We have studied the isothermal relaxation behavior of 500 nm Cu films with SiNx passivation and barrier layers on Si substrates. Oxygen content in the Cu films was varied by deposition in various oxygen partial pressures in the range 10-10 to 10-4 Torr. The substrate curvature method was used to investigate the thermomechanical behavior of the Cu films. Isothermal relaxation experiments were performed in the temperature range 50-175°C. Comparison with constitutive creep deformation equations shows that the relaxation data in this temperature range are well described by power law behavior. Under certain isothermal conditions related to temperature, stress, and thermal history, anelastic recovery was observed– i.e. while in a tensile stress state and with the temperature held constant, the overall film stress was seen to increase over a relatively short time scale.