Page 253 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

THERMAL SENSORS 233

example, types B, E, J, K, N, R, S, and T. Typically, they can operate from —100 to
7
+2000 °C with an accuracy of between 1 and 3 percent for a full-scale operation (FSO).
Here, we are mainly interested in whether a temperature sensor can be integrated in
a silicon process to become either a temperature microsensor or part of a silicon-based
MEMS device. Table 8.2 summarises the typical properties of conventional temperature
sensors and, more importantly, whether they can be integrated into a standard integrated
circuit (1C) process.
As is apparent from Table 8.2, it is possible to integrate resistive temperature sensors
such as the platinum Pt-100. However, the deposition of platinum or the thermistor oxide
is a nonstandard IC process and therefore requires additional pre- or post-IC processing
steps. The inclusion of nonstandard materials during, for example, a CMOS process,
which is 'intermediate' CMOS, is generally regarded as highly undesirable and should be
avoided if possible.
It is possible to fabricate silicon resistors in standard silicon IC process, as described
in Chapter 4. For example, five or more resistors can be made of doped silicon in a
standard bipolar process, such as a base resistor, emitter resistor, or an epi-resistor, and
two or three resistors can be made in a CMOS process (see Figure 4.15). The resistivity
of a single crystal of silicon varies with temperature and doping level, as illustrated in
Figure 8.6, and the lightly doped silicon provided the highest TCR. In practice, it is diffi-
12 -3
cult to make single-crystal silicon with an impurity level below ~10 cm ; therefore, it
will not behave as an intrinsic semiconductor with a well-defined Arrhenius temperature-
dependence because the intrinsic carrier concentration is about 10 10 cm -3 at room temper-
-3
18
ature. In highly doped silicon resistors (~10 cm ), the temperature-dependence approx-
imates reasonably well to the second-order polynomial given in Equation (8.1). Never-
theless, the temperature-dependence of a silicon resistor is nonlinear and depends upon
the exact doping level, making it less suitable for use as a temperature sensor than other

Table 8.2 Properties of common temperature sensors and their suitability for integration. Modified
from Meijer and van Herwaarden (1994)

Property Pt resistor Thermistor Thermocouple Transistor
Form of output Resistance Resistance Voltage Voltage
Operating range Large -260 to Medium —80 to Very large —270 Medium —50
(°Q + 1000 + 180 to +3500 to +180
Sensitivity Medium 0.4%/K High 5%/K Low 0.05 to High ~2 mV/K
1 mV/K
Linearity Very good Very nonlinear Good ±1 K Good ±0.5 K
<±0.1 K
Accuracy:
-absolute High over wide High over small Not possible Medium
range range
-differential Medium Medium High Medium
Cost to make Medium Low Medium Very low
Suitability for 1C Not a standard Not a standard Yes Yes-very easily
integration process process

'The sensitivity diminishes significantly below — 100°C.