Low-Loss SAW Devices for Sensing Liquid Phase Based on Acoustic Model Conversion

This work achieves the regulation of acoustic modes by metal modification on the surface of piezoelectric crystals and successfully designs a low-loss surface acoustic wave (SAW) device for liquid-phase sensing. The wave equations in piezoelectric crystals and metal-modified piezoelectric crystals were solved using the effective dielectric constant method, and the main acoustic modes propagated in the piezoelectric crystals before and after metal modification were obtained. By calculating the transmission depth, it was found that the energy dissipation decreased after metal modification, thereby reducing device losses. Meanwhile, the optimized device structure design parameters were extracted employing the finite element method and coupling mode theory. On this basis, a delay-line patterned SAW device using the LiTaO3/SiO2 structure and operating at 80 MHz was photolithographically developed, and the measured insertion loss was around 10.5 dB. Then, a 500-nm aluminum is sputtered onto the acoustic propagation path between the interdigital transducers prior to SiO2 deposition to perform surface metal modification on the device. The measured results show that after aluminum metal modification, the insertion loss of the SAW device is reduced to 6.9 dB. Interestingly, after metal modification, the sensitivity of SAW devices to liquid-phase environments decreased by about 83%. After fully immersing in the liquid-phase environment, the device loss only increased by 3.7 dB, which is much lower than the loss generated by unmodified devices in the liquid-phase environment. The low-loss SAW device obtained through modal analysis and structural optimization in this article has great potential for application in liquid-phase sensing.