High thermostability in CoFeB/MgO/Ta multilayers by {Pt(t)/Ta(t)}_n superlattice buffer layers

Ta/CoFeB/MgO/Ta multilayer known as the fundamental building block for magnetic tunneling junctions, has been studied for decades. However, its thermostability is normally less than 300 C-degrees, which cannot meet the requirements for subsequent semiconductor processes. In this study, buffer layers of {Pt(t)/Ta(t)}n superlattices were introduced into CoFeB/MgO/Ta multilayers grown by magnetron sputtering. The growth period n and the single layer thickness t in the superlattice have been systematically explored. And it is found that the sample can maintain distinct perpendicular magnetic anisotropy (PMA) even after a 440 C-degrees post-annealing treatment at n = 3 and t = 1.5 nm. This has been attributed to that the superlattice buffer layer might serve as a diffusion barrier, reducing Ta atoms from diffusing into the CoFeB/MgO interface. Less Ta at the interface results more FeOx formed and enhances the hybridization of Fe 2p and O 3d orbitals which increases the thermostability of PMA. This suggests that superlattice buffer layer provides a new approach to improve the materials' thermostability.