The thermal behaviour of sputtered Cu/Ta bilayer films on Si-substrate (Cu is the top layer) was investigated in the temperature range of 563-713 K. Samples were heated in ultra high vacuum conditions, while the Auger intensities were monitored. The time evolution of the Ta and Cu Auger signals was interpreted as Ta grain boundary diffusion through the Cu film and Ta accumulation on the copper surface. On the basis of the method developed by Hwang and Baluffi, the activation energy of the Ta grain boundary diffusion in copper was determined (Q = 0.7 +/- 0.2 eV). At 713 K a 10-h heat treatment caused a complete degradation of the sample, silicon appeared on the top surface and silicide formation was detected. (C) 2003 Elsevier B.V. All rights reserved.