We have successfully observed the process-induced microstructure of the ultrafine CoFeB-MgO-based magnetic tunnel junctions with a perpendicular easy axis (p-MTJs) down to junction size of less than 20 nm using scanning transmission electron microscope (STEM) tomography combined with energy-dispersive X-ray spectroscopy (EDX). This made it possible to examine 3-dimensionally the precise structural information inside the entire MTJ. By this, the root cause of the change in the resistance of the MTJ was found to be the altered region consisting of Fe-dipping attached to the edge of the MgO and ultrathin TaO layer uniformly formed on the MTJ surface. This altered region is caused by etching during patterning of the MTJ, and the resistance value of the MTJ changes depending on the degree of adhesion of insulating TaO and Fe atoms at the edge of the MgO barrier layer. Furthermore, it was found that the unevenness of the MgO/CoFeB interface inside the MTJ was not a factor that caused the change in the thickness of the MgO barrier layer in the current process. Our results show that STEM tomography is an effective tool for failure analysis to clarify the relationship between the detailed structure of fine MTJs and magnetic properties. Unlike the conventional cross-sectional STEM observation, since the STEM tomographic observation includes the whole view of the MTJ, this enables higher resolution observation as the MTJ size decreases without overlooking the cause of failure inside the MTJ.
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