Nanotwinning-assisted structurally stable copper for fine-pitch redistribution layer in 2.5D/3D IC packaging

The redistribution layer (RDL) is crucial for fanning out circuits and for 2.5D/3D IC packaging. As device density increases, RDL CD/pitch must shrink. During RDL circuit fabrication, the etching process is necessary to remove the exposed sputtered Cu seed layer and Ti barrier layer. Unfortunately, this process unavoidably etches and roughens the Cu surface of the RDL. As the post-plating resting time (q-time) increases, the Cu surface oxidizes, resulting in even worse surface roughness after etching. This roughness causes contact resistance and electromigration issues when the RDL linewidth shrinks. This paper presents a composite Cu structure that combines fine grains and nanotwins, which is structurally stable and can effectively resist surface damage even after extended periods, offering a promising solution for achieving finer RDL in advanced packaging technologies with a longer electromigration lifetime than regular coarse-grain lines.