A smart single-chip micro-hotplate-based chemical sensor system in CMOS-technology

A novel monolithic chemical (gas) sensor fabricated in industrial CMOS-technology combined with post-CMOS micromachining is presented, which comprises metal-oxide (SnO₂)-covered micro-hotplates and the necessary driving and signal-conditioning circuitry. The operating temperature of the SnO₂ resistor is between 200 and 350°C. The on-chip temperature controller regulates the temperature of the membrane up to 350°C with a resolution of 0.5°C. A special heater design to achieve temperatures up to 350°C with 5 V supply voltage was developed, which ensures homogeneous temperature distribution (1-2% maximum temperature fluctuation). Temperature sensors, on and off-membrane (near the circuitry), show excellent thermal isolation between the heated membrane area and the circuitry-area on the bulk chip (chip temperature rises by max 6°C at 350°C membrane temperature). A logarithmic converter was included to measuring the SnO₂ resistance change upon gas exposure over a range of four orders of magnitude.