Shinjiro Umehara

Transmission electron microscopy study on the crystallization and boron distribution of CoFeB/MgO/CoFeB magnetic tunnel junctions with various capping layers

High-resolution transmission electron microscopy and electron energy-loss spectroscopy (EELS) were used to study the microstructural properties of CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) with various capping layers. Crystallization of CoFeB layers was strongly dependent on the capping materials, and was affected by B diffusion. With NiFe-cap MTJs, CoFeB crystallized from the cap interface and formed a fcc structure; on the other hand, with Ta- and Ti-cap MTJs, CoFeB crystallized from the MgO interface and formed a bcc structure. EELS analysis showed that B mainly diffused to the capping layers and rarely to the MgO layers with increasing temperature. With Ti-cap MTJs, B diffusion caused hcp-Ti crystals to form an amorphous structure and CoFeB crystallized at lower temperature.

Lower-temperature crystallization of CoFeB in MgO magnetic tunnel junctions by using Ti capping layer

Effects of capping materials on magnetoresistance (MR) properties of MgO magnetic tunnel junctions (MTJs) with a CoFeB free layer were investigated. MR ratios of samples with various capping materials showed a difference in annealing temperature dependence. MTJ with a Ti capping layer annealed at 270°C showed a MR ratio 1.4 times greater than that with a conventional Ta or Ru capping layer. Secondary ion mass spectroscopy and high-resolution transmission electron microscopy images revealed that crystallization of CoFeB was remarkably affected by adjacent materials and the Ti capping layer adjoining CoFeB acted as a boron-absorption layer. These results suggest that the crystallization process can be controlled by choosing proper capping materials. Ti is one of the effective materials that accelerate the crystallization of CoFeB layers at lower annealing temperature.

Highly scalable STT-MRAM with MTJs of top-pinned structure in 1T/1MTJ cell

We report on spin transfer torque magnetoresistance random access memory (STT-MRAM) with magnetic tunnel junctions (MTJs) that have a top-pinned stacking structure. By adopting the top-pinned structure, in which a pinned layer and an antiferromagnetic layer are deposited above the MgO tunnel barrier, we can relieve the current limitation caused by driving power asymmetry of the transistor in a 1T/1M structure without an additional current path to make a reverse connection between the transistor and the top side of the MTJs, resulting in the cell area being reduced by about half.