(A) surface-micromachined capacitive microaccelerometer

Abstract

The thesis presents the analysis, design, fabrication, testing and uncertainty evaluation of a surface-micromachined capacitive accelerometer, where grid-type electrodes are surrounded by a perforated proof-mass, suspended by cantilever beam springs. From an electromechanical analysis of the accelerometer, analytic design formulae for mass, stiffness, frequency, amplitude and sensitivity have been developed. In the design process, the sizes of the accelerometer are decided to satisfy the performance requirements for an automotive airbag application as well as the size constraints from the microfabrication process. From a 4-mask surface-micromachining process, the designed accelerometers have been fabricated with on-chip test structures for residual stress, Young``s modulus and micromachining accuracy, from which the maximum residual stress of 1.5MPa, the Young``s modulus of 94.0±1.0GPa and the minimum micromachining errors of ±0.4 ㎛ are measured, respectively. From the test of the fabricated accelerometer, the natural frequency of 5.8±0.17kHz and the sensitivity of 0.28mV/g have been measured and compared well with the estimated values of 5.7±0.3kHz and 0.29±0.1mV/g, respectively. The parasitic capacitance of the detection circuitry has been measured as 3.34±1.16pF. From the uncertainty analysis of the present accelerometer, we find that the uncertainty in the beam-width becomes the major source of the uncertainty in the natural frequency estimation, while the uncertainties in parasitic capacitance, inter-electrode gap and resonant frequency influence the sensitivity uncertainty in the portions of 75%, 14% and 11%, respectively.