Micromachined thin film solid state electrochemical CO2, NO2 and SO2 gas sensors

Among the many physical and chemical strategies used to make air pollution gas sensors for COx, NOx, SOx monitoring, those employing electrochemical detection means offer the highest selectivity, long lifetimes, low drift and low costs of manufacture. Targeting the three gases CO2, NO2 and SO2, new solid-electrolyte-based, selective potentiometric gas sensors were fabricated in both bulk and thin film versions. For all six bulk and thin film sensors, the EMF responses were related to the analyzed target gas pressure by the Nernst law. For each of the target gases CO2, NO2 and SO2, our thin film micro-sensor exhibits excellent gas-sensing sensitivity, response time, selectivity and recovery compared to the bulk sensor. The carbon dioxide micro-sensor characteristics obtained at 250 degrees C were 45 +/- 3 mV/decade for the sensitivity, 10 to 30 s for the response time and 60 s for recovery time. The nitrogen dioxide micro-sensor characteristics obtained at 200 degrees C were 48 +/- 3 mV/decade for the sensitivity, 2 to 10 s for the response time and 10 s for recovery time. The sulfur dioxide micro-sensor characteristics obtained at 250 degrees C were 50 +/- 3 mV/decade for the sensitivity, 1 to 2 s for the response time and 5 s for recovery time. In comparison, the bulk sensor CO2 characteristics obtained at 250 degrees C were 50 +/- 3 mV/decade for the sensitivity, 60 to 300 s for the response time and 900 s for recovery time. To verify selectivity in air samples, the sensitivity of each gas detector with respect to the gases CO2, NO2 and SO2, oxygen, CO, and humidity were evaluated. The CO2 sensor was insensitive to the later three gases, but there was a measurable EMF drift produced for NO2 at concentrations above 10 ppm and for SO2 at concentrations above 80 ppm. Similarly, the NO2 sensor operating at 200 degrees C was insensitive to all other gases except for SO2 at concentrations above 80 ppm. Finally, the SO2 sensor operating at 250 degrees C was insensitive to all other. We optimized our processing to obtain a dense strongly adhering deposited solid electrolyte thin film. XPS and XRD studies show that the chemical composition and structure of the deposited solid electrolyte thin-film and targets for RF magnetron sputtering were the same to within experimental error. For brevity, complete results are presented only for the CO2 micro-sensor. The results for the other two gas micro-sensors are essentially the same and will be published elsewhere. (C) 1999 Elsevier Science S.A. All rights reserved.