Mechanisms of crystallization of CoFeB-based TMR stacks under laser annealing

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CoFeB/MgO based layer stacks have been extensively investigated as model systems for understanding spin-dependent phenomena and the tunneling magnetoresistance (TMR) effect, as well as due to their suitability for applications such as magnetic recording media, sensors, or microwave sources for communication applications. Such devices rely on a crucial thermal treatment, necessary to maximize the TMR ratio by ensuring the crystallization of the layers, together with an appropriate boron migration, as well as setting the reference magnetization through the exchange bias (EB) effect. Whereas this can be done via standard vacuum annealing in the presence of a magnetic field, a laser-based approach presents several advantages. In particular, the EB can be set locally, allowing to establish different reference magnetization directions across a single wafer and therefore to implement multidimensional magnetic field sensors with minimum magnetic hysteresis. For this purpose, a profound understanding of the changes of the thin film structural properties induced by the laser irradiation, including crystallization of the layers and diffusion mechanisms, is required. With this proposal, this laser based procedure will be introduced as a tool to induce structural modifications of CoFeB single layers, as well as CoFeB layers integrated in complex layer systems such as, for instance, magnetic tunnel junctions. The challenges underlying the characterization of the thin films involved in TMR devices will be addressed by combining ellipsometric and magneto-optical spectroscopy techniques, which have been proven in our previous work to be extremely sensitive to structural changes, too. The in-situ optical characterization of the layers during the laser annealing process, along with simulations toward the temperature dynamics of the irradiation, will furthermore allow to parameterize in detail a model of the heat transfer dynamics of the laser annealing on these thin films. Finally, a comprehensive investigation comparing oven and laser annealing, bridging the gap between continuous plane layer systems and completely microfabricated TMR devices, will be performed, to acquire the applicability of laser annealing for local enhancement of the TMR device response. This will be done in the context of the structural properties responsible for setting the EB and to obtain large TMR yields in a rather small temperature window and opposing dependencies with regard to crystallization and diffusion.