Enhanced Sensing Characteristics in MEMS-based Formaldehyde Gas Sensor

In this study, the proposed sensor integrates a sensing layer, a heating device, and electrodes on the substrate. The micro heater is integrated in the sensor to provide instantaneous and precise temperature control capability. The electrodes are fabricated to connect resistance meter for measuring variation of electrical conductibility of the sensing layer. The grain size of the NiO thin film is almost to be nanometer level, and therefore both the sensitivity and the lowest sensing limit of the device are enhanced due to the enlarged area of the catalyst grains contacting with the surrounding gas. The experimental data show that decreasing thickness of sensing layer in the sputtering process significantly increases the sensitivities of the gas sensor and improves its lowest detection limit capability (0.7 ppm). Although we can further improve lowest detection limit by co-sputtering with NiO/Al2O3 (40 ppb), it needs to consider that selectivity will be reduced. The integrated micro heater simplifies the experimental set-up and can be realized using a simple fabrication process. The presented microfabricated formaldehyde gas sensor with a self-heating NiO/Al2O3 thin film is suitable not only for industrial process monitoring but for safeguarding the human health in buildings.