Page 311 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

BIO(CHEMICAL) SENSORS 291

1 Air 0.625 Air 1.25 Air 2.5 Air 5.0 Air 6.25
ppm ppm ppm ppm ppm
N0 2 N0 2 NO 2 NO 2 NO,
1.2 j
1.0 H
L-" X. Pt doped
/
0.8 - r~

fl fi -
V Pd doped
^-
0.4-
^«.
0.2 - | *• «, Undoped
V
— i i
0 50 100 150 200 250
Time (minutes)
Figure 8.58 Typical response of doped and undoped resistive tin oxide gas microsensors to pulses
of NO 2 in air. From Pike (1996)

a simple back-propagation neural network. It is particularly exciting to note that, using
this dynamical approach, a single microsensor can predict the concentration of a binary
mixture of gases from the different rate kinetics.
In the past few years, there has been an enormous increase in the number of research
groups from Germany, Korea, and China reporting on the fabrication of microhotplate-
based resistive gas sensors. These show some general improvements in the device perfor-
mance, such as a lower power consumption, greater robustness, and so on. Much of this
recent interest has stemmed from the fact that Motorola (USA) set up a fabrication facility
to make a low-cost CO gas sensor with Microsens (Switzerland) in the mid-1990s that
was based on a suspended poly silicon microhotplate design (Figure 8.54(b)). The device
was aimed at the automotive market with a nominal price of €1. Since then, the company
has been relocated to Switzerland and become independent. The main competition to
such silicon gas sensors is from the commercial screen-printed thick-film-based planar
devices, such as those sold in medium volume by Capteur Ltd (UK).
A variety of different materials have been studied for use in solid-state resistive gas
sensors. These materials are not only semiconducting oxides (e.g. SnO 2, ZnO, GaO, and
TiO 2) that tend to operate at high temperatures but also organometallic materials such
as phthalocyanines that operate around 200 °C and organic polymers that operate near
room temperature (Moseley and Tofield 1987; Gardner 1994). However, the successful
application of these other materials in gas sensors has not yet been realised. Instead, some
of these materials - conducting polymers, in particular - are being used as nonspecific
elements within an array to detect vapours and even smells (Gardner and Bartlett 1999).
Details of these devices, or so-called electronic noses, are given in Chapter 15 on Smart
Sensors.