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IN MOTION

MEMS actuators fabricated for optical and mechanical applications

One of the key abilities of MEMS devices is that they can produce mechanical motion on a very small scale. This provides the means to create compact actuators with motions in the range of nanometers to millimeters and with the ability to generate forces up to the milli-newton range. MEMS actuators are generally based on either electrostatic or thermal operation, although piezoelectric, magnetic and even hydraulic types also have been demonstrated. An electrostatic actuator relies on the potential difference between a movable and a fixed surface and exerts a force on the movable surface. These actuators are very low power (microwatts) but require high voltage, sometimes in excess of 100 V, to generate a sufficient electric potential. Their response time is fast - tens to hundreds of microseconds - but they can be limited in their maximum displacement. Such actuators have a "pull-in" voltage limit whereby they fully deflect after moving a certain portion of their range, making this type of actuator most suitable for binary positioning. The well-known comb drive actuator is of this type. Thermal actuators, on the other hand, use Joule heating to produce thermal expansion. They have only a slightly slower response time than electrostatic actuators and can be operated over their entire range of motion.

One of the first MEMS projects at SwRI was an internally funded development study of several of these micro-actuators and their use in switching applications. Actuators were designed based on both electrostatic and thermal methods, with a variety of force-displacement characteristics and the ability to produce both horizontal and vertical motion. Also developed were two novel actuators that have since been patented by SwRI - a single-material, in-plane, bi-directional thermal actuator and a vertical thermal actuator based on a bi-metal design.

One switching device produced with the actuators from this project was an optical switch. It was designed to switch light between any of several input collimated or parallel optical fibers to any output fiber. The switch was based on a two-dimensional array of micromirrors mounted on the end of electrostatic out-of-plane actuators. The mirrors, which are smaller than the head of a pin, were fabricated flat, but were flipped and latched into position by a hinge mechanism on the end of the actuator arm. The switch was scaleable to allow additional input or output fiber channels. Such a switch can be used as a cross-connect in fiber optic communications systems or in optical scientific instrumentation.

Several sponsored programs for development of unique microstructures and sensors have benefited from this internal research. Some of the actuator designs have been used directly in the development of the SwRI "materials lab on a chip." The handling and testing techniques, design procedures and measurement tools developed under the internal research project laid the groundwork for many subsequent MEMS projects at SwRI.

Published in the Winter 2004 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.

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