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

We report on the design, implementation and characterisation of a thick-film gas sensor deposited for the first time by screen-printing technique onto a micromachined hotplate, the microheater maintains a film temperature as high as 400°C with <30 mW of input power. 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 optimise the radial temperature gradient up to 1200°C/mm. A newly developed scheme for temperature measurement was adopted for on-line adjustment of the film temperature through aconventional low-power proportional integral (PI) regulator. Deposition of sensing layers based on semiconductor oxides, such as SnO2 was achieved by computer-aided screen-printing. The films were then fired through the microheater itself to guarantee thermodynamic stability for long time exploitation. The response of the device to CO, CH4 and NO2 at concentrations typical for indoor and outdoor applications was recorded by measuring the film resistance through ultra high impedance CMOS circuit.