Single-Crystal-Silicon Continuous Membrane Deformable Mirror Array for Adaptive Optics in Space-Based Telescopes

In this paper, we present a single-crystal-silicon (SCS) continuous membrane deformable mirror (DM) as a corrective adaptive-optics (AO) element for space-based telescopes. In order to correct the polishing errors in large aperture (~8 m) primary mirrors, a separate high-quality surface DM array must be used. Up to 400000 elements and a mirror stroke of ~100 nm are required for the correction of these polishing errors. A continuous membrane mirror formed by the the SCS device layer of a silicon-on-insulator (SOI) wafer is used to achieve a high-quality optical surface and to minimize the additional diffractive effects in the optical system. To achieve substantial local deformation needed to correct high-order errors, we use a highly deformable silicon membrane of 300-nm thickness. This thin membrane is able to deform locally by 125 nm at an operating voltage of 100 V with a pixel pitch of 200 mum. The resonance frequency of a pixel is 25 kHz with a low Q-factor of 1.7 due to squeeze-film damping. The device is fabricated by processing the microelectromechanical system (MEMS) and electronic chips separately and then combining them by flip-chip bonding. This allows optimization of the MEMS and electronics separately and also allows the use of an SOI layer for the mirror by building the MEMS bottom up. A small prototype array of 5times5 pixels with 200-mum pitch is fabricated, and we demonstrate single pixel and multiple pixel actuation

I. Introduction

In this paper, we present a single-crystal-silicon (SCS) continuous membrane deformable mirror (DM) array with electrostatic actuation for applications in adaptive optics (AO) [1], [2]. AO used to correct the atmospheric distortion requires large strokes of up to several micrometers. For vision science applications, microelectromechanical system (MEMS) mirrors with strokes even higher than 10 m are required. In space-based telescopes, where atmospheric distortion is a nonissue, imperfections of the optical system are the main cause of image degradation. Such optical systems require only short-stroke AO to correct the imperfections such as local defects of a few nanometers, large curvature flaws of several hundred nanometers, and component misalignment. For these types of corrections, the continuous face-sheet mirrors as described in this paper, perform better than the segmented mirrors that degrade the image quality through attenuation and diffraction.