Interconnect damage by electromigration: Experiment and numerical simulation

Electromigration is a severe reliability issue for polycrystalline aluminum -based interconnects. The phenomenon is amplified by taking exposed interco nnects at an elevated temperature. The intense electric currents they carry would drive atoms to diffuse, and leave hillocks or sinks along the interc onnects. Experimental observations revealed that the hillocks grow against the surrounding grains in four stages: the incubation, the jump start, the deceleration, and the final stabilization. An analytical model is proposed to address features such as fluctuating mass flows into grains, partition o f mass Bows through the surface and the defect layers, stresses induced by the mass how, and resistance against the grain boundary sliding. The incuba tion time and the protruding height are determined. The incubation time is proportional, while the protruding height is roughly inversely proportional , to the square of the grain size. Numerical simulations are exploited to q uantify the experimental phenomena. To design against the electromigration, one needs to raise the incubation time of an interconnect beyond its servi ce life. To comply with this criterion, a polycrystal line with large and u niform grains (if not a bamboo line or even a single crystal line) with sma ll variation in grain boundary diffusivities should be pursued. (C) 1998 Ac ta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved .