Electromigration study of copper lines on steps prepared by a plasma-based etch process

Abstract

The electromigration phenomenon of the copper line etched with a plasma-based process over the SiNx step has been investigated. Two important factors, i.e., the dielectric topography and the stress temperature, were examined using the accelerated isothermal electromigration method. The activation energy of 0.73 eV to 0.89 eV indicates two possible mass transport pathways: interfacial and copper surface diffusions. The copper line on the SiNx step has a shorter lifetime and a smaller activation energy than the copper line on the flat surface has. For the former, voids were formed at the cusp region and perpendicular to the current flow direction. For the latter, voids were formed in series and parallel to the current flow direction. The “neck” structure at the cusp region, which is a result of the inappropriate etching condition, further decreased the lifetime and the activation energy. The lifetime of the “neck-free” copper line over the SiNx step was estimated to be 7.1 × 109 s under the high-speed IC operation condition. The thermal stress mismatch between the copper layer and TiW barrier layer as well as the underneath dielectric layer facilitated the void formation. The step effect on the lifetime was reduced when the test temperature was high because of the change of the local stress. In summary, the topography and the test temperature are critical factors for the copper line’s lifetime.