Measurement of the activation energy for electromigration in polycrystalline copper films under clean surface, ultra-high vacuum (<10(-10) Torr) conditions has been carried out. An isothermal electrical resistance method was used and a value of 0.47 +/- 0.03 eV was obtained. This result suggests that under clean surface conditions surface diffusion is the primary damage mechanism for electromigration in copper, since the measured energy lies in the range of values obtained theoretically and experimentally for surface diffusion on copper. Scanning electron microscopy studies taken subsequent to the resistance measurements support this conclusion.