A Digital CMOS Architecture for a Micro-Hotplate Array

A monolithic gas sensor array fabricated in industrial CMOS technology combined with post-CMOS micromachining is presented. The device comprises an array of three metal-oxide-coated micro-hotplates with integrated MOS transistor heaters and the needed driving and signal-conditioning circuitry. Three digital PID controllers enable individual temperature regulation for each hotplate. The operating temperature of the SnO2metal-oxide sensors may amount up to 350degC. A serial interface and the temperature control units have been implemented digitally. Emphasis was put on designing a modular system with the required analog circuitry reduced to a minimum. With its small overall size of 5.5times4.5\ mm2, its digital interface and its good hotplate thermal efficiency of 6degC/mW, the system represents a significant development on the way to low-cost mobile gas sensor systems. The limit of detection at constant temperatures has been assessed to be below 1 ppm for CO and approximately 100 ppm for CH4. The mainly digital implementation with a maximum sampling rate of 9.3 kHz for all three sensors offers the advantage to apply a power-saving mode and temperature modulation techniques to enhance the analyte discrimination capability