Effects of Boundary Design on the Performance of MEMS Traveling-Wave Ultrasonic Motors

The effects of boundary design on the performance of MEMS traveling-wave ultrasonic motors, including effects on mode matching, quality factor, and dead-zone voltage, are investigated in this work. The stator modes under different boundaries are analyzed by finite element simulation; this reveals that the boundary must have a spatial period of π/2 to ensure mode matching. Anchor loss and squeeze-film damping are then simulated to analyze the quality factor. With the flexible tethers, the two B13 modes are separated by 920 Hz, whereas the rigid boundary with a hole array keeps the two modes consistent in both frequency and mode shape, confirming the above condition. Squeeze-film damping is found to dominate the quality factor, and the measured quality factors of the two designs are 84.6 and 87.3, respectively, indicating that the flexible tethers offer no advantage over the rigid boundary. Residual deformation simulation and experiment show that enlarging the stiffness relief holes reduces the maximum warpage from about 4 μm to about 2 μm, and lowers the dead-zone voltage from 2.0–2.5 V to 1.5–2.0 V. These results provide a design guideline for the boundary of MEMS traveling-wave ultrasonic motors.