Electromigration study of magnetic thin films for the electrical reliability of spin-valve read heads

Electromigration-induced failure (EIF) lifetimes of magnetic thin films used in giant magnetoresistive (GMR) spin-valve materials have been studied to predict the fundamental electrical reliability of GMR spin-valve read head. Magnetic thin films exhibited large failure lifetimes (time to failure, TTF) compared to highly conductive materials such as Cu and Al thin films. In addition, TTF of magnetic thin films was found to have a lognormal distribution and to depend on materials, thermally induced stress, applied dc current density (J), and film geometry. The activation energy and the current density factor n were extracted from the "Black equation." The calculated activation energies for NiFe, CoFe, Co, NiFe-CoFe, and Cu thin films were determined to be 0.8 ± 0.15, 0.9 ± 0.1, 1.4 ± 0.2, 1.17 ± 0.7, and 0.5 ± 0.2 eV, respectively, depending on applied dc current density. The current density factor "n" value" determined from the slope of a plot /ln (J) versus /ln (TTF) at constant temperature was 3.1 ∼ 3.3 for NiFe, 6.1 for CoFe, 4.6 ∼ 5.5 for NiFe-CoFe (or CoFe-NiFe), 3.4 ∼ 4.2 for NiFe-Cu-NiFe and 1.7 for Cu, respectively. Typical electromigration failures such as voids and hillocks were clearly observed at the cathode and center regions of electromigration tested samples.