The electrical resistivity of thin films of Ta and TaN has been examined as a function of film thickness and other processing parameters. The films were deposited by magnetron sputtering of Ta cathodes with Ar, or mixtures of Ar and N-2 for the case of TAN. Ta films deposited on silicon dioxide or Cu surfaces were always beta phase, and showed little resistance increase as the thickness was reduced. The TaN films were deposited in a broad range of compositions, depending on reactive gas flow and system configuration, and showed strong increases in resistivity as thickness was decreased. Ta films deposited on TaNx with x equal to or greater than 1, an electrical resistivity greater than 300 mfg cm, and with thickness greater than 2 nm were alpha phase, and showed significant increases in resistivity as thickness was reduced. The application and electrical properties of these films as diffusion barriers in interconnect structures depends strongly on the thicknesses at the contact points at the bottoms of vias and only weakly on the phase of the Ta. The resistivity of the barrier films on via and trench sidewalls is high enough to be irrelevant to circuit performance.