This work investigates the thermal stability and physical and barrier properties for three species of amorphous silicon-oxycarbide (alpha-SiCO) dielectric barrier films, deposited by plasma-enhanced chemical vapor deposition (PECVD), to copper (Cu) diffusion using octamethylcyclotetrasiloxane precursor and helium (He) carrier gas with and without oxygen (O-2) reaction gas. The alpha-SiCO dielectric barrier film deposited by PECVD without O-2 reaction gas exhibits a low k-value of 2.8, thermally stable at temperatures up to 550 degreesC, excellent moisture resistance, and superb Cu barrier property until 400 degreesC. With the addition of O-2 reaction gas during the dielectric deposition process, the dielectric constant of the alpha-SiCO dielectric barrier films increases with increasing flow rate of O-2 reaction gas. Increasing flow rate of O-2 reaction gas during the deposition of the alpha-SiCO dielectric barrier films also degrades the thermal stability and moisture resistance of the dielectric barrier films. Moreover, the addition of O-2 reaction gas also results in a degraded Cu barrier property of dielectric films. (C) 2004 The Electrochemical Society.