Tantalum nitride (TaN) ultra-thin films deposited by Atomic Layer Deposition (ALD) are efficient diffusion barriers for copper interconnects embedded in silica matrix. The present paper reports a methodical investigation of the first stages of TaN ALD growth on either dense SiO2 or nanoporous SiOCH surfaces. The deposited TaNx phases, film microstructure, chemistry and resistivity were characterized at different steps of ALD growth (from 0 to 160 ALD-cycles). While a granular morphology combining orthorhombic-Ta3N5 (o-Ta3N5) and cubic-TaN (c-TaN) was obtained on SiOCH, a continuous film mainly composed of o-Ta 3 N 5 was found when deposited on dense SiO2. In both cases, TaNx films were not completely-crystallized. The electrical behavior is shown to be mainly driven by film morphology rather than by the intrinsic conductivity of the deposited phases. Growth modes on both surfaces are then discussed in terms of surface chemical reactivity: two growth timelines are proposed to explain the observed phenomena (film continuity, deposited phase, conductivity). As this methodical approach shows the major interest for a deposition on SiO2 substrate, a surface functionalization of SiOCH has also been performed. It was determined that a 3 nm-thick SiO2 layer capping the SiOCH surface is enough to recover the required properties for optimal TaN deposition.