1 of 6 A 2.5 MHz 2D Array with Z-Axis Electrically Conductive Backing M. Greenstein, P. Lum, H. Yoshida, M.S. Seyed-Bolorforosh* Hewlett-Packard Laboratories, P.O. Box 10350, Palo Alto, CA 94303 * Currently at General Electric Medical Systems, P.O. Box 414, Milwaukee, WI 53201 Abstract The design, fabrication and initial testing of a pro- totype fully l/2 sampled, 2500 element 2D phased array is presented. The array utilizes a unique Z-axis electrical conductivity backing layer, to provide both acoustic attenuation and electrical interconnect for the signal channels. The electrical interconnect is designed to be in the acoustic shadow of the trans- ducer elements so as to minimize the foot print of the array. A modular, demountable Pad Grid Array inter connect is used to connect to the backing of the array. Results are presented for measurements of the single element properties of electrical impedance, pulse echo waveform and spectrum, directivity and cross talk.? I. INTRODUCTION There has been much discussion in the recent liter- ature about the need and desirability of 2D acoustic phased arrays for medical imaging. Most of the work discussed has been simulations of the arrays because of the difficulty in actually building such arrays. The theoretical aspects of 2D arrays have been discussed [1], [2]. There has also been discussion about the use of 2D arrays to enable 3D ultrasound imaging [3], and phase aberration correction [4]. Extensive experimen- tal work has been done on coarsely sampled 2D arrays [5], [6], as well as multilayer approaches to 2D arrays [7]. Additional theoretical work has been done on the design and the performance of sparse 2D arrays [8] in an attempt to reduce the number of elements, and thus the required number signal channels. There has not been extensive discussion on the fab- rication of fully sampled, λ/2 pitch 2D phased arrays. Such arrays are difficult to build and the number of potential signal channels exceeds the capability of most present day commercial medical ultrasound machines. The interconnection of such densely sam- pled arrays is also a substantial challenge. This paper discusses the design, fabrication and initial single ele- ment characterization of a fully λ/2 sampled 2D phased array, and the demountable interconnect sys- tem used with the array. II. ARRAY DESIGN AND FABRICATION A. Design The present 2D array was designed with the follow- ing objectives. The design center frequency was chosen to be 2.5 MHz, motivated by the penetration depth for cardiac applications, the obtainable spatial resolution, and the relative ease of fabrication. An active aperture of 15 mm was chosen to match the typical acoustic window between the ribs for transthoracic ultrasound measurements. The array was sampled at λ/2 in order to provide full capability for steering and focussing, resulting in a 50 x 50 array with 2500 active elements. A Z-axis electrically conductive backing layer was developed in order to allow for the electrical intercon- nect to come through the backing layer, in the acoustic shadow of the elements, and mate to a modular and demountable interconnect system. This approach allows the array to be tested as a separate module before committing it to the interconnect. The array was designed with a PZT piezoelectric layer with inactive oversized guard elements sur- rounding the active elements. These inactive guard elements, at the periphery of the array, protect the interior active elements from physical damage. The transducer elements were diced to provide acoustic isolation and then back filled with an epoxy and glass microsphere mixture to increase the mechanical robustness of the individual elements. In Figure 1 a partial top view of the array is shown, drawn to scale. At the upper left corner is a large square inactive guard element, shown hatched. The top row and the left column are also peripheral inactive guard ele- ments. The active elements are shown with a small black square in the middle of each element. This black square represents the cross sectional end of the Z-axis electrical conductor in the backing. The active ele- ments are 250 x 250 μm in cross section. For the piezoelectric layer, PZT-5H [9] was chosen for its acoustic properties and for its machinability. The matching layer was fabricated from epoxy impreg- nated graphite [10]. The backing layer was chosen for its acoustic properties and for its Z-axis electrical con- ductivity. The acoustic properties of the backing layer were determined by an electrically insulating epoxy matrix with Tungsten powder to increase the attenua- Internal Accession Date Only