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

This paper presents a monolithic chemical gas sensor system fabricated in industrial CMOS-technology combined with post-CMOS micromachining. The system comprises metal-oxide-covered (SnO2) micro-hotplates and the necessary driving and signal-conditioning circuitry. The SnO2 sensitive layer is operated at temperatures between 200 and 350degreesC. The on-chip temperature controller regulates the temperature of the membrane up to 350degreesC with a resolution of 0.5degreesC. A special heater-design was developed in order to achieve membrane temperatures up to 350degreesC with 5 V supply voltage. The heater design also ensures a homogeneous temperature distribution over the heated area of the hotplate (1-2% maximum temperature fluctuation). Temperature sensors, on- and off-membrane ( near the circuitry), show an excellent thermal isolation between the heated membrane area and the circuitry-area on the bulk chip (chip temperature rises by max 6degreesC at 350degreesC membrane temperature). A logarithmic converter was included to measuring the SnO2 resistance variation upon gas exposure over a range of four orders of magnitude. An Analog Hardware Description Language (AHDL) model of the membrane was developed to enable the simulations of the complete microsystem. Gas tests evidenced a detection limit below 1 ppm for carbon monoxide and below 100 ppm for methane.