Micromachined piezoelectric ultrasonic imaging transducer

A MEMS ultrasonic transducer, based on thin film PZT membrane has been designed and successfully fabricated. This paper presents the design, fabrication, analysis, and testing results of the membrane devices. The transducers are square membranes, where silicon nitride is the supporting layer, with a piezoelectric capacitor structure of lead zirconium titanate (PZT), deposited on top. The PZT structure enables the actuation and sensing of ultrasonic waves. The thin film PZT was spin-coated on the device using a metal-organic deposition method, resulting in PZT thickness of 0.4 μm. The average remanent polarization (+Pr) of the PZT was measured at 23.5 μC/cm² with an average dielectric constant (K) of 1552. The membranes were individually released by a deep reactive ion etching (DRIE) of the back of the Si substrate, resulting in a structure with minimal crosstalk between the different elements when fabricated in arrays. Modal analysis was performed for the transducers on ANSYS. Simulation and experimental results match perfectly. The acoustic impedance, phase and impulse response for the transducers were calculated using KLM and Reeder-Winslow design, models. The transducers, fabricated in four different sizes ranging from 100 μm×100 μm to 1500 μm×1500 μm, were tested for impedance characteristics and electrical resonance modes were found at 15.6 MHz for the smallest membrane and 8.7 MHz for the largest one. Finally, the transducers were pulsed in a perfluorocarbon high dielectric strength solution, and echoes were received from an air-solution interface. This work shows the potential of thin film PZT for ultrasonic imaging transducers. The novel microfabrication process minimizes the transducer size and the spacing between individual devices while minimizing crosstalk. The use of the PZT allows for maximum sensitivity. A two dimensional array, like the one demonstrated in this work can be used for coronary artery imaging.