Flexural-standing-wave elastic force motor using ZnO and PZT thin film on micromachined silicon membranes for wristwatch applications

Ultrasonic motors for low electrical power applications are considered for portable devices such as an electronic wristwatch. For the study of a standing wave type motor, a modular approach was proposed, where stators and rotors are added in an assembly step. The mode conversion principle called the 'elastic fin motor' and its extension to an 'elastic force motor' suitable for a flat profile motor are discussed. Fabrication methods and the mechanical response of silicon diaphragms covered by PZT (PbZrxTi1-xO3) and ZnO (zinc oxide) piezoelectric thin films are compared. A closed loop position control using an integrated piezoelectric stress detection is proposed and demonstrated. The target torque of 1 mu N m at a mechanical power of a few mu W has been achieved. A long-term test during 2 months of continuous operation proved an acceptable reliability for the specified applications.