Cu-ion-migration-induced the porous low-k dielectric breakdown was studied in alternating polarity-bias conditions using a metal–insulator–metal (MIM) structure with Cu as a top electrode. The experimental results indicated that Cu ions migrate into a dielectric film under a positive polarity stress, leading to a shorter time to failure (TTF). Additionally, the TTF obtained in the alternating-polarity test increased with decreasing the stressing frequency, indicating that the backward migration of Cu ions during the reverse-bias stress. Meanwhile, this Cu backward migration effect is effective as the stressing time of a negative polarity stress is larger than 0.1 s. When the frequency is decreased to 10^-2 Hz, the measured TTFs were higher as compared to a direct-current (DC) stress condition. Finally, the electric-field acceleration factor of porous low-k dielectric breakdown became larger under  Cu-ion-migration case.