Brillouin scattering from acoustic excitations in TiN films on high speed steel—A stiffening system

Abstract

Brillouin scattering measurements are presented of surface acoustic waves in TiN films of various thicknesses on high speed steel. Because of its relatively high elastic moduli as compared with those of steel, TiN has a stiffening effect on the surface, causing the surface acoustic wave (SAW) to increase in velocity, merge into the bulk wave continuum, and become a pseudo-SAW. In the limit of large film thickness this pseudo-SAW evolves into the Rayleigh wave for TiN. A Green’s function method, invoking the surface ripple mechanism for the inelastic scattering of light, is used to calculate the Brillouin spectrum for scattering from these surface acoustic modes, and reveals details of the acoustic excitations of stiffening thin films not previously appreciated. A comparison between the measured and calculated dispersion relation for TiN thicknesses ranging from 20 to 4180 nm reveals that the elastic moduli of the thicker films are close to those of bulk TiN, but the effective elastic moduli of the thinner films are found to decrease with reducing film thickness. This conclusion is reinforced by backscattering measurements of Brillouin spectra at incident angles between 50° and 80° for a film thickness of 350 nm. Compositional variations at the interface have been investigated using x-ray photoelectron spectroscopy in an effort to understand this reduction in the elastic constants.