Page 316 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

296 MICROSENSORS

disposable single-shot sensing element - the enzyme is coupled to a conducting polymer
in an electrochemical cell. The sensing strips are fabricated using screen printing, rather
than silicon microtechnology, and sold in their millions by companies such as Medisense
(UK). Other coatings can be used to detect lipids, peptides, and so on, but to date, their
commercial success has been somewhat limited by either the long-term stability of the
electrode or the selectivity of the biological coatings.
Finally, there is considerable need in the automotive industry for gas sensors that
can monitor the engine combustion process either in-line or at-line — this is extremely
demanding and rules out conventional CMOS devices that operate only up to a temperature
of about +125°C. However, there is a field-effect diode made from SiC that can be
operated at more than 700 °C and responds in milliseconds (Svenningstorp et al. 2000,
2001). Figure 8.62 shows the response of a MISiC Schottky diode to the exhaust gases
from a car engine (Savage et al. 2000). This device enables the real-time monitoring of
the combustion process in each cylinder as it fires in turn and could well be used as a
diagnostic sensor.

8.6.3 Others

There are a number of other principles of transduction that can be used to make chemical
microsensors. For example, the most obvious type of sensor to make is a capacitive one
because the device requires little power and fits in well with CMOS technology. Early
work by Gopel on polymeric capacitors had a limited success because of the relatively

Figure 8.63 (a) Array of gas-sensitive polymeric capacitors on a CMOS chip (from Baltes and
Brand 2000) and (b) two electrode geometries to discriminate between polymer dielectric constant
and swelling changes