Influence of external swelling stress on the frequency characteristics of a volatile organic compound (VOC) sensor based on a polymer-coated film bulk acoustic resonator (FBAR)

Abstract

Film bulk acoustic resonators (FBARs) are widely applied as gravimetric sensors for gas and biochemical analysis. For these devices, the sensing layer is usually coated on to the FBAR for qualitative analysis based on the frequency response according to the Sauerbrey relationship. Various polymers have been used as the sensing films for volatile organic compound (VOC) monitoring. The most significant works are focused upon developing sensing materials and optimizing the piezoelectric film to improve the performance of the sensors. However, the influence of the swelling stress upon the sensing film during vapor absorption on resonant frequency of the FBAR is often ignored. In this work, the influence of the swelling stress upon the frequency characteristics of the sensors based on polymer-coated FBARs have been investigated in detail. The elastic coefficient and acoustic wave velocity changes of the piezoelectric (AlN) layer in the FBAR in the presence of stress loads were calculated by the first-principle methods. In combination with the Mason model of the sensors, the resonant frequency shifts of the FBAR under swelling stress were obtained. The results show that the frequency shifts of the FBAR are enhanced with increased swelling stress that induce negative impacts upon the FBAR sensor-based operation. In order to improve the vapor sensing performance, the optimal thickness of the sensing film must be evaluated.