Hydrogenated amorphous a-SiC:H and a-SiCN:H with different nitrogen-concentration dielectric barrier films are prepared using the plasma-enhanced chemical vapor deposition technique （RF-PECVD）. The chemical and structural nature, mechanical property, dielectric constant and copper diffusion property of these films are characterized by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy measurement, nano-indentation, Agilent 4294A precision impedance analyzer, Auger electron spectroscopy and field emission transmission electron microscopy. The results indicated that the dielectric constand of the film can be adjusted btween the values of 38 and 52 by controling the NH3/3MS ratio. With the increasing of NH3/3MS ratio, the concentration of Si-N and C-N bond structure is gradually increase and micro-structure of the film becomes more dense, which is the mechanism of the improvement in the mechanical property, thermal stability and copper diffusion property of the a-SiC:H based film produced by N-doping.