Development of a low-power thick-film gas sensor deposited by screen-printing technique onto a micromachined hotplate

Increasing request for extensive environmental monitoring is driving both research and market to the achievement of miniaturized low-power gas detectors to be used in portable measuring systems. The main limitation to the effective implementation of battery operated microsystems based on Taguchi gas sensors (TGS) is the high power consumption of the heater designed to keep the chemoresistive film at temperatures usually greater than 300 °C. We report on the design, implementation and characterization of a thick-film gas sensor deposited for the first time by screen printing technique onto a micromachined hotplate: the microheater maintain as high a film temperature as 450oC with less than 50 mW of input power (Fig. 1). The microheater consists of a dielectric stacked membrane equipped with embedded polysilicon resistors acting as heating element as well as temperature sensing elements. Extensive finite-element computer simulations were carried out during the design step to optimize the radial temperature gradient up to 1200oC/mm. A newly developed scheme for temperature measurement was adopted for on-line adjustment of the film temperature through a conventional low-power proportional integral (PI) regulator. Deposition of different sensing layers based on semiconductor oxides such as SnO2 and TiO2 and was achieved by computer-aided screen-printing. The films were then fired either in a conventional oven or through the microheater itself to guarantee thermodynamic stability for long time exploitation. The response of the device to CO, CH4 and NOx at concentrations typical for indoor and outdoor applications was recorded by measuring the film resistance through ultra high impedance CMOS circuit and the performance is compared to traditional devices fabricated via thick-film methodology. The feasibility of an application specific integrated circuit (ASIC) matching temperature regulation and front-end electronics is finally discussed