The introduction of functionally improved materials such as magnetic heterostructures in magnetic tunnel junctions (MTJs) is a major driver to enable the continued down-scaling of circuit density and performance enhancement in logic and memory devices beyond the roadmap. In this talk, I will discuss current research advances in multifunctional and complex material systems, especially in engineering and patterning of complex stacks of thin films at the atomic scale.
Understanding and control of the factors affecting the interfacial phenomena at the CoFeB|MgO interface in a MTJ, from which the perpendicular magnetic anisotropy (PMA) of the CoFeB originates, are crucial to the realization of MTJ’s full potential. Efficient manipulation of PMA using an applied voltage, known as the voltage-controlled magnetic anisotropy (VCMA) effect, offers significant energy savings over electric-current-controlled alternative memory devices. Ab initio studies in the literature revealed a dependence of the VCMA effect on the oxidation state of interfacial Fe atoms in an Fe/MgO interface and on the heavy metal insertion layers at the CoFeB/MgO interface. This work addresses experimentally the dependence on the VCMA effect of oxide and metallic insertion layers in the MTJs. For oxide insertion layers, an atomic layer deposited lead zirconium titanate (PZT) layer in a MgO/PZT/MgO structure resulted in a 40% increase in the VCMA coefficient, despite the PZT layer being amorphous. For metallic insertion layers, Mg was found to be the most effective and a 1.1-1.3nm layer improved the VCMA coefficient by more than a factor of 3 and gave rise to the highest perpendicular magnetic anisotropy and saturation magnetization, as well as to the best CoFe and MgO crystallinity. These results demonstrate that a precise control over the material’s type and oxidation level in the MTJ is crucial for the development of electric-field-controlled perpendicular magnetic tunnel junctions with low write voltages.
Finally, to address the high density integration challenges of MTJs, a generalized methodology, combining thermodynamic assessment and kinetic verification of surface reactions, will be presented to define atomic layer etching processes that enable the patterning of these complex magnetic and noble metal heterostructures, including etching efficacy, directionality, and selectivity at the atomic scale.