Oxidation of Polycrystalline Copper Thin Films at Ambient Conditions

Qualitative and quantitative studies of the oxidation of polycrystalline copper (Cu) thin films upon exposure to ambient air conditions for long periods (on the order of several months) are reported in this work. Thin films of Cu, prepared by thermal evaporation, were analyzed by means of X-ray photoelectron spectroscopy (XPS) to gain an understanding on the growth mechanism of the surface oxide layer. Analysis of high-resolution Cu LMM, Cu2p3/2, and O1s spectra was used to follow the time dependence of individual oxide overlayer thicknesses as well as the overall oxide composite thickness. Transmission electron microscopy (TEM) and spectroscopic ellipsometry (SE) were used to confirm the results obtained from XPS measurements. Three main stages of copper oxide growth were observed: (a) the formation of a Cu2O layer, most likely due to Cu metal ionic transport toward the oxide−oxygen interface, (b) the formation of a Cu(OH)2 metastable overlayer, due to the interactions of Cu ions with hydroxyl groups present at the surface, and (c) the transformation of the Cu(OH)2 metastable phase to a more stable CuO layer. These three stages were found to occur simultaneously and to be mutually dependent on each other. The findings of this study may provide guidance in choosing the optimal conditions to fabricate and store copper-based ultra-large-scale integrated (ULSI) circuits.