FOCUSING PERFORMANCE OF EPOXY-BACKED AND AIR-BACKED POLYVINYLIDENE FLUORIDE FRESNEL ZONE PLATES

In this paper the focusing performance of epoxy- and air-backed polyvi nylidene fluoride (PVDF) Fresnel zone plate (FZP) transducers are inve stigated. This work is motivated by an interest in constructing integr ated, polymer FZP transducers for imaging applications on a silicon su bstrate using integrated circuit fabrication techniques. While it is w ell known that using an impedance matched backing facilitates broadban d transducer performance, to the authors' knowledge the effects on the acoustic beam patterns of polymer FZPs have not been reported. A mode l for the ideal zone plate is also presented, and a general pressure t ransfer function is derived in order to generate theoretical beam prof iles. Epoxy- and air-backed, three-zone FZPs were constructed using 11 1 mu m thick PVDF. The devices were tested in an ultrasound scanning t ank while operating at 5 MHz. The focal plane profile for the epoxy-ba cked device was in good agreement with that predicted for the ideal zo ne plate. In addition, the epoxy backing was found to dampen higher or der radial mode excitation. The radial modes are believed to be respon sible for the narrow main lobe and elevated side lobes found in the fo cal plane profile for the air-backed device.