FEM Modeling of Sensitive Layer Swelling Effect on Microbalance Gas Sensor Based on TFBAR Resonator

摘要

The electromechanical response of a chemical gas sensor based on a TFBAR (Thin Film Bulk Acoustic Resonator) structure coated with a sensitive polymer layer was simulated with a FEM (Finite Elements Method) software (COMSOL Multiphysics?). The principle of sensing is based on the change of the mechanical properties of the polymer due to the gas adsorption. This will cause an additional mechanical load inducing a shift in the electrical input admittance curve of the resonator which decreases the resonance frequency of the fundamental thickness mode. This work presents a comparison between two adsorption models describing non swelling and swelling mechanisms by using FEM simulation. The presented structure is a piezoelectric capacitance Al/ZnO/Al with thicknesses of 0.2 μm /6 μm /0.2μm respectively above which a thin film (0.4 μm) of CH2Cl2-sensitive Polyisobutylene (PIB) polymer is coated. The structure vibrates at a resonance frequency of 491.2 MHz with a maximum mechanical displacement of 2.3 nm in the fundamental thickness mode. The FEM simulation has shown a sensor sensitivity of 3 Hz/ppm. The comparison between the two models shows that the model which considerates the swelling effect is more appropriate for describing the real sensitivity of the device.