Research On Key Technologies Of Surface-Micromachined Electrostatic Actuated MEMS Deformable Mirrors

Deformable mirrors(DMs) are key components of adaptive optics(AO) system. The performance of DMs determines the performance of AO system, as a result, study of DMs is one of the important research fields in AO research fields. Recently, Studies of Electrostatically actuated microelectromechanical systems(MEMS) DMs have became more and more popular for their advantages of small volume, low power dissipation, high actuator density, fast response speed and good compatibility with integrated circuit, etc.A series of key issues of electrostatically actuated MEMS DMs based on surface micromachining fabrication process are intensively studied in this dissertation. The main research works and conclusions are as following:1. Study of electrostatic actuators with large stroke. A) Due to the small stroke and low reliability of traditional square shaped clamped-beam actuator, a hexagonal shaped clamped-beam actuator is developed. Theoretical calculations and finite element analysis(FEA) simulation is applied on the developed actuator. Then, it is designed, fabricated and measured and the results verify the analysis conclusion. B) An improved method to calculate the electrostatic force of any type of actuator is proposed. By using this improved method, the electrostatic force of three-layer repulsive actuator, two-layer repulsive actutor and vertical comb drive attractive actuator are calculated. The close results with FEA simulaiton show that the proposed improved method is feasiable and accurate. C) A pseudo-three-layer electrostatic repulsive actuator is proposed that combines the advantages of two-layer and three-layer repulsive actuators. The results show that the former actuator offers higher performance than the latter two in terms of driving force and stroke. The pseudo-three-layer actuator structure is compatible with two-layer and three-layer actuators. D) The two-layer repulsive actuator of a previously-publised 19-element segmented DM is connected in the pseudo-three-layer mode and a large stroke of 4.5μm is obtained.2. Study of the impact of release holes on the DM. A) The dynamic characteristic of the actuator is modeled and the impact of release holes on the dynamic characteristic is analyzed. Some corresponding structures are designed and fabricated to verify the theoretical results. B) The impact of release holes on the bending stiffness of continuous mirror facesheet is analyzed. A 45-element continuous DM is designed and fabricated and its performances are summarised by theoretical analysis and experimental measurement. Finally, a correction of static aberration is demonstrated and the defects of DM are summarised.3. A 140-element continous DM with good practical application potential in AO systems is developed and some key techniques of it are intensively studied. A clamped-beam actuator and a design-based approach to planarization are applied in this DM. As to the problem of residual stress-induced surface buckling, a physical model of the mirror including a variety of influenced factors is estabilished, the effect of stress compensating film thickness on the characteristics of mirrors is analyzed, and a layer of 40 nm Gr stress compensating film and Au reflection film is fabricated by shadow mask deposition process. Finally, the fabricated MEMS DM is packaged and tested.4. Analysis, optimization and improvement of some other problems during the study and development of MEMS DMs. A) A novel segmented tip-tilt piston DM is proposed. For this type of DM, there are only one actuator for each mirror element. Compared with traditional tip-tilt piston DM, the number of actuators of the developed DM reduced two thirds. Experimental results show that the novel DM is almost able to achieve continuous deformation. B) The field electron emission of electrostatic attractive actuators and repulsive actuators is researched and the corresponding key parameters are achieved by experimental measurement, which can accelerate the reliability of MEMS devices. C) In aspect of fabrication process and optical design, two methods for improving the fill factor of segmented DM are presented. Theoretical analysis and Flip-Chip(FC) bonding are researched. This study will improve the practicality of segmented DM.In conclusion, a series of key technical problems of MEMS DMs such as the increasement of stroke and the fill factor, the improvement of surface quality and reliablity, as well as the decreasement of residual stress are intensively analyzed in theory, simulation and experiment. A 140-element MEMS DM prototype with continuous mirror is firstly produced in our country. Almost all parameters of the MEMS DM meet the requirments of AO systems.The results of this dissertation would be helpful for the design and fabrication of MEMS DMs with high performance and good practical potentioal. The devloped MEMS DM prototype in this paper is expected to be applied in small AO systems, and may stimulate the development of modern AO technology.