Structural Analysis of CoFeB/MgO-Based Perpendicular MTJs with Junction Size of 20 nm by STEM Tomography

M. Niwa, K. Kimura, T. Naijo, A. Oshurahunov, S. Nagamachi, H. Inoue, H. Honjo, S. Ikeda, T. Endoh

Research output: Contribution to journalArticlepeer-review


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.

Original languageEnglish
Article number9138472
JournalIEEE Transactions on Magnetics
Issue number2
Publication statusPublished - 2021 Feb


  • Etching process
  • MgO barrier
  • junction size
  • magnetic tunnel junction (MTJ)
  • scanning transmission electron microscope (STEM) tomography
  • spin-transfer torque magneto resistance random access memory (STT-MRAM)
  • tunnel magnetoresistance (TMR)

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Electrical and Electronic Engineering


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