Scaling of the copper interconnect structures requires dielectric barrier materials with a gradually lower dielectric constant that still have adequate copper and moisture barrier properties. In this work, we study the PE-CVD deposition of a-SiCO:H films using octamethyl cyclotetrasiloxane (OMCTS) as a precursor, targeting a dielectric constant of 3.5 or below. The effect of process parameters on film characteristics and chemical bonding structure was investigated. Copper and moisture barrier properties were studied for a selection of films. For an a-SiCO:H film with a dielectric constant as low as 3.4, the intrinsic TDDB lifetime at 1MV/cm exceeds the 10years specification, meaning that it is a copper barrier, at a thickness of 30nm. A Young’s modulus of 14.8±0.8GPa was measured, and a hardness of 1.8±0.1GPa. The plasma etch selectivity to a commercial porous a-SiCO:H film with dielectric constant of 2.5 is 1:2.3. However, moisture barrier properties are doubtful, as was shown by 2 different evaluation methods based on spectroscopic ellipsometry.