Study On Capacitive MEMS Accelerometer Fabricated By Four Layers Si-Si Bonding

Capacitive MEMS accelerometer has the advantage of high resolution, large dynamic bandwidth and low temperature drift. So, it is widely used in the areas like inertia navigation, micro gravity test and the oil prospect which have more resolution requirements. Generally speaking, there are two kinds of capacitive accelerometer structures, the comb structure and the sandwich structure. The comb structure is more easily to fabricate but the aspect ratio of the capacitors is limited by the deep reactive ion etching (DRIE) process to～25:1. On the contrary, though the fabrication process of sandwich capacitive accelerometer is much more complex, it is more likely to get the sub-micro g level resolution.Firstly, the structure of the accelerometer is designed. The whole sensor is made of four silicon wafers bonded while the top and bottom wafers are the fixed electrodes and the cantilever-mass structure is made on the middle two wafers. The mass is suspended by 8 highly symmetric cantilevers, four on the top side and four on the bottom side. Thus structure can effectively constrain the cross-axis sensitivity. The size of the chip is 6.8mm×5.6mm×1.72mm. Its mass' size is 3.2mm×3.2mm x 0.86mm and as heavy as 17.5mg. The stiffness of the cantilevers is 337.6N/m with a sensing gap of 3.5μm. After vacuum packaged, mechanical noise will be lower than 1 micro-g and the difference capacitive output of 1g is as large as 7.52pF.Secondly, the critical problems of the fabrication of the accelerometer are solved. The symmetric cantilever-mass structure is made by fusion bonding of two wafers. Then, another two wafers are bonded by glass-melting bonding as the fixed electrodes and this bonding also enables wafer-level vacuum package. For the sandwich structure, the PAD of the mass structure is usually difficult to make. The PADs of top electrode and the mass together are made by just one metallization process after all the bonding processes are finished.Finally, the hermetic, Q value and the static output of the accelerometer are tested. The helium leakage of the sensor is lower than 0.1×10^-9atm cc/sec. The sensitivity of the difference capacitive output is about 6.5pF/g. The Q value is 146 and the resonant frequency is 745Hz. All the data shows that equipped with a suitable ASIC interfacing circuit, the accelerometer can get the sub-micro g resolution.