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

基于CoFeB/MgO的层堆叠已经被广泛地研究作为用于理解自旋相关现象和隧穿磁阻（TMR）效应的模型系统，以及由于它们对于诸如磁记录介质、传感器或用于通信应用的微波源的应用的适用性。这种器件依赖于关键的热处理，通过确保层的结晶以及适当的硼迁移，以及通过交换偏置（EB）效应设置参考磁化，来最大化TMR比。虽然这可以通过在磁场存在下的标准真空退火来完成，但基于激光的方法具有几个优点。特别地，EB可以被局部地设置，从而允许跨单个晶片建立不同的参考磁化方向，并且因此实现具有最小磁滞的多维磁场传感器。为此，需要深刻理解激光照射引起的薄膜结构特性的变化，包括层的结晶和扩散机制。有了这个建议，这种基于激光的过程将被引入作为一种工具，以诱导CoFeB单层的结构修改，以及CoFeB层集成在复杂的层系统，如，例如，磁性隧道结。在TMR器件中所涉及的薄膜的表征的挑战将通过结合椭圆偏振和磁光光谱技术来解决，这在我们以前的工作中已经被证明是非常敏感的结构变化，太。在激光退火过程中层的原位光学表征，沿着对辐照的温度动力学的模拟，将进一步允许详细地参数化这些薄膜上的激光退火的热传递动力学的模型。最后，比较烘箱和激光退火，桥接连续平面层系统和完全微制造的TMR器件之间的差距，将进行全面的调查，以获得激光退火的适用性的TMR器件响应的局部增强。这将在负责设置EB的结构性质的背景下进行，并在相当小的温度窗口中获得大的TMR产率，并且相对于结晶和扩散具有相反的依赖性。

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.