Silicon Membrane Mirrors With Electrostatic Shape Actuators

NASA's Jet Propulsion Laboratory, Pasadena, California

Efforts are under way to develop deformable mirrors equipped with microscopic electrostatic actuators that would be used to maintain their reflective surfaces in precise shapes required for their intended applications. Unlike actuators that depend on properties of materials (e.g., piezoelectric and electrostrictive actuators), electrostatic actuators are effective over a wide temperature range. A mirror of the present type would be denoted a MEMS-DM (for microelectromechanical system deformable mirror). The reflective surface of such a mirror would be formed on a single-crystal silicon membrane that would be attached by posts to a silicon actuator membrane that would, in turn, be attached by posts to a rigid silicon base (see figure).

The Silicon Actuator Membrane would serve partly as an electrode that could be deformed by electrostatic attraction to the electrodes on the base. The deformation would be transferred to the mirror membrane.

The actuator membrane would serve as the upper electrode of a capacitor. Multiple lower electrodes, each occupying a conveniently small fraction of the total area, would be formed on an electrically insulating oxide layer on the base, thereby defining a multiplicity of actuator pixels. The actuator membrane would be corrugated in a pattern that would impart mechanical compliance needed for relaxation of operational and fabrication-induced stresses and to minimize the degree of nonlinearity of deformations. The compliance afforded by the corrugations would also help to minimize the undesired coupling of deformations between adjacent pixels (a practical goal being to keep the influence coefficient between adjacent pixels below 10 percent).

The mirror and actuator membranes and posts would be fabricated partly by surface and partly by bulk micromachining of silicon. Other micromachining techniques that are in common use in the integrated-circuit industry could be used to integrate driver and control electronic circuits into the actuator structure. The base, actuator, and mirror layers would be assembled by use of the process described in "Wafer-Level Membrane-Transfer Process for Fabricating MEMS," which appears on page 58.

The center-to-center distance of adjacent pixels could be as small as about 100 µm. A typical design would call for a center-to-center distance of 200 µm and a maximum deflection of about 2 µm. Calculations for a representative example of such a design, in which the actuator in one pixel generated a force of 10-4 N, yielded an estimated actuator-membrane deflection of 0.03 µm and mirror-membrane deflection of 0.4 µm. The adjacent-pixel influence coefficient in this example was found to be less than 10 percent.

This work was done by Eui-Hyeok Yang of NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Physical Sciences category. NPO-21120.