A Spherically-Shaped PZT Thin Film Ultrasonic Transducer with an Acoustic Impedance Gradient Matching Layer Based on a Micromachined Periodically Structured Flexible Substrate

Guo-Hua Feng,Wei-Fan Liu

doi:10.3390/s131013543

Abstract

This paper presents the microfabrication of an acoustic impedance gradient matching layer on a spherically-shaped piezoelectric ultrasonic transducer. The acoustic matching layer can be designed to achieve higher acoustic energy transmission and operating bandwidth. Also included in this paper are a theoretical analysis of the device design and a micromachining technique to produce the novel transducer. Based on a design of a lead titanium zirconium (PZT) micropillar array, the constructed gradient acoustic matching layer has much better acoustic transmission efficiency within a 20–50 MHz operation range compared to a matching layer with a conventional quarter-wavelength thickness Parylene deposition. To construct the transducer, periodic microcavities are built on a flexible copper sheet, and then the sheet forms a designed curvature with a ball shaping. After PZT slurry deposition, the constructed PZT micropillar array is released onto a curved thin PZT layer. Following Parylene conformal coating on the processed PZT micropillars, the PZT micropillars and the surrounding Parylene comprise a matching layer with gradient acoustic impedance. By using the proposed technique, the fabricated transducer achieves a center frequency of 26 MHz and a −6 dB bandwidth of approximately 65%.

Highlights

High-frequency (>20 MHz) focused ultrasonic transducers appear in many applications, including high-resolution medical imaging, therapeutic treatment, and nondestructive material testing [1,2,3,4]
Most transducers created by using the piezoelectric micromachined ultrasonic transducer or capacitive micromachined ultrasound transducer schemes are based on a planar substrate, which typically limits their geometric designs
A conventional acoustic impedance matching layer can be made with the following materials: (a) suspended particles surrounded by a polymer matrix prepared separately and bonded to the piezoelectric ceramic layer; (b) a composite of solid and polymer filler materials with specific acoustic impedance

Summary

Introduction

High-frequency (>20 MHz) focused ultrasonic transducers appear in many applications, including high-resolution medical imaging, therapeutic treatment, and nondestructive material testing [1,2,3,4]. Sensitive broadband ultrasonic transducers require efficient transfer of acoustic energy between the active piezoelectric material and the propagating medium. A conventional acoustic impedance matching layer can be made with the following materials: (a) suspended particles surrounded by a polymer matrix prepared separately and bonded to the piezoelectric ceramic layer; (b) a composite of solid and polymer filler materials with specific acoustic impedance. These composites are made separately and must bond to the active piezoelectric ceramic layer; (c) a piezocomposite (e.g., 1–3 composites) made of a piezoelectric ceramic and a passive conformal polymer. The depth of the grooves determines the thickness of the integrated impedance matching layer, and a conformal filler material is used to fill the grooves [5,6,7,8,9]

Methods

Results

Conclusion