TiN and TaN thin films are proposed as barrier layers between copper interconnects and low dielectric constant (low-k) polymers. As the barrier layer thickness is scaled down, the uniformity and morphology of these films will be severely affected by the nitride-polymer interface and become an important issue for the reliability of the whole interconnect structure. In order to evaluate nitride formation and the interfacial chemistry we deposited TiN and TaN on fully cured low-k polymers by two different techniques: reactive evaporation of the metal in nitrogen ambient and by ion assisted reactive deposition using a low energy (100 eV) nitrogen ion beam during evaporation. Photoelectron spectra were recorded in situ for metal coverages from 0.1 nm until bulk like metal or metal nitride spectra were obtained. Nitride concentrations, extracted from the photoelectron spectra, show that even though very similar nitride films are produced by both techniques for thicker films (>5 nm) we only find significant amounts of nitride at the interface in the ion assisted case. Thinner films formed in nitrogen ambient were very similar to those where the pure metal was deposited and were dominated by the formation of compounds with carbon and oxygen from the polymer. This shows that the composition of barrier layers can be drastically altered near the polymer interface. Low energy ions in contrast allow the growth of more homogeneous films which can significantly improve the reliability of copper based high density interconnects.