Microhotplate with Pt Temperature Sensor

The conclusion from the results of this chapter is, that a sihcon island fabricated by ECE is not absolutely necessary, if a relative temperature difference of 5% within the active area between the electrodes is acceptable. A microhotplate with a dielectric membrane and a polysilicon heat spreader in the center features sufficient temperature homogeneity. Moreover, the tin-oxide droplet serves as additional heat spreader and smoothes out temperature gradients. 

In conclusion, a simple KOH-etching process without ECE is applicable for future microhotplate designs, although the best temperature homogeneity is achieved with the silicon island heat spreader. The island remains an important design feature, especially for the use of thin-film sensitive layers, where the additional heat spreading effect of the sensor materials is small. 

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Fig. 4.12. Micrograph of the microhotplate with Pt temperature sensor Table 4.4. Design parameters of the high-temperature microhotplate... 

Fig. 4.14. Schematic ofthe microfabrication process for the high-temperature microhotplate with Pt temperature sensor...

To overcome the temperature limits of CMOS integrated systems that are imposed by, e.g., the degradation of the CMOS metallization, a microhotplate with Pt-temperature sensor was also monolithically integrated with circuitry so that the hotplate operating temperature range could be extended to 500 °C (Sect. 5.2). The read-out of the comparatively low Pt temperature sensor resistance required the integration of a fully differential amplifier architecture. 

The third microhotplate introduced in Sect. 4.3 was designed to extend the operation temperature limit imposed by the CMOS-metallization contacts in the heated area. A new heater design was devised, and a microfabrication sequence that enables the realization of Pt temperature sensors and Pt-electrodes was developed. This microhotplate was also monolithically integrated with circuitry as presented in Sect. 5.2, and operating temperatures of up to 500 °C have been achieved. 

This chapter includes two different sensor system architectures for monolithic gas sensing systems. Section 5.1 describes a mixed-signal architecture. This is an improved version of the first analog implementation [81,91], which was used to develop a first sensor array (see Sect. 6.1). Based on the experience with these analog devices, a complete sensor system with advanced control, readout and interface circuit was devised. This system includes the circular microhotplate that has been described and characterized in Sect. 4.1. Additionally to the fabrication process, a prototype packaging concept was developed that will be presented in Sect. 5.1.6. A microhotplate with a Pt-temperature sensor requires a different system architecture as will be described in Sect. 5.2. A fully differential analog architecture will be presented, which enables operating temperatures up to 500 °C. 

The limit for the operating temperature of CMOS-microhotplates can be extended by using the microhotplate that was presented in Sect. 4.3. We now detail high-temperature microhotplates with Pt-resistors that have been realized as a single-chip device with integrated circuitry. While the aluminum-based devices presented in Sect. 4.1 were limited to 350 °C, these improved microhotplates can be heated to temperatures up to 500 °C. As the typical resistance value of the Pt-resistor is between 50 and 100 Q, a chip architecture adapted to the low temperature sensor resistance was developed. The system performance was assessed, and chemical measurements have been performed that demonstrate the full functionality of the chip. 

The main goal of another microhotplate design was the replacement of all CMOS-metal elements within the heated area by materials featuring a better temperature stability. This was accomplished by introducing a novel polysilicon heater layout and a Pt temperature sensor (Sect. 4.3). The Pt-elements had to be passivated for protection and electrical insulation, so that a local deposition of a silicon-nitride passivation through a mask was performed. This silicon-nitride layer also can be varied in its thickness and with regard to its stress characteristics (compressive or tensile). This hotplate allowed for reaching operation temperatures up to 500 °C and it showed a thermal resistance of 7.6 °C/mW. 

The discrete microhotplates were packaged and bonded in a DIL-28 package for temperature sensor cahbration. A Pt-lOO-temperature sensor was attached to the chip package in close vicinity to the sensors. The chips were then caHbrated in an oven at temperatures up to 325 °C with the help of the Pt-100 resistor. A second-order polynomial was extracted from the measurements for each temperature sensor providing the temperature coefficients i and a2. ... 

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