Atomic layer deposition (ALD) of ultra-thin barrier layers is a key process for implementation of Cu/low k interconnects. Low k dielectric surfaces are generally characterized by weak surface bonds which have to be properly activated for barrier formation. In this study, we investigated the surface activation of low k dielectrics by atomic hydrogen, nitrogen and ammonia beams for ALD of Ta/TaN barrier layers. In-situ x-ray photoemiision spectroscopy (XPS) together with differential FTIR were used to examine the beam modification of the surface and the subsequent growth on the low k surface. The evolution of the low k surface chemistry revealed an initial transient growth region controlled mainly by the substrate surface chemistry. The initial chemisorption was found to be through formation of Ta-O bonds on organosilicates (OSG) and charge transfer complexes on aromatic SiLK. This led to a significant slower initial nucleation on the OSG low k surface without beam activation. The atomic beams, particularly the nitrogen and amine radials, were found to serve as effective reducing agents in initiating and enhancing the precursor chemisorption. This led to an improvement of the barrier film quality and an increase of the deposition rate at a temperature compatible with low k dielectrics (< 400C). In the subsequent linear growth region, atomic hydrogen species was able to reduce the chlorine content under appropriate temperature and with sufficient purge. The effect of porosity in low k dielectrics on barrier formation will be discussed.