ABSTRACT
The analysis and modeling of SAW resonator devices based on the coupling-of-modes (COM) theory are described, integrating the effect of polymer coating so that the sensor effects can be accounted for in the device transfer function. Based on the perturbation method, the effects of film coating are included in determining the parameters for the model. The COM parameters are, therefore, modified and its simple analytical approaches are presented. The model is validated using the experimental data of a two-port SAW resonator device fabricated on ST-X quartz substrate. The experimental results for a device coated with Parylene C are compared with the simulation results of the proposed model. The comparative results of the electrical characteristics and the frequency sensitivity to film thickness show a good agreement which proves the validity of the model. This analysis and model will provide insight into the influence of the device design parameters on the sensor performance and help in practical design and optimization of SAW-based chemical sensor systems.
Subject(s)
Acoustics/instrumentation , Models, Theoretical , Polymers/chemistry , Xylenes/chemistry , Computer Simulation , Quartz , Reproducibility of ResultsABSTRACT
The paper presents an equivalent circuit model of the polymer coated surface acoustic wave (SAW) resonators by combining coupling-of-mode (COM) description of SAW resonators and perturbation calculation of SAW propagation under polymer loading. An expression for the motional load produced by polymer coating is deduced in terms of COM parameters and polymer characteristics. In addition, expressions for the shifts in resonance frequency and attenuation due to polymer loading are obtained. Simulation results are presented for one-port and two-port resonator devices coated with viscoelastic thin polymer film. The influence of polymer film on resonator response is studied with regard to variations in film thickness and shear modulus. The model simplifies understanding of polymer-coated SAW sensors.
Subject(s)
Acoustics/instrumentation , Computer-Aided Design , Electronics/instrumentation , Micro-Electrical-Mechanical Systems/instrumentation , Models, Theoretical , Polymers/chemistry , Transducers , Computer Simulation , Electric Impedance , Equipment Design , Equipment Failure Analysis , Scattering, RadiationABSTRACT
Simplified relations for the changes in SAW velocity and attenuation due to thin polymer coatings and vapor sorption are presented by making analytic approximations to the complex theoretical model developed earlier by Martin et al. [Anal. Chem. 66 (14) (1994) 2201-2219]. The approximate velocity relation is accurate within 4% for the film thicknesses up to 20% of the acoustic wavelength in the polymer film, and is useful for analyzing the mass loading, swelling and viscoelastic effects in SAW vapor sensors. The approximate attenuation relation is accurate within 20% for very thin films, (less than 2% of the acoustic wavelength in the film). Based on these relations, a new procedure for determination of polymer viscoelastic properties is described that exploits the frequency dependence of the velocity and attenuation perturbations, and employs multifrequency measurement on the same SAW platform. Expressions for individual contributions from the mass loading, film swelling and viscoelastic effects in SAW vapor sensors are derived, and their implications for the sensor design and operation are discussed. Also, a new SAW comb filter design is proposed that offers possibility for multimode SAW oscillator operation over a decade of frequency variation, and illustrates feasibility for experimental realization of wide bandwidth multifrequency SAW platforms.
