<title>Single-element and linear-array transducer design for ultrasound biomicroscopy</title>

Abstract

Ceramic ultrasonic transducers with a frequency response <50MHz using lead zirconate titanate (PZT) layers in the 5-4Otm range have been difficult to achieve by bulk or thin film techniques. Advances in the production of small sized PZT ceramic powder allow the development ofthinner ceramic layers for this application. Sol gel composite thin film technology also provides a new technique for producing ceramic coatings of a thickness that successfully bridges the gap between traditional thin film and bulk techniques. So! gel composite PZT layers of 5-7Om have been coated on substrates (aluminum, platinized silicon, stainless steel, .. .) that can withstand the thermal processing of the ceramic. The thickness mode response of a thin piezoelectric layer supported by a thick substrate has been modeled from first principles using complex material constants. The LevenbergMarquart non linear regression technique has been used to extract the thickness mode elastic stiffness, dielectric constant and piezoelectric constant of the PZT, and the elastic stiffness of the substrate from the layered structure. This non-destructive technique allows for a reliable assessment of the quality of a coating prior to the fabrication of a transducer. The elastic stiffness of the substrate is not lossy enough for the required broadband response of an imaging transducer. However, aluminum can be preferentially etched, releasing the ceramic coating. Therefore it is possible to transfer the PZT film to a more suitable backing material. A processing sequence for single element PZT transducers in the frequency range of 50-200MHz has been developed. Characterization of transducers has been performed using pulse-echo techniques and by creating real time B-scan images of agar phantoms and biological tissue. Methods for patterning the PZT composite coatings are being developed with the intent to fabricate a linear array in the 40-60 MHz frequency range. Due to the very fine patterning and high concentration of cuts required for a high aspect ratio linear array, the limits of conventional etching techniques are surpassed. Laser micromachining using a frequency doubled Nd:YAG and a KrF excimer laser have the ability to pattern the array structure. In both cases, laser cuts <1Om wide have been achieved.