The passivation effect of ultraviolet light annealing on sodium;contaminated metal oxide semiconductor (MOS) devices was studied. MOS devices were intentionally contaminated with sodium during the interconnect formation process after the gate electrode was formed. After aluminum pad formation, these devices were irradiated with ultraviolet light (wavelength 250 to 370 nm) at elevated temperatures. Sodium-induced threshold voltage of active and parasitic MOS transistors returned to original values during the annealing. It is assumed that positive sodium ions in the oxides are neutralized with electrons excited by ultraviolet light. This passivation effect becomes more prominent as the temperature becomes higher, suggesting that the diffusion of sodium ions in the oxides is closely related to the passivation effect. A model for this temperature-dependent passivation effect is presented. Bias-temperature testing revealed that this passivation effect is remarkably stable for 100 h even at 200 degrees C.