Abstract
Gap asymmetry in differential capacitors is the primary source of the zero bias output of force-balanced micro accelerometers. It is also used to evaluate the applicability of differential structures in MEMS manufacturing. Therefore, determining the asymmetry level has considerable significance for the design of MEMS devices. This paper proposes an experimental-theoretical method for predicting gap asymmetry in differential sensing capacitors of micro accelerometers. The method involves three processes: first, bi-directional measurement, which can sharply reduce the influence of the feedback circuit on bias output, is proposed. Experiments are then carried out on a centrifuge to obtain the input and output data of an accelerometer. Second, the analytical input-output relationship of the accelerometer with gap asymmetry and circuit error is theoretically derived. Finally, the prediction methodology combines the measurement results and analytical derivation to identify the asymmetric error of 30 accelerometers fabricated by DRIE. Results indicate that the level of asymmetry induced by fabrication uncertainty is about ±5 × 10−2, and that the absolute error is about ±0.2 μm under a 4 μm gap.
Highlights
The development of micro-electro-mechanical-systems (MEMS) usually involves the fabrication of small structures with relative errors larger than that observed in traditional fabrication technology.evaluating the level of structural error and its influence in micro-fabrication has significance in the design of new devices and improvement of processes
Cigada used the electrical method to measure the dynamic behaviors of a MEMS gyroscope with fabrication error [1]; Wittwer predicted the error effect on compliant mechanisms from the perspective of dimensions and materials [2]; and
Aside from direct measurements or prediction methods, design methodologies have been proposed to reduce the dependence of device performance on microfabrication errors; these methodologies include optimization [4,5,6,7] and robust design technologies [8,9,10]
Summary
The development of micro-electro-mechanical-systems (MEMS) usually involves the fabrication of small structures with relative errors larger than that observed in traditional fabrication technology. Evaluating the level of structural error and its influence in micro-fabrication has significance in the design of new devices and improvement of processes. Aside from direct measurements or prediction methods, design methodologies have been proposed to reduce the dependence of device performance on microfabrication errors; these methodologies include optimization [4,5,6,7] and robust design technologies [8,9,10]. Error prediction in the current work was indirectly carried out to evaluate gap asymmetry in differential capacitors of micro accelerometers fabricated by deep reactive-ion etching (DRIE). Identifying the level of asymmetry in structures is urgently needed before differential sensors with new performance requirements can be designed.
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