Development of SAW-based multi-gas sensor for simultaneous detection of CO2 and NO2

A 440 MHz wireless and passive surface acoustic wave (SAW)-based multi-gas sensor integrated with a temperature sensor was developed on a 41° YX LiNbO3 piezoelectric substrate for the simultaneous detection of CO2, NO2, and temperature. The developed sensor was composed of a SAW reflective delay lines structured by an interdigital transducer (IDT), ten reflectors, a CO2 sensitive film (Teflon AF 2400), and a NO2 sensitive film (indium tin oxide). Teflon AF 2400 was used for the CO2 sensitive film because it provides a high CO2 solubility, with good permeability and selectivity. For the NO2 sensitive film, indium tin oxide (ITO) was used. Coupling of mode (COM) modeling was conducted to determine the optimal device parameters prior to fabrication. Using the parameters determined by the simulation results, the device was fabricated and then wirelessly measured using a network analyzer. The measured reflective coefficient S11 in the time domain showed high signal/noise (S/N) ratio, small signal attenuation, and few spurious peaks. The time positions of the reflection peaks were well matched with the predicted values from the simulation. High sensitivity and selectivity were observed at each target gas testing. The obtained sensitivity was 2.12°/ppm for CO2 and 51.5°/ppm for NO2, respectively. With the integrated temperature sensor, temperature compensation was also performed during gas sensitivity evaluation process.