A theoretical study of the VOC sensor based on polymer-coated diaphragm embedded with FBAR

Film bulk acoustic resonators (FBAR) are widely applied in mass, bio-sensing and pressure sensors. In this work, a theoretical study of the VOC sensor based on polymer-coated diaphragm embedded with FBAR is presented. The sensing film swells and leads to the deformation of diaphragm when it is exposed to organic vapor. The FBAR is used to transform the deformation into a measurable output frequency. A model of the interaction between diaphragm embedded FBAR and sensing layer is presented. In this model, the stress distribution of the FBAR area under vapor loads is obtained using the finite element analysis (FEA) based on the equivalence principle of polymer swelling. Then, the longitudinal elastic coefficient variation of piezoelectric film in FBAR under the bending stress is deduced using the first principle methods. Next the longitudinal wave velocity of the piezoelectric film is also obtained. Finally the response frequencies of the FBAR are calculated based on Mason model using ADS software. The simulation results show that the output resonant frequency shift of vapor sensor varies linearly with concentration of vapor and the sensitivity of this sensor for chloroform is approximately 2.5 Hz/ppm. To improve the performance of kind of vapor sensor, this analysis methods will be useful for optimizing the design parameters of FBAR and sensing film.