Physical Microhotplate Characterization

For thermal characterization and temperature sensor calibration a microhotplate was fabricated, which is identical to that on the monoHthic sensor chips, but does not include any electronics. The functional elements of this microhotplate are connected to bonding pads and not wired up to any circuitry, so that the direct access to the hotplate components without electronics interference is ensured. The assessment of characteristic microhotplate properties, such as the thermal resistance of the microhotplate and its thermal time constant, were carried out with these discrete microhotplates. 

A schematic view of the microhotplate functional elements is presented in Fig. 4.4. A resistive temperature sensor is embedded in the heated area of the microhotplate. The resistance is measured in a four-point measurement The calibration procedure of the temperature sensor will be explained in the next section (Sect. 4.1.4). The heating power dissipation is determined using also a four-point configuration. The external wiring of the heater typically adds another 5% to the heater resistance, which has to be eliminated for an accurate measurement of the dissipated power. A heating current, /heat, is applied, and the voltage drop, Vheat. across the heater is measured on chip. 

In order to determine the thermal time constant of the microhotplate in dynamic measurements, a square-shape voltage pulse was applied to the heater. The pulse frequency was 5 Hz for uncoated and 2.5 Hz for coated membranes. The amplitude of the pulse was adjusted to produce a temperature rise of 50 °C. The temperature sensor was fed from a constant-current source, and the voltage drop across the temperature sensor was amplified with an operational amplifier. The dynamic response of the temperature sensor was recorded by an oscilloscope. The thermal time constant was calculated from these data with a curve fit using Eq. (3.29). As already mentioned in the context of Eq. (3.37), self-heating occurs with a resistive heater, so that the thermal time constant has to be determined during the cooHng cycle. 

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