SH-BAW devices with abnormal mass-loading effect for chemical sensing

Surface acoustic wave (SAW) devices are promising for chemical and biological sensing applications. This work studies the basic operating principles and the physical behaviors of the "Rayleigh"-SAW and the "Shear Horizontal (SH-)" bulk acoustic wave (BAW), particularly in relation to the chemisorption process. A complete 3D delay line SAW model is developed and performed by the finite element analysis, and a methodology was introduced for characterizing the transmission characteristics (S21) of these devices. Notably, our investigation unveils an intriguing phenomenon in the behavior of SH-BAW in response to loading mass. We observed an anomalous shift in the central frequency, which increases as the chemical adsorbate concentration rises. Leveraging these insights, we designed and constructed a SAW-based gas sensor, and the vinyl-terminated polydimethylsiloxane was synthesized for the detection of chloroform, a challenging pollutant to identify. Through a comparative study, we illustrate distinct responses of Rayleigh-SAW and SH-BAW devices to accumulated loading mass and gaseous contaminants. These experimental results validate and corroborate our simulations. This work demonstrates a unique mass-loading effect exhibited by SH-BAW devices, which differs from the existing theories. These findings offer the opportunity to refine and enhance models for accurately describing the functionality of delay line SAW sensors, thereby contributing to improved sensor reliability.