LATTICE ELECTROMIGRATION IN NARROW AL-ALLOY THIN-FILM CONDUCTORS AT LOW-TEMPERATURES

At low temperatures, electromigration in polycrystalline Al thin-film conductors has been considered to occur predominantly along grain boundaries. As conductor widths decrease below the average grain size, however, other transport mechanisms will become important. Here we examine electromigration transport mechanisms in narrow AlSiCu conductors in the temperature range 190 to 290 degrees C using the stripe drift technique. For conductors with widths between 0.9 and 2.75 mu m both the absolute values of the drift velocity and the activation energy for drift are consistent with a lattice diffusion mechanism. Over this linewidth range the Al microstructure ranges from near-bamboo to approximately 20 mu m long polycrystalline segments. The independence of the drift data from the linewidth shows that steady state transport is controlled by the bamboo regions of the conductors, which results from the slower rate of diffusion of Al through the lattice compared to along grain boundaries.