Metal oxide-based monolithic complementary metal oxide semiconductor gas sensor microsystem

A fully integrated gas sensor microsystem is presented, which comprises for the first time a micro hot plate as well as advanced analog and digital circuitry on a single chip. The micro hot plate is coated with a nanocrystalline SnO2 thick film. The sensor chip is produced in an industrial 0.8-mum CMOS process with subsequent micro-machining steps. A novel circular micro hot plate, which is 500 x 500 mum(2) in size, features an excellent temperature homogeneity of +/-2% over the heated area (300-mum diameter) and a high thermal efficiency of 6.0 degreesC/mW. A robust prototype package was developed, which relies on standard microelectronic packaging methods. Apart from a microcontroller board for managing chip communication and providing power supply and reference signals, no additional measurement equipment is needed. The on-chip digital temperature controller can accurately adjust the membrane temperature between 170 and 300 degreesC with an error of +/- 2 degreesC. The on-chip logarithmic converter covers a wide measurement range between 1 MOmega and 10 MOmega. CO concentrations in the sub-parts-per-million range are detectable, and a resolution of +/-0.1 ppm CO was achieved, which renders the sensor capable of measuring CO concentrations at threshold levels.