CoFeSiB–Pd multilayers and co-deposited alloy films exhibiting perpendicular magnetic anisotropies after heat treatment up to 500 °C

Ferromagnetic layers exhibiting perpendicular magnetic anisotropy (PMA) are essential components in magnetic random access memory cells. Various Co- and Fe-based multilayers (MLs) have been considered because the magnetic properties of MLs can be easily controlled by changing the ferromagnetic and nonmagnetic layer thicknesses and number of bilayers. However, MLs generally suffer from the loss of PMA after high-temperature heat treatments (beyond approximately 400 °C) during device processing, which are associated with back-end-of-line device fabrication. Moreover, ML deposition may limit the throughput of the film mass production. Here, we investigate the PMA stability of sputter-deposited CoFeSiB/Pd ML as well as CoFeSiB–Pd co-deposited alloy (CA) films exposed to various annealing temperatures. Although the ML films exhibited PMA, regardless of the annealing temperature up to 500 °C, the CA films started to exhibit strong PMA after 400 °C, which was maintained up to 500 °C. To understand the magnetic property changes, we performed microstructural analyses. The ML films in the as-deposited state exhibited well-defined layered microstructures and evolved into polycrystalline alloys at 500 °C. In contrast, the CA films in the as-deposited state exhibited an amorphous microstructure but transformed into polycrystalline alloys at 500 °C, similar to their ML counterparts. Both Co and Pd, the major constituents of the films, underwent interdiffusion and formed a Co–Pd ordered phase, which contributed to the appearance of PMA.