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

CoFeB/MgO基层叠作为模型系统被广泛研究，用于理解自旋相关现象和隧道磁阻(TMR)效应，以及它们在磁记录介质、传感器或用于通信的微波源等应用中的适用性。这类器件依赖于关键的热处理，通过确保层的结晶、适当的硼迁移以及通过交换偏置(EB)效应设置参考磁化来最大化TMR比率。虽然这可以通过在磁场存在的情况下通过标准真空退火来完成，但基于激光的方法有几个优点。具体地说，Eb可以在本地设置，从而允许在单个晶片上建立不同的参考磁化方向，从而实现具有最小磁滞的多维磁场传感器。为此，需要深入了解激光辐照引起的薄膜结构性质的变化，包括薄膜的晶化和扩散机制。通过该提议，将引入这种基于激光的过程作为一种工具，以诱导CoFeB单层以及集成在诸如磁隧道结等复杂层系统中的CoFeB层的结构改变。我们将结合椭圆偏振技术和磁光光谱技术来解决TMR器件中涉及的薄膜表征所面临的挑战，这两种技术在我们以前的工作中也已被证明对结构变化非常敏感。激光退火过程中各层的原位光学表征，以及对辐照温度动态的模拟，将进一步允许对这些薄膜上的激光退火热动力学模型进行详细的参数化。最后，我们将对连续平面层系统和完全微细加工的TMR器件之间的差距--烘箱和激光退火法进行全面的比较研究，以获得激光退火法在局部增强TMR器件响应方面的适用性。这将在负责设定EB的结构性质的背景下进行，并在相当小的温度窗口和关于结晶和扩散的相反依赖关系中获得较大的TMR产率。

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.