Electromigration and diffusion of Cu have been investigated for polycrystalline Al(0.25 at. % Cu) conductor lines. In situ measurements of the evolution of Cu concentration profiles along 200 μm long, 10 μm wide conductor lines with 1.5 μm thick SiO2 passivation during electromigration have been obtained by synchrotron-based white x-ray microbeam fluorescence. The apparent effective charge ZCu* of Cu in Al(Cu) has been found to be -8.6±1.0. The evolution of Cu concentration profiles can be manipulated by controlling the direction and magnitude of the current flow at different temperatures. The effective grain boundary diffusivity DCueff has been determined by fitting the time dependent experimental Cu concentration profiles. The results show Arrhenius behavior of DCueff=D0 exp(-Q/kT) for T=275–325 °C with D0=10-(2.3±1.6) cm2/s and Q=0.76±0.19 eV. © 2001 American Institute of Physics.