Research On The Effects Of Adhesives On Silicon Resonant Accelerometer And Isolation Structure Design

Packaging is an important part in the manufacturing process of MEMS sensors.On the one hand,it can provide MEMS chips with functions such as mechanical protection,electrical connection,and heat dissipation.On the other hand,the mismatch of the package structure will also cause the package effect,which leads to the deformation of the sensitive structure and affects the performance of the sensor.This paper cuts in terms of the silicon resonant accelerometers’ packaging structure,and studies the influence of adhesive on the packaging effects.The main contents are as follows:(1)In view of the viscoelasticity of the die attach adhesive,the samples were made of silica gel,epoxy resin and modified epoxy resin commonly used for MEMS die attaching.The viscoelasticity of die attach adhesives is measured using dynamic mechanical analysis.During the stress relaxation tests,the relaxation modulus of the adhesives at different temperature are obtained.According to the principle of time-temperature superposition,the stress relaxation master curve at20℃ is constructed by shifting these relaxation curves at different temperature and then fitted with Prony series to obtain the viscoelastic parameters.Based on the packaging structure of the silicon resonant accelerometer,a laminated model containing a viscoelastic die attach adhesive was established.Then,the stress and warpage of the laminated model are calculated by taking into parameters of three die attach adhesives.After that,the effect of the relaxation modulus and CTE of the adhesive on the thermal stress and warpage are studied.The results show that the relaxation modulus of the adhesive is the main factor affecting the stress and warpage of the packaging structure.(2)According to the viscoelastic parameters obtained in the test,a simulation model of the silicon resonant accelerometer package was established.By finite element analysis,the thermal stress and warpage of the packaging model under two different loads which include long-term storage and thermal cycles at different temperatures are calculated.the differences between the analysis results of the viscoelastic model and the elastic model are compared,and the rationality of the viscoelastic model is verified through the thermal cycle experiments.Then the influences of the bonding layer on the output of the resonator under the thermal cycle and long-term storage circumstances are studied.The properties of different adhesives under different environmental conditions is summarized from the finite element analysis,which provide references for the selection of low stress,weak packaging effect die attach adhesives with high bonding performance and high reliability.(3)A series of isolation structures is proposed to optimize the structure so as to reduce the impact of packaging effects on the sensitive structure of the accelerometer.Through the thermal stress and impulsive response analysis of the package structure,the isolation effects and impact resistance capability are obtained.The single beam-frame isolation structure has the most significant isolation effect on thermal stress,and the thermal stress of the resonator end is reduced from 0.72 MPa without isolation structure to 0.18 MPa at-40℃,but it will cause the deflection of the accelerometer’s sensitive axis during the impact,which affects the stability of the accelerometer.The double beam-frame structure can maintain the direction of accelerometer’s sensitive axis during the impact,but the thermal stress in the double beam is large,as well as the stress at the end of the resonator is reduced to 0.22 MPa,which means the thermal stress isolation effect is not as good as that of the single beam model.The ‘Ri’-shaped isolation structure has both good package stress isolation effect,impact resistance and symmetry.It can reduce the stress at the end of the resonator to 0.20 MPa,which can effectively reduce the thermal stress in the resonator.In this study,the temperature coefficient of the resonator frequency is reduced from 34.90ppm/℃ to9.24ppm/℃,which means the isolation structures can effectively suppressing the influence of the packaging effect on the frequency output of the accelerometer.