Backside copper metallization of GaAs MESFETs is studied. A thin Ta layer of 40 nm was sputtered on a GaAs substrate as the diffusion barrier before copper film metallization. X-ray diffraction data show that the Cu/Ta films with GaAs were very stable up to 500 °C. After annealing at 300 °C for two hours, the copper-metallized MESFETs showed very little change in terms of their device characteristics. The changes in the electrical parameters and RF characteristics of the devices after annealing were of the same order as those devices without Cu metallization. These results show that Ta is a good diffusion barrier for Cu in GaAs devices and that Cu/Ta films can be used for the backside copper metallization of GaAs devices.