By using Kelvin test structures, the via reliability with respect to electromigration failure of tungsten and aluminum vias under dc, pulse-dc, and ac stressing have been studied. Our results indicate that although W-plug vias can eliminate the step coverage problem, this metallization system is not ideal because the intermetallic contact represents an undesirable flux divergence location for electromigration. A1 vias are more reliable than W-plug vias with respect to electromigration failure. The unidirectional 50% duty factor pulse-dc lifetime is found to be twice the dc lifetime in the low-frequency region (< 200 Hz) and four times the dc lifetime in the normal frequency region (>10 kHz). The via lifetimes under bidirectional stressing current are found to be orders of magnitude longer than dc lifetimes under the same stressing current density for both W and A1 vias. All the observations are in agreement with a vacancy relaxation model.