Titanium monoxide ultra-thin coatings evaporated onto polycrystalline copper films

We evaporated polycrystalline copper thin films of thickness between 10 and 100nm on silicon substrates with their native oxide under ultra-high-vacuum conditions. Some of them were exposed to air for a period ranging from 1 day to 2 weeks. X-ray photoelectron spectroscopy (XPS) revealed a clean copper surface with a trace of oxygen. These films that were exposed to air presented oxides in the state Cu(II), the amount of CuO depended on the time that the film was exposed to air. Subsequently, we deposited TiO ultra-thin films on polycrystalline copper substrates. Both these thin films were formed by electron beam evaporation. XPS spectra showed that the surface of the titanium monoxide (TiO) films was contamination-free. An evaporation of 0.3 nm of TiO reduced the native oxide of the copper substrates from Cu(II) to Cu(I) or Cu(0) and transformed the TiO into TiO2 at the interface. Low-energy ion spectroscopy showed that the complete coverage of the substrates depends on the thickness of the copper films. For 10 nm copper thin films the complete coverage occurred at 1.5 nm of TiO, and for 100 nm it occurred at 2.0nm of TiO. In samples exposed to air, the complete coverage occurred at a film thickness slightly higher than those treated under ultra-high-vacuum conditions. r 2004 Published by Elsevier Ltd.