Fabrication of high-frequency single-element ultrasonic transducers using lithium niobate

The design and fabrication of high frequency single element ultrasonic transducers present a multitude of challenges for the transducer engineer, from size constraints to electrical impedance matching. This paper discusses the trade-offs involved in a procedure used to fabricate transducers with center frequencies in the 25 MHz - 100 MHz range using 36 degree rotated Y-cut lithium niobate (LiNbO3) as the active element. Transducers of varying dimensions were built according to an f-number range of 2 - 3.5. A (lambda) /4 silver epoxy matching layer with an acoustic impedance of 7.3 Mrayls was used. Desired focal depths were achieved with use of an acoustic lens. Silver epoxy backing with an acoustic impedance of approximately 5.9 Mrayls was also used in all designs. All transducers were designed around a 50(Omega) send and receive circuit. Electrical tuning of the transducer to the receive circuitry was achieved by using an RF transformer. All transducers were tested in a pulse-echo arrangement using a Panametrics 5900PR pulser, a Wavetek function generator and a LeCroy digital oscilloscope. The bandwidth, insertion loss, and depth of focus were measured. Several transducers were fabricated with -6dB bandwidths ranging from 62% to 74%, and two-way insertion loss values ranging from -14dB to -22dB.