A Spherically Focused Capacitive-Film Air-Coupled Transducer

Abstract

We have developed, fabricated, and tested spherically focused capacitive‐film air‐coupled ultrasonic transducers, which 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. The backplate is patterned with 40 μm depressions having 80 μm spacings. A spherically deformed 6 μm aluminized Mylar film, conforms to the spherical backplate, completing the transducer. Two transducers have been 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 transducers have frequency spectra centered at 805 kHz with — 6dB points at 440 and 1210 kHz. In the focal plane, the beam diameter is 2.7 mm (1 cm transducer) and 1.32 mm (5 cm transducer), when excited with a broadband signal. The focal zone extends from 17 to 34 mm (1 cm transducer) and from 46 to 57 mm (5‐cm transducer). Both transducers’ diffraction behaviors have been successfully modeled as focused piston radiators.