Megapixel wavefront correctors

Optical-quality microelectromechancal deformable mirrors (DMs) and spatial light modulators (SLMs) are described. With such mirrors, the shape of the reflective surface can be modified dynamically to control an optical wavefront. A principal application is to compensate for aberrations and thereby improve image resolution in telescopes or microscopes: a process known as adaptive optics. iDMs are an enabling component for adaptive optics Over several years, researchers at Boston University and Boston Micromachines Corproation have developed manufacturing processes that allow production of continuous and segmented deformable mirrors. We have produced mirror arrays with up to 22,500 actuators, 3 .5p.m of useful stroke, tens of picometer position repeatability, >98% reflectivity, and flatness better than l5nm RMS. Challenges to manufacturing optical quality micromachined mirrors in particular have been addressed: reducing surface roughness, increasing reflectivity, and eliminating post-release curvature in the mirror. These silicon based deformable mirrors can modulate spatial and temporal features of an optical wavefront, and have applications in imaging, beam-forming, and optical communication systems. New developments in DM design are discussed, and manufacturing approaches to micromachined DM and SLM production are presented, and designs that will permit scaling to millions of actuators are introduced. Introduction In many optical systems, it is desirable to alter the optical wavefront dynamically. Prominent examples include adaptive optical systems, in which performance degradation due to the existence of aberrations in the system' s components or beam path is overcome by measuring and compensating the effect of aberrations on the wavefront. Other examples include display systems, in which dynamic modulation of amplitude can be used to produce high-quality projected images. (e.g. Texas Instruments DLP). A third category of dynamic wavefront control involves holographic beam shaping, which has been used as a tool in holographic optical tweezers. In all of these systems, wavefront control is achieved by subdividing the optical wavefront and adjusting the phase or amplitude of the beam using a spatially distributed 1 Corresponding Author: Manufacturing Engineering Department, Boston University, 15 Saint Mary's St., Boston, MA, 02215, tgbbu.edu 2 Boston Micromachines Corporation Mechanical Engineering Department, Boston University Advancements in Adaptive Optics, edited by Domenico Bonaccini Calia, Brent L. Ellerbroek, Roberto Ragazzoni, Proceedings of SPIE Vol. 5490 (SPIE, Bellingham, WA, 2004) 0277-786X/04/$15 · doi: 10.1117/12.549393 1472 array of actuators beneath a continuous membrane mirror (DM) or an array of uncoupled mirror segments (SLM). For some of these applications, the production tools of microelectromechanical systems (MIEMS) offer the promise of rapid advancement. MEMS technology is well known for delivering products with attributes that are important in wavefront control: compactness, scalability, nanometer-scale actuation resolution, low cost, and low power consumption. In this paper, several MIEMS micromirror devices produced at the Boston University Precision Engineering Research Laboratory, and in collaboration with Boston Micromachines Corporation are described. The devices that are described in this paper can be divided into two categories that are delineated not so much by final application as by manufacturing approach. Devices have been made using silicon surface micromachining or aluminum surface micromachining. In silicon surface micromachining, MEMS stmctures are constructed by depositing, lithographically patterning, and then etching alternate layers of structural and sacrificial thin films on a polished silicon wafer. This patterned thin film approach uses polycrystalline silicon as a stmctural material, and silicon dioxide as a sacrificial material, and borrows much of its tooling and process-knowledge base from the semiconductor fabrication industry. Upon completion of the build-up of layers, the wafer is diced and the sacrificial materials are dissolved, leaving a fully assembled silicon device. In practice, the processes used in MIEMS fabrication have diverged considerably from their roots in semiconductor fabrication, and many adjunct processes have been incorporated into the MEMS toolkit to improve electromechanical performance or to allow additional functionality. For development of optical devices, a primary advantage of silicon surface micromachining is that devices can be made using standardized processing tools (e.g. foundries), and the starting point is a flat, smooth, high-quality wafer. A disadvantage is that the processes used to create free-standing MEMS structures generally do not yield optically flat structures over spans of more than a few tens of micrometers, due to intrinsic material stresses and stress gradients. Also, deposited thin-films of polycrystalline silicon are neither reflective nor smooth, and their thinness makes them difficult to polish or coat. A disadvantage particular to silicon surface micromachining is that the thin film deposition and annealing processes require temperatures in processing that exceed 1000°C. As a result, these devices cannot be integrated directly with complementary metal oxide semiconductor (CMOS) electronics, which are limited to processing temperatures lower than 35O°C. Instead, such silicon MIEMS devices must be wirebonded to packages or flip-chip bonded to a separate electronics drivers. These hybrid integration approaches do not scale well for more than a few thousand connections. Many optical wavefront correctors do not require so many actuators, but those that do must use an alternative manufacturing approach. Proc. of SPIE Vol. 549