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Towards fabrication of high frequency ultrasonic transducers using soft mold process

: Gebhardt, Sylvia; Günther, Paul; Neubert, Holger; Fröhlich, Selina; Walter, Susan; Herzog, Thomas; Heuer, Henning


Institute of Electrical and Electronics Engineers -IEEE-:
IEEE International Ultrasonics Symposium, IUS 2016 : September 18-21, 2016, Tours, France
Piscataway, NJ: IEEE, 2016
ISBN: 978-1-4673-9897-8
ISBN: 978-1-4673-9898-5
Art. 7728555, 4 S.
International Ultrasonics Symposium (IUS) <2016, Tours>
Fraunhofer IKTS ()
ultrasonic transducer; high frequency; Soft-Mold-Technologie; micromolding; PZT

For high resolution imaging high frequency ultrasonic transducers are needed which can be operated well above 15 MHz. Therefore, fine scale 1-3 piezocomposites are required. Due to the brittleness of the ceramic, the limited blade width and the increasing effort convention aldice-and-fill technique becomes increasingly insufficient as pillar size and kerf become smaller. Micromolding techniques turned out to be suitable for the fabrication of fine scale piezoceramic arrays with free design of ceramic pillars and randomized spatial distribution. The soft mold process offers the opportunity to fabricate fine scale 1-3 piezocomposites with free design of the piezoceramic part. The basis of this approach are master molds, which are structured by microsystems technologies like LIGA, SU8 and deep reactive ion etching (DRIE). That allows a high variety of rod size, shape, spacing, and arrangement. The combination of silicon industry based microstructuring and ceramic molding is possible through soft plastic templates taken from the master mold, which are reusable. A ceramic slurry based on lead zirconate titanate (PZT) is filled into the soft molds under vacuum and there in dried. After demolding array structures are debindered, sintered and filled with an epoxy polymer. Ceramic body and excessive epoxy are removed by grinding and the resulting 1-3 piezocomposite lapped to the desired thickness, electroded and poled. In this contribution, we report on the development of a 20 MHz ultrasonic transducer. Different designs of piezoceramic rods and spacings have been tested and characterized to shift spurious modes to frequencies approximately twiceas working frequency. Ceramic pillars based on triangle and half hexagon cross section with 32 μm edge length, 160 μm height and 13 μm spacing tend to topple during sintering. Best results have been obtained using round pillars in hexagonal arrangement with sintered dimensions of 34 μm diameter, 160 μm height and 10 μm spacing. 1-3 piezocomposites made of these structures show coupling coefficients kt = 0.6 at the thickness resonance of 20 MHz. First lateral resonance is measured at 37 MHz far enough away from thickness mode. A single element ultrasonic probe has been manufactured by selection of appropriate backing and matching layers. The composition of the matching layers were adjusted according to match acoustical impedance of water and grinded down in order to meet quarter wavelength criteria. For characterization of sensitivity and bandwidth pulse-echo curves have been measured. Experimental results will be presented in detail and will be compared to modelled data.