An oxygen plasma treatment of the TiN diffusion barrier prior to Al deposition of an AlSiCu/TiN/Ti structure is studied and compared to an air break process in order to determine the reactions occurring at the Al/TiN interface as well as the mechanisms responsible for the improved barrier performance using an ex situ and an in situ oxygen plasma, respectively. Various experimental techniques such as grazing angle x-ray diffraction, elastic recoil detection and field emission scanning electron microscopy were used. The ternary and quaternary phase diagrams were used to predict the phases formed. It is first observed that an oxygen plasma increases the oxygen concentration at the Al/TiN interface as well as in the TiN barrier. It is conjectured that first the oxygen reacts with Ti in order to form Ti oxides in the TIN grain boundaries and on the TiN surface. The Al film then reacts with this Ti oxide layer in order to form an Al2O3 layer upon sintering which in turn acts as an additional diffusion barrier at the Al/TiN interface. It is also determined that the Al2O3 phase formation is increased after an oxygen plasma treatment of the TiN diffusion barrier. This Al oxide layer as well as the Ti oxides in the TiN grain boundaries could;reduce the Al and Ti intermixing as observed by the reduction in the formation of TiAl3. The reduced Al diffusion in the barrier would then stop Si outdiffusion from the substrate. In addition, the penetration of Si would also be blocked at the Al/TiN interface by the Al2O3 layer. The decreased Si penetration in the upper layers in addition to the lower TiAl3 formation are the mechanisms proposed to explain the decreased formation of Ti(7)A(5)Si(12) after an in situ oxygen plasma treatment of the TiN diffusion barrier. It is concluded that an oxygen plasma-treated TiN barrier is more efficient than an air-exposed TiN one due to the lower degree of Ti7Al5Si12 formation which also explains the reduced risks of junction spiking. The advantage of the in situ oxygen plasma treatment is therefore related to better process control in addition to higher throughput.