Interfacial reactions of Cu/Ta/Si multilayers after thermal treatment were investigated using transmission electron microscopy. The Cu and Ta films were deposited onto Si wafer by ionized metal plasma technique. The samples were then annealed at 400, 500, 550 and 600 degreesC in purified Ar atmosphere for 30 min. The effect of oxygen in the atmosphere on the thermal stability is studied. An interlayer of Ta oxide was observed between Cu and Ta after annealing at 400, 500 and 550 degreesC. It is evident that oxygen as residual gas from furnace ambient can diffuse through Cu grain boundaries to form the Ta oxide layer. After annealing at 600 degreesC, Si reacted with Ta to form TaSi2 at the interface of Ta and Si, in the meantime Cu3Si with surrounding SiO2 formed in the Si substrate. The thermal stability of the Cu/Ta/Si samples was also examined in a two-step annealing treatment of 400 degreesC for 30 min, followed by 600 degreesC for 30 min. Even though interlayers of crystalline Ta-Cu oxide and Ta silicide were formed, Cu silicides were not observed. Formation of TaOx interlayer at the first stage of 400 degreesC annealing may inhibit Cu diffusion into the Si substrate in the second stage of the 600 degreesC annealing process.