Thin film adhesion investigations with the acoustic microscope

We propose an application of the continuous wave scanning acoustic microscope set up (CW-SAM) to interface characterization. The basic principle is to create a stress in a film-substrate bond via acoustic waves and detect the response of the bond to this non-destructive ultrasonic excitation. The surface acoustic wave velocity is strongly affected by the layer-substrate boundary condition, that is to say by adhesion. A CW-SAM with a high numerical aperture spherical lens is used in the quantitative mode to propagate surface modes in the specimen and measure their velocity as a function of frequency (dispersion relation). By comparison of the absolute position and shape of experimental and calculated dispersion curves for various boundary conditions (perfect bonding, weak interface and perfect detachment) at the interface, we extract information on the adhesion of the film on the substrate. Both perfect bonding and perfect detachment cases have been emphasized with this technique. To reach the intermediate adhesion states we have made samples with artificial adhesion defects by varying the substrate treatment before the deposition