Measurement of CoFeB configurational ordering in magnetic random-access memory stacks using extended x-ray absorption fine structure spectroscopy

Using extended x-ray absorption fine structure spectroscopy (EXAFS), we demonstrate a quantitative method to evaluate the degree of structural order in thin CoFeB-based structures used in spin-transfer-torque magnetic random access memory (STT-MRAM). This method allows us to access a large-area scale (≈1 cm²) for characterization of the relative degree of crystallization of ultrathin CoFeB (≈2 nm) when the composition and thickness of surrounding materials are varied and the presence of other ordered materials complicates XRD analysis. Our EXAFS-based analysis was applied to multiple combinations of metal- and oxide-capped CoFeB layer structures grown on an MgO layer. We show how specific combinations of free layer capping and underlayers can significantly enhance or lower the relative degree of local order of a thin CoFeB layer. The impact of the capping metal layer thickness and the effect of oxide-based capping layers (metal oxides and MgO) are also presented. Among the elemental metals evaluated in the current work, we find zirconium to be the most efficient in enhancing the crystallization of a 2 nm thick CoFeB layer. We also find that the ability to enhance ordering is not dramatically reduced when using metal oxides rather than pure metal capping. Future STT-MRAM stack optimization can be guided by this comprehensive study and by additional measurements using the proposed quantitative method of assessment.