The Failure Mechanism And The Application In Microwave Field Of RF MEMS Switches

The main bottleneck restricting the application of the MEMS switch is the reliability issue.The design of the MEMS switch involves mechanics,materials science,electromagnetism,thermodynamics,surface micro machining and other fields.To improve the reliability of the MEMS switch various factors should be taken into consideration.The purpose of this paper is to analyze the MEMS switch failure mechanism and give the corresponding improvement measures,at the same time studying the typical microwave application of the MEMS switch.The main work of this paper is as follow.1.The study of the direct contact clamped beam MEMS switches' mechanical and electrical performance is presented,including the derivation of the drop-down voltage and the lumped equivalent circuit of the MEMS switch when it is in up or down state.Modeling and simulation are accomplished by the COMSOL software,indicating that the drop-down voltage is about 40 V.The HFSS software is adopted to simulate the RF performance of the switch.The influences brought by key design parameters such as beam length,beam width and the height of the beam above the electrode on the insertion loss and isolation of the switch are verified.2.Failure mechanism analysis of the direct contact clamped beam MEMS switch.The switch failure caused by technological factors was found through the experiment and process adjustment was carried out.The influence of residual stress on the elastic coefficient of the switch is studied,and the residual stress is released by punching on the beam.Moreover,the temperature effect on the reliability is also studied,including contact adhesion and switch beam deformation and the improving methods are given respectively.3.Design of a MEMS switch reliability test platform.The design and implementation of a MEMS switch test platform is introduced.The test platform can carry out the life test of the MEMS switch and generate pull down voltage with arbitrary waveform.The test platform provides a convenient test method for the reliability study of the MEMS switch.4.Typical applications of MEMS switches in microwave circuits.The application of MEMS switch in microwave circuit is studied by designing the linear phase shifter and the tunable bandpass filter based on MEMS switches.The four bit phase shifter based on MEMS switches is simulated and tested,achieving operating frequency range 2.1GHz~2.3GHz,in band insertion loss <3dB,return loss <-10 dB,VSWR<1.25,0~180 degree phase shift,12 degree phase shift step and the shift accuracy is within±3 degree at the center frequency.The simulation of the tunable bandpass filter based on MEMS switches is carried out,achieving center frequency f0 1.5~2GHz,passband bandwidth 150 MHz,in band insertion loss <2d B,return loss <-10 d B,stopband attenuation?30dB(when f?1.15f0 or f?0.85f0)and the in band ripple is 0.2dB.In summary,In this paper the mechanical properties and failure mechanism of the direct contact clamped beam MEMS switch are studied.The failure factors such as process,residual stress and temperature are studied and the corresponding improving methods are given.This paper introduces the basic principle and design scheme of a MEMS switch test platform which could be used as a test method for the failure mechanism research.Finally,the design examples of MEMS switches in microwave circuits are presented.