Hotplate-based monolithic CMOS microsystems for gas detection and material characterization for operating temperatures up to 500°C

Two monolithic CMOS microsystems for gas detection and material characterization fabricated in industrial 0.8-μm CMOS technology combined with post-CMOS micromachining are presented. The first microsystem comprises an array of three microhotplates, and three single-ended proportional temperature controllers (one controller per microhotplate), which regulate the microhotplate temperature up to 350°C using a polysilicon resistor as a temperature sensor on the microhotplate. The second microsystem comprises one microhotplate, and a fully differential proportional temperature controller, which regulates the microhotplate temperature up to 500°C using a platinum (Pt) resistor as a temperature sensor on the microhotplate. For gas sensing applications, the microhotplates were covered with tin dioxide (SnO₂), which changes its conductivity upon exposure to gaseous analytes. Gas tests evidenced a detection limit of 0.2 ppm for carbon monoxide (CO). For material characterization, the microhotplates were covered with ammonium nitrate (NH₄NO₃), which exhibits a melting point of 169.6°C according to the CRC handbook of chemistry and physics. The measured melting point was 168±2°C. Temperature sensors, on- and off-membrane (near the circuitry), showed excellent thermal isolation between the heated membrane area and the circuitry area on the bulk chip (overall chip temperature rose by max 3°C at 500°C microhotplate temperature).