Mode-Selective Electrode Configuration for Multifrequency Operation in PMUTs: Microfabrication and Characterization

Multi-frequency operation in ultrasonic transducers balances the trade-off between resolution and penetration depth in ultrasonic imaging. Single top-electrode PMUTs couple strongly to the fundamental mode but weakly to higher-order modes because uniform electrode coverage does not conform to the higher-mode strain profile. Dual concentric ring-and-disk (C–C) electrodes access multiple modes yet reduce fundamental-mode coupling due to misalignment between the segmented coverage and the fundamental strain profile. In this work a circular square (C-Sq) dual top electrode is introduced to strengthen coupling in both the fundamental and higher order modes for multifrequency operation. The configuration pairs a circular outer electrode with a square inner electrode on a circular membrane. The topology aligns with the strain profiles of the fundamental and a selected higher mode enabling mode selective excitation by electrode switching. The proposed C-Sq dual electrode design and reference single electrode and dual electrode concentric ring and disk (C-C) devices were fabricated using the PiezoMUMPs multi-user MEMS process for fair comparison. Electrical impedance analysis, laser doppler vibrometry, and acoustic measurements show a 37% increase in fundamental mode electromechanical coupling relative to C-C dual electrode PMUT and a 27% improvement for the higher mode relative to single electrode PMUTs. Membrane displacement increases by 67% and acoustic pressure at the fundamental resonance is enhanced by 13%. The proposed C-Sq PMUT supports efficient, mode selective multifrequency operation for biomedical imaging and sensing.[2025-0152]