Grain growth study in aluminum films and electromigration implications

Studies of grain structure, size, distribution, and geometric arrangement are gaining importance especially from the point of view of thin-film interconnects for micron and submicron semiconductor technologies. We have studied the effect of annealing on the grain growth characteristics of pure Al films. As-deposited films exhibit a uniform grain size distribution between 500 and 800 Å. With heat treatment, the average grain size increases with the largest grains exhibiting a parabolic growth with time, limited to a size in the neighborhood of 1 μ. Smaller grains, however, cannot be eliminated and about 30% of the grains are less than 2500 Å across. The grain boundaries of the smaller grains have a higher oxygen content than the rest of the film. Lines defined by liftoff exhibit a characteristic grain boundary seam along the length of the line. The implications of the above study in terms of processibility and reliability with respect to electromigration are also discussed.