Sensitivity of acoustic microscopy for detecting three-dimensional nanometer gaps embedded in a silicon structure

The sensitivity of acoustic microscopy for detecting three-dimensional defects in a Si structure is reported. Circular, nanometer gaps with diameters ranging from 5 to 1000 microm were embedded in Si disks by a direct bonding technique, and these were visualized using acoustic microscopy. The limits of detection for the gap thickness and diameter were observed simultaneously in samples with gaps of 4 and 140 nm. The behavior of the sensitivity in detecting the gaps can be explained by a simple analytical model. It is shown experimentally and theoretically that the gap thickness and diameter are not independent variables as regards detection. The sensitivity of acoustic microscopy is governed by the three-dimensional features of the embedded defects.