Measuring thin-film elastic constants by line-focus acoustic microscopy

The determination of elastic constants of bulk specimens and anisotropic films deposited on anisotropic substrates from V(z) measurements obtained by using a line-focus acoustic microscope is discussed. The procedure has three essential components: (1) V(z) curve measurement as a function of direction of wave mode propagation in the sample at fixed frequency and/or as a function of the frequency or film thickness for a fixed direction, (2) use of a measurement model for parametric studies of V(z) curves, and (3) determination of elastic constants by systematically comparing wave-mode velocities obtained from the measurement model and the V(z) measurement. Some examples are presented for transition metal nitride films used as hard protective coatings of softer surfaces. Attention is also directed to certain advantages of using multiple modes. It is shown that multiple-mode results reduce the region in a plane of material constants for which a deviation function between theoretical and experimental results has prescribed small values. Finally a brief discussion is presented of some recent results obtained at much lower frequencies by time resolved acoustic microscopy