MEMS deformable mirrors for adaptive optics

This dissertation describes the development of MEMS (MicroElectroMechanical Systems) deformable mirrors, using thin film surface-micromachining fabrication technology. Such deformable mirrors are used for phase aberration correction in adaptive optics systems. The actuation elements used in these deformable mirrors are electrostatic fixed-fixed actuators exhibiting surface-normal motion. Continuous membrane mirrors, segmented mirrors with piston motion as well as segmented mirrors with tip-tilt motion were investigated. Prototype actuators and mirrors were first fabricated in a three-layer polysilicon foundry process. Detailed characterization of actuators fabricated in this foundry process indicated sufficiently good yield, position repeatability and frequency response. Preliminary examination of mirrors indicated difficulties caused by nonplanar topography, residual stress-induced deformation and stiction. A planarization design strategy was developed and implemented, resulting in a notable improvement in planarity. Geometric and numerical models were developed to predict the topography generated by conformal thin film deposition processes. Experimental studies were conducted to determine the nature and magnitude of the residual stress gradient in polysilicon films. This gradient is important because it causes the thin film structures to bend out of plane, resulting in some initial deformation of the mirrors. A custom fabrication process for high-stroke, low-stress mirrors was designed and fabrication was done at a commercial fabrication facility. Large (100 and 400 element) arrays of continuous and segmented polysilicon mirrors were fabricated in the custom process. These mirrors have a stroke of 2 {dollar}mu{dollar}m, surface planarity of {dollar}lambda{dollar}/2 to {dollar}lambda{dollar}/6, and have demonstrated yield of close of 100%. Print-through and residual stress limit the optical quality of these mirrors. Preliminary experiments demonstrate the feasibility of using these MEMS deformable mirrors for optical wavefront correction. In a first experiment, a nine-element continuous mirror was used to change the focal plane of a collimated laser beam. In a second experiment, real time correction of dynamic wavefront tilt was demonstrated using a single tip-tilt mirror segment with closed-loop feedback control. This paves the way for future experiments incorporating real time adaptive correction of optical wavefronts using the larger arrays of deformable mirrors.