Modeling And Simulation Of A High Frequency SAW Gas Sensor Based On Finite Element Method

Surface acoustic wave (SAW) devices are microelectromechanical systems(MEMS) that are used for signal processing by manipulating surface waves on piezo-electric substrate through interdigital transducer (IDT).Because of their versatility,small size, and ease of fabrication, they have broad applications ranging from pro-fessional radar and communications systems to consumer areas such as TVs, pagers,mobile phones, and sensors. SAW sensor is a new important division of SAW devices,its distinctive advantages of high precision, high resolution, digital output, small massand size, low power consumption, noise resistance, working in harsh environmentmake it a promising sensor for various applications. At present, SAW sensors havebeen widely used in gas detection, temperature measurement, humidity measurement,pressure and torque measurement, wireless measurement and biological and medicalareas.To satisfy the need for high performance SAW sensors, the sensors advance tohigh frequency, to develop high frequency SAW sensors, it requires precise under-standing of the second order effects that have been previously ignored in the design ofrelatively low frequency devices. As the frequency of operation increases, the widthof electrodes becomes smaller while thickness of electrodes is not reduced accord-ingly, the electrode thickness is no longer negligible compared to the electrode width,accordingly the second order effects caused by the electrode become outstanding. Forthe high frequency SAW sensors, it requires suitable model to quantitatively analyzethe second order effects introduced by the high ratio of the electrode thickness andelectrode width. Furthermore, it depends heavily to the prototype experiments to ex-tract some parameters of sensors, the FEM response analysis has the capability toextract these parameters replacing of the prototype experiments. Hence some param-eters of SAW sensor IDTs such as resonance frequency, anti-resonance frequency andthe electrical admittance could be obtained without the prototype experiments. By thesimulation, it can remarkably reduce the number of prototype experiments, which in aresult reducing the period and cost of the sensor development. Moreover, the param-eters extracted from the simulation can give important reference for further design of the whole sensor system.A delay line type high frequency SAW gas sensor was preliminarily designed,which gave a specific object for the modeling and simulation of SAW sensor. Someassumptions and simplifications were made for the designed SAW sensor, and bas-ing on these assumptions and simplifications a finite element periodic model of SAWsensor IDT was developed. In the simulation, first the SAW propagation propertyon the free piezoelectric surface was analyzed using the non-electrode model. Next,a series of models with varying electrode parameters were conducted modal analy-sis, the second order effects of electrode were quantitatively analyzed under differentelectrode parameters. And based on the modal analysis, the function of the sensor'selectrical admittance with the working frequency is analyzed through the harmonicanalysis, and the resonance and anti-resonance frequencies were obtained. Finally,to analyze the in?uence of aperture to the admittance of the IDT and to estimate thereal admittance of the IDT, a simple three dimensional model was built, the relationof the aperture and the admittance was analyzed and the admittance of the IDT wasestimated.