Improving the Vibration Mode of Cantilever Beam-Based PMUT with Post Processing Soft Interconnecting Strategy

To improve the asynchronous vibration mode in the cantilever beam-based piezoelectric micromachined ultrasonic transducer (PMUT), a novel soft spring interconnection design is proposed. In this case, reservoir structures are incorporated to facilitate liquid injection, while the gaps serve as microchannels for post-processing sealing. Driven by capillary forces, PDMS microfluidics autonomously fills the gaps in the proposed cantilever beam-based PMUT. After curing, the filled PDMS forms soft interconnecting springs between the cantilever beams, introducing mechanical coupling that effectively suppresses shear motion and improves the PMUT vibration mode. Due to the low Young's modulus of PDMS, the soft spring minimally affects the original mechanical performance of the PMUT. Moreover, as a post-processing strategy, this treatment neither alters the existing fabrication process nor interferes with the stress release of the device. Laser Doppler vibrometry (LDV) and acoustic pressure measurements demonstrate that the treatment transforms the vibration mode from asynchronous to synchronous. Additionally, the treated PMUT exhibits enhanced emission sensitivity and significantly reduced ringdown time. Thus, the proposed soft interconnecting PMUT is highly suitable for pulse-echo applications requiring high signal-to-noise ratios, minimal blind areas, and high fabrication consistency.