P(VDF-TrFE) 고분자 압전막을 이융한 고주파 초음파 집속 트랜스듀서의 제작 및 특성 해석

Abstract

Polyvinylidene fluoride trifluoroethylene [P(VDF-TrFE)] is a copolymer material synthesized by monomers of VDF and TrFE. It has low acoustic impedance so that the acoustic matching with water or biological tissue is good, and the thin film which is required for high frequency ultrasound generation is easily obtained. In this study, two [P(VDF-TrFE)] ultrasonic transducers were fabricated using two different methods which are the spin-coating method and the bonding with CCP (Copper Clad Polyimide) method. In the spin coating method, the P(VDF-TrFE) solution is directly deposited on the backing rod which has a concave surface with an appropriately designed curvature for focusing. The transducer by this method has a very simple structure of the P(VDF-TrFE)/backing material. On the other hand, the transducer by bonding with CCP method has a complicated structure of the P(VDF-TrFE)/bonding layer/Cu/Polyimide/backing material. Beside of a piezoelectric layer, a bonding layer that is a dielectric layer exists between two electrodes in this structure. The fabrication processes of the P(VDF-TrFE) transducer by the above mentioned two methods have been established through this study. The X-ray diffraction (XRD) patterns of the deposited P(VDF-TrFE) films annealed in different times are analyzed to confirm the suitable annealing time. And it was shown that 4 hours is good for the crystallization of polymer. The electromechanical characteristics of the fabricated transducer were analyzed by using the KLM equivalent circuit model. For the application of the KLM equivalent circuit to the transducer with a dielectric layer between two electrodes, a method to calculate the clamped capacitance in the circuit model has been newly proposed. The simulated waveforms and power spectra of the pulse echo responses were compared with the results of measurement. Consequently, it was shown that the experimental results of waveforms and power spectra are well agreed with simulation results. Besides, the acoustic fields along the lateral directions were measured using a needle hydrophone. And the measurement results are compared with the simulation result. The acoustic field patterns showed that the ultrasonic beam is obviously reinforced at the focal point and the concave structure has a strong focusing effect.