Electromigration-Induced Failure Characteristics of Spin-Valve Multilayers for Metallic-Based Spintronic Devices

摘要

Electromigration (EM) failure characteristics of patterned NiFe/Cu/NiFe spin-valve (SV) multilayers (MLs) with or without thin Co diffusion barrier inserted at the interface between NiFe and Cu layers have been investigated by applying a constant dc current with different current densities to predict the electrical and magnetic stabilities of metallic SV-based spintronic devices. It was experimentally verified that the Ni–Cu intermixing caused by the EM-induced Cu interdiffusion to the top or bottom NiFe layer plays a dominant role in determining the EM-induced failure characteristics of the NiFe/(Co)/Cu/(Co)/NiFe SV multilayered devices. In addition, it was clearly demonstrated that the interlayer (indirect) exchange coupling field and the magnetic moment of the NiFe/(Co)/Cu/(Co)/NiFe SV-MLs are strongly dependent on the EM-failure induced Ni–Cu intermixing directly relevant to the Cu spacer interdiffusion. The failure mechanism of NiFe/(Co)/Cu/(Co)/NiFe SV multilayered devices showed “bimodal failure characteristics.” The critical current density $(J_{rm c})$ for such a bimodal failure mechanism was found to be determined at $J_{rm c} = hbox{7} times hbox{10}^{7} hbox{A/cm}^{2}$. When $J leq J_{rm c}$, the failure was mainly caused by the electrostatic force (or electron wind force) accelerating an interdiffusion through grain boundaries that leads to forming typical EM failures such as voids and hillocks. Whereas, when $J ≫ J_{ rm c}$, a melting or a vaporization dominantly accelerated by the Joule heating played more significant role and caused the catastrophic failures. The NiFe/Co/Cu/Co/NiFe SV-MLs showed a much longer mean time-to-failure $(t_{50})$ than that of NiFe/Cu/NiFe SV-MLs. This experimental result implies that an ultrathin Co insertion layer is effective to improve the EM-induced failure lifetime due to its restraining effects as a diffusion barrier to prevent Cu interdiffusion to both the top and bottom NiFe layers. Furthermore, this result directly verified that Ni–Cu intermixing is the dominant factor in determining the EM characteristics of the NiFe/Cu/NiFe SV multilayered spintronic devices.