Ultra-fine piezoelectric composites for high frequency ultrasonic transducers

This paper presents a new technique called interdigital phase bonding (IPhB) that has been developed to make 2-2 PZT composites of ultra-fine pitch for use as transducers and arrays operating at frequencies >30 MHz. By using the IPhB method, a composite structure with a pitch that is less than the dicing saw blade thickness can be created. The flexibility of the approach is demonstrated by presenting composites of two different dimensions. Samples have been fabricated using a conventional dicing saw (Disco DAD-2H/6TM) and a 50 μm thick blade. The most ultra-fine composite consists of 15 μm wide PZT bars (CTS 3203HD) embedded in an epoxy matrix (EPOTEK 301) with a pitch of 25 μm resulting in a volume ratio of 61 percent. Such a sample is suitable for the fabrication of ultrasonic transducers and arrays with central frequencies of up to 85 MHz., however single element transducers working at central frequencies of 50-60 MHz are shown. Electrical impedance and acoustic pulse echo measurements have been taken to characterize the transducer performance. Results show that the longitudinal electromechanical coupling coefficient is greater than 0.6, and that there are no noticeable lateral resonances in the frequency range of 55-150 MHz. This agrees with theoretical predictions based on velocity dispersion curves calculated from the general 2-dimensional Lamb modes.