We estimated segregation energy for compound impurities at the grain boundary in very narrow Cu wires using the ab initio calculation. From the calculation results, we clarified that the impurities such as FeO, Fe(ClO), TiO and Ti(ClO) have a strong segregation tendency at the Cu grain boundaries. The FE-TEM observation results of the Cu wires with Fe impurity atoms confirmed that the impurity atoms led to smaller grain sizes than the impurity-free case. The resistivity was measured as a function of the Cu wire width for various Fe impurity concentrations and the wire resistivity became higher with increased impurity concentration. (C) 2017 The Electrochemical Society. All rights reserved.