Simulation Of A Piezoelectric MEMS Accelerometer Using 3D Finite Element Modeling

With the developments of microelectronic technologies and the advancements in the technologies of device design and fabrications,piezoelectric MEMS accelerometers have gradually replaced traditional mechanical accelerometers due to their smaller size,lighter weight and higher sensitivity.Therefore,it is of great significance to study the factors affecting the performance of piezoelectric MEMS accelerometers.This thesis uses the finite element method to analyze a ring-shaped MEMS piezoelectric accelerometer.In order to obtain more accurate simulation results,first the software CATIA is used to construct the geometric model of the accelerometer,then Hyper Mesh is used for build fine meshes,and finally the mesh data is imported into Free Fem++ for programming and simulation.First,the influences of the temperature field and the lateral acceleration on the accelerometer's output charges and its sensitivity are studied.Then,the influences of fabrication defects as well as cracks in the structure on the sensitivity and the natural frequency of the accelerometer are also studied.The following conclusions are obtained:First,besides thermal stresses,the temperature dependence of the elastic constant and piezoelectric coefficients of the material are also considered.It is found the output charge of the accelerometer increases with the increase of temperature and the influence of thermal stresses is dominant;in addition,the sensitivity of the accelerometer also slowly increases as the temperature rises.It is calculated that when the temperature rises from 25°C to 65°C,the sensitivity of the accelerometer increases from 10.27 p C/g to 10.47 p C/g,which is an increase of about 2%,so the accelerometer is usually limited to work within a certain temperature range.Second,according to our simulation results,the lateral sensitivity of the accelerometer is much smaller than the axial sensitivity,and the calculated value of the transverse sensitivity ratio is 0.0448%,it meets the engineering requirements,indicating that the accelerometer's structural design is reasonable.Third,it is found fabrication defects have an important impact on the performance of the accelerometer.When the positions of the defects are different,they have different effects on the sensitivity and natural frequency of the device.Among the defects such as the slope of the edge of the proof mass,the uneven thickness of the silicon film,the slope of the side of the supporting frame,and the deviation of the proof mass from the central position,the uneven thickness of the silicon film will have a significant impact on the sensitivity and natural frequency of the accelerometer.The calculation results show that when the inclination angle caused by the uneven thickness of the silicon film is 1°,the sensitivity of the accelerometer is reduced by nearly 70%,and the natural frequency is increased by more than twice.Therefore,the processing accuracy of the silicon film must be strictly controlled in the processing accelerometer.Therefore,the processing accuracy of the silicon film must be strictly controlled in the processing accelerometer.Finally,cracks in the silicon film of the accelerometer also have impacts on the performance of the accelerometer.When there are circular and radial cracks in the silicon film near the central proof mass,the sensitivity of the accelerometer increases to a certain extent and the natural frequency decreases.