Precipitation of Al2Cu in blanket Al-Cu films

We have used x-ray absorption spectroscopy to study the formation and dissolution of $\mathrm{\theta -}{\mathrm{Al}}_{\mathrm{2}}\mathrm{Cu}$ precipitates in blanket Al-Cu films. In one series of experiments, we examined films deposited at different temperatures and average Cu concentrations. For a given temperature, there is a Cu concentration above which precipitates form. This effective solvus agrees with the equilibrium solvus at high temperatures, but exceeds the equilibrium values at low deposition temperatures. The formation of precipitates correlates with a pileup of Cu in the part of the film which was deposited first. This pileup is explained by a model involving precipitate growth at grain boundaries and grain growth during deposition. We also measured the kinetics of precipitation formation and dissolution in $\mathrm{Al}\text{−0.5}$ w/o Cu. In the range 200–270 °C, the precipitation kinetics show an activation energy of 0.54 eV, which is lower than that for grain-boundary diffusion of Cu in Al. Precipitate dissolution over the range 300–400 °C shows an activation energy of 1.37 eV, consistent with lattice diffusion. These results may be useful in designing heat treatments which will minimize the occurrence of precipitates in integrated-circuit interconnects when process corrosion could be a problem, yet leave the material with precipitates before use, when electromigration becomes an issue.