Ru was evaluated as a new underlayer material for depositing ultrathin chemical vapor deposited (CVD) Cu seed layers, and the morphology and adhesion property of CVD Cu films on Ru were compared to that of Ta. CVD Cu films deposited on Ru showed higher nuclei density than Cu films deposited on Ta. Relatively low deposition temperatures of 90 degrees C and high precursor partial pressures of 160 mTorr yield high density of Cu nuclei. Smooth, 20 nm thick, continuous CVD Cu seed layers were deposited onto Ru underlayers at this optimized condition. CVD Cu adheres well to Ru because a fluorinated impirity layer was not formed interface. Using Ru underlayers and optimizing deposition conditions permits the fabrication of thin, continuous CVD Cu seed layers for Cu interconnects of future generations. (c) 2006 The Electrochemical Society.