Design, Fabrication and Characterization of a Spherically Focused Capacitive Air-Coupled Ultrasonic Transducer

We report the development, fabrication, and testing of spherically focused capacitive air-coupled ultrasonic transducers, that need no mirror, zone plate, or similar external device. To achieve native focusing, we have employed a flexible copper/polyimide backplate that permits a conformal fit to a spherical fixture whose radius determines the focal length. A spherically deformed aluminized Mylar foil, conforms to the spherical backplate, completing the transducer. Two devices have been fabricated and tested, one having a 1-cm diameter and 2.54-cm focus, and another with a 5-cm diameter and 5.1-cm focus. Both devices have frequency spectra centered at 840 kHz with -6 dB points at 310 and 1200 kHz. Using a quasi-point receiver, the beam diameter is found to be 2.7 mm (1-cm device) and 1.32 mm (5-cm device) in the focal plane, when excited with a broadband signal. The monochromatic diffraction behavior of the 1-cm device agrees with model calculations based on a focused piston radiator to within the measurement resolution of about 2%. Features as small as a 250-micron wire have been successfully imaged using the 5-cm devices in a confocal geometry, demonstrating resolution at the diffraction limit for this device. Further examples of defect imaging in honeycomb composites and in wood are also presented.