Abstract
For conventional copper (Cu) damascene processes employed in integrated-circuits fabrication technology, a seed layer is required prior to electroplating of copper metallization. As a result, the surface roughness of the seed layer is expected to affect the structure and quality of the electroplated Cu layer. For 45 nm technology node, the thickness of Cu seed layer will be 30-50 nm with less than 1 nm surface roughness. However, the surface roughness of Cu films after annealing at 200 °C for 1 minute would be 7.5 nm. A rough Cu surface would cause voiding problem between Cu seed layer and electroplated Cu, leading to electromigration reliability issue. In the present study, we have proposed a method to improve the surface roughness of copper thin films by electric current induced surface smoothing. Due to fast surface diffusion of copper atoms, the surface morphology of copper thin film can be modified by electrical current induced atomic migration. Our results show that the surface morphology and film texture of the electrically stressed Cu thin films change notably with time according to the atomic force microscopy (AFM) and X-ray diffraction (XRD) analysis. The roughness of copper thin film was found to improve from the initial 7.5 nm to 4 nm after 30-minute electrical current stressing, and eventually down to 1.4 nm after 10 hours of electrical current stressing. In general, no long range mass transport is expected for metal films stressed by alternating electrical current. However, the surface roughness of Cu films is reduced after alternating electrically stressing. The mechanism of electric current induce surface smoothing is speculated to be related to the ratchet effect.