Fabrication processes for MEMS deformable mirrors in the next generation telescope instruments.

摘要

This dissertation advances three critical technology areas at the frontier of research for micro electro-mechanical systems (MEMS) deformable minors (DMs) needed for next generation telescopes (NGTs). High actuator-count MEMS deformable minors are needed for future ground-based large astronomical telescopes. Scaling up the current MEMS DMs to unprecedented numbers of independent actuators---up to 10,000 on a single DM---will require new electrical connection architecture for the actuators in order to replace the wire-bonded scheme that has been used to date. A through-wafer via interconnection fabrication process for MEMS DMs is developed to offer a path to transform the frontier of high actuator count MEMS micromirrors.;In a class of NGTs instrument known as the Multi-Object Adaptive Optics (MOAO), the correction made by the DM of the wavefront phase error over the entire telescope field view is not accessible to the sensing unit. To achieve compensation, precise, single step "open-loop" commands must be developed for the DM. Due to the nonlinear relationship between applied voltage and actuation displacement at each actuator, and the mechanical coupling among actuators through the mirror membrane, such open-loop control is a formidable task. A combination of mirror surface modeling and sparse actuator empirical calibration is used to demonstrate open-loop control of MEMS deformable minors to the accuracy of closed-loop control over the entire available DM stroke. Shapes at the limit of achievable minor spatial frequencies with up to 2.5microm amplitudes have been achieved within 20nm RMS error accuracy of closed-loop control. The calibration of a single actuator to be used for predicting shapes results in an additional 14nm RMS surface error compared to parallel calibration of all actuators in the deformable minor.;The ubiquitous reflective coatings for MEMS deformable minors are gold and aluminum. Emerging adaptive optics application require broadband optical coatings usable from the near visible to near infrared, i.e. silver. Many of the reflective coatings of interest have high stress resulting in added curvature on deformable mirror. A thick epitaxial polysilicon MEMS mirror that can tolerate thicker reflective coatings with higher stresses has been fabricated. A thin film deposition process to counteract the stress in the reflective coating was also developed. The deformable mirror has low stress (∼10 nm flatness), high reflectivity (>95%), 1024 actuated segments, that is usable over a wavelength range from visible to IR with a protective layer for the silver coating and stress-reducing layer between the silver and mirror layer to obtain the desired flatness.