Page 309 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

BIO(CHEMICAL) SENSORS 289

Figure 8.56 Photographs of two examples of silicon resistive gas sensors: (a) array of three micro-
diaphragms, each 1100 jim x 3500 um and about 0.6 um thick with two sets of sensing electrodes
per cell and (b) single microdiaphragm of 1500 um square with a drop of doped tin oxide located
on top of a single set of sensing electrodes and a single 750 um square microheater. Both devices
are mounted on a DIL header with 0.1" spacing

three mechanisms is given by

2
4
4
(T - To) + b conv (T - T 0) + c rad (T - T 0 ) (8.55)
with a, b and c being constants. The actual contributions from each of these three mecha-
nisms has been determined by running a device (SRL108) in a vacuum, and Figure 8.57(b)
shows that the results are a good fit to the terms in Equation (8.51) (Pike and Gardner
1997).
It can be seen that devices operated at about 350 °C lose most of their heat through
convection to air and a negligible amount in radiation. In this case, the DC power
consumption of the microhotplate is typically 120 mW at 300 °C or 60 mW per resis-
tive sensor. The thermal response time of the microhotplate was measured to be 2.8 ms
for a 300 °C change in operating temperature (Pike and Gardner 1997). Both the power
consumption of the device and its thermal time constant will scale down with the size of
the diaphragm; hence, power consumptions and time constants of less than 10 mW and
1 ms, respectively, are quite realizable.
Figure 8.58 shows the characteristic response of an undoped and a doped tin oxide
resistive gas microsensor operated at a constant temperature of 367 °C to ppm pulses of
NO2 in air at 38% relative humidity (RH). The doped devices clearly show a higher
response to NO 2 and it should be noted that the resistance here increases in the presence
of the oxidising gas. The resistance falls in the presence of reducing gases such as CO or
hydrogen. The rise time of a tin oxide sensor tends to be faster than its decay time; this
becomes more apparent when detecting larger molecules such as ethanol. The response is
also not well approximated by a first-order process; therefore, an accurate model of the
dynamic response requires a multiexponential model (Llobet 1998).
However, the fast thermal response time of the microhotplate permits the rapid modula-
tion of its operating temperature - this can be used to reduce the average power consump-
tion of the device by a factor of approximately 10 when powering up for only 100 ms