A low-drift silicon MEMS resonant accelerometer

Gavcar, Hasan Doğan
This thesis presents the design, fabrication, and experimental verification of low temperature drift silicon resonant accelerometers for tactical grade applications. The working principle of a silicon resonant accelerometer is based on force sensing, in which the sensor output is a frequency proportional to the input acceleration. The stress-insensitive sensor design prevents the thermal stress produced by the mismatch of the thermal expansion coefficients (CTE) of glass and silicon from transmitting to the DETF resonators. In the scope of this thesis, three different silicon resonant accelerometer structures are sequentially designed, fabricated, characterized, and tested with the frequency readout circuit. The first sensor structure consists of two differential DETF resonators and a large proof mass connected to a stationary outer frame. The second sensor structure utilizes microlevers to magnify the inertial force acting on the DETF resonators in addition to the sensor frame of the first sensor structure. Although specially designed stress release beams are implemented in the first and second sensor structures, they suffer from thermal stress caused by the stationary outer frame after fabrication. In the third sensor design, the sensor structure is placed on a specially designed single anchor to overcome the stress problem encountered in the first two sensor designs. The FEM simulations are performed to analyze and optimize the mode shapes, force sensitivity, and temperature sensitivity of the sensor structures. The designed resonant accelerometers are fabricated using the aMEMS1 process with wafer-level hermetic encapsulation. The fabricated sensor chips are integrated with the capacitive preamplifier and digital signal processor (DSP)-based phase-locked-loop (PLL) system implemented on a Zurich Instrument HF-2 lock-in amplifier. The functionalities of the accelerometers are verified by the resonance and 4-point tumble tests, and their performances are experimentally evaluated in terms of thermal sensitivity and bias stability by temperature and Allan variance tests. Test results show that the single-anchor silicon resonant accelerometer achieves a bias instability of 1.25 µg, a bias stability of 2.8 µg, a bias repeatability of 6.6 µg, a velocity random walk of 6.1 µg/√Hz for the sensor bandwidth of 33 Hz at room temperature, and a maximum bias change of 4.3 mg for the temperature range from -40 °C to +85 °C. It has a measurement range up to ±60 g with a 500-ppm deviation from the linear scale range. Compared to the commercial capacitive MEMS accelerometer developed by Mikrosistemler, the silicon resonant accelerometer developed in this work exhibits at least a 20-fold improvement in bias temperature sensitivity, a 7-fold improvement in bias instability, a 112-fold improvement in bias stability, and a 2-fold improvement in noise floor. More importantly, although the bias of the commercial capacitive MEMS accelerometer tested in this study moves 1.7 mg in 50 minutes at constant 25°C, the bias of the silicon resonant accelerometer shifts less than 0.25 mg in 10 hours at ambient temperature.


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Citation Formats
H. D. Gavcar, “A low-drift silicon MEMS resonant accelerometer,” Ph.D. - Doctoral Program, Middle East Technical University, 2023.