Relationship between copper concentration and stress during electromigration in an Al(0.25 at. % Cu) conductor line

Abstract

Synchrotron-based x-ray microbeam fluorescence and diffraction have been used for in situ measurements of Cu concentration and biaxial stress in a 200-μm-long, 10-μm-wide Al(0.25 at. % Cu) conductor line with 1.5-μm-thick SiO2 passivation during electromigration. Measurements over 48 h with T=300 °C and j=1.5×105 A/cm2 show that a stress gradient of 3 MPa/μm develops over the upstream 130 μm of line length where Cu concentration drops below 0.15 at. %, and a 10-μm-long void develops at the cathode end of the line, but little change in stress occurs over the downstream 70 μm of line length where Cu concentration remains above 0.15 at. %. These experimental results have been reproduced by a finite element model in which the downstream Cu transport is accompanied by a counter flow of Al in the upstream direction, and downstream Al motion is blocked where the local Cu concentration is above ∼0.15 at. %. Defect mediated coupling between Al and Cu diffusive flows, e.g., Cu–vacancy binding, is proposed as the cause for the counterflow of Al when the Cu concentration is above the critical concentration, and as the mechanism by which Cu reduces the rate of electromigration damage in Al(Cu) conductor lines.