Integrated MEMS technologies for adaptive optics

Image resolutions of modern optical systems are many times limited by wavefront aberrations due to turbulence in the optical media. Adaptive Optics (AO) is a technology that utilizes deformable mirrors (DM) to correct the wavefront distortion, thereby enhancing the image resolution. In this research, we investigate the design and fabrication of micromechanical-deformable-mirror arrays for AO applications. The mirror arrays are produced using surface micromachining techniques developed for the fabrication of Microelectromechanical Systems (MEMS).;Because many AO applications require large arrays (100s-1000s of segments) of closely-spaced deformable mirrors that need to be controlled individually, it is highly desirable that the DM arrays can be integrated with CMOS control electronics. In this research, we develop a CMOS-compatible fabrication process for MEMS DM arrays, in which polycrystalline-silicon-germanium (poly-SiGe) and polycrystalline-germanium (poly-Ge) are used as the structural and sacrificial materials, respectively.;One major challenge of using poly-SiGe as the structural material is to reduce the high strain gradient in as-deposited poly-SiGe films, because the low-thermal-budget requirement for post-CMOS integration prohibits the use of a high-temperature annealing step. In this research, we demonstrate a means to use bilayer films to modify curving effects in the SiGe platforms that carry the deformable mirrors.;The AO applications also require that the micromechanical deformable mirrors can be controllably moved distances that are relatively large for MEMS (i.e. 10-20 mum). In this research, we demonstrate a means to utilize strain gradients in poly-SiGe to form mirror-support structures that lift the deformable mirrors away from the substrate by large distances (i.e. 10-50 mum), creating room for large mirror movements.;Using the technologies developed in this research, we demonstrate a 37-segment deformable-mirror array that is fabricated using a micromachining process that can potentially be carried out on top of a CMOS integrated circuit built with selection- and drive-electronics for the mirrors. The thermal budget of the demonstrated process is below the maximum allowed for integration with a CMOS 0.25 mum foundry technology. The deformable-mirror array, which has 37 three-degree-of-freedom segments forming an aperture 3.5 mm in diameter, was designed specifically for use in adaptive-optics applications to vision science. The DM achieves a maximum stroke of 15-17 mum and a maximum tip/tilt angle of 15.7 mrad (0.9 degree) at a maximum actuation voltage of 68 V. The frequency bandwidth of the DM array is approximately 200 Hz. These specifications meet the requirements for vision-science AO applications.