MEMS Micromirror Devices for Optical Applications

MEMS Micromirror devices find numerous applications in the field of projection displays, adaptive optics, MEMS scanners, multi-object spectroscopy, optical switches, coherence tomography and projection lithography. Most of these devices are used for angular deflection of light by torsional deflection of micromirror or axial modulation of light by piston motion of micromirror. Motions in both cases is achieved by electrostatic actuation. At CSIR-CEERI, single bridge digital micromirrors devices are designed and fabricated using surface micromachining and electroplated gold for multi-object spectroscopy applications.

For obtaining large deflection angles with single digit actuation voltage, an innovative double bridge micromirror design where anchor post and actuation beams are hidden under the top reflective plate has also been developed. In this case a large micromirror aperture can be created by placement of micromirrors in close proximity. A fill factor of more than 95% is achievable using the improved design compared to a fill factor of less than 40% in single bridge design.

Since electrostatic force is a short distance force, maximum actuation distance is limited to few microns (5-10 micron) only. For large distance actuation (beyond 10 microns), electrothermal actuation is mostly preferred. Joule’s heating is used for expansion of bimorph structures having different thermal expansion coefficients. At CSIR-CEERI, large deflection bimorph actuator based analog micromirror devices are also being developed for very large actuation. A sandwiched structure of Silicon Dioxide-Au-Silicon Dioxide is being developed for very large scale deflection.

Figure : (a) 3×3 array of surface micromachined double bridge digital micromirror, (b) Pull-in behaviour of fabricated digital micromirrors, (c) LDV image of double bridge micromirror in    torsion mode, (d)LDV image of double bridge micromirror in piston mode