Fabrication and Characterization of MEMS-Based Resonant Organic Gas Sensors

Polymer coated thermal-piezoresistive micromechanical resonant silicon nanobalances have been utilized for detection and concentration measurement of volatile organic compounds in gas phase. Polyglycolic acid, which is the main polymer ingredient of the shipley-1813 photoresist, was used as the absorbent coating layer. Experiments have shown that polymer thickness determines the achievable sensitivity towards the gas molecules. The main challenge is to coat the devices with a thick layer of polymer while still maintaining their high-Q resonance. Multiple polymer coating approaches have been demonstrated. Following polymer deposition, in order to find the sensitivity of the devices, they were tested by exposure to regular gasoline and toluene vapor. The best measurement results were obtained by utilizing the photoresist already present on the resonant structures from the fabrication process. A maximum frequency shift of 3600 ppm (55 kHz) was obtained from a 15.5 MHz resonator with a ~ 1.5 μm thick coating upon exposure to nitrogen saturated with toluene vapor at room temperature. Based on the measurement results, minimum detectable concentration of toluene in the gas phase for such devices is in the range of a few ppm.