Equivalent Model Extraction And Application Of RF Micro-Electromechanical System

The equivalent model,as a tool for refining the internal laws in engineering research,simplifies the description of actual physical phenomena,has extremely important research value in the complex system of micro-electromechanical systems with three-dimensional,micro-scale and multi-physical effects.With the help of the microelectromechanical equivalent model,we can learn more about the working mechanism of micro devices at different research levels,and simulate and calculate the working characteristics of the device faster and better.Usually after the reasonable equivalent model is proposed,the parameters of the equivalent model need to be extracted by fitting the device performance curve and other methods.After obtaining the model parameters,the equivalent model can be applied to quickly optimize the required MEMS design.The paper first briefly introduces the definition and classification of MEMS,and then introduces the core RF MEMS system discussed in this paper,including piezoelectric surface acoustic wave devices,piezoelectric bulk acoustic devices,electrostatically actuated micro devices,etc.The important role and working principle in the RF communication system are also described.Then it introduces several different levels of models of RF MEMS.After that,the research work on piezoelectric filters,piezoelectric resonators and electrostatic MEMS switches are reviewed in related fields in recent years,which provides reference and inspiration for the research work in this paper.For the piezoelectric filter design at the circuit level,the MBVD equivalent model is most effective.This paper first studies the method of extracting the parameters of MBVD model from the given piezoelectric resonator performance curve.Secondly,a multi-mode piezoelectric bandpass filter is designed by using this model.A common mode suppression circuit is designed based on the commercial balun equivalent circuit,which greatly improves the filter performance.It achieves an insertion loss less than 1.8dB and the out-of-band rejection at 1.1 GHz higher than 50 dB.Then,based on the L-type matching network,a piezoelectric notch type band-reject filter is designed.The influence of each parameters of the components on the filter performance is analyzed in detail.Finally,a SAW resonator is fabricated and measured.The result verifies the theoretical design.It shows the in-band rejection is higher than 27 dB and the input to output impedance is changed from 50 Ohms to 72.4 Ohms.For the internal design of the periodic piezoelectric resonator at the component level,a COM phenomenological model considering internal reflection is adopted to meet the requirements of fast device simulation for normal cases.However,the research in this paper finds that there are mutual coupling effects between multiple modes excited inside the resonator.In order to introduce this effect,the COM model including coupling between modes is established based on the traditional COM model and the extraction steps of its model parameters are discussed.Based on this COM model,the method of quickly implementing the spurious mode suppression on a specific structure is discussed.Finally,the above research was repeated on the temperature-compensated SAW resonator structure with silicon dioxide layer,and the best suppression angle in the range of 10%³0%SiO₂ thickness and 3%⁵%copper electrode thickness is obtained.For the internal design of the aperiodic piezoelectric resonator at the component level,the finite element model is most effective.In order to solve the problem that the large-scale aperiodic finite element model occupies too much computational resources,a hierarchical cascading finite element model is adopted,which can greatly reduce the calculation time and storage space for the specific structure.Based on this cascading model,the scattering characteristics of a surface acoustic wave discontinuity structure structed on 128-YX Lithium Niobate substrate are analyzed.Based on the analysis results,a transmission line equivalent model is finally established.For the component-to-circuit level electrostatic MEMS switch and its application research,a commercial MEMS switch is analyzed by equivalent circuit model and full-wave simulation finite element model.The parameters of the equivalent circuit are extracted from the measured characteristic curve of the switch.Then,based on this MEMS switch,a 4-bit switch line phase shifter is designed.The key T-section impedance matching problem is solved in the design and the full-wave simulation of the whole phase shifter is established.Finally,the phase shifter component was fabricated and tested.The test results show that the average phase shift error of the phase shifter is 0.54°,which satisfies the design specification.