Study On The Packaging Of High-g MEMS Accelerometer

High-g MEMS accelerometer sensor is the key device for measuring acceleration during high shock process. The acceleration is so high (up to tens to several hundreds times of gravity) that there are difficulties in packaging during the manufacturing of the devices, but packaging is essential to the reliability of them. Jn this thesis, the systemic research on the packaging of an advanced high-g piezoresistive MEMS accelerometer with double beams was conducted. Two main contents of the thes s were as follows:i) The finite element analysis of the packaging structure for the accelerometer, including the modal analysis, static analysis and dynamic analysis of both the packaging structure and packaging materials, were conducted for optimization. The vibration characteristics of the packaging structure were studied in the modal analysis, while the response analysis were conducted under static and half-sine dynamic accelerations of 100,000 g's (1g = 9.8m/s2). The detecting theory of Wheatstone Bridge circuit was applied in the calculation of the response analysis.The influence of elastic modulus of the potting resin in the package was systemically studied. Results of the modal analysis showed that tre natural frequency of the same mode increased with the elastic modulus (E) of the potting resin. When E < 1 GPa, the potting resin was sensitive to motion and the packaging structure behaved as the vibration of the potting resin. For the common potting resin (E > 1 GPa), the natural frequency of the package structure was mainly dominated by the header. Stress analysis under dynamic load showed that the equivalent stress in both the die attachment adhesive and the sealing ring adhesive between the chip and the silicon protective cap decreased with the increase in the elastic modulus of potting resin. When the elastic modulus of potting resin was above 5GPa, the maximum equivalent stress in the sealing ring adhesive tended to be stable, and the natural frequencies of the package were slightly higher than those of the header, so this kind of potting resin was suitable for the potting of :he accelerometer chip. It was showed by dynamic response analysis that the influence of potting resin on the output voltage of the accelerometer could be ignored, and the response curves under different elastic modulus of the potting resin were overlapped with each other proximately.The influence of header materials were also analysed. It was found that the higher the specific modulus (E/p) of the header material, the higher natural frequencies of the packaging structure and the smaller the stress in the other materials in the package. The natural frequencies of the vibration mode in the acceleration direction for the header of aluminum alloy, titanium alloy or stainless steel wen all about 150 kHz, much higher thanthe basic frequency of the cantilever beam (81.58 kHz), therefore the accelerometer was more robust to high shock loading.During the period of shock loading, the response of the accelerometer was caused by outside shock load, consisting of static response and transient response. The former was related to the harmonic acceleration excitation with the same half-period and amplitude with the outside acceleration pulse. The latter was related to the natural vibration characteristics of the cantilever beam. After the shock loading, the response of the accelerometer was damped free vibration. The peak values of the output signals were well linear with the acceleration load. The output sensitivity of the accelerometer under static acceleration could be up to 1.28 V/g. The simulated output voltage was close to both the analytical solution and the test result.ii) The packaging process of the high-g MEMS accelerometer was studied. It was focused on the advanced wafer level packaging technique, which is the trend for the packaging of the high-g MEMS accelerometer, essential to the industrialization of the MEMS accelerometer.The single chip packaging method was studied, and the process techniques related to all the process steps...