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

基于CoFeB/MgO的层堆栈作为理解自旋依赖现象和隧道磁电阻（TMR）效应的模型系统，以及由于它们适用于磁记录介质，传感器或通信应用的微波源等应用，已被广泛研究。这种装置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。虽然这可以通过存在磁场的标准真空退火来完成，但基于激光的方法具有几个优点。特别是，EB可以在局部设置，允许在单个晶圆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，需要深入了解激光照射引起的薄膜结构性质的变化，包括层的结晶和扩散机制。根据这一建议，这种基于激光的程序将作为诱导CoFeB单层结构修饰的工具，以及集成在复杂层系统（例如磁隧道结）中的CoFeB层。TMR器件中涉及的薄膜表征的挑战将通过结合椭偏和磁光光谱技术来解决，这在我们之前的工作中已经被证明对结构变化也非常敏感。在激光退火过程中对层的原位光学表征，以及对辐照温度动力学的模拟，将进一步允许详细参数化这些薄膜上激光退火的传热动力学模型。最后，将对烘箱退火和激光退火进行全面的比较研究，弥补了连续平面层系统和完全微加工TMR器件之间的差距，以获得激光退火在局部增强TMR器件响应方面的适用性。这将在负责设置EB的结构性质的背景下完成，并在相当小的温度窗口内获得大的TMR产率，以及关于结晶和扩散的相反依赖关系。

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.