Miniaturized Thin-Film Piezoelectric Traveling Wave Ultrasonic Motor
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For many small-scale systems, compact rotary actuators are highly attractive. Many current millimeter-scale motor technologies, such as electrostatic motors and electromagnetic motors, operate at high speeds (on the order of 105 RPM) but low torque, usually pico- or nano-newton-meters. In order to drive large loads at speeds closer to 100 to 1000 RPM, gearing would be required, which drastically increases system complexity and size. Electromagnetic motors, which are effective at the macro-scale, become less practical at the millimeter-scale due to unfavorable scaling of energy density and complex fabrication. Electrostatic micro-motors require approximately 100 V for operation and produce limited torque. Traveling wave ultrasonic motors (TWUM) can provide micro- to milli-newton-meters of torque at low speeds and fill a necessary place within the millimeter-scale rotary motor landscape. Using recent developments in high quality piezoelectric film deposition and microfabrication techniques, TWUM can be made an order of magnitude smaller than currently possible.
The fabrication process for the TWUM is described within, with a focus on stator fabrication and the enabling fabrication methods developed for the manufacture of TWUM, including backside vapor-HF release, deep reactive ion etch footing release, and photoresist deep-trench refill. Design and characterization of the traveling wave stator component, both disc and ring are described. Disc stators, 1 to 3 mm in diameter, exhibited traveling waves up to 1 μm in out-of-plane amplitude with quality factors in air of 95. The design process for ring stators with mechanical impedance transformer tethers is presented. The tethers are designed to allow large motion at the stator perimeter, while tethering the stator to the anchored substrate. This mechanical impedance transformer tether allowed for an in increase in standing wave amplitude by over 100% compared to straight tethers. TWUM were demonstrated and characterized, and represent the smallest TWUM currently reported, at 2 to 3 mm in diameter and less than 1 mm thick. Motor performance characteristics are presented, with speeds exceeding 2000 RPM while consuming 4 mW of power at 10 V. These millimeter-scale motors have potential applications in fields such as fuzing, medical imaging, micro-robotics, and sensor steering and calibration.