Effect of Cu at Al grain boundaries on electromigration behavior in Al thin films

The distribution of copper in aluminum thin films is examined with respect to how the copper can influence electromigration behavior. Al-Cu thin films annealed in the single phase region, to just below the solvustemperature, have 0-phase Al2Cu precipitates at the aluminum grain boundaries. The grain boundaries between precipitates are depleted in copper. Al-Cu thin films heat treated at lower temperatures, within thetwo phase region, also have 0-phase precipitates at the grain boundaries but the aluminum grain boundariescontinuously become enriched in copper, perhaps due to the formation of a thin coating of 0-phase at the grain boundary. Here, it is proposed that electromigration behavior of aluminum is improved by addingcopper because the 0-phase precipitates may hinder aluminum diffusion along the grain boundaries. It was also found that resistivity of Al-Cu thin films decrease during accelerated electromigration testing prior to failure. Pure Al films did not exhibit this behavior. The decrease in resistivity is attributed to theredistribution of copper from the aluminum grain matrix to the 0-phase precipitates growing at the grain boundaries thereby reducing the number of defects in the microstructure.