Mechanism of high purity Cu thin film deposition from a new F-free Cu complex, Cu(EDMDD)(2) have been studied using H-assisted plasma CVD (HAPCVD). The species of Cu(EDMDD) is the dominant neutral radical dissociated from Cu(EDMDD)2 by electron impact. In situ measurements by Fourier transform infrared (FTIR) spectroscopy show that H atoms are quite effective in removing impurities in Cu films. The simplified version of important surface reaction is Cu(EDMDD)+H -> Cu+H(EDMDD). Cu films deposited by HAPCVD have a low as-deposited resistivity of 1.84 mu Omega cm and is 20 nm thick in trenches 0.5 mu m wide and 2.73 mu m deep. (c) 2005 Elsevier B.V. All rights reserved.