Abstract
A dual-frequency ultrasound transducer (DFUT) is usually preferred for its numerous advantageous applications, especially in biomedical imaging and sensing. However, most of DFUTs are based on the combination of fundamental and harmonic operations, or integration of multiple different single-frequency ultrasound transducers, hindering perfect beam alignment and acoustic impedance matching. A novel single-element DFUT has been proposed in this paper. A small piezoelectric membrane is used as the high-frequency ultrasound transducer, which is stacked on a large non-piezoelectric elastic membrane with a groove used as the low-frequency capacitive ultrasound transducer. Such a capacitive-piezoelectric hybrid structure is theoretically analysed in details, based on the electrostatic attraction force and converse piezoelectric effect. Both the low and high resonance frequencies are independently derived, with a maximum deviation of less than 4% from the finite element simulations. Besides, a lumped-parameter equivalent circuit model of combining both the capacitive and piezoelectric ultrasound transducers was also described. Based on our dual-frequency structure design, a high-to-low frequency ratio of about 2 to more than 20 could be achieved, with easy and independent controllability of two frequencies, and the high-frequency operation shows at least an order-of-magnitude displacement sensitivity improvement compared with the conventional harmonic operations.
Highlights
Benefiting from deep penetration and high resolution, dual-frequency ultrasound transducers (DFUTs) have been studied in recent years for applications in medical therapy and imaging, such as non-destructive testing, transdermal drug release, photoacoustic imaging, and acoustic cavitation enhancement [1,2,3,4,5]
Almost all present stacked DFUT designs are based on piezoelectric technology which is largely limited by the strict acoustic impedance matching, low electromechanical coupling factor, and weak compatibility with CMOS process
Apart from the basic capacitive micromachined ultrasound transducer (CMUT) operation mode, an additional mechanical regime is entered: only the central piezoelectric laminated layers will vibrate when the DFUT is excited at the resonance frequency of the central laminated layers
Summary
Benefiting from deep penetration and high resolution, dual-frequency ultrasound transducers (DFUTs) have been studied in recent years for applications in medical therapy and imaging, such as non-destructive testing, transdermal drug release, photoacoustic imaging, and acoustic cavitation enhancement [1,2,3,4,5]. Zemp et al designed a dual-frequency capacitive micromachined ultrasound transducer (CMUT) 2D array with interlaced low-frequency and high-frequency elements [7]. Though they achieved favorable co-registered beams, the pitch between composite elements is just on a scale of λ for high-frequency (6.5 MHz) operation and the linear array design based on such composite elements would be challengeable for the perfect dual-frequency beam alignment. Manh et al designed a dual-frequency capacitive-piezoelectric hybrid ultrasound transducer consisting of a low-frequency piezoelectric stack and a high-frequency CMUT served as an outer matching layer for the low-frequency band [13]
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