Page 319 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

BIO(CHEMICAL) SENSORS 299

Table 8.18 Relative characteristics of the gas microsensors
Sensing Transducer Sensitivity Power/operating Unit CMOS-
material temp. (°C) cost compatible
or not?
Metal oxide (best for Resistive High High/300 High SOI only
combustible gases) with
on-chip
anneal
Conducting polymers QCM High All very Medium No
(best for polar low/ambient
VOCs)
Resistive Medium Low Yes
MOSFET Medium Low Yes
Calorimeter Low High Yes, with
silicon
etch
Catalytic metals (best MOSFET Low Medium/200 Low Yes, with
for reactive gases) etch
Pellistor Low High/500 High No
Solvating polymer SAW High All very Medium Nonstandard
coatings low/ambient
(best for nonpolar QCM High Medium No
VOCs)
Capacitive Medium Low Yes
Resist Medium Low Yes
Calorimetric Low Medium Yes, with
etch
Cantilever Low High Yes, with
etch

The power consumption of the device, shown in Figure 8.64(d), is about 150 mW at
an operating temperature of 450 °C. The microheater has an active area of 570 u,m
square.
27
The catalyst-coated bead was formed electrochemically using a lyotropic self-
assembling material as a template (Attard et al. 1997). The resulting structure is made
up of a regular array of 5.5-nm pores in palladium and has a very high surface area,
2
approximately 1000 m /g (Figure 8.65 (a,b)). Preliminary results of the micropellistor
are shown in Figure 8.65(c). Its response to 2.5 percent methane in air (Lee et al.
2000) suggests that commercial silicon micropellistors may become available in the near
future.
In conclusion, there are a wide variety of different materials and different types of
microsensors reported, which respond to gases and vapours - including those based on
acoustic principles and described in other chapters. Table 8.18 summarises some of the key
characteristics of the different types of gas microsensors and their potential for integration
into a standard process.

27
Made from two phases, literally soaplike.