The Removal of Copper Oxides by Ethyl Alcohol Monitored In Situ by Spectroscopic Ellipsometry

As an interconnect material, copper has the disadvantage of not forming self-limiting oxides, which can negatively affect device performance and reliability. Undesired oxide layers need to be removed by in situ cleaning, before the copper is subjected to subsequent depositions. We have used ethyl alcohol as a vapor phase reducing agent to remove copper oxides formed on electroplated copper films upon exposure to the ambient. Spectroscopic ellipsometry has been used to monitor the reduction process in situ. Ex situ characterization using X-ray photoelectron spectroscopy and atomic force microscopy support in situ measurements. While oxide removal can be achieved at temperatures as low as 130°C, independent of oxide layer thickness and composition, it occurred more efficiently at 200°C, showing compatibility with the low temperature processing requirements of low dielectric constant materials. The initial reaction involves the reduction of to species followed by a second phase consisting of conversion to elemental copper, producing a clean metal surface. Reduction of to species is the rate-limiting step as evidenced by enhanced sensitivity to the reaction temperature.