Passivation layers of 200 Angstrom sputtered Cr-O as well as reactive sputtered Cr-N-O were studied with respect to the passivation capability against thermal oxidation of Cu in flowing nitrogen and flowing oxygen ambients. In a flowing N-2 ambient, both Cr-O and Cr-N-O passivation layers were able to prohibit oxidation of Cu at temperatures up to 700 degrees C. In an O-2 ambient, the passivation capability of Cr-N-O layer was found to be 500 degrees C, which is 150 degrees C higher than that of Cr-O layer. The superiority of the passivation capability of the Cr-N-O layer is presumably due to decoration of the surface defects and grain boundaries with nitrogen, which provide fast paths for oxygen and copper diffusion.