Enhanced Sensitivity of SAW Gas Sensor Based on High Frequency Stability Oscillator

This paper presents a 158 MHz surface acoustic wave (SAW) gas sensor based on a novel SAW oscillator. As the oscillator element, a SAW delay line on ST-X quartz substrate with low insertion loss (less than 8 dB) and single mode selection capability was developed. Low insertion loss is implemented by electrode width control single phase unidirectional transducer (EWC/SPUDT) configuration. Single mode selection can be simply accomplished by comb transducer which is means of combination of frequency selectivity of two interdigital transducers (IDTs). Coupling of modes (COM) modeling is performed to predict device performance prior to fabrication. The measured frequency response S₁₂ matched well with simulated results. The experimental results show that the baseline noise of the fabricated oscillators was typically up to ~0.7 times 10^-7 in a laboratory environment with cooling step. The oscillator was successfully applied to gas sensor coated self-assembled composite monolayer as sensor material for dimethyl-methyl-phosphonate (DMMP), The sensitivity for low DMMP concentrations detection was evaluated as ~25 Hz/mg/m³, and the threshold detection limit was up to 0.5 mg/m³.