MTL Annual Research Report 2012

Design and Modeling of a PZT Thin-film-based Piezoelectric Micromachined Ultrasonic Transducer

Although new software techniques enable higher-resolution medical ultrasound imaging, commercial ultrasonic transducer technology has remained largely unchanged for a few decades.  Current transducers are fabricated from bulk PZT using assembly steps that are labor-intensive and limit individual transducers to millimeter-sized features.  With micro-fabrication technology, micro-scale transducers can be easily manufactured at very low cost, but their acoustic power and efficiency may be compromised.  We revisit a piezoelectric micro-machined ultrasonic transducer (PMUT) based on a lead zirconate titanate (PZT) thin film with a view to improve acoustic performance.  Our initial findings show that the inherently high piezoelectric coupling of thin-film PZT produces the deflection necessary for high acoustic pressure applications without significant power requirements or application of a DC bias voltage if the design can be optimized. With its high acoustic pressure output and small size, a PMUT could be used for deep penetration and non-invasive medical imaging, e.g., intracranial monitoring of head injuries.

Our group has derived the equivalent circuit for a bimorph PMUT ^[1] .  This configuration sandwiches a PZT between top and bottom electrodes and actuates it with an applied voltage across the electrodes.  Adding a structural support layer, such as silicon, creating a multimorph device increases the model’s complexity. With separate definition of mechanical and electrical neutral axes, the equivalent circuit derivation extends to include the multimorph design ^[2] .  With this advance, transduction behavior of the PMUT can be more accurately predicted, designs more easily optimized, and results validated with a complete model.  An analytical solution for deflection based on electrode coverage has been derived and the optimum electrode coverage for maximum deflection has been determined.  Based on the modeling results, fabrication of an optimized PMUT design is now underway. Our eventual goal is to incorporate PMUT elements into 1D and 2D arrays with a small form factor to enable high resolution medical imaging.

Figure 1: Cross-sectional view of PMUT element. Design is currently being fabricated.

1. F. Sammoura and S.-G. Kim, “Theoretical modeling and equivalent electric circuit of a bimorph piezoelectric micromachined ultrasonic transducer,” IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, to be published. [↩]
2. F. Sammoura and S.-G. Kim, “Modeling of the neutral axes of a circular piezoelectric micromachined transducer in transmit and receive mode,” Tech. Dig. of Solid-State Sensors and Actuators Workshop, to be published. [↩]