This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. JOURNAL OF MICROELECTROMECHANICAL SYSTEMS 1 A Novel PZT-Based Traveling-Wave Micromotor With High Performance and Unconstrained Coaxial Rotation Feng Qin, Gang Dai , Xiangyu Sun , Qinyin Xu, Yijia Du , and Jingfu Bao Abstract—Traveling-wave ultrasonic micromotor is a promis- ing technology for microactuation and nanopositioning. A special designed annular stator was proposed with the combination of two separate buffer structures to achieve high performance. Traveling-wave propagation with moving antinodes was observed and analyzed. The fabricated stator exhibits large displacement of 4.71 μm at 1 V and high quality factor of 220 with the central frequency of 101.6 kHz. The assembled rotor was actuated, and controllable rotate speed was achieved. The measured maximum torque reached to 2.63 μNm at 1.3 V and 101 kHz. Besides, unconstraint rotation with no lateral slip was found during the entire rotation process, indicating stable, uniform, and highly reliable actuation of the micromotor. [2018-0026] Index Terms—Traveling wave ultrasonic micromotor, large out-of-plane displacement, high quality factor, unconstraint rotation. I. I NTRODUCTION O WING to the great development in Microelectromechan- ical systems (MEMS) technology, fast progress has been achieved in miniaturization of integrated functional devices in recent years. Applications such as micro-robotic engi- neering and auto calibration and focusing have become hot spots. As a consequence, micro-control, nano-positioning and self-calibration technologies have gained increasing attention. Therefore, micro actuators, especially traveling wave ultra- sonic micromotor (TWUM), with high rotate speed, high output torque and high operational resolution have become more important. Comparing with other micromotors such as electrostatic micromotors and electromagnetic micromotors, piezoelectric TWUM micromotor is one of the most promising micro actuators which has lower excitation voltage, higher output Manuscript received February 8, 2018; accepted May 6, 2018. Subject Editor R. Ghodssi. (Corresponding author: Yijia Du.) F. Qin is with the School of Electronic Engineering, University of Electronic Science and Technology of China, Chengdu 611731, China, and also with the Microsystem and Terahertz Research Center, China Academy of Engineering Physics, Chengdu 610299, China. G. Dai, X. Sun, and Y. Du are with the Microsystem and Terahertz Research Center, China Academy of Engineering Physics, Chengdu 610299, China (e-mail: duyijia@mtrc.ac.cn). Q. Xu is with the Nanjing Engineering Research Center of Micro-Nano Technology, Nanjing 210032, China. J. Bao is with the School of Electronic Engineering, University of Electronic Science and Technology of China, Chengdu 611731, China. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JMEMS.2018.2834541 torque, energy density and control precision [1], [2]. This can be ascribed to the high dielectric constant and excellent electromechanical conversion ability of piezoelectric materials. Moreover, piezoelectric TWUM is highly scalable due to its fully integrated MEMS fabrication process, which broadens its applications in compact functional microsystems. Taking advantage of inverse piezoelectric effect of piezo- electric materials, TWUM creates two standing wave modes on the layered membrane, known as the stator. The two standing wave modes are orthometric with typically the same frequency, same shape but a quarter wavelength difference on phase. After superimposition of the two standing waves, a traveling wave has been created on the stator, where the surface follows elliptical motion trail. When the piezoelectric TWUM works as an actuator, a rotatable component called rotor must be applied on the stator, with a vertical preload. As a result, the elliptically moved stator surface provides a driving force to the rotor through frictional force between the interfaces. Moreover, to a certain motor-rotor system, the driving force and rotate speed can be easily adjusted by changing the vertical preload [3]. Traditionally, bulk piezoelectric materials are often employed in ultrasonic micromotors by simply sticking the materials on the surface of the stator. However, problems such as large volume, complicate assembling technique and incompatibility with standard MEMS process are inevitable [4]–[6]. Recently, newly developed micromotors based on piezoelectric thin film have been widely studied [7]–[9]. Integrated MEMS processes combining film deposition and multi-layer patterning are applied to fabricate this kind of micromotor. Piezoelectric thin films are usually deposited by sputtering. Oriented growth of the film with spontaneous polarization leads to better piezoelectric performance. In the meantime, a more stable piezoelectric/substrate interface is formed resulting in a better reliability of the devices [10], [11]. Moreover, the thin film based micromotor is feasible for different structural designs of the stator owing to the integrated film deposition and patterning techniques. For this reason, small- scale piezoelectric micromotor with multi-degrees-of-freedom actuation can be easily obtained, which enables the integration and miniaturization of micro-actuators for future microsystem research [12]–[14]. However, thin film piezoelectric micromotors are fac- ing a problem of low output torque, which is considered 1057-7157 © 2018 IEEE. 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