Design of focused single element (50-100 MHz) transducers using lithium niobate

This paper discusses two fabrication procedures used to build LiNbO₃ single element ultrasonic transducers with center frequencies in the 50-100 MHz range. Transducers of varying dimensions were built according to an f-number range of 2.5-3.0. A quarter wavelength silver epoxy matching layer (Z_a=7.3 MRayls), and a silver epoxy backing (Z_a=5.9 MRayls), were used in all designs. The desired focal depths were achieved by either casting an acoustic lens on the transducer face or press-focusing the piezoelectric into a spherical curvature. The lens material EPO-TEK 301 (Z_a=3.1 MRayls) was modeled as a second matching layer. Parylene (Z_a=2.6 MRayls) was used as the second matching layer in the press-focused transducer design. For designs that required electrical impedance matching, a low impedance transmission line coaxial cable was used. All transducers were tested in a pulse-echo insertion loss arrangement, whereby the center frequency, bandwidth, insertion loss, and focal depth were measured. Numerous transducers were fabricated with center frequencies in the 50 to 100 MHz range. The measured -6 dB bandwidths and two-way insertion loss values ranged from 50% to 70% and 12.5 dB to 23.0 dB, respectively. Both fabrication procedures proved successful in producing very sensitive, high frequency single element ultrasonic imaging transducers. The press-focused devices displayed significantly lower insertion loss levels than lensed devices. The Parylene matching layer provided a better acoustic match to the load medium water (Z_a=1.5 MRayls) and a reduced attenuation compared to the lens, which may account for the discrepancy insertion in loss levels. Finally, transmission line tuning demonstrated a 3 dB reduction in insertion loss and a 6% increase in device bandwidth