Using Electrostatic Spring Softening Effect to Enhance Sensitivity of MEMS Resonant Accelerometers

Micromachined resonant accelerometers (MRAs) are very attractive for high-precision measurement applications due to their high sensitivity, frequency output, and large dynamic range. The sensitivity is one of critical metrics in MRA sensors. In order to enhance this metric, a micro-lever is usually used. However, the micro-lever may suffer from the attenuation of effective amplification ratio and increasing the structure complexity. Lowering spring stiffness of suspension structure and enlarging the proof mass are alternative methods, but these methods are restricted in real devices due to the trade-offs for the device sizes, cross-axis sensitivity, robustness and fabrication yield. This paper presents a novel method to improve sensitivity of MRAs by using electrostatic spring softening effect without sacrificing the mechanical robustness or enlarging footprint size of the device. Besides, this method can on-line adjust the sensitivity of MRAs, which is another potential advantage for specific applications. The prototype has been designed, fabricated and characterized in both open-loop and close-loop ways. In agreement with the proposed device model, the experiment results demonstrated that the sensitivity of the prototype device increased from 492.7 Hz/ ${g}$ ( ${g}$ being the gravity acceleration) to 2277 Hz/ ${g}$ by using a sensitivity enhancing bias voltage of 67V in a linear scale range of $\pm 1~{g}$ . Moreover, the prototype’s equivalent input acceleration noise reduces down from 5.5 ug / $\surd $ Hz to 1.6 ug / $\surd $ Hz within 1Hz bandwidth, and the bias-instability is down from 6.1 ug to 2.2 ug .