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Unformatted text preview: ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 51, no. 2, february 2004 211 Characterizing Ultra-Thin Matching Layers of High-Frequency Ultrasonic Transducer Based on Impedance Matching Principle Haifeng Wang and Wenwu Cao Abstract —The quarter-wavelength ( /4) acoustic match- ing layer is a vital component in medical ultrasonic transducers, which can compensate for the large acous- tic impedance mismatch between the piezoelectric material and the human body. At high frequencies ( 100 MHz), the /4 matching layers become extremely thin, and the charac- terization of their properties becomes very challenging. We report a method to measure the phase velocity and attenu- ation of ultra-thin layers using the /4 matching principle, in which the acoustic impedance of the thin layer is between the substrate and water. The method has been successfully used to characterize epoxy films on glass substrate. The experimental results show good agreement in the phase- velocity measurement between our proposed method and the conventional ultrasonic spectroscopy method, but the attenuation measurement is sensitive to the properties of the substrate and water medium as well as the alignment of the sample. I. Introduction I n order to obtain higher sensitivity and broader band- width, ultrasonic transducers often require one or more quarter-wavelength ( λ /4) acoustic matching layers be- tween the piezoelectric ceramic and the propagating medium. Epoxy resin, parylene, and composite materials have been widely used as matching layers in the medi- cal transducer industry for many years . Recently, high frequency ultrasonic transducers (30–100 MHz) have be- come essential to further improve the quality of clinical ultrasound images , . Because matching materials have strong frequency-dependent acoustic properties, es- pecially at high frequencies, the knowledge of their prop- erties at the desired operating frequency is important to high-frequency transducer design. The methods used to characterize the matching layers can be classified into two categories: direct measurement and resonance measurement. For the direct measurement method, in which the phase velocity is determined by the time delay between two successive reﬂected (or transmit- ted) signals, and the attenuation is determined by the am- plitude ratio of the signals, transducers with center fre- quency at least four times that of the quarter-wavelength Manuscript received April 8, 2003; accepted October 4, 2003. This research was sponsored by the NIH Grant for the Ultrasonic Trans- ducer Engineering Resource (Grant number P41-RR11795-06). The authors are with the Materials Research Institute, The Pennsylvania State University, University Park, PA 16802 (e-mail: [email protected])....
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- Fall '10
- Frequency, Wavelength, phase velocity, glass substrate, Haifeng Wang