Flexures for large stroke electrostatic actuation in MEMS

Abstract

The stroke of a microelectromechanical systems (MEMS) stage suspended by a flexure mechanism and actuated by electrostatic comb-drives is limited by pull-in. A method to analyze the electrostatic stability of a flexure mechanism and to optimize the stroke with respect to the footprint of flexure mechanisms is presented. Four flexure mechanisms for large stroke are investigated; the standard folded flexure, the slaved folded flexure, the tilted folded flexure and the Watt flexure. Given a certain stroke and load force, the flexures are optimized to have a minimum wafer footprint. From these optimizations it is concluded that the standard folded flexure mechanism is the best flexure mechanism for relatively small strokes (up to ±40 μm) and for larger strokes it is better to use the tilted folded flexure. Several optimized flexure mechanisms have been fabricated and experimentally tested to reach a stroke of ±100 μm. The displacement of the fabricated stages as a function of the actuation voltage could be predicted with 82% accuracy, limited by the fairly large tolerances of our fabrication process.

Deflection of an Eccentric Tooth of a Comb Drive in an Electrostatic Field

“…In some designs, the approximate nature of the guidance causes the teeth to have a lateral or rotational offset. For instance, in a tilted folded flexure with a length of 1000 µm and a tilt angle of five degrees, the lateral displacement is about 20 nm and the rotation about 0.7 mrad for a longitudinal displacement of 100 µm [5]. The precise values depend strongly on the specific guidance used and the configuration.…”

A large-stroke planar MEMS-based stage with integrated feedback

“…our design, we would expect an eigenfrequency of 1186 Hz. We have seen before that fairly large deviations exist in the eigenfrequency of MEMS devices [69]. From the frequency sweep measurement data, we can obtain the Q-factor based on the width of the peak at the first eigenfrequency: Q = ω 0 /∆ω −3 dB .…”

“…For stability, an equilibrium should exist in the lateral direction of the flexure mechanism. This equilibrium is given for every displacement in actuation direction x in [69],…”

“…To prevent pullin, the flexure mechanism should be compliant in actuation direction and stiff in lateral direction. In previous work we have studied several flexure mechanisms and concluded that the tilted folded flexure is the optimal flexure mechanism for the strokes we are aiming at, between ±50 ñm and ±100 ñm [69].…”

Design and optimization of large stroke flexure mechanisms

“…For most exure-based mechanisms, underconstrained intermediate bodies dramatically decrease support sti ness due to coupling between external loads on the end e ector and loads in the underconstrained degrees of freedom. Especially for large de ection angles, this results in high compliance [48] in the load carrying directions. However, for the bu er y hinge, the instant center of rotation of each intermediate body and the end e ector coincide and do not vary much over the range of motion, and therefore external loads on the end e ector result in only limited reaction moments in the degrees of freedom of the intermediate bodies.…”