PRECIPITATION OF AL2CU IN BLANKET AL-CU FILMS

We have used x-ray absorption spectroscopy to study the formation and dissolution of theta-Al2Cu precipitates in blanket AI-Cu films. In one series of experiments, we examined films deposited at different tempe ratures and average Cu concentrations. For a given temperature, there is a Cu concentration above which precipitates form. This effective so lvus agrees with the equilibrium solvus at high temperatures, but exce eds the equilibrium values at low deposition temperatures. The formati on of precipitates correlates with a pileup of Cu in the part of the h im which was deposited first. This pileup is explained by a model invo lving precipitate growth at grain boundaries and grain growth during d eposition. We also measured the kinetics of precipitation formation an d dissolution in Al-0.5 w/o Cu. In the range 200-270 degrees C, the pr ecipitation kinetics show an activation energy of 0.54 eV, which is lo wer than that for grain-boundary diffusion of Cu in Al. Precipitate di ssolution over the range 300-400 degrees C shows an activation energy of 1.37 eV, consistent with lattice diffusion. These results may be us eful in designing heat treatments which will minimize the occurrence o f precipitates in integrated-circuit interconnects when process corros ion could be a problem, yet leave the material with precipitates befor e use, when electromigration becomes an issue. (C) 1997 American Insti tute of Physics.