| MEMS package residual stress is induced by thermal mismatch of different materials during chip packaging process.It will seriously deteriorate the reliability and long-term storage stability of high precise MEMS accelerometer.However,there is not a clear theory to explain the variation mechanism of package residual stress and how it affects the long-term storage stability of MEMS accelerometer,thus it is hard to absolutely solve the problem of drift of micro-accelerometer during long-term storage.A deep investigation of package residual stress is helpful to improve the stability and quality of micro-accelerometer.The main topics covered in this dissertation include:(1)In order to settle the problem of ultra-low package residual stress measurement,a novel method combining on-wafer stress magnifying structure and micro-Raman spectroscopy was present.Based on theoretic model and 3D FEM,the design of stress magnifying structure was elaborately analyzed and three design rules of its geometrical dimensions were summarized.Besides,a MEMS fabrication process based on SOI wafer was proposed,and the samples of stress magnifying structure were successfully fabricated.The stress measurement results validate the feasibility of our proposed method.(2)The variation rule and mechanism of package residual stress during long-term storage was investigated.By using this novel package stress measuring method and accelerated test,the package residual stress was monitored to obtain its variation rule during long-term storage.Then,its mechanism was investigated from two aspects:the stress relaxation of chip adhesive and silicon material.The relaxation module of chip adhesive was measured at different high temperature,and then its relaxation module and relaxation time at room temperature was deduced.Besides,the long-term stress variation of silicon wafer and unpackaged micro chip was monitored by using accelerated test and Raman spectroscopic technique.The research result theoretically confirmed that the variation of package residual stress during long-term storage is induced by stress relaxing of chip adhesive rather than silicon material.(3)With the understanding of variation mechanism of package residual stress,the long-term output drift of parallel-plate capacitive micro-accelerometer was analyzed by taking adhesive relaxation module and time as the main parameter.To exploit package residual stress’influence on micro-accelerometer’s output,a formula of deformed-plate capacitance was established.Then,the 3D FEM was conducted to simulate the deformation of chip and output drift of accelerometer during long-term storage under the impact of stress relaxation of adhesive. |
