In this monograph, the authors report the current advancement in high
frequency piezoelectric crystal micromachined ultrasound transducers
and arrays and their biomedical applications.
Piezoelectric ultrasound transducers operating at high frequencies
(>20 MHz) are of increasing demand in recent years for medical imaging
and biological particle manipulation involved therapy. The performances
of transducers greatly rely on the properties of the piezoelectric
materials and transduction structures, including piezoelectric coefficient
(d), electromechanical coupling coefficient (k), dielectric permittivity
(e) and acoustic impedance (Z). Piezo-composite structures are
preferred because of their relatively high electromechanical coupling
coefficient and low acoustic impedance. A number of piezo-composite
techniques have been developed, namely "dice and fill", "tape-casting",
"stack and bond", "interdigital phase bonding", "laser micromachining"
and "micro-molding". However, these techniques are either difficult to
achieve fine features or not suitable for manufacturing of high frequency
ultrasound transducers (>20 MHz). The piezo-composite micromachined
ultrasound transducers (PC -MUT) technique discovered over
the last 10 years or so has demonstrated high performance high frequency
piezo-composite ultrasound transducers.
In this monograph, piezoelectric materials used for high frequency
transducers is introduced first. Next, the benefits and theory of piezo
composites is presented, followed by the design criteria and fabrication
methods. Biomedical applications using PC -MUT and arrays
will also be reported, in comparison with other ultrasound transducer
techniques. The final part of this monograph describes challenges and
future perspectives of this technique for biomedical applications.
