Quantitative analysis was used to monitor time-resolved acoustic spectroscopic measurements of atomic diffusion, at nm length-scales, which occurred at the interface between sputter-deposited W and Nb films. The extent of diffusion at the interface was determined by comparing experimental, simulated and theoretical transfer functions between acoustic arrivals. The experimental and simulated transfer functions used the spectral content of successive reflected acoustic waves, and the theoretical transfer function was based upon the transfer matrix of an equivalent stratified interface region. The combination of theoretical, simulated and experimental analyses made it possible to separate signals having distinct differences between as-deposited interfaces, and interfaces which were diffused to experimentally determined extents of 0.8 and 1.4nm. A comparison of predicted and measured diffusion depths, for this diffusion couple, indicated that bulk diffusivities were not suitable for describing nm-scale interface diffusion.

Measurements of Nanometer Scale Interface Diffusion between Tungsten and Niobium Thin Films using High Frequency Laser Based Ultrasound. C.J.K.Richardson, M.J.Ehrlich, J.W.Wagner: Journal of the Acoustical Society of America, 2000, 107[4], 1987-93