Researches On High Frequency (?100MHz) Ultrasonic Transducers:Fabrication And Application

High frequency (?100MHz) ultrasound has been widely employed in various aspects including medical imaging and single-beam acoustic tweezer. When comparing to conventional ultrasound, hight frequency ultrasound provides a better spatial resolution and is capable in trapping and manipulating smaller microparticles. However, many challenges remains in the fabrication of high frequence ultrasonic transducers with good performances, such as the preparation of ultra-thin piezoelectric element; the issues of impedance matching in both electrical and acoustic aspsects; and the focusing of the aperture with low F-number. Based on the understanding of piezoelectric materials and the fabrication process of ultrasonic transducers, in this thesis, we presented the fabrication and application of high frequency ultrasonic transducers. The main contents are as follow:1. Lithium niobate (LiNbO3) single crystal high frequency (?100MHz) ultrasonic transducer36 ° Y cut lithium niobate single crystal carries excellent piezoelectric properties, low dielectric constant and high Curie temperature which makes it suitable for fabrication of high frequency ultrasonic transducer. However, two major technical challenges remains in fabricating the LiNbO3 high frequency ultrasonic transducer:1) reducing the thickness of LiNbO3 element, and 2) tightly focusing the transducer. In our work, LiNbO3 single crystal piezoelectric films with average thickness of 30?m, 13 ?m and 9 ?m were obtained through conventional lapping method. Based on these piezoelectric films,100MHz single element ultrasonic transducers and 200MHz/ 300MHz needle type single element ultrasonic transducers were fabricated and tested. The results show that such transducers exhibit high sensitivity and wide -6dB bandwidth (> 40%), thus suitable for applications in high-resolution imaging and single-beam acoustic tweezers research.2. High frequency focused ultrasonic transducers for high resolution ultrasonic imaging and non-contact manipulation of microparticlesBased on the lithium niobate high-frequency ultrasonic transducers, researches on high frequency ultrasound imaging and single-beam acoustic tweezers (SBAT) were presented in detail. Spatial resolution test shows that such LiNbO3 ultrasonic transducers exhibit excellent axial and lateral resolution. For transducers with center frequency higher than 200MHz, the measured spatial resolution is in the order of a few microns. Imaging experiments of fish eyes and polystyrene beads were carried out. Due to the difference in acoustic impedance and the high spacial resolution of ultrasonic transducers, the microstructure of object could be identified clearly. The high frequency ultrasonic transducers could be utilized in the imaging of biological tissues or structures which requires high resolution but low penetration depth such as blood vessels and skin.Due to the excellent performance and highly focused ultrasound ultrasonic beam, the lithium niobate ultrasonic transducers are suitable for SBAT applications. The trapping experiments demonstrated the capability to trap and manipulate microspheres with comparable dimension to a single cell. Meanwhile, the relationship between the excitation frequency and the particle was investigated. The experiment results demonstrated that the ratio of wave length and particle size is an important determinant of acoustic trapping. Base on this feature, SBAT could be developed as a tool in selectively manipulating microparticles or cells at certain size by carefully designing the acoustic beam shape and acoustic wave length of SBAT. 3. Metal-polymer acoustic impedance matching layers design for high frequency ultrasonic transducers.Matching the acoustic impedance of high-frequency ultrasound transducers to an aqueous loading medium remains a challenging for fabricating high-frequency transducers. The traditional matching layer design has been problematic to establish high matching performance given requirements on both specific acoustic impedance and precise thickness. Based on the models from both mass-spring method and microwave matching network analysis, we employed metal-polymer layers for the matching effects. Both methods hold promises for guiding the metal-polymer matching layer design. We fabricated a 100 MHz LiNbO3 transducer to validate the performance of the both matching layer designs. In the pulse-echo experiment, the transducer with the matching layer increased the echo amplitude by 84.4% and the -6 dB bandwidth from 30.2% to 58.3% compared to the non-matched condition, demonstrating that the matching layer design method is effective for developing high-frequency ultrasonic transducers.4. Modification of microstructure on PZT films for ultrahigh frequency transducerPZT piezoelectric films are materials commonly used for ultrasonic transducer fabrication. It can be prepared though chemical solution deposition method, thus, the thickness can be effectively controlled. However, due to its high dielectric constant, such PZT films are not suitable for fabricating high frequency ultrasonic transducers. In this work, porous PZT films in range of several micrometers were fabricated using a chemical solution deposition (CSD) method modified with polyvinylpyrrolidone (PVP) as a pore-foaming agent. The crystalline phase, microstructure and electrical properties of the porous films were investigated as a function of PVP contents, molecular weights and annealing temperatures. It was found that introduction of pores into the films effectively decreased the dielectric constant, and the electrical properties were closely associated with the porosity. Ultrasonic transducers were fabricated using these porous film. The center frequency was measured to be 222.4MHz and the -6dB bandwidth was 67.8%.