The effect of the thickness of low-k polymer dielectrics on electromigration characteristics has been investigated. The dielectrics used in this study were several kinds of polyimides and SiLKTM-G. Low-pressure chemical vapor deposition (LPCVD)-SiO2 was used as a reference. It was found that the microstructure of Al thin films deposited on SiO2 and various polymer structures are almost the same. However, the electromigration (EM) lifetime of the Al/low-k polymer was approximately one order of magnitude shorter than that of Al/SiO2, and the measured lifetime decreased logarithmically with the thickness of polymer materials. The short EM lifetime of Al/low-k polymer was directly related to the temperature increase of IU lines due to Joule heating, since the thermal conductivity of the employed polymer films are approximately one order lower than that of SiO2. The poor thermal conduction in the polymers could also explain why EM lifetime decreased with the thickness of polymer dielectric layers and in order to apply low-k polymer to a future interlayer dielectric (ILD) that will have three to seven levels, the thickness effect of Al/polymer due to the Joule heating should be of serious concern.