【作者】 许晓慧；

【导师】 褚家如；

【作者基本信息】 中国科学技术大学 ， 精密仪器及机械， 2008， 博士

【摘要】 自适应光学技术在地基天文望远镜、星载/机载相机、激光武器、激光通讯和医学检测等领域有着广泛的应用前景。变形镜作为自适应光学系统的核心部件,其微型化是自适应光学系统微型化和集成化的关键。使用MEMS技术制造的微变形镜由于其具有体积小、成本低、响应快以及集成度高等传统变形镜不具备的特点,已成为变形镜技术发展的重要方向。其中,连续薄膜式微变形镜通过独立控制各致动器单元的位移来控制镜面形状,镜面变形连续且填充比高,利于实现自适应光学系统的高精度补偿,已成为国内外微变形镜研究的热点。本文以PZT厚膜驱动的单晶硅连续薄膜式微变形镜为研究对象,以提高连续薄膜式压电微变形镜的冲程、降低工作电压、微型化器件为目标,对连续薄膜式压电微变形镜的结构设计方法、压电微变形镜的加工制造技术、变形镜的驱动和性能表征以及变形镜的形变自检测技术等方面进行了系统的研究。在微变形镜的结构设计方面,以压电材料的逆压电效应与镜面弹性负载的相互耦合为基础,提出了两种压电微变形镜结构。根据板壳理论和压电方程建立了两种结构的理论模型,研究了变形镜的结构参数(主要有PZT直径、电极直径、PZT/Si厚度、镜面厚度等)对致动器和镜面冲程的影响,讨论了各参数之间的相互耦合关系;此外,分析了致动器和镜面之间的固定方式以及PZT/Si之间的界面接合层对致动器和镜面形变的影响,最后给出了合理的压电微变形镜的结构参数。在压电微变形镜的加工制造方面,主要涉及三个方面:压电厚膜致动器阵列、镜面以及两者的集成。致动器阵列加工中的一个关键问题是硅基PZT厚膜的制备,本论文以商业化的PZT陶瓷片和SOI基片的接合技术以及PZT陶瓷的湿法刻蚀减薄技术为途径,成功制备出了厚度可控(20～100μm)、结构致密以及性能优异的硅基PZT厚膜。在硅基PZT厚膜的基础上再辅以MEMS加工工艺,制得压电微致动器阵列。镜面基于SOI技术制备,以保证镜面有较好的厚度一致性和较低的残余应力;镜面和致动器阵列之间通过中间层技术进行固定以保证足够的结合强度,并且中间层技术的引入在一定程度上补偿了致动器阵列的不平整度。最后根据设计的工艺方案,制备了10×10阵列的MEMS压电微变形镜样机。在变形镜的驱动控制方面,以Maxim公司集成化的单片多路D/A芯片以及基于MOS管的放大电路,设计并制作了多通道电压源,用于微致动器阵列的驱动。在性能测试方面,用激光多普勒微振动测量仪测试了压电变形镜样机的变形特性,致动器在装配镜面前后的冲程分别为4.5μm和3.8μm,压电位移迟滞分别为9%和13%,采用“归零登山法”后,致动器和镜面的迟滞得到明显改善,分别为3.7%和1%。装配镜面前后的基频谐振频率分别为72KHz和21KHz,镜面影响函数约为30%。用WYKO光学轮廓仪测试了致动器和镜面单元的初始形变和表面粗糙度,镜面的平面度用实验室自制的Michelson干涉仪表征。在变形镜形变自检测方面,通过分割PZT上电极同时实现了PZT膜的驱动和位移传感功能,中心部分为驱动电极,外圈部分为传感电极。当电压加在驱动电极上时,致动器发生形变,从而在外圈的PZT上产生应力,并在传感电极上产生压电电荷,该压电电荷量在一定程度上反映了PZT致动器的形变量,进而定量分析了压电电荷量和致动器(镜面)形变之间的相互关系。最后,制备了集成传感器的压电微致动器阵列,并进行了性能测试。实验结果表明,该传感器的灵敏度约为4pC/nm,和理论值基本相符。用目前的电荷检测电路,可以实现的位移检测分辨率约为50nm。传感器的集成工艺简单,且集成后不影响变形镜的性能。基于以上研究,本论文在以下方面具有创新之处:1)提出了基于PZT厚膜驱动的连续薄膜式MEMS压电微变形镜,并建立变形镜的理论模型。和传统的体压电变形镜相比,具有低工作电压、高动态响应、大冲程、高阵列密度、低成本等优点;2)提出了基于环氧接合以及湿法刻蚀减薄技术的硅基PZT厚膜制备方法,所成膜性能优异,且工艺简单,对设备要求低,能在多种衬底上集成;3)提出了压电微变形镜的形变自检测功能的集成技术,工艺简单,集成度高,有望实现自适应光学系统的微型化。更多还原

【Abstract】 Adaptive optics plays a significant role in the correction of wave aberrations and is applied in various fields such as astronomical observation,laser weapon,laser communication and retina imaging.The aberrated incoming image can be compensated by the adaptive optics,which is mainly composed of the deformable mirror（DM）and a wavefront sensor.Recently,MEMS continuous face-sheet deformable mirror has been deeply investigated,because it offers the advantages of high resonance frequency,low actuation voltage,low cost as well as small volume and weight compared with the traditional deformable mirror and have tremendous potential to realize the adaptive optical system integrated on a single chip.The profile of the continuous face-sheet deformable mirror is controlled by an underlying array of actuator.Pushing one actuator produces a localized deflection of the mirror surface,termed the influence function.The deflection typically extends to adjacent actuators where it changes the mirror surface height by a fraction of the peak deflection.This fraction is termed the coupling coefficient.Then it is possible to realize the high-order Zernike modes compensation using continuous face-sheet deformable mirror.This dissertation mainly focuses on the PZT thick film actuated MEMS continuous face-sheet deformable mirror to achieve large mirror stroke,high device stiffness and good mirror surface flatness.Accordingly,several topics,including the DM structure deigns,the fabrication techniques,the driving and measurement techniques as well as the self-sensing technology of the DM,are investigated in the dissertation.Two designs of the deformable mirror working in d₃₁mode are proposed.These two designs are nearly identical in structure.The only difference between the two designs is that one design employs a full sheet PZT thick film for all the actuators and the other one uses patterned PZT thick film for each actuator.To aid in the DM design, an analytical model based upon the theory of plates and shells is developed to determine the optimal DM membrane dimensions.The main parameters for optimization are the PZT/electrode diameter,the PZT/Si thickness and the mirror thickness.In addition,the influence of the post diameter on the mirror deflection is analyzed using finite element method,and the effect of the bonding layer between the PZT/Si interface and actuator/mirror interface are also discussed.Finally,the DM structure dimensions are selected according to both the optimal results and the feasibility of fabrication techniques.As for the fabrication process of the deformable mirror,it mainly includes three parts:the actuator array,the membrane mirror and the assembly of the actuator array and mirror.The key technique in the actuator array fabrication is the preparation of PZT thick film on silicon.The reported preparation methods of PZT films such as the screen printing and sol-gel methods are usually difficult in meeting thickness and piezoelectricity.Therefore,in this dissertation,a hybrid process combining PZT-Si bonding and wet etching technology is developed to prepare silicon based PZT thick film whose electromechanical properties are comparable to the bulk materials.The mirror was fabricated using SOI wafer so that the mirror may have good thickness uniformity and low residual stress.The assembly of the actuator array and the mirror is realized by using an additional interfacial layer which is also beneficial for the final mirror surface flatness.Finally,a prototype of deformable mirror consisting of a 36-μm-thick single-crystal-silicon membrane which is supported by a 10×10 actuator array was fabricated.The deformable mirror is driven by a home-build multi-channel dc-voltage supplier which consists of several D/A chips（Maxim Co.）and a series of voltage amplifiers.The deflection of the PZT actuator and mirror were measured with a laser Doppler vibrometer（MLD-821,Neoark Co.）.The PZT actuator produces a stroke of 4.5μm at 100V and a piezoelectric displacement hysteresis of about 9%.The resonance frequency of the PZT thick film actuator was measured at about 72 KHz. When a 36-μm-thick silicon membrane mirror was assembled,the mirror has a maximum deflection of 3.8μm with a displacement hysteresis of about 13%and a resonant frequency of 21 KHz.The hysteresis loop was greatly eliminated by using the method of straying on the same segment.Base on this strategy,the displacement hysteresis for the actuator alone and the mirror are reduced to 3.7%and 1%, respectively.The measured influence function is approximately 30%.The influence function refers to the interactuator coupling or cross-talk between adjacent pixels.The initial deformation and surface roughness of the actuator and mirror were measured by a WYKO profile.The mirror surface flatness was characterized by a home-build Michelson interferometer.Bulk PZT thick film actuator integrated with displacement sensor,termed self-sensing actuator,is also investigated.The PZT film is used as not only an actuating layer but also a displacement sensing layer,which is achieved by dividing the metal layer on the top surface of the PZT film into two parts:central top electrode for actuation and outer annular electrode for displacement detection.When the actuator moves,the piezoelectric charge is induced in the outer annular PZT due to piezoelectric effect.The total amount of accumulated charge is proportional to the stress acting on the PZT,which is in turn proportional to the actuator displacement. By collecting the piezoelectric charge,the actuator displacement can be detected.A theoretical model is proposed to determine the structure parameters of the sensor and predict the sensor sensitivity.Experiments were performed on the sensor integrated PZT thick film actuator,and the measurement results show that the integrated piezoelectric sensor has a displacement sensitivity of approximately 4pC/nm which is close to the theoretical prediction.Using the current test circuit,a displacement resolution of about 50nm is achieved.In addition,the integration of displacement sensor into the actuator requires no additional fabrication process and has no influence on the DM performance.The following points are pioneering work of this dissertation:①The MEMS continuous face-sheet deformable mirror based on PZT thick film actuator is proposed for the first time and correspondingly an analytical model is developed for parameter optimization.Compared with the commercial piezoelectric-stack actuators that are widely employed in commercial DM,our DM requires lower operating voltage and less power consumption for producing the same magnitude of mirror deflection.In addition,our DM costs less and can achieve high actuator density and large aperture more easily due to MEMS batch fabrication;②PZT thick films have been deposited on silicon substrate by combining PZT-Si bonding and wet etching technology.The measured electromechanical properties of the PZT thick films are comparable to the corresponding bulk ceramics.This method separates the PZT wafer fabrication from the target substrate and allows integrating the PZT thick films onto many kinds of substrates;③a self-sensing PZT thick film actuator was developed for the MEMS deformable mirror.By collecting the piezoelectric charge induced in the outer annular PZT,the actuator displacement can be detected.The sensor integration requires no additional fabrication process and does not influence the performances of the DM.It may provide the potential for the microminiaturization of the adaptive optics system.更多还原