In₂O₃-based micro gas sensor for detecting NO_x gases

In this study, NO_x micro gas sensors for monitoring the indoor atmosphere of automobile were fabricated using MEMS (microelectromechanical system) technology and a sol-gel process. The sensing electrode and micro heater were designed to have a co-planar typed structure in a Pt thin film layer. The thermal characteristics of a micro heater array were analyzed using a finite element method (FEM). The chip size of the gas sensor was approximately 2 mm × 2 mm. Indium oxide as a sensing material for NO_x gas was synthesized by a sol-gel process with indium isopropoxide as a precursor. Field emission Scanning electron microscopy and x-ray diffraction showed that particle size of the synthesized In₂O₃ was approximately 17-45 nm. The maximum gas sensitivity as the relative resistance (R _s = R _gas/R _air) was observed at 275°C with a value of 8.0 at 1 ppm NO₂ gas. The response (80% saturation) and recovery times were within 1 min. The sensing properties of NO₂ gas exhibited linear behavior with increasing gas concentration. The sensing mechanism of the gas sensor was explained by the variations in the electron depletion layers and the adsorption of gas molecules on the In₂O₃ particle surface. These results suggest that in the future, MEMS-based gas sensors can be used as automotive-exhaust-gas sensors.