Copper metallization for GaAs was evaluated by using Cu/Ta/GaAs multilayers for its thermal stability. A thin Ta layer of 30 nm thickness was sputtered on the GaAs substrate as the diffusion barrier before copper film metallization. As judged from the results of sheet resistance, X-ray diffraction, Auger electron spectroscopy and transmission electron microscopy, the Cu/Ta films on GaAs were very stable up to 500 °C without Cu migration into GaAs. After 550 °C annealing, the interfacial mixing of Ta with GaAs substrate occurred, resulting in the formation of TaGa2 and TaAs2, and the diffusion of Ga and As through the Ta layer formed the Cu3Ga and Cu3As phases at the Cu/Ta interface. After 600 °C annealing, the reaction of GaAs with Ta and Cu formed TaAs and Cu3Ga, as a result of Cu migration and interfacial instability.