Single Chip Gas Sensor Array for Air Quality Monitoring

This paper describes the fabrication of a four element gas sensor array for monitoring air pollutants, namely CO, CO₂, NO₂, and SO₂. The four micro-heaters share a single suspended SiO₂ diaphragm, utilizing thermal proximity to achieve low power consumption (~10 mW for 300 °C). Plasma enhanced chemical vapour deposition SiO₂ diaphragm is demonstrated to give higher yield compared with thermally grown SiO₂. Sensor array elements are fabricated by customizing each element to sense a specific gas, using different sensing materials. Optimized thin films of ZnO, BaTiO₃-CuO doped with 1% Ag, WO₃, and V₂O₅ are used for selective sensing of CO, CO₂, NO₂, and SO₂. Four sensors can be independently controlled to operate at different temperatures to get high selectivity for test gases. The sensor array is packaged on Kovar header, and then characterized for gas sensing. It is demonstrated that the sensors exhibit good sensitivity and selectivity. We report a maximum repeatable response to CO (~78.3% for 4.75 ppm), CO₂ (~65% for 900 ppm), NO₂ (~1948.8% for 0.9 ppm), and SO₂ (~77% for 3 ppm) at operating temperatures of 330 °C, 298 °C, 150 °C, and 405 °C, respectively.