Polarized evolution of interfacial intermetallic compounds (IMCs) in interconnects under electromigration (EM)

Abstract

In situ observation of electromigration (EM) has been carried out on cross-sectioned copper/tin–copper/copper (Cu/Sn0.7Cu/Cu) line-type interconnects. The surface vertical variation after EM is measured by step profiler, which indicates serious mass migration during EM. The EM rate is calculated using two distinct methods, in situ mark movement and the growth of anode intermetallic compounds (IMCs). Defects and IMCs grains in the bulk solder are used as marks to measure the atomic flux. It is calculated that the atomic flux under a current density of 4.77 × 10³ A cm⁻² and a temperature of 60 °C is 5.241 × 10¹² cm⁻² s⁻¹. Calculation based on the growth of the anode IMCs layer thickness determines the atomic flux as 4.114 × 10¹² cm⁻² s⁻¹. The top morphology of IMCs layers is studied by etching away the bulk solder. It is found that the initial scalloped IMCs in the as-reflowed interconnect evolves towards different directions as a serrated IMCs layer with deep grooves forms at the cathode side, while a thick planar IMCs layer forms at the anode side. This phenomenon is defined as polarized evolution. The mechanisms for the polarized evolution rely on the fast diffusion of Cu atoms along the IMCs grain boundaries. The planar IMCs at the anode is due to the stacking of Cu atoms, which fills the grooves by reacting with Sn to form Cu₆Sn₅ IMCs. This polarized evolution threatens the reliability of interconnects since a thick planar IMCs layer tends to be brittle, while a thin serrated IMCs may suffer from accumulated voids.