Electromigration in microbumps with Cu-Sn intermetallic compounds

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.