Research On The Packaging Stress And Stability Of Capacitive Micro-Accelerometer

Packaging technology is one of the key technologies of micro accelerometers.The output drift issue induced by packaging effect has become one of the biggest obstacles to the application in high-precision fields for micro accelerometers.In this dissertation,a comb capacitive micro accelerometer is selected to investigate the relationship between the output stability of the sensor and packaging effects systematically.The packaging model including silicon chip,glass,packaging adhesive and ceramic substrate is established to study the output drifts of the accelerometer experiencing temperature change,long-term storage at room temperature,thermal cycles at different temperatures and high-temperature bake process in different periods respectively by simulation methods and experimental tests.Based on the relationship of the packaging material properties and the stress and deformation states of the sensitive components,the mechanisms of the output drifts of the accelerometer in different environment conditions are interpreted deeply.The main topics covered in this dissertation include:(1)The analytical expression of thermal deformation of the glass upper surface is established based on a thermal deformation theory of multilayer structures,and the analytical results are verified by finite element methods(FEM).Then the key displacements of the glass which affect the bias and sensitivity of the accelerometer are analyzed according to the structure characteristics,and the influences of the adhesive material parameters and the overflow phenomenon on the key deformation are calculated and compared using the analytical expression.(2)The viscoelasticity of the packaging adhesive is measured using dynamic mechanical analysis(DMA).A series of stress relaxation tests are performed on the adhesive samples to obtain their time and temperature dependent modulus,then the master curve is constructed by shifting relaxation curves at different temperatures to the reference temperature along the log time scale axis according to time-temperature superposition.The viscoelastic property is described using Maxwell model,and the viscoelastic parameters are acquired by fitting the master curve with Prony function.(3)The thermal bias drift induced by the adhesive overflow phenomenon which appears during the procedure of attaching the die onto the adhesive layer is investigated based on the material property test.The accelerometers with different levels of overflow are modeled to study the deformation of sensitive component induced by temperature change.The thermal drift is acquired by an analytical method using the structure deformation and the calculation of the differential capacitance,and is compared with the experimental result.The principle of thermal drift is qualitatively analyzed by considering the influence of the overflow on the deformation of the glass upper surface.At last,several methods of improving the thermal stability of accelerometer are proposed and discussed.(4)The long-term stability of the accelerometer is investigated based on the time and temperature dependence of the packaging adhesive.The accelerometers subjected to temperature change,long-term storage at room temperature,thermal cycles at different temperatures and high-temperature bake process in different periods respectively are simulated and their bias and sensitivities are calculated according to the deformed structures.The corresponding experimental tests are conducted to verify the correctness of the simulation results.Based on the relationship of the adhesive viscoelasticity and the stress and deformation states of the sensitive components,the mechanism of the output drifts of the accelerometer in different environment conditions is interpreted deeply.Finally,the selection principle for the packaging adhesive used in die attach of the accelerometer is put forward according to the analysis results.Based on the research contents,the innovation points in this dissertation are as follows:(1)The influence of the adhesive overflow on the thermal drift of the accelerometer is assessed in this dissertation.This phenomenon and its effects on sensor performances have rarely been reported openly.The size of the overflow adhesive is measured and its influence and rule on the thermal bias stability is studied with the consideration of position between the overflow and the stiffness error of the sensor.(2)The mechanisms of the long term drifts of the accelerometers under different environmental tests are clarified in this dissertation,including long-term storage at normal temperature,different temperature cycles,and high temperature experiments at different stages.Bases on the adhesive properties and the stress and deformation of the package structure,the variation and its trend of the key parameters for the accelerometer in environmental tests are analyzed.(3)The selection principle for the adhesive used in die attach of the accelerometer is put forward according to the analysis results.The most critical parameter impacting on the thermal bias stability of the adhesive is achieved by comparing the influence of different material parameters on the structure of key deformation,and the factors affecting the long-term drifts of the sensor are summarized in different environment conditions.In consequence,the selection principle of packaging adhesive for improving the overall stability of the micro accelerometer is proposed.In this dissertation,the thermal stress and deformation of the accelerometer are studied by an analytic model of multilayer structures,which is of theoretical significance for the study of the thermal stability of other micro devices.Besides,since adhesive materials are widely used in the packaging of micro sensors,the research methods and research results have an important engineering significance in the investigation and improvement for output stability of other micro devices.