Abstract

This paper investigates a resonant microaccelerometer that measures acceleration using a built-in micromechanical resonator, whose resonant frequency is changing due to an acceleration-induced axial force. From a structural analysis, design equations for the resonant microaccelerometer have been developed and have included analytic formulae for the resonant frequency, sensitivity, nonlinearity and maximum stress in the mechanical structures. On this basis, the sizes of the accelerometer are designed for a sensitivity of 10−3g/Hz over the bandwidth of 100Hz, while satisfying the maximum nonlinearity of 5%, the minimum shock endurance of 100g, the detection range of 5g and the size constraints placed by microfabrication process. A set of the resonant accelerometers has been fabricated by an integrated use of bulk-micromachining and surface-micromachining techniques. From on-chip test structures, the Young’s modulus of polysilicon is measured in the range of 60∼100GPa. The residual stress of polysilicon structures was reduced to 362.1kPa by the annealing process performed for 2 hours at 1000 C in N2 atmosphere. From a static test of the resonant accelerometer, a frequency shift of 860Hz has been measured corresponding to the deflection of the proof-mass by 4.3±0.5μm; thereby obtaining the detection sensitivity of 0.92±0.11 × 10−3g/Hz. Uncertainty in the resonant frequency output has been evaluated to identify important issues involved in the design, fabrication and testing of the resonant accelerometer.

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