Design of continuous membrane surface micromachined silicon deformable mirror for adaptive optics

Abstract

Adaptive Optics (AO) Technology controls the phase of the optical wave front phase in real time and corrects it by, closed-loop manner. A AO system consists of a correcting element deformable mirror (DM), wave front sensor used for measuring the wavefront deviations and a real-time controller to correct the phase by iterative algorithms. The Adaptive Optics technology finds its varied applications in long range imaging for resolution enhancement, low-power laser communications and astronomy. A deformable mirror is an adaptive element with a reflective surface that changes shape when the control signal is applied. By introducing the correct mirror shape, a distorted wave front can be improved. In this paper, the electromechanical design, dynamic behaviour and fabrication details of two-layer surface micro machined silicon based Deformable Mirror (DM) is described. The surface micromachined MEMS DM is fabricated using polysilicon as the mirror membrane which is mechanically supported to an array of electrostatic parallel plate actuators by posts. Deflection characteristics, modal, damping and pull-in behaviour of the actuators have been analytically studied and simulated for membrane measuring 3.3 mm X 3.3 mm X 3 µm supported by 10 X 10 array of actuators to achieve a stroke of 2.5 µm, resolution of 10 nm and frequency bandwidth 1 kHz. The fabrication process steps of DM has been worked out using Poly MUMPS (Multi-user foundry Process).