This work investigated the diffusion barrier properties of ionized metal plasma (IMP) deposited Ta between Cu and SiO2. When Cu and Ta layers were formed by IMP sputtering, it was found that the Cu has the equilibrium f.c.c. structure with the grain size of 80 nm whereas Ta is in a metalstable tetragonal (beta-Ta) form with a grain size of around 10 nm. With the help of sheet resistance measurement, X-ray diffraction, cross-section transmission electron microscope analysis, Rutherford backscattering spectroscopy, secondary ion mass spectroscopy, and scanning electron microscopy, the Ta barrier layer was observed to fail at temperature above 650degreesC due to the reactions among Cu, Ta and O and formation of CuxTayOz. The phase transformation of beta-Ta into the stable phase (alpha-Ta), in the presence of Cu was encountered when annealing the sample at above 800degreesC. The role of oxygen was also found to be important in the phase transformation, in the reactions and it seems to have a strong effect on the thermal stability of the barrier layer.