Measurements of nanometer scale interface diffusion between tungsten and niobium thin films using high frequency laser based ultrasound

This article presents the use of a quantitative analysis technique to describe time-resolved acoustic spectroscopy (high frequency laser based ultrasound) measurements of atomic diffusion on nanometer length scales occurring at the interface between sputter-deposited tungsten and niobium films. The extent of diffusion at the tungsten-niobium interface is determined by comparing experimental, simulated, and theoretical transfer functions between acoustic arrivals. The experimental and simulated transfer functions use the spectral content of successive reflected acoustic waves and the theoretical transfer function is based on the transfer matrix of an equivalent stratified interface region. This combination of theoretical, simulated, and experimental analyses makes it possible to separate signals with distinct differences between the as-deposited interface and those interfaces diffused to an experimentally determined 0.8-nm and 1.4-nm extent. Comparison of predicted and measured diffusion depths for this diffusion couple indicates that bulk diffusivities are not appropriate for describing nanometer scale interface diffusion. (C) 2000 Acoustical Society of America. [S0001-4966(00)01904-4].