Electroplating characteristics of eutectic Sn-Cu ions for micro-solder bump on a Si chip

The fabrication of fine bumps on a Si chip is an important issue due to the trend of smaller sized and multi-functioning electronics. In this study, a Sn-Cu near eutectic solder bump was fabricated by electroplating. A Si wafer was used as a substrate, while layers of the Under Bump Metallization (UBM) of Al/Cu/Ni/Au (400/300/400/20 nm in each) were coated onto the Si wafer by electron beam evaporation. The bumps on the UBM were plated by a direct current, and the bump size was 20 × 20 × 10 μm with a 50 μm pitch. Characteristics of the electroplated bumps were examined by XRD, EDS and EPMA. A polarization curve was established to find a potential range of electrodeposition of Sn-Cu. By plating with a reduction current density of 1 A/dm ² for 23 min, a near eutectic Sn-Cu bump was obtained. The bump height increased in current density, namely from 2.25 μm at 0.5 A/dm ² to 6.58 μm at 2 A/dm ² from 10 min of plating. In the electroplated state, a β-Sn and Sn-Cu intermetallic compound (IMC) coexisted in the bumps. Cu ₃Sn and Ni ₃Sn ₄ IMCs were discovered by XRD analysis along the interface between the bump and the UBM.