Electromigration Reliability of Cu₃Sn Microbumps for 3D Heterogeneous Integration

3D heterogeneous integration (HI) and advanced packaging (AP) technologies require small volume, high-density interconnects for stacking discrete chips for which the reliability of interconnects becomes crucial. Intermetallic compounds (IMCs) based μbumps have been shown to outperform solder-based μbumps concerning their resistance to electromigration (EM) related failures, which is a key index to assess the interconnect reliability. Cu-Sn solid-liquid interdiffusion (SLID) bonding is an attractive low-cost wafer-level bonding technology for rapid manufacturing of full Cu₃Sn IMC μbumps, However, SLID requires melting of Sn during the bonding process which poses risks and design challenges in manufacturing. Due to Sn squeeze-out during the bonding process, Sn melt could react with redistribution layers (RDLs) or metallization layers and form IMCs at undesired locations resulting in early failures thereby compromising the reliability. The Sn-squeeze out issue during bonding is addressed in this work by designing test structures with equal and unequal lateral dimensions of μbumps in the top and bottom wafers. The effects of Sn-squeeze out on the EM resistance and reliability are compared in both designs. Significant improvement in the Sn-squeeze out and corresponding EM resistance was observed in the test structures manufactured with unequal lateral dimensions of μbumps in the top and bottom wafers. FE element simulations were carried out to gain insights and assess the impact of Sn squeeze-out on the reliability and functionality of the Cu₃Sn μbumps.