A temperature compensated biaxial eFM accelerometer in Epi-seal process

Abstract

This paper reports on the implementation of a vacuum-encapsulated bi-axial resonant accelerometer utilizing the electrostatic frequency modulation (eFM) technique. A novel flexure structure is designed to enable fully-decoupled in-plane displacement of the proof-mass to minimize cross-axis sensitivity. A differential readout scheme leveraging pairs of high quality factor (>12k) free-free beam resonators mitigates the first-order nonlinearities due to temperature effects and unwanted response to proof-mass spurious modes. The fabricated device measures a scale factor of 45.8 Hz/g with negligible cross-axis sensitivity (<2 %). Furthermore, the smooth characteristics of temperature coefficients of frequency (TCf) of the resonators enable an accurate temperature sensor using the common-mode frequency of the resonator pair. Applying a temperature compensation scheme utilizing the common-mode frequency output and pre-characterized TCfs, the accelerometer demonstrates superior performance measuring a VRW of 5.8 μg/√Hz and BI of 5.7 μg with a scale factor stability of 0.38 % and zero-g-output variation of 8.3 mg over temperature range from –20 °C to 80 °C.