Ta(Si)N thin films were deposited by TCP (transformer coupled plasma) assisted MOCVD (metal organic chemical vapor deposition) using PDEAT (pentakisdiethylaminotantalum) and SiH4 below 350℃. The characteristics of deposited films were analyzed with plasma characteristics and Cu diffusion barrier properties. The films were deposited with variables of substrate temperature, $H_2/(Ar+H_2)$ ratio, pressure, and input power for plasma generation. The plasma properties of plasma temperature (Te), plasma potential (Vp) and plasma density (Ne) were characterizes by Langmuir probe and OES (optical emission spectroscope). The plasma dissociation efficiency was most effective factor for composition and resistivity of the films. The carbon was incorporated in PECVD TaN films in the form of Ta-C bond and resistivity of the films decreased with the Ta-C content increased. The more source was dissociated the more Ta-C bonding reaction occurred, carbon in the film was increased and resistivity was decreased. PECVD TaN film of 30nm thickness deposited at 320℃ showed a resistivity below 1000μΩcm and was stable for Cu during heat treatment up to 600℃. With an appropriate negative bias on substrates, step coverage was improved as well as diffusion barrier property against Cu was improved. Ta-Si-N films of Si added TaN were amorphous and show improved diffusion barrier property for Cu but also increased resistivity with increase in Si content. The Ta-Si-N film of 30nm thickness with Si content of 17.5% showed a resistivity of 1610μΩcm and was stable for Cu up to 700℃ for 50min.