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 μm). 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 μm), 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 μm 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 μm 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.