Focusing performance of epoxy- and air-backed polyvinylidene fluoride Fresnel zone plates

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Abstract
In this paper the focusing performance of epoxy- and air-backed polyvinylidene fluoride (PVDF) Fresnel zone plate (FZP) transducers are investigated. This work is motivated by an interest in constructing integrated, polymer FZP transducers for imaging applications on a silicon substrate using integrated circuit fabrication techniques. While it is well known that using an impedance matched backing facilitates broadband transducer performance, to the authors’ knowledge the effects on the acoustic beam patterns of polymer FZPs have not been reported. A model for the ideal zone plate is also presented, and a general pressure transfer function is derived in order to generate theoretical beam profiles. Epoxy- and air-backed, three-zone FZPs were constructed using 111 μm thick PVDF. The devices were tested in an ultrasound scanning tank while operating at 5 MHz. The focal plane profile for the epoxy-backed device was in good agreement with that predicted for the ideal zone plate. In addition, the epoxy backing was found to dampen higher order radial mode excitation. The radial modes are believed to be responsible for the narrow main lobe and elevated side lobes found in the focal plane profile for the air-backed device.

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