Finite element analysis of multi-layer composite hybrid and single crystal medical ultrasound transducers
Increasing transducer bandwidth and signal-to-noise ratio (SNR) is fundamental to improving the quality of medical ultrasound images. In previous work, we have proposed the use of multi-layer 1-3 PZT/polymer composites to increase both, but have encountered significant fabrication challenges. These difficulties include making the bond thickness between the layers small relative to the ultrasound wavelength and aligning the posts of the composite to increase coupling coefficient. We have routinely achieved a bond thickness of less than 5 microns but aligning the posts is more complicated. Finite element (PZFlex) simulations show that the pulse echo SNR and bandwidth degrade significantly with misalignment of the posts. Alignment of greater than 90% of the post pitch (i.e. tolerance of 10-20 μm) is required to obtain significant increases in SNR and bandwidth relative to conventional transducer arrays. This will be a difficult tolerance for large-scale production. Thus, we have developed a multi-layer composite hybrid array that will not require post alignment. Starting from a 2 MHz 3 layer PZT-5H sintered transducer, cuts are made only through the top layer and back-filled with epoxy forming a composite layer on top of 2 ceramic layers referred to as a hybrid transducer. Finite Element (PZFlex) simulations show that for a 2 MHz phased array element with a single matching layer, the 3 layer hybrid structure increases the pulse echo SNR by 16 dB over that from a single layer PZT element and -6 dB pulse echo fractional bandwidth from 58% for the PZT element to 75% for the hybrid element. Analogous FEM simulations of single crystal material (PZN-8% PT), showed increased SNR by only 3.1 dB, but a -6 dB bandwidth of 108%.
