Page 317 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

BIO(CHEMICAL) SENSORS 297

poor sensitivity and high noise levels. However, more recent research by Baltes (Baltes
and Brand 2000) at ETH (Zurich) has been reported in which polymeric capacitors are
integrated on to a CMOS chip to enhance the signal gain, and, with different electrode
geometries (Figure 8.63(a)), improve the selectivity.
Interestingly, they report on the use of a pair of electrodes, as shown in Figure 8.63(b),
to enhance the selectivity - an approach similar to that proposed by Gardner (1995) with
resistive microsensors, and which is based on the fact that there are two mechanisms
by which the polymers can work. When the analyte dissolves into the nonconducting
polymer, it changes the dielectric constant of the material and hence the capacitance
changes. However, the polymer also swells as it absorbs the analyte, which provides a
second competing mechanism. Now, the capacitor with the narrow electrodes will not
measure the swelling effect as the electric field is contained within the film, whereas the

Figure 8.64 (a) A conventional commercial pellistor (City Technology Ltd, UK), (b) cross section
of a silicon planar pellistor (SRL162g), (c) photograph of a platinum microheater and gold electrode
area, and (d) measured power consumption up to 700 °C. From Lee et al. (2000)