Development of 40-MHz Ultrasonic Transducers via Soft Mold Process

This paper reports on the fabrication of 1-3 piezocomposites for ultrasonic transducers with operating frequencies up to 40 MHz based on recent developments of the soft mold technique. Compared to the established dice-and-fill technique, the soft mold process allows for the manufacturing of 1-3 piezocomposites with higher variability of pillar design and distribution as well as smaller structural size. Consequently, spurious modes generated by the lateral composite layout can be pushed to higher frequencies, which allows for increased operating frequency. Different designs of circular piezoceramic pillars in hexagonal arrangement with decreasing diameters and spacings have been developed and characterized in order to shift spurious modes to frequencies approximately twice the desired working frequency. The influence of the lateral composite layout on resonance modes is investigated by analyzing the produced transducers with respect to their electrical impedance spectra. Experimental results are explained in detail and compared to modeled data. Results show that by downsizing pillar diameter from 30 to 15 μm and pitch from 40 to 19 μm, the first spurious mode could be shifted from ~42 to ~78 MHz. Thereby, the soft mold process proves to be suitable for the fabrication of 40-MHz ultrasonic transducers based on 1-3 piezocomposites.