Development of 20-MHZ PMN-PT single crystal phased-array ultrasound transducers for biomedical imaging applications

Abstract

Ultrasound diagnostic has high potential for biomedical imaging applications including biomedical studies and clinic use. When comparing with other diagnostic tools, ultrasound transducer has its advantages such as compact, non-invasiveness and non-harmful, but the resolution of ultrasound is relatively low. In this thesis, an ultrasound transducer with high lateral and axial resolutions was developed by increasing its center frequency and -6 dB bandwidth. To achieve the goal, a high-frequency (a center frequency of 20-MHz) phased-array ultrasound transducer fabricated using an active layer of lead magnesium niobate-lead titanate (PMN-0.3PT) single crystal was developed. It should be noted that this single crystal material has not commonly used in commercial ultrasound transducers. Compared to the traditional piezoelectric ceramics, lead zirconate titanate (PZT), PMN-0.3PT is a piezoelectric single crystal with better piezoelectric properties, such as piezoelectric constant ( d33 > 1000 pC/N), electromechanical coupling coefficient ( k33 ≥ 0.8) and clamped dielectric constant ( ε33 ≥ 1300). These outstanding piezoelectric parameters are beneficial to minimize the aperture size and enhance both -6 dB bandwidth and sensitivity of the transducer, respectively. Thus, the axial and lateral resolutions of phased-array transducer fabricated using the PMN-0.3PT material is expected to enhance for biomedical imaging applications. Similar to the typical array transducer, the structure of the developed phased-array transducer has three main parts: an active layer for transmitting and receiving the acoustic pulse; a double quarter wavelength-thick matching layer for maximizing the acoustic pulse transmission efficiency; and a backing layer for absorbing the back reflected acoustic pulse to widen the -6 dB bandwidth of the transducer. The PMN-PT array was fabricated by the typical mechanical dicing method and independent electrodes were made by photolithography with wet etching. The 80 µm -thick array with a pitch of 75 µm (~1λ) and an element width of 55 µm was casted on the backing layer made by a conductive epoxy, E-solder. A flexible circuit with 64 electrode traces was aligned and adhered on the array. Besides, the double quarter wavelength matching layer, made by a mixture of aluminum oxide and epoxy for the first layer and a pure epoxy for the second layer, was also adhered on the array. The performance of developed phased-array transducers has shown a center frequency of 22. By using the built-in FFT function of the oscilloscope, the pulse-echo response in the frequency domain illustrated a -6 dB bandwidth of 91%. The sensitivity of the phased-array transducer was represented by an insertion loss of 29 dB. Comparing with the commercial or reported array transducers within the specific high-frequency range (20 MHz to 50 MHz), the -6 dB bandwidth of the developed array prototype is the widest and its sensitivity is also comparable. The small aperture size ((L) 5 mm × (W) 5 mm × (H) 2.2 mm) and wide -6 dB bandwidth (>70%) of the developed transducers are highly appropriate for accomplishing high-resolution imaging in the biomedical and clinical applications.