CMOS microhotplate sensor system for operating temperatures up to 500 °C

A monolithic gas sensor microsystem fabricated in industrial CMOS-technology combined with post-CMOS micromachining is presented, which comprises a new type of microhotplate for operating temperatures up to 500 °C and specifically designed on-chip circuitry. These high temperatures can be achieved by using carefully optimized heater geometries and layouts, micropatterned Pt-temperature sensors and Pt-electrodes as well as a post-CMOS local passivation. Moreover, the on-chip Pt-temperature sensor requires a fully differential low-noise amplifier as a consequence of its low overall resistance. A logarithmic converter for measuring the metal-oxide resistance changes upon gas exposure and a bulk chip temperature sensor were co-integrated. The monolithic system was characterized, and operating temperatures up to 500 °C were achieved and controlled at ±2 °C precision. The correlation between input reference voltage and microhotplate temperature was found to be linear, which is advantageous for temperature modulation. A detection limit of less than 1 ppm CO (40% r.h.) was established using a nanocrystalline Pd-doped (0.2 wt.%) tin-oxide thick film as sensitive material.