Within Die Coplanarity Improvement Strategies for Electroplated Cu Pillars

In wafer level packaging, within die Co-Planarity (CoP) of Cu Pillar (CuP) heights has always been critical for ensuring good electrical connections and low stresses in the final package. The recent market trend towards advanced packages imposes even more stringent CoP demands, thus warrants fundamental understanding of CoP drivers as well as innovations in plating technology. Different die layouts and variable pillar dimensions present their own unique challenges for electroplating. They fall broadly into two classes. While deposition rate is a strong factor in driving the CoP in both classes, beyond that there are stark differences. CoP of Class 1 structures is dominated by the locations of the Cu pillars within the die, and only slightly affected by Cu concentration and convection of the electroplating solution. CoP of Class 2 structures is dominated by the size differences of the pillars rather than their locations within the die, and they are highly sensitive to both Cu concentration and convection. In this paper we review the factors influencing CoP for these two classes and present the strategies for CoP improvement in each case.