Fabrication of single crystal based phased array medical imaging ultrasonic transducers

Abstract

Ultrasound imaging is one of the most used methods for medical diagnostics. Conventional PZT transducers usually can only give a -6dB bandwidth of about 60 -70%, which is not wide enough for second harmonic imaging. Relaxor-based ferroelectric single crystal, Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT), is receiving great deal of attention for application in medical imaging ultrasonic transducers because of its high piezoelectric coefficient and electromechanical coupling factor. It has been demonstrated that the PMN-PT single crystal phased array transducers have a wider bandwidth (>100% at -6dB), higher sensitivity and better resolution than conventional transducers made of PZT. In this thesis, development of PMN-PT single crystal phased array transducer with wide bandwidth and high sensitivity is carried out. PMN-28%PT single crystal oriented along <001> direction is selected to fabricate the phased-array transducer. Array elements are spaced at 0.26 mm with a center frequency around 3 MHz. A KLM model based PiezoCAD simulation was used to help the acoustic design of the transducer, where the effect of the matching layers with different thicknesses and acoustic impedances, the effect of backing materials with different acoustic impedances, and also the effect of electromechanical coupling factor of driving material are studied. Based on the simulation results, two PMN-PT phased array transducers, with A/4 and A/8 matching layers, were fabricated. Both transducers show 90% above bandwidth, which is a big increase compared to the conventional PZT phased array transducers (about 70%). The substitution of the PZT ceramic with PMN-PT single crystal with higher electromechanical coupling factor, as the active element in ultrasound transducer, improves the transducer performance. Compared to the well-accepted double quarter wavelength matching system, the novel double A/8 matching scheme also shows the trend to further improve the bandwidth of the transducer from my research. Because of the relatively lower sound velocity of PMN-PT single crystal compared to PZT, the aspect ratio becomes a problem when using PMN-PT to substitute PZT. The 1 -3 composite may solve this problem by bringing much high aspect ratio and better coupling factor. In order to further increase the bandwidth, some unique designed transducer structure and process are also tackled in my work. A new transducer with 1-3 composite PMN-PT active elements will be developed in the future study.