A feasibility study of piezoelectric micromachined ultrasonic transducers fabrication using a multi-user MEMS process

We study the feasibility of fabricating piezoelectric micromachined ultrasonic transducers (PMUTs) using a commercially available multi-user MEMS process (MUMPs). MUMPs provide a low cost and easily accessible option for MEMS researchers, but the resulting device performance is usually sacrificed. In this work, we investigate the potential limitations of MUMPs PMUTs using an experimental approach. The square-shaped PMUT structure that we fabricated has a dimension of 500 μm by 500 μm. It consists mainly of a 10 μm silicon layer and a 0.5 μm aluminum nitride piezoelectric layer. Vibrometer measurements revealed that the PMUTs resonate at 600 kHz in air. Acoustic experiments showed a resonance frequency of 1.2 MHz in immersion, which was caused by a higher order vibration mode. Transmit and receive experiments using a hydrophone and an off-the-shelf transducer demonstrated the functionality of PMUTs in immersion, with measured transmit and receive sensitivity at 0.95 kPa/V and 1.35 μV/Pa, respectively, for a 1.8 mm by 1.8 mm element. Limitations of the multi-user process that restrict the operating frequency and output pressure are discussed. More specifically, the maximum fundamental modal frequency is limited to about 2 MHz in immersion by the process design rule, and the maximum output pressure is restricted by a maximum driving voltage of 25Vpp because of potential dielectric breakdown of the piezoelectirc layer. Acoustic experiments also showed ringing that affects the axial resolution of the devices. The ringing limits the usefulness of the devices in pulse-echo applications. We conclude that MUMPs PMUTs are more suitable for applications that rely mainly on the device receive performance.