It was noted that this process occurred along grain boundaries in polycrystalline thin-film conductors when the grain size was considerably greater than the width of the conductor. As the device size was reduced, the microstructures became near-bamboo like and alternative diffusion mechanisms operated within the bamboo grains. Drift velocity data were examined here in order to study the electromigration mechanisms in bamboo grains. Measurements of the temperature- and width-dependences of the drift velocity of thin-film conductors with bamboo microstructures were considered. It was found that the activation energy for drift was consistent with lattice diffusivity in Al. There was no width dependence of the drift velocity between 0.6 and 2.7; thus indicating a negligible flux of atoms along the conductor edges. These measurements were compared with the available data for various conductor microstructures, and with drift data for bulk Al. The drift velocities of conductors with bamboo microstructures, obtained from various sources, revealed very good agreement. They were similar to the drift velocities which were observed in bulk Al. It was concluded that the predominant mode of electromigration in thin-film conductors with a bamboo microstructure was the same as that in bulk Al, and involved lattice vacancy diffusion.

A.S.Oates: Journal of Applied Physics, 1996, 79[1], 163-9