Research On Low-frequency MEMS Capacitive Accelerometer With E-shaped Beam

With the advantages of small size,light weight,high reliability,and easy integration,MEMS accelerometers have been widely used in commercial fields such as consumer electronics and automotive electronics.Recently,MEMS accelerometers are gradually moving into the medium and high precision field,and have made some achievements.However,when facing earthquake monitoring and oil prospecting,MEMS accelerometers still have problems in noise floor,cross-axis interference and sensitivity.Therefore,a low-frequency MEMS capacitive accelerometer with E-shaped beam is proposed for seismic monitoring.Specifically,the main work is as follows:Firstly,in order to reduce noise floor,an accelerometer with large proof mass,low resonant frequency and high quality factor by using large size mass,tandem folded beam and glass capping with cavities is achieved,which can be used for seismic monitoring.In order to improve the rejection ratios of spurious modes and suppress cross-axis interference,using the ANSYS software simulation,the traditional serpentine folded beam structure is changed into an E-shaped folded beam with an intermediate frame.The mathematical model of four shapes of electrodes is established and analyzed to determine a displacement-capacitance array transducer based on triangular electrodes,which takes advantage of the special advantage to improve the sensitivity of transducer.According to the characteristics of seismic signals superimposed by large DC and small AC,a dual-channel closed-loop interface circuit model is developed.Using the periodic output of the displacement-capacitance array transducer,the feedback driver pulls the proof mass to the nearest equivalent null position of the displacement cycle instead of origin position in the absence of acceleration,reducing the effect of noise caused by the current required for long-distance feedback.Secondly,according to the functional requirements of the device,the silicon wafer including insulating layer 1,ohmic contact layer,metal layer 1,insulating layer 2,metal layer2 and oscillator structure is realized using process steps such as photolithography,plasma enhanced chemical vapour deposition,reactive ion etching,sputtering and deep reactive ion etching.Large and deep cavities are etched on the top and bottom glasses,which reduces the slide-film damping of the device.The glass-silicon-glass structure combination is realized by metal and glue wafer-level bonding respectively.Finally,using picosecond laser cutting machine to dice the three-layer wafer.The pad signal is exported to the PCB with a wire bonding machine,and the device package is completed with the photosensitive resin cover.Aiming at the performance of the capacitive accelerometer,three test schemes including signal on-off,oscillator structure,and capacitance output are designed,and a complete device test process is built with probe station,LCR meter,infrared microscope,roll-over table and other equipment.Some tests are carried out on the finished device,and the experimental results are analyzed.