Synchrotron-based X-ray micro-beam fluorescence and diffraction were used to make in situ measurements of Cu concentration and biaxial stress in a 200µm-long, 10µm-wide Al-0.25at%Cu conductor line with 1.5µm-thick SiO2 passivation during electromigration. Measurements over 48h, at 300C and 1.5 x 105A/cm2, showed that a stress gradient of 3MPa/µm developed over the up-stream 130µm of line-length where the Cu concentration dropped to below 0.15at%, and a 10µm-long void developed at the cathode-end of the line, but little change in stress occurred over the down-stream 70µm of line-length where the Cu concentration remained above 0.15at%. These experimental results were reproduced by using a finite element model in which the down-stream Cu transport was accompanied by a counter-flow of Al in the up-stream direction, and down-stream Al motion was blocked where the local Cu concentration was above about 0.15at%. Defect-mediated coupling between Al and Cu diffusive flows (e.g. Cu–vacancy binding) was proposed to be the cause of the counter-flow of Al when the Cu concentration was above the critical concentration, and to be the mechanism via which Cu reduced the rate of electromigration damage in Al(Cu) conductor lines.

Relationship Between Copper Concentration and Stress during Electromigration in an Al-0.25at%Cu Conductor Line. H.K.Kao, G.S.Cargill, F.Giuliani, C.K.Hu: Journal of Applied Physics, 2003, 93[5], 2516-27