Electromigration in microbumps with Cu-Sn intermetallic compounds

Abstract

The electromigration test of the microbump interconnects with Cu/Cu₆Sn₅/Cu structure is reported in this study. This Cu₆Sn₅ intermetallic compound layer was single-crystal like. The diameter of the microbumps in die-to-die stacking was 30 μm. Test vehicles were applied by a current density of 2.2×10⁵ A/cm² and settled on a hotplate at 150°C. The resistances of the microbumps were simultaneously monitored by the four point probe during the test procedure. The Cu₆Sn₅ transformed into Cu₃Sn in the early stage and porous Cu₃Sn generated in the Cu₆Sn₅ layer at later stages. The morphology of porous Cu₃Sn could be caused by the current direction. The decomposition of Cu₆Sn₅ into Cu₃Sn and Sn occurs during the EM test. The migration of Sn atoms to the periphery of the under-bump-metallization was forced by the current stress, which may be the mechanism of the pore formation with the pore volume of 41.4%. More porous Cu₃Sn was found in the cathode end which was an evidence of the EM effect. Finite element analysis was used to do some calculation based on theoretical stoichiometry and microstructures in the test results. The resistivity of porous Cu₃Sn structure which was about 30 μΩ-cm which was three times larger than that of Cu₃Sn. The porous Cu₃Sn may threaten the reliability issues of microbumps.