Electromigration transport mechanisms in Al thin-film conductors

Electromigration occurs along grain boundaries in polycrystalline thin-film conductors when the grain size is significantly larger than the conductor width. As circuit feature sizes have been reduced, microstructures have become near bamboo and alternate diffusion mechanisms must operate within the bamboo grains. In this article we examine drift velocity data to determine electromigration mechanisms in bamboo grains. We present measurements of the temperature and width dependence of the drift velocity of thin-film conductors with bamboo microstructures. The activation energy for drift is consistent with lattice diffusion in Al. There is no width dependence of the drift velocity in the range 0.6-2.7 mu m, indicating a negligible flux of atoms along the conductor edges. We compare these measurements with data available for a variety of conductor microstructures, and with drift data for bulk Al. The drift velocities of conductors with bamboo microstructures obtained from a variety of sources show very good agreement, and are similar to drift velocities observed in bulk Al. We conclude that the dominant mode of electromigration in thin-film conductors with bamboo microstructures and bulk Al is the same, and occurs by lattice vacancy diffusion. (C) 1996 American Institute of Physics.