1 Copyright 1999 Society of Photo-Optical Instrumentation Engineers This paper was published in the 1999 SPIE Conference Proceedings and is made available as an electronic reprint with permission of SPIE. Single print or electronic copies for personal use only are allowed. Systematic or multiple reproduction, or distribution to multiple locations through an electronic listserver or other electronic means, or duplication of any material in this paper for a fee or for commercial purposes is prohibited. By choosing to view or print this document, you agree to all the provisions of the copyright law protecting it. _____________________________________________________________________________________ Composite ultrasound transducer arrays for operation above 20 MHz Timothy Ritter, K.Kirk Shung, Xuecang Geng, Pat Lopath, Richard Tutwiler, and Thomas Shrout NIH Resource Center for Medical Ultrasonic Transducer Technology Penn State University 205 Hallowell Building University Park, PA 16802 ABSTRACT Methods for fabricating and modeling high frequency 2-2 composites and arrays are presented. The composites are suitable for arrays and small aperture single element devices operating above 20 MHz. Coupling coefficients above 0.65 and lateral mode frequencies near 60 MHz were achieved with this composite. Backing and matching materials were prepared to provide up to 70% bandwidth and coaxial cable was used to impedance match the elements to a 50 ohm source. A TPX lens was fabricated and bonded to the face to provide focusing in the elevation direction. Three prototype 4 element 30 MHz linear arrays were designed and built. The designs were analyzed in a time domain finite element analysis program and excellent agreement between theory and experiment was achieved. Keywords: transducers, piezoelectric composites, ultrasound arrays, high frequency arrays, finite element analysis 1. INTRODUCTION Ultrasonic imaging applications are being driven to higher and higher frequencies. Very high frequency ultrasonic imaging systems, critical for improved diagnostic applications in dermatology and opthalmology, await the development of arrays operating above 20 MHz. These high frequency arrays require small spatial scales (<10 μm) which cannot be achieved using conventional fabrication techniques. Novel fabrication methods and innovative materials are therefore required. Prototype arrays developed in this frequency range include a pair of 20 MHz PZT arrays [1][2], a 100 MHz array incorporating a sapphire lens and thin film ZnO [3], and a piezoelectric polymer array with built-in transmit and receive circuitry [4]. In addition, O’Donnel et al described the operation of a 20 MHz phased array imaging system for catheter use [5][6]. Linear arrays, which typically do not use beam-steering, can tolerate a much larger element pitch than phased arrays. It is therefore practical to focus first on the development of linear arrays with 1λ to 2λ pitch. Although integrating the array with the electronics offers advantages, a more conventional and flexible approach of coupling the array elements to a 50 ohm imaging system has been adopted. Broad bandwidth (minimum of 40%) is desired, both to suppress grating lobes and to improve the axial resolution. Crosstalk levels of near –30dB are considered acceptable for a linear array not incorporating Doppler. Finally, mild elevational focusing is desired for improved resolution in the elevation direction.