Development of a 20 MHz annular-array - a balancing act between optimized design and technological opportunities

Scanning Acoustic Microscopes for nondestructive imaging commonly use strong focusing single element probes with center frequencies up to 230 MHz. Depending on the test scenario and the materials a resolution down to 10 mu m is reachable. This high resolution means a small extension of the sensitivity zone in lateral and axial direction and leads to the necessity of multiple C-Scans for the inspection of a volume. Using an annular-array transducer, the focal point can be shifted in axial direction. Thus, multiple scans in different depth can be replaced by a single scan. To achieve a comparable resolution to strong focused single element transducers it is necessary to increase the center frequency of annular-array transducers. So we present a prototype of a probe, that contains a planar annular-array transducer with a center frequency of 20 MHz and an aperture of 5.4 mm, separated into four quartered rings (16 elements). The characterization of the sound field, using a spherical reflector, shows a sensitivity zone with a lateral extension of 300 mu m for focusing without a lens