High-temperature microhotplate

Fig. 4.11. Cross-sectional schematic of the high-temperature microhotplate... 

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

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. 

Another issue is the membrane buckling due to internal and thermal stress of the CMOS layers, which are not optimized for high-temperature operation. The buckling of the microhotplate can generate severe problems in the adhesion of the sensitive layer. In a temperature-pulsed operation mode, the repeated bending of the membrane could cause a rehabihty problem [120]. 

During the last years, so-called microhotplates (pHP) have been developed in order to shrink the overall dimensions and to reduce the thermal mass of metal-oxide gas sensors [7,9,15]. Microhotplates consist of a thermally isolated stage with a heater structure, a temperature sensor and a set of contact electrodes for the sensitive layer. By using such microstructures, high operation temperatures can be reached at comparably low power consumption (< 100 mW). Moreover, small time constants on the order of 10 ms enable applying temperature modulation techniques with the aim to improve sensor selectivity and sensitivity. 

---