Increasing the thermal tolerance of reference layer for STT-MRAM manufacturing

Spin-transfer torque magnetoresistive random-access memory (STT-MRAM) writes information by flipping the magnetization direction of a free layer in a magnetic tunnel junction using a spin-polarized current. Compared to conventional MRAM, STT-MRAM’s advantages include high performance and scalability with low power consumption.

Magnetic tunnel junctions consist of a barrier sandwiched between a free layer and a reference layer. Compatibility with CMOS back-end-of-line, a type of fabrication process, requires both the free and reference layers to withstand annealing at 400 degrees Celsius. Honjo et al. developed a new method to enhance the thermal tolerance of a Co/Pt based reference layer in a CoFeB/MgO magnetic tunnel junction.

The authors exposed argon plasma to the platinum buffer layer under the Co/Pt reference layer. They found that this surface modification treatment gives the reference layer more robustness during annealing.

With a cross-sectional scanning transmission electron microscope, the authors determined that, contrary to expectations, their method didn’t change the surface roughness. Instead, X-ray diffraction analysis showed that their surface modification treatment causes a crystallographic change in both the buffer layer and the reference layer. The argon ions penetrated the buffer layer and increased the lattice spacing between Pt sites, while decreasing lattice spacing of the Co/Pt layer. The increase in perpendicular anisotropy enhances the thermal tolerance of the reference layer.

Author Hiroaki Honjo said they achieved this thermal tolerance without using a new material or a special physical vapor deposition machine, which should help make mass production of STT-MRAMs easier.

Source: “Effect of surface modification treatment of buffer layer on thermal tolerance of synthetic ferrimagnetic reference layer in perpendicular-anisotropy magnetic tunnel junctions,” by H. Honjo, S. Ikeda, H. Sato, M. Yasuhira, and T. Endoh, Journal of Applied Physics (2019). The article can be accessed at https://doi.org/10.1063/1.5112017 .