley-VCH, which provides the most comprehensive review of
3
sensors to date . There are relatively few books that have been published specifically
on the topic of actuators. More commonly, actuators are often described within books on
either transducers or, perhaps, instrumentation. Therefore, we recommend the introductory
texts on Transducers by Norton (1989) and the more advanced instrumentation reference
book edited by Noltingk (1995).
4
In this chapter, we are concerned with miniature sensors, so-called microsensors ,
which are fabricated using predominantly the bulk- and surface-micromachining technolo-
gies described in Chapters 5 and 6, respectively. Again, there are a number of textbooks
already published, which report on the topic of microsensors, but there are very few on
5
microactuators . For example, we recommend the book on Silicon Sensors by Middelhoek
and Audet (1989) and Microsensors by Gardner (1994). The subsequent sections provide
an overview of the field of microsensors, and as stated above, the emerging field of IDT
microsensors is covered separately in Chapter 13.
Some sensing devices have a part or all of the processing functions integrated onto the
same silicon substrate. We refer to these devices as smart sensors. We reserve the label
of 'intelligent' for devices that have in addition some biomimetic function such as self-
diagnostic, self-repair, self-growth, and fuzzy logic. The topic of smart (and intelligent)
sensors is dealt with in Chapter 15.
There have been rapid developments in the field of microsensors during the past 10
years, and a sharp increase has taken place in the size of the world market, which has
become some billions of euros today (see Chapter 1). Here, we focus upon the main types
of microsensors, which have powered this sensing revolution, together with some of the
emerging new designs.

8.2 THERMAL SENSORS

Thermal sensors are sensors that measure a primary thermal quantity, such as temper-
ature, heat flow, or thermal conductivity. Other sensors may be based on a thermal

2
This includes books on the topic of transducers (where a sensor is an input transducer).
3
Wiley-VCH regularly publish books called Sensors Update to supplement the original volume series.
4
Most microsensors are based on silicon technology; however, the term refers to devices with one dimension
in the micron range.
5
Published proceedings of meetings are not regarded here as textbooks.