Interdiffusion in copper-aluminum thin film bilayers. II. Analysis of marker motion during sequential compound formation

Isolated W islands, 150 Å in diameter, have been deposited between Cu and Al thin film bilayers to serve as inert diffusion markers. Marker displacements have been measured consecutively by Rutherford backscattering spectroscopy during the sequential growth of CuAl₂, CuAl, and Cu ₉Al₄ intermetallic compounds upon annealing in the temperature range 160-250 °C. The intrinsic interdiffusion coefficients of Al and Cu in each of these compounds have been determined by applying an analysis of marker motion in a binary diffusion couple to the measured displacement data. Moreover, the prefactor and activation energy of the individual diffusivities have been calculated as shown below by measuring the marker motion as a function of temperature. For CuAl₂, D _0Al =0.4 cm²/s, Q_Al =1.25±0.05 eV, D_0Cu =9.5 cm²/s, Q_Cu =1.40±0.05 eV. For CuAl, D_0Al =1.5×10^-7 cm²/s, Q_Al =0.7±0.05 eV, D_0Cu =1×10^{- 2} cm²/s, Q_Cu =1.1±0.05 eV. For Cu ₉Al₄, D_0Al =1.7×10^{- 3} cm²/s, Q_Al =1.20±0.05 eV, D _0Cu =2.4×10^-2 cm²/s, Q _Cu =1.30±0.05 eV. These values agree quite well to those chemical interdiffusion coefficients published in the literature for bulk samples. A discussion on sequential compound formation has been given on the basis of these measured values.