Thermal sensors

Thermocouples

A working thermocouple is shown in Figure 2. It consists of a sensing junction, at temperature Ta, and a reference junction, at temperature Tb. The voltage developed by the thermocouple is measured with a high resistance voltmeter.

Figure 2.

The open circuit voltage (i.e. as measured by an ideal voltmeter with infinite input impedance) is related to the temperature difference (Ta - Tb ), and the difference in the Seebeck coefficients of the two materials (Pa - Pb ); equation 1:

V = ( Pa - Pb )( Ta - Tb )
Equation 1.

V will typically be of the order of millivolts, or tens of millivolts, for metal thermocouples with temperature differences in the order of 200degC.

Semiconductor materials often exhibit a better thermoelectric effect than metals. It is also possible to integrate many semiconductor thermocouples in series, to make a thermopile, which has a larger output voltage than a single thermocouple on its own. However the high thermal conductivity of silicon makes it difficult to maintain a large temperature gradient ( Ta - Tb ).

Therefore, one application of silicon micromachining is to thermally isolate the sensing element from the bulk of the silicon wafer. This may be done by fabricating the device on bridges or beams machined from silicon.

Thermoresistors

r=R(1+aT+bT2)
Equation 2.

Where R is the resistivity of the material at a reference temperature (0degC), and a and b are constants specific to the metal being used. Platinum is often used, as its resistance variation is particularly linear with temperature (i.e. b is particularly small).

As metal thermoresistors generally have relatively small resistances, and their rate of change of resistance with temperature (temperature coefficient of resistance, or TCR) is not particularly large, they require the use of a resistance bridge network to detect the signal.

Semiconducting thermoresistors (or thermistors) can be formed from metal oxides or silicon. These are generally not as accurate or stable as platinum thermoresistors, but are cheaper to manufacture and are potentially easier to integrate with microelectronic circuitry on the same substrate.

The temperature coefficient resistivity of a thermistor is highly non-linear and negative, and quite dependent on the power being dissipated by the device. The resistivity is typically expressed relative to the resistivity at 25degC with no power being dissipated by the device, and can be from 500 Ohm and 10MOhm.

Due to the negative TCR, it is possible for the resistor to go into a self-heating loop: current flowing through the resistor heats it up, the resistivity drops, more current flows, it gets hotter, etc. However, the large TCR does make it possible to couple thermistors directly to amplifier circuits without the requirement for a bridge configuration. The nonlinearity would typically be dealt with by calibration of the device.

Microengineering techniques can be used in a variety of ways to enhance thermal sensors. As mentioned above, they can be used to thermally isolate the sensing element from the rest of the device. Also, arrays of sensors can be produced to give signals that are larger than one sensor on its own would produce. If the device is small and thermally isolated, then its response time (the time the sensor takes to heat up / cool down in response to changes in the temperature of the environment) can be quite fast. With silicon based devices there is, of course, all the potential benefits that could come if electronics were integrated onto the chip (e.g. calibration done on- chip, self-testing).

Thermal flow-rate sensors