The diffusion barrier properties of Co/TaN bilayer as Cu adhesion layer/diffusion barrier on the Si and low k (k=2.5) substrates were investigated. The barrier was prepared by using ion-beam sputtering technique. The barrier property was investigated by sheet resistance, X-ray diffraction (XRD), and Auger electron spectroscopy (AES) and electrical measurements. No obvious Cu diffusion and agglomeration was observed after annealing at 500°C for 30 min. Results show that Co/TaN has a good potential as cost effective adhesion/diffusion stack in copper metallization.