A monolithic fully-differential CMOS gas sensor microsystem for microhotplate temperatures up to 450°C

A fully-differential monolithic gas sensor microsystem fabricated in industrial CMOS-technology combined with post-CMOS micromachining is presented, which comprises a metal-oxide-covered (SnO₂) microhotplate for operating temperatures up to 450°C by using a platinum (Pt) resistor as the microhotplate temperature sensor (polysilicon resistor in previous designs), a rail-to-rail differential difference amplifier (DDA) for analog proportional temperature control (room temperature-450°C±2°C), a logarithmic converter for measuring the SnO₂-resistance change upon gas exposure over a range of four orders of magnitude, and a low-noise amplifier (LNA) for the readout of the platinum resistor. 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 maximum 3°C at 450°C microhotplate temperature). Gas tests evidenced a detection limit below 1 ppm for carbon monoxide.