Acoustic Transmission and Image Contrast of Tilted Plate Specimens in Transmission Acoustic Microscopy

Abstmct-The results of a study on acoustic transmission and image contrast of a tilted plate specimen in transmission scanning acoustic microscopy (SAhI) are presented. Both the transmission angular spectra and the angular contrasts, as a function of the axial location and the tilt angle of the specimen, are analyzed using the Fourier decomposition technique. The angle-dependent transmission coefficient of a water-plate specimen-water system and acoustic resonance, as well as mode conversion (from longitudinal wave to shear wave) are included in the formulation. An excellent agreement between the theoretical predictions and the experimental results obtained with aluminum plate specimens at 150 MHz has been obtained. Acoustic imaging of tilted specimens is then treated as an application of the analysis. It is shown that transmission acoustic microscopy for an acoustically thick specimen is dominated by angular contrast rather than axial contrast as in reflection acoustic microscopy. It is found that both acoustic resonance and mode conversion in the specimen play an important role in determining the contrast of the acoustic image. However, the effect of mode conversion on the image contrast is found to be negligible for a small tilt angle in a transmission SAM that utilizes small numerical aperture lenses. It is also found that by selecting an appropriate tilt angle for a given specimen, higher transmitted acoustic power, andlor greater angular contrast can be facilitated to delineate additional details in the acoustic image.