Micromachined resonant multiple gas sensor

This paper introduces a novel resonant MEMS sensor for carbon dioxide and humidity detection and discusses its performance. The sensor is composed of a resonant silicon nitride diaphragm and a nanostructured gas sensitive film with high adsorption capability and specificity to the analyte. The diaphragm is driven into transverse vibrational resonance by means of Lorentz electromagnetic forces. Upon exposure to the gas analyte, the functional material adsorbs gas molecules inducing a dimensional and mass change in the material and hence a measurable change in the resonant frequency of the sensor. We systematically study the performance of the sensor as a function of geometrical parameters such as membrane size and thickness. Two different adsorbents were used in conjunction with the resonant sensor to detect H2O vapor and CO2. A thin film of polystyrene sulfonic acid (PSSA) H+ was used to detect moisture while a thin layer of dispersed single wall carbon nanotubes (SWCNT) was used to sense CO2. Results indicate high performance for low dew point moisture detection and CO2 measurements within a broad range of concentrations (−60 to 15 °C dew points, 0-20 CO2 vol.%) with resolutions as low as 1 ppm water vapor and 0.7 vol.% CO2.