Local strain measurements during electromigration

Abstract

This paper reviews the measurements of local strain distribution in individual 10 μm-wide 200 μm-long passivated Al conductor lines using white-beam x-ray microdiffraction. Local strain changes were examined in real-time when the Al lines were subjected to temperature changes as well as to electrical current stressing. Results of thermal strain measurements show that the stress in the Al lines can be treated as equibiaxial, and that the passivation does not hinder the overall expansion of the conductor line along the film normal. Results of electromigration strain measurements show relief of the initial biaxial compressive stress near the cathode end of the conductor line, and increase of compressive stress near the anode end. A linear stress gradient was observed when steady-state electromigration was reached. Steady-state measurements at various current densities below the threshold value, jth=1.6×105 A/cm2, yield an effective valence Z*=1.6 for Al electromigration, and transient-state measurements as a function of time allow us to estimate the effective grain boundary diffusivity Deff≅8×10−11 cm2/sec for Al self-diffusion at 260 °C. Results of measurements with j=6×105 A/cm2, which is much higher than the threshold value, show fast stress relaxation near the anode end after about 15 hours of stressing, most likely due to fracture of delamination of passivation by the electromigration-induced compressive stress there.