Investigation of pseudo lateral field excited acoustic wave gas sensors with finite element method

Abstract

To improve the weak piezoelectric excitation of lateral field excited (LFE) acoustic wave sensors in air, a novel bulk acoustic wave device, named pseudo LFE acoustic wave sensor, is proposed in this paper. The commercially available finite element software, COMSOL Multiphysics, is adopted to analyze pseudo LFE acoustic wave gas sensors and further calculate their sensitivities to electric conductivity variation of the polyisobutylene (PIB) sensing film. An AT-cut quartz plate with a thickness of 185 µm is chosen as the piezoelectric substrate due to its tiny temperature coefficient to frequency for thickness shear mode (TSM). After the mesh convergence analysis, a mesh level is decided for following calculations. In frequency response analysis, a bare pseudo LFE sensor shows a pure TSM and a stronger excitation than a bare LFE sensor. The experiments also verify this result. In sensitivity analysis, a gradual conductivity increase of the sensing film from 0 to 3 S/m causes a significant resonance frequency decrease of the pseudo LFE sensor. Besides, the pseudo LFE sensor exhibits an excellent sensitivity to the film conductivity variation, about 30 times as high as the QCM sensor. Finally, the electrode dimensions of pseudo LFE sensors are discussed for optimizing device performances. One can find ringed floating electrodes exhibit greater sensitivity than circular ones. Moreover, a larger gap width between the LFE electrodes also contributes to larger sensitivity but weakens piezoelectric excitation. The results conclude that the pseudo LFE sensor exhibits a satisfactorily large admittance and an excellent sensitivity to external electrical fluctuation.