Mass Loading Characteristics of One-Port SAW Resonator With Sensing Film Attached Reflector Electrodes

We investigated the mass loading effect caused by a thin polymer film attached only on the reflector electrodes of a one-port surface acoustic wave (SAW) resonator and explored the possibility of devising low-loss SAW sensors. One-port SAW resonators have a miniaturized device footprint and are well known for their high unloaded quality factor (Q). However, despite these advantages, they are less popular for sensing applications than other types of SAW devices due to insufficient space on the device surface for coating a sensing medium without affecting the stability of the resultant sensor response. Accordingly, we fabricated typical one-port SAW resonators operating at the resonant frequency of similar to 130 MHz and patterned positive photoresist (considered as typical sensing film) only onto the reflector electrodes. It was observed that mass loading of the film on the device reflectors shifted the resonant frequency (f(0)) of the main cavity resonance toward the resonant frequency of the interdigital transducer (f(IDT)) to allow more acoustic wave energy from the IDT to contribute toward exciting the main cavity resonance. The relatively lower mismatch between f(0) and f(IDT) of the resonator composed of polymer thin film-coated reflectors overcame the viscous damping of SAW contributed by the intrinsic loss in the polymer to increase the Q of the film-coated resonator. The resultant humidity sensor demonstrated a sensitivity of 155.6 Hz/% RH with good repeatability.