Low Stress and High Ductility Copper Electroplating Additive Development

Copper electroplating is one of the most important metallization technologies to produce metal contacts in the fabrication of crystalline silicon solar cells. It has been adopted in three areas, including: Interdigited Back Contact cell (IBC); Heterojunction with Intrinsic Thin layer (HIT); and Fully Plated Front Contact solar cells. Due to cost concerns, significant changes have been made in the silicon manufacturing process leading to thinner Si wafers.  This change brings many challenges for copper metallization. One of the biggest challenges is to maintain a low internal stress of the copper deposit.  This is key to avoid solar wafer warpage and maintain good adhesion. Copper deposits with high ductility are also desired to prevent or reduce the potential for the copper deposit to crack under stress over time.  Ideal copper deposits exhibit low internal stress and high ductility. However, development of this plating chemistry is a challenge for the material supplier, as improvement in one attribute typically comes at the expense of another.  Accordingly, there is need to develop organic additives for copper electroplating baths that alleviate internal stress in copper deposits as well as providing good ductility. In this paper, the performance of new acid copper electroplating bath capable of achieving both low internal stress and good ductility will be highlighted. Additive impact on copper deposit material properties and correlation between these properties and performance will be discussed