Electroplating nanotwinned copper for ultrafine pitch redistribution layer (RDL) of advanced packaging technology

Copper is the most common interconnected materials in the field of microelectronic packaging, and it is widely used in various advanced package technologies, such as redistribution layer (RDL), copper pillar (CuP) and under bump metallization (UBM) etc. However, the rapid development of packaging technology, decreasing size and increasing Joule heat make a great challenge on the mechanical properties and thermal stability of copper interconnected materials. It was widely reported that the nanotwinned copper had very high strength and excellent conductivity, which was quite suitable for the next generation interconnected materials. However, different from electroplating thin films, the achievement of uniformity and flatness for electroplating ultra-fine pitch nanotwinned RDLs would be difficult since the line width was as low as 1.5µm. Through the study of growth mechanism and additives effect, we had invented an acid nanotwinned copper electrolyte as reported previously. So in our study, the evaluation of modified nanotwinned copper electrolyte was conducted in order to clarify its performance of on the electroplating ultra-fine pitch RDLs. Also, three kinds of commercial RDL electrolytes were served as comparisons. In detail, different electrolytes were evaluated from the aspect of microstructure, hardness, thermal stability, uniformity and flatness. Firstly, the morphology of electroplating copper film was characterized by focus ion beam (FIB), and then the hardness was measured. The thermal stability was revealed by the hardness evolution before and after heat treatment test. Finally, the 1.5µm, 4pm and 1.5µm RDLs were electroplated on a patterned wafer, and their uniformity and flatness were calculated. Both uniformity and flatness showed a decreasing tendency basically when the line width kept decreasing. The uniformity and flatness were 6.73~13.89% and 2.32~4.17% respectively for nanotwinned copper electrolyte, which was at the upper level compared to the commercial RDL electrolytes.