Structure effects on the electrical reliability of fine-pitch Cu micro-bumps for 3D integration

Recently, flip chip solder bumps have been replaced by the fine-pitch solder micro-bump due to the miniaturization of electronic devices and the high performance requirement, and so on. Because of the fast decreasing size of the micro-bump and increasing power consumption needs in logic through-Si via applications, the significance of electromigration among major reliability issues have been increased. There are several important electrical current-induced reliability issues such as current crowding, polarity effect and thermomigration. And the excessive intermetallic compound (IMC) growth and Kirkendall voiding or micro voiding in micro-bump can degrade the mechanical reliability as well as electrical reliability. Therefore, the understanding of fundamental IMC growth mechanism is essential. This study systematically investigated the effects of bump structures such as solder height and UBM structure on the IMC growth kinetics and electromigration performance of Cu micro-bump. Quantitative analyses on the IMC growth kinetics during in-situ electromigration test were performed in a scanning electron microscope chamber under current stressing conditions with current density of 1.5 × 10⁵ A/cm² at 150°C. Under high temperature and electric current stressing, the IMCs growth is accelerated by electron wind force. And the IMC phase transition time became shorter because IMC growth rates increased. In Cu/Sn-Ag and Cu/Ni/Sn-Ag system, the effect of current crowding and Joule heating was negligible in fully IMC-transformed micro-bump. Finally, microvoid formation mechanisms during IMC growth in the Cu/Ni/thin Sn system were discussed in detail.