Interfacial reactions of Cu/TaNx/Si and silicon nitride/Cu/TaNx/Si multilayers after thermal treatment at 500 and 700 degreesC under an ambient with residual oxygen were investigated using an energy-filtered TEM (EFTEM). The Cu and TaNx films were deposited onto the Si (0 0 1) wafer by ionized metal plasma (IMP) technique. An interlayer of TaOxNy was observed between Cu and TaNx diffusion barrier in the Cu/TaNx/Si sample after 500 degreesC annealing. It is evident that oxygen diffused through the Cu grain boundaries and promotes the oxidation of the Ta nitride barrier layer to form the TaOxNy. It is also found that the as-deposited TaNx (x similar to 0.5) film with nano-crystalline microstructure would transform into Ta2N structure with large grain character after 500 degreesC heat treatment. After 700"C annealing, not only the TaNx barrier layer transformed into Ta2N but the silicidation of Cu to Cu5Si and TaNx to Ta5Si3 occurred. However, no TaO(x)Ny interlayer was observed. This may result from the preferable oxidation of Cu3Si that may suppress the oxidation of TaNx. Nevertheless, in the silicon nitride capped (silicon nitride/Cu/TaNx/Si) case, there was no TaOxNy interlayer observed in the 500 degreesC annealed specimen. And the interfacial reaction in the silicon nitride/Cu/TaNx/Si annealed specimen at 700 degreesC also showed much less severe extent than the sample without capping. Experiments show that the oxygen in the ambient enhances the oxidation at 500 degreesC and silicidation at 700 degreesC.