Thermal effects in scanning acoustic microscopy for fine resolution applications

A novel scanning acoustic microscopy technique for achieving high resolution acoustic images by employing thermal effects and image subtraction has been studied and demonstrated. Experiments were performed on a perspex block patterned with a machined grid on the reverse surface, and on a buried channel in similar material. If was found that using the image subtraction technique, short periods of sample heating can lead to a stronger pattern selectivity, because of the strong temperature dependency of the elastic parameters of the polymer. In previous SAM techniques improvement in signal has been achieved through the use of special liquids as acoustic coupling media between the acoustic lens and the sample. The reported technique retains water as the coupling medium and the acoustic impedance matching is performed by varying the elastic parameters of the sample itself through direct heating. The temperature increase in the sample decreases the velocity of propagation of acoustic waves in the solid, and brings the acoustic impedance close to that of water. A theoretical model, including expressions for the acoustic aberrations, depth dependence and acoustic impedance matching has been derived. Examples of the results obtained are presented.<>