Bulk Chip Temperature Sensor

The fabrication of the sensor system was described in Sect. 4.1.2, since this microsystem also features a circular microhotplate. A micrograph of the complete microsystem (die size 6.8 x 4.7 mm ) is shown in Fig. 5.2. The microhotplate is located in the upper section of the chip. The analog circuitry and the A/D and D/A converters are clearly separated and shielded from the digital circuitry. The bulk-chip temperature sensor is located close to the analog circuitry in the center of the chip. The distance between microhotplate and circuitry is comparatively large owing to packaging requirements, as will be explained in Sect. 5.1.6. 

The schematic of the temperature sensor on the bulk chip is shown in Fig. 5.3. The bulk chip temperature is measured via the voltage difference between a pair of diode-connected pnp-transistors (parasitic transistors as available in the CMOS process, collectors tied to substrate) working at different current densities. Transistor Qi is biased with a current of 40 pA, and transistor Q2 is biased with a current of 10 pA. 

The voltage difference between the emitter-base voltages (AVeb) is proportional to the absolute temperature (T), and is given by Eq. (5.1)  

The performance of the temperature sensor on the bulk chip is shown in Fig. 5.4. The ambient temperature was swept from -40 to 120 °C in steps of 5 °C, and the temperature controllers were switched off. A two-point calibration at 0 °C and 80 °C was performed. The measured sensitivity is about 128 pV/°C, and the accuracy is better than 1.5 °C. 

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Fig. 5.4. Output of the bulk-chip temperature sensor as a function of the ambient temperature...

The dc level of the logarithmic converter can be changed with the reference current (/ref) or with the common-mode voltage (Fcm). The bulk-chip temperature sensor can be used to compensate for the temperature dependence of the logarithmic converter. The performance of the logarithmic converter is shown in Fig. 5.6. 

This microsystem also comprises three 10-bit successive approximation analog-to-digital converters (ADCs) that are used for reading out the microhotplate temperature, the bulk chip temperature, and the sensor resistance, three programmable offset... 

A cross-sectional schematic of a monolithic gas sensor system featuring a microhotplate is shown in Fig. 2.2. Its fabrication relies on an industrial CMOS-process with subsequent micromachining steps. Diverse thin-film layers, which can be used for electrical insulation and passivation, are available in the CMOS-process. They are denoted dielectric layers and include several silicon-oxide layers such as the thermal field oxide, the contact oxide and the intermetal oxide as well as a silicon-nitride layer that serves as passivation. All these materials exhibit a characteristically low thermal conductivity, so that a membrane, which consists of only the dielectric layers, provides excellent thermal insulation between the bulk-silicon chip and a heated area. The heated area features a resistive heater, a temperature sensor, and the electrodes that contact the deposited sensitive metal oxide. An additional temperature sensor is integrated close to the circuitry on the bulk chip to monitor the overall chip temperature. The membrane is released by etching away the silicon underneath the dielectric layers. Depending on the micromachining procedure, it is possible to leave a silicon island underneath the heated area. Such an island can serve as a heat spreader and also mechanically stabihzes the membrane. The fabrication process will be explained in more detail in Chap 4. 

A problem of the calorimetric sensing mode is its cross-sensitivity to changes in ambient temperature. The realization of an additional temperature sensor on the bulk chip solves this problem. The signal-to-noise ratio of the calorimetric mode... 

The singled-ended analog architecture comprises a temperature sensor on the bulk chip, three single-ended analog proportional microhotplate temperature controllers... 

The nanogram-sized PBT sample on the chip sensor crystallizes at high temperatures about 2 orders of magnitude faster than does the milligram-sized DSC sample. The larger surface area-to-volume ratio for the smaller chip-based calorimeter sample results in significant deviations from bulk behavior. 

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