Design and Fabrication of Humidity Sensors Based on Ceramic Thin Films

This research utilizes Micro-Electro-Mechanical Systems (MEMS) technology to develop a resistance-based humidity sensor fabricated on an aluminum oxide (Al₂O₃) substrate. Microfabrication techniques are first employed to pattern the temperature and humidity sensing electrodes on the Al₂O₃ substrate. Electron Beam Evaporation (EBE), followed by a lift-off procedure, is then used to pattern platinum (Pt) and chromium (Cr) electrodes for temperature and humidity sensing, respectively. Finally, radio frequency (RF) sputtering is performed to deposit thin layers of titanium dioxide (TiO₂) and tungsten trioxide (WO₃) on the electrodes as sensing layers. The working temperature and humidity are measured concurrently by monitoring the resistance signals of the device using LabVIEW software. The effects of the sputtering and annealing times on the structures of the two ceramic films are explored. In addition, the corresponding changes in the temperature and humidity sensing performance are evaluated and compared. The results show that the TiO₂ film has a better moisture expulsion capability than the WO₃ film. Consequently, the sensor incorporating the TiO₂ film shows a superior sensitivity and faster response time. The maximum sensitivity is observed in the sample annealed at 450°C for 4 h owing to the formation of optimally-sized pores that enhance moisture absorption and removal.