Fabrication of a sandwich type three axis capacitive MEMS accelerometer

This paper presents a three axis capacitive MEMS accelerometer including individual lateral and vertical accelerometers in a same die. The three axis capacitive MEMS accelerometer is fabricated by utilizing a glass-silicon-glass multi-stack formed by a fabrication process depending on the double glass modified silicon on glass process (DGM-SOG), where the structural layer is selected to be 35 μm thick <;111> silicon. The fabrication process uses the Au-Si eutectic bonding in the last step of the formation of the glass-silicon-glass multi-stack, eliminating the stiction risk of the suspended silicon structures to the top glass with the voltage free feature of the Au-Si eutectic bonding method, allowing to implement 2 μm capacitive gaps for the vertical accelerometers without any pull in during fabrication, while matching 2μιη capacitive gaps of the lateral accelerometers. The top glass wafer allows implementing not only a top electrode for the vertical accelerometer, but also a cap for the entire structure. The fabricated three axis MEMS capacitive accelerometer die is 12×7×1 mm³. Measured capacitances of the lateral and the vertical accelerometers are 9.4 pF and 7.4 pF, but their capacitance change is similar to each other, as they are approximately measured as 90 fF/V and 100 fF/V, respectively. The Brownian noise of the lateral and vertical are estimated to be 6 Mg/√Hz and 7.6 Mg/√Hz, respectively.