Research On Electrical Features And Applications Of RF MEMS Movable Devices

With the trend of miniaturization and integration of electronic devices,micro-electro-mechanical system(MEMS)technology is showing a vast prospect for application.MEMS is featured with the advantages of small size,low power consumption and compatibility with integrated circuit technology.RF MEMS movable device is the core of reconfigurable network.It has become a hot and chanllenging research topic in recent years due to its excellent performance and wide application at the component level.With the properties of low insertion loss,high isolation,high linearity and high power processing capability,MEMS switches are widely used in signal routing and control(attenuation,phase shift,etc.)in the whole RF system.MEMS resonators are widely used as frequency reference and selection components in RF system.Therefore,both MEMS switches and MEMS resonators are considered as representatives of RF MEMS movable devices.Deep study to the core electrical characteristic parameters of the devices takes important significance in RF system.For the RF MEMS switch,driving voltage produces impact to the power efficiency.High power consumption is unacceptable for mobile communication devices.Besides,high driving voltage requires additional transformer circuit,which will reduce the device compatibility.Consequently,the driving voltage is extremely important for the RF MEMS switch.For the resonator,quality factor is the core parameter.Resonator with high quality factor can help oscillator getting better phase noise performance.Besides,it also helps filter get lower insertion loss and better frequency selectivity.In sensors,the high quality factor corresponds to the accurate sensitivity.In addition,due to the inevitable introduction of feedthrough capacitor in the process of resonator packaging,parasitic capacitive feedthrough is also a key parameter for MEMS resonators.Moreover,the parasitic feedthrough is also a key index which determins the quality factor and transmission characteristics when the resonator is used as a liquid sensor.For this reason,this dissertation focuses on the driving voltage of RF MEMS switch,the quality factor and parasitic feedthrough of RF MEMS resonator.The main contents of this dissertation are as follow:1.The driving voltage of RF MEMS switch is studied deeply.A graphene composite switch beam structure and another switch beam based on stress compensation mechanism are proposed.Compared with the ordinary single metal switch beam,proposed multi-layer composite switch beam can effectively reduce the residual stress of the beam structure to lower the elastic coefficient,resulting in the reduce on switch pull-down voltage.The switch beam structure design use silicon nitride as the structure layer.Residual stress of the switch beam is reduced by the principle of stress compensation.The fabrication process of RF MEMS switch is also studied.The reactive ion etching parameter of the switch sacrificial layer is optimized with the corresponding process optimization scheme provided.The RF MEMS switch based on the stress compensation mechanism is fabricated and driving voltage is tested.Based on the study of thin film materials,the graphene sandwich low stress thin film was proposed and fabricated.Based on the process,the low driving voltage and high reliability MEMS switch with graphene sandwich film as beam structure material is successfully fabricated.2.In order to improve the quality factor of MEMS resonator,the energy loss mechanism of TPoS(Thin-film Piezoelectric-on-Silicon)resonator has been researched.Two TPoS resonators are proposed.By improving the supporting beam structure of the resonator,proposed resonator effectively reduces the anchor loss and improves the quality factor of the resonator,comparing with the original resonator without improving the supporting beam.The first TPoS resonator reflectes the dissipative energy by adding reflection block structure in the supporting beam and adopting T-shaped beam structure on the anchor part of the supporting beam.Based on Bragg reflection mechanism,the reflection block structure has utilized multi-layer periodic arrangement of high and low effective acoustic impedance material layers to reflect energy,achieving the goal of reducing the anchor loss and improving the quality factor.The second TPoS resonator is designed to improve the support beam structure based on the two-stage vibration isolation theory.The suspension frame structure is designed on the support beam as the vibration isolation structure.Part of the acoustic energy dissipated from the resonant body to the anchor part through the support beam can be constrained within the suspension frame structure,which reduces the anchor loss of the resonator and improves the quality factor.3.In order to solve the problem of large parasitic capacitive feedthrough in the process of packaging of MEMS resonators or in liquid sensor applications,this dissertation has studied the suppression of parasitic feedthrough in TPoS resonators.Based on the piezoelectric characteristics of TPoS resonator working in even order width extensional mode,a dual interdigital electrodes structure is designed.By introducing differential input and output structure,TPoS resonator can realize the fully-differential configuration by single device to realize the common mode suppression and differential mode amplification of TPoS resonator.Based on this common mode and differential mode,a TPoS resonator with double interdigital electrode structure based on fully-differential configuration is proposed.This resonator possesses good common mode suppression and differential mode amplification.After test in transmission characteristics of the resonator in different input and output configurations,experimental results show that insertion loss has been improved,and parasitic feedthrough has been suppressed in fully differential configuration.To sum up,this dissertation has studied the RF MEMS switches and resonators,which are representative devices of the movable RF MEMS equipment.Corresponding improvement methods are proposed according to the key performance parameters,that is,the driving voltage of the switch,quality factor and the parasitic capacitive feedthrough of the resonator.The theory and experiment are taken into combination in this dissertation.The improved methods are systematically studied from theoretical exploration,simulation analysis,test verification and other aspects to achieve the improvement of performance parameters.