Thin film micro carbon dioxide sensor using MEMS process

Pt/Na⁺ ion conductive ceramic thin film/Pt/carbonate (Na ₂CO₃:BaCO₃ = 1:1.7 mol) system CO₂ micro gas sensor was fabricated and the sensing properties were investigated. The Na⁺ ion conductive thin film was prepared by RF magnetron sputtering method. The thin film micro carbon dioxide sensor was prepared by using silicon process combined with MEMS technology. A NASICON thin film (2000-2500 Å) as main layer of the device was formed on N/O/N film using magnetron sputtering system. In order to simplify the device process, both the heater and lower electrode were formed on the same plane. And carbonate thin film as a sensing layer was formed on the upper electrode by sputtering sodium-barium carbonate target (Na₂CO₃:BaCO₃ = 1:1.7 molarratio). The area of the sensing layer was 0.55 mm × 0.4 mm and that of total device was 3.2 mm × 4.5 mm. The study of NASICON thin film conductivity was carried out using AC impedance spectroscopy. The properties of NASICON thin film for various temperatures were investigated. The NASICON thin film showed a higher ionic conductivity (0.43S/cm) compared to that of bulk NASICON material. The Nernst's slope of 57 mV per decade for CO₂ concentrations from 1000 to 10,000 ppm was obtained at operating temperature of 400°C. The power consumption of the fabricated sensor was about 52 mW at 400°C. As results, it is supposed that the fabricated sensor could be applied for monitoring CO₂ gas in environment.