Local Study of Thin-Film Adhesion by Surface Acoustic Waves and Subsurface Acoustic Microscopy Imaging

Abstract
Four thin-film adhesion characterization methods are proposed. Two of them are surface acoustic wave based and the two others emphasize the complementary nature between acoustic microscopy, scratch test and tensile experiments. A continuous wave scanning acoustic microscope was used to propagate surface modes in the specimen and to measure the surface acoustic wave velocity as a function of the frequency. Characteristic equations have been calculated for generalized Lamb wave and generalized surface skimming compressional wave modes as a function of interface boundary conditions and frequency. Experimental surface acoustic wave velocity dispersion curves and numerically evaluated ones have been compared in shape and absolute position. It has been shown that this comparison allows one to characterize the interface preparation quality or adhesion property of thin films deposited by Physical Vapour Deposition (PVD). Two original coupling techniques were developed to increase the comprehension of intermediate adhesion states and thin-film delamination initiation and propagation characteristics. The first applies tensile stress to thin-film specimens directly under the microscope and the other creates a typical scratch at the top of the layered sample before surface and interface acoustic microscopy observations.