Acoustic-microscopy measurements of the elastic properties of TiN/(VxNb1-x)N superlattices.

Abstract

The elastic constants of (${\mathrm{V}}_{0.6}$${\mathrm{Nb}}_{0.4}$)N and (${\mathrm{V}}_{0.3}$${\mathrm{Nb}}_{0.7}$)N films have been determined from anisotropic surface acoustic wave (SAW) dispersion data measured by line-focus acoustic microscopy. Using the acoustic microscope, measurements of SAW velocities have also been carried out on the TiN/(${\mathrm{V}}_{0.6}$${\mathrm{Nb}}_{0.4}$)N (001) and TiN/(${\mathrm{V}}_{0.3}$${\mathrm{Nb}}_{0.7}$)N (001) and (111) superlattice films, all grown on cubic-crystalline MgO substrates. The phase velocities, measured as functions of the angle of propagation, display the expected anisotropic natures of (001) and (111) cubic symmetries. Dispersion curves of SAW's propagating along the symmetry axes have been obtained by measuring wave velocities for various film thicknesses and frequencies. From the elastic constants of homogeneous TiN, (${\mathrm{V}}_{0.6}$${\mathrm{Nb}}_{0.4}$)N, and (${\mathrm{V}}_{0.3}$${\mathrm{Nb}}_{0.7}$)N films, the effective elastic constants of TiN/(${\mathrm{V}}_{0.6}$${\mathrm{Nb}}_{0.4}$)N and TiN/(${\mathrm{V}}_{0.3}$${\mathrm{Nb}}_{0.7}$)N superlattice films and the corresponding phase velocities of SAW's have been calculated. The calculated dispersion curves show very good agreement with the experimental results. The dependence of the elastic properties of the superlattices on the TiN fraction, on the superlattice period, and on the crystallographic orientation has been investigated. The results show no elastic anomalies within measurement errors, indicating that previously reported hardness enhancements were not related to elastic property changes.