Design, fabrication and static test of a resonant microaccelerometer

Abstract

This thesis 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. In the present accelerometer, the electrostatic excitation direction of the microresonator is taken to be perpendicular to the acceleration input direction; thereby reducing the coupling effect found in the previous designs. Design equations for the resonant microaccelerometer have been developed and have included analytic formulae for resonant frequency, resolution, nonlinearity and maximum stress. On this basis, a set of feasible designs has been developed for two different types of the resonant accelerometer: a fixed-fixed beam type and a cantilever beam type. The sizes of accelerometers are decided for a resolution of $10^{-3}g$ in the detection range of 5g, while satisfying the bandwidth requirement of 300-600Hz, the maximum nonlinearity of 5%, the minimum shock endurance of 100g 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℃ in $N_2$ atmosphere. From a static test of the cantilever beam resonant accelerometer, a frequency shift of 860Hz has been measured for the proof-mass deflection of 4.3±0.5㎛; thereby obtaining the detection resolution of 0.92±0.11×$10^{-3}$g/Hz and the sensitivity of 1.10±0.13×10^3Hz/g. Analytic and experimental uncertainties in the ccelerometer performance have been analyzed and discussed.