Using Kelvin test structures, electromigration performances of selective CVD tungsten filled vias under dc, pulsed dc, and ac current signals have been studied. The metallization consists of AI-Cu/TiW multilevel metals. The via electromigration lifetime exhibits a current polarity dependence. The via ac lifetimes are found to be much longer (more than 1000 x) than dc lifetimes under the same peak stressing current density. The via lifetimes under pulsed dc stress of 50% duty factor are twice the dc lifetimes at low-frequency regions (< 200 Hz) and 4-5 times the dc lifetimes at high-frequency regions (> 10 kHz). The results are in agreement with the vacancy relaxation model.