Mems Micro-deformable Mirror Design And Thin Film Residual Stress Control Study

MEMS-based deformable micromirrors receive ever-increasing attentions for Adaptive Optics (AO). Due to the advantages of simple control, high integration and mass production, surface-micromachined segmented piston-like deformable micromirrors driven by electrostatic force become the focus of research among various types of micromirrors. However, they suffer from the process-induced residual stresses in MEMS thin films, resulting in poor working performances and reliability. As a result, it is necessary to control the residual stresses effectively for the successful manufacturing of MEMS deformable micromirrors.Firstly, to the forementioned deformable micromirrors, a behavioral model was accomplished with system-level method, and the effects of squeeze film damping of the air were discussed in detail. Then, mechanical and optical performances were simulated and analyzed for the designed structures. After that, a two-layer polysilicon surface micromachining process was constituted and the mask layout of the micromirrors was designed. At last, first manufacturing was carried out in our lab's cleanroom and several unreleased deformable micromirrors were obtained. As a result, the designed deformable micromirrors have a pull-in voltage of 6.3V, resonant frequency of 27.898kHz, stroke of 0.67μm, respondence time of about 10μs and stabilization time of about 415μs. They meet the demand for common AO applications.Secondly, a systematic analysis was performed on the origins, measuring techniques and controlling techniques for residual stresses in MEMS thin films with finite element method to investigate how the performances of MEMS deformable micromirrors are influenced and how the residual stresses can be controlled to an acceptable extent. The results show that the existence of tensile residual stress can improve the stiffness of supporting beams, resulting in larger pull-in voltage and resonant frequency than the design. But totally opposite findings are for compressive residual stress. On the other hand, larger deformation of mirror plate will be induced as the residual stresses increasing, making the far-field diffraction performance of deformablemicromirrors degraded.Finally, some suggestions for future research are given on how to improve the fill factor and stroke of the surface-micromachined segmented piston-like deformabie micromirrors. Also, advices are provided for solving the problems of sacrificial layer release and residual stresses control during the manufacturing process of deformabie micromirrors.