Development of a tilt actuated micromirror for applications in laser interferometry

A silicon micromirror with 3x3 mm² surface area and a thickness of 100 μm has been designed and realized for the future space mission LISA (Laser Interferometer Space Antenna). The mirror is electrostatically actuated. The tilt movement of the mirror is provided by torsional load of the mirror suspension. 3D FEM simulations have been used for optimization of the layout of the mirror device. A torsion angle of ± 1.9 mrad is achieved at a driving voltage of U=200V. The demanding requirements on the laser interferometer in the mission LISA in respect to mechanical stability, noise performance and especially piston effect, (i.e. the requirement that under rotation of the mirror no significant z-movement of the reflection surface occurs) are fulfilled with a new design and fabrication concept for the micromechanical device. The piston-effect is avoided by a rotational axis of the micromirror which coincides exactly with the surface of the mirror. This is achieved by using a symmetric SOI-wafer (Silicon on Insulator) with handle and device wafer having exactly the same thickness. The mirror plane is formed by the handle wafer. The suspending beams are realized from both, the handle and the device wafer of SOI-wafer. Thus the central axis of the beams coincides with the reflecting plane. In addition, the z-displacement of the mirror under rotation due to the attracting electrostatic force is minimized by optimization of the beams and the counter electrode using FEM simulation. Fabricated devices are characterized by special interferometric optical measurements.