MEMS-based formaldehyde gas sensor integrated with a micro-hotplate

This paper presents a novel micro-fabricated formaldehyde gas sensor with enhanced sensitivity and detection resolution capabilities. The device comprises a quartz substrate with Pt heaters as a micro-hotplate and deposited formaldehyde-sensing layer on it. A sputtered NiO thin film is used as the formaldehyde-sensing layer. A specific orientation of NiO becomes more apparent as the substrate temperature increases in the sputtering process, which helps the formation of NiO material with a correct stoichiometric ratio. The gas sensor incorporates Pt heating resistors integrated with a micro-hotplate to provide a heating function and utilizes Au inter-digitated electrodes. When formaldehyde is present in the atmosphere, oxydation happens near the sensing layer with a high temperature caused by the micro-hotplate and causes a change in the electrical conductivity of the NiO film. Therefore, the measured resistance between the inter-digitated electrodes changes correspondingly. The application of a voltage to the Pt heaters causes the temperature of the micro-hotplate to increase, which in turn enhances the sensitivity of the sensor. The nanometer scale grain size of the sputtered oxide thin film is conducive to improving the sensitivity of the gas sensor. The experimental results indicate that the developed device has a high stability (0.23%), a low hysteresis value (0.18%), a quick response time (13.0 s), a high degree of sensitivity (0.14 Ω ppm⁻¹), and a detection capability of less than 1.2 ppm.