Hydrogenated amorphous silicon-carbon alloy thin films (alpha-SiC:H) deposited from C2H2 and SiH4 by the hot-wire chemical-vapor deposition (HWCVD) technique on low-k hydrogen silsesquioxane (HSQ) layers show effective barrier properties against Cu diffusion. These a-SiC:H films with different thicknesses were deposited on HSQ films and the leakage current in a metal-insulator-semiconductor device structure such as Cu/a-SiC:H/HSQ/Si/Al was determined. It was observed that HWCVD a-SiC: H acts as a very efficient diffusion barrier layer on HSQ with an effective dielectric constant of the combined stack much lower than that of SiO2. Secondary-ion-mass spectroscopy analysis indicates that an a-SiC: H film of less than 10 nm would provide the desired barrier effect. (c) 2006 American Vacuum Society.