Integrated three-axis accelerometers with nanometer scale capacitive gaps and signal conditioning interface IC

Abstract

This dissertation focuses on implementing multi-axis capacitive MEMS accelerometers with high dynamic range that sense acceleration in wide frequency range (> 10 kHz) with considerable accuracy (< 100 μg/√Hz) by utilizing high aspect ratio (>100:1) nano-gap (< 300 nm) microstructures. Such feature provides an increased electro-mechanical coupling that enables improved operational bandwidth while achieving low-noise without altering device geometry. Furthermore, the use of nano-gap enables adjustment of air-damping so that the quasi-static (i.e. non-resonant) accelerometers can be operated in low-pressure level (1~10 Torr) without instability issues. Doing so paves the path toward the single-chip sensor fusion by enabling integration of quasi-static accelerometer with gyroscopes in low-pressure environment on a common silicon substrate. Both in-plane and out-of-plane accelerometers are designed and fabricated using the HARPSS process, and interfaced with a switched-capacitor signal conditioning IC to characterize their performances. The measurement results showed the sensor can achieve operational bandwidth higher than 8.5 kHz, and noise levels of 221 μg/√Hz and 72 μg/√Hz for in-plane and out-of-plane devices respectively. The figure-of-merit (FOM) defined as the ratio of device bandwidth over noise density for the presented designs are orders of magnitude higher than that of other commercially available MEMS accelerometers. To realize sensing gap in the nanometer range, a dedicated fabrication process (i.e. HARPSS), which gap size is determined by the thickness of thermally grown sacrificial layer is used. However, using such process makes it difficult to implement shock stop, which requires smaller gap size than sense electrode to prevent excessive proof-mass movement under high levels of accelerations, as it requires an increased number of optical masks as well as fabrication steps. To resolve these issues, a novel sloped-electrode, which enables creating different effective gap sizes by simply adjusting its geometry, is proposed and consecutive measurements were performed to validate the effectiveness of the scheme. The changing capacitance from sensor element is converted into an electrical signal using a low-noise switched-capacitor (SC) interface circuit. Correlated double sampling (CDS) technique is introduced to eliminate inherent flicker noise of the amplifier, which is the dominant noise source of the circuit, and an extensive analysis was conducted to suppress other noise sources and attain high capacitive resolution. Measurement results showed that the presented readout IC achieves more than 10 times better noise performances compared to the previous circuit that was used to interface MEMS accelerometer. Furthermore, to minimize the effect of capacitance mismatches in the MEMS accelerometer, a precision calibration circuit that employs a time-averaged charge-tuning technique was incorporated into the readout circuit, achieving a resolution level of sub-aF and a wide calibration range of 300 fF.