Microscale Actuators Drive Macroscale Optical Element

Daniel S. Burgess

A team of engineers has reported the development of a rigid-body motion generator based on a two-dimensional array of deformable micromembrane actuators. The generator has a variety of potential optical applications, including use with tunable filters in spectroscopy, imaging, telecommunications and displays.

Xingtao Wu, a researcher at Optron Systems Inc. in Bedford, Mass., explained that process incompatibilities between micromachining and conventional optical fabrication heretofore have hampered the development of tunable optical filter technologies based on microelectromechanical systems (MEMS). A MEMS wet-etching step, for example, can chemically consume the dielectric coatings that are deposited on a surface to give it the desired optical performance. Moreover, if a MEMS element has an aspect ratio that is higher than that of conventional optical substrates, the result can be material incompatibilities because the thermal-dependent stresses in a multilayer dielectric coating can buckle thin MEMS elements.

The motion generator features a two-dimensional array of deformable micromembrane actuators. The researchers speculate that such an optical element may be suitable for use in a variety of applications, including spectroscopy, imaging, telecommunications and displays.

The engineers from Optron, Integrated Micromachines Inc. of Monrovia, Calif., The University of Akron in Ohio and Massachusetts Institute of Technology in Cambridge believe their approach resolves these problems. The motion generator uses thousands of microscale MEMS actuators to drive a conventional, macroscale optical element.

The principle is analogous to cooperative action by a large group of individuals to accomplish a great feat. "One can visualize the picture of millions of ants trying to move a big loaf of bread. They work collaboratively underneath the big piece ... each holding the bread on their shoulders," Wu said. "And when the order is given to lower the bread, all of them instantly lower their body position by twisting the legs in an identical move. The big bread on their shoulders is then moved to a new, and lower, position."

In the demonstration, the team first created the micromembrane actuator array in silicon and polymer using standard deposition, dry etching and chemical mechanical polishing techniques. The result was a series of 3248 4-µm-high polymer support posts atop a 0.25-µm-thick metallized polyimide membrane. The optical element -- a 200-µm-thick, 2.7-mm-square silicon mirror coated with a spin-on adhesive -- was placed atop the actuators and thermally bonded to the posts. Tests of the performance of the array were conducted in a Michelson interferometer setup at a 5-kHz, 200-V actuation voltage ramp, and no failure mode was observed over 100 million cycles.

Wu said the demonstration indicates that electrostatic actuation is suitable for driving macroscale optical elements, offering positioning accuracy and a compatibility with traditional integrated circuit fabrication methods. Initial application development efforts will include refining the actuation scheme for use with tunable optical filters for telecommunications laser diodes or for spectral imaging.

Optron is seeking industrial partnerships to further develop the technology, the researcher said.