应用于激光微显示中高速扫描的压电MEMS微镜

High Speed Scanning Piezoelectric MEMS Micro Mirrors for Laser Micro Displays

设计并制作了一款直径为1.1 mm、频率为31.11 kHz的MEMS压电微镜,用于需要高速扫描且小尺寸的应用场景。该器件所需模态为扭转模态,与其他模态分离情况好,不会出现耦合。实验结果显示,电压为32 V时光学扫描角40.66°,品质因子1155。改变PZT极性,实验得到了薄膜材料的铁电性质影响。另外,完成了MEMS微镜在0℃~100℃不同温度下角度的变化实验,偏离不超过±1°。仿真模拟、实验结果和理论计算结果三者拟合情况好,表明该设计的微镜具有较高的可控性和稳定性,为实现高精度扫描提供了有力支持。该MEMS压电微镜在AR/VR等领域中具有潜在的应用前景。

In recent years,with the concept of Metaverse getting great attention,under the promotion of the development of Augmented Reality(AR),Virtual Reality(VR)and Mixed Reality(MR)technologies,the Laser Beam Scanning(LBS)including micro-mirrors of MEMS has been widely used in the field of micro-displays.The LBS technology enables high-resolution,high-brightness micro-displays.MEMS mirrors are important optical devices in LBS due to their compact structure,small size,and fast response speed.Of all the methods available for driving mirrors,piezoelectric driving has a high driving efficiency and response speed,and can be driven in microseconds.At the same time,piezoelectric materials are invertible,so they can be driven repeatedly without too much energy.These properties make piezoelectric materials suitable for fabrication of miniaturized display piezoelectric mirrors.Common piezoelectric thin film materials used in MEMS mirrors include AlN,AlScN,and piezoelectric ceramics(PZT).In the fabrication of piezoelectric mirrors,AlN and AlScN are compatible with semiconductor processes with fine linearity,high stability and low thermal drift.However,AlN and AlScN have lower piezoelectric coefficient,smaller driving forces,and require higher voltages,which are not suitable for large angle mirrors.PZT is a commonly used piezoelectric material.Although not compatible with semiconductor processes,its high electrical response,high piezoelectric coefficient,and high stability make it suitable for fabrication of miniaturized display piezoelectric mirrors.To meet the requirements of laser micro display applications,a MEMS piezoelectric mirror with diameter of 1.1 mm,optical angle of 40°and frequency of 31.11 kHz is designed and fabricated,which is suitable for high-speed scanning and small size application scenarios.The large angle piezoelectric MEMS mirror is designed,in which the torsion beam thickness is 40μm,the length is 1.1 mm,the width is 200μm.The results are validated by theoretical calculations and simulation analyses.By using inverse piezoelectric effect,the static angle of PZT multi-layer composite beam with one end fixed and one end free boundary condition under applied voltage is calculated.At the same time,the angle of resonance is calculated by solving the second-order equation of the mass-damped spring.After the calculation,the result is essentially as required.The simulation results show that the required mode of the device is torsion mode,which is well separated from other modes and does not have coupling,and when the target angle is 40°and the frequency is 31.08 kHz,the maximum von Mises stress of the mirror does not exceed the fracture limit of silicon,thus proving the reliability of the design.In the experiment,the electrical properties of 2μm PZT thin film material were first tested.It was found to have an intrinsic piezoelectric coefficient d33 of 192.2 pm/V and a capacitance of 2.06 nF,meeting the requirements of large-angle driving forces.The complete device was successfully fabricated by multistep lithography.The test voltage-angle curve shows that under the excitation of 32 V and 31.11 kHz,the mirror can reach an angle of 40.66°,and the thin film does not break down and the structure is not damaged.Tests on the frequency response curves show a quality factor of 1155.At the same time,resonant frequency drifts are observed at different voltages due to the nonlinearity of the MEMS mechanics.In addition to these tests on device performance under different conditions such as voltage polarity and temperature changes,this study also tested voltage curves applied under different polarities to verify the impact of PZT thin film ferroelectricity on device performance and reminded that in future work,voltage direction should always be consistent(such as using a biased square wave signal)to avoid polarization reversal caused by changes in voltage direction leading to a decrease in angle.Also,we take the curve of device performance as a function of temperature.Within the working range of 0℃~100℃,the angle change does not exceed 1°.In the simulation experiment of the designed micro-mirror,the experimental results are compared with the theoretical calculation results,and it is found that the three fit well,indicating that the designed micro-mirror has a high controllability and stability,and provides strong support for the realization of high-precision scanning.In further research,the present study plans to introduce piezoresistive or piezoelectric angular feedback modules and implement closed-loop control to improve the performance of such devices.This will provide higher accuracy and stability for piezoelectric MEMS mirror applications and useful references for the development of micro display technologies.