Electromigration in isolated 0.25-1. 0 mu m wide pure Cu fine lines has been investigated using both resistance and edge displacement techniques in the sample temperatures range 221-394 degrees C. The Cu lines, sandwiched with a top and bottom Ta layer, were 200 mu m long and were connected to underlying W bars on each end of the Cu line. Transmission electron microscopy revealed that the microstructures of the 1 and 0.25 mu m wide lines were polycrystalline and bamboo-like grain structures, respectively. During electromigration, voids formed at the cathode end while hillocks,stew at the anode end. The failure lifetimes were correlated to the sizes of voids formed in the vicinity of the cathode end of the line, and a distribution of multiple failure modes was found. A low value of electromigration activation energy, 0.81 +/- 0.05 eV, and a Linear relationship between failure lifetime and metal linewidth were found, despite the fact that these narrow lines were close to a perfect bamboo-like grained structure. These results suggest that the mass transport of Cu during electromigration primarily occurs along the sidewall surfaces of the lines.