MEMS Deformable Mirror Based On PZT Thick Film

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:â‘ The 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;â‘¡PZT 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;â‘¢a 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.