Investigation on the galvanic corrosion of copper during chemical mechanical polishing of ruthenium barrier layer

With the development of ultra-large scale integrated circuits, ruthenium has been used as one of the most promising barrier materials for copper interconnects by replacing the conventional Ta/TaN bilayer film. However, during the polishing process of Ru/Cu structure using KIO₄ slurries, the galvanic corrosion of copper could happen due to the large difference of the corrosion potentials between ruthenium and copper. This paper investigates the galvanic corrosion of copper of the Ru/Cu galvanic couple in KIO₄-based slurries. The results reveal that, in KIO₄ slurries without the addition of effective inhibitors, the galvanic corrosion of copper can greatly accelerate the oxidation and the corrosion of copper, and as a result, more reaction products, such as Cu(IO₄)₂/Cu(IO₃)₂ and CuO, can be rapidly formed on the copper surface, by which the material removal rate (MRR) of copper increases and the defects of copper could be aggravated. According to the theory of galvanic corrosion, two types of methods for controlling the galvanic corrosion of copper are proposed, i.e. the 1st one is using other oxidizer instead and the 2nd one is adding appropriate inhibitors. For the 1st method, KIO₃ is used for demonstration and the effectiveness is verified. For the 2nd method, the combination of benzotriazole (BTA) and a non-ionic surfactant is tested. With the addition of BTA and the non-ionic surfactant, the theoretical galvanic current density decreases by one order of magnitude, and the MRR of copper slightly decreases after copper is connected to ruthenium to form a Ru/Cu galvanic couple. The passivation mechanism of BTA and the non-ionic surfactant on the copper surface can be proposed as follows: firstly the added BTA can form hydrophobic Cu-BTA complex on the copper surface; then the hydrophobic polypropylene oxide segments of the non-ionic surfactant can be effectively absorbed on the hydrophobic Cu-BTA complex as a supplement. These two parts are integrated as a complete passivating film on the copper surface to protect it from corrosion and excessive mechanical abrasion.