Date of Award

Summer 7-2016

Embargo Period

5-11-2017

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Electrical and Computer Engineering

Advisor(s)

Mark H. Kryder

Abstract

The objective of the research conducted herein was to develop L10-ordered materials and thin film stack structures with high perpendicular magnetic anisotropy (PMA) for spin-transfertorque magnetoresistive random access memory (STT-MRAM) applications. A systematic approach was taken in this dissertation, culminating in exchange coupled L10-FePt and L10- MnAl heterogeneous structures showing great promise for developing perpendicular magnetic tunnel junctions (pMTJs) with both high thermal stability and low critical switching current. First, using MgO underlayers on Si substrates, sputtered MnAl films were systematically optimized, ultimately producing a Si substrate/MgO (20 nm)/MnAl (30)/Ta (5) film stack with a high degree of ordering and large PMA. Next, noting the incompatibility of insulating MgO underlayers with industrial-scale CMOS processes, attention was turned to using conductive underlayers. TiN was found to excel at promoting growth of L10-MnAl, with optimized films showing improved magnetic properties over those fabricated on MgO underlayers. Although the MnAl films grown on TiN underlayers on Si substrates demonstrated good magnetic properties, it was found that the high deposition and ordering temperatures contributed to high film roughness. In an effort to reduce ordering temperature and surface roughness of L10- MnAl films, adding other materials (Ni, C, and SiOx) to the MnAl film in conjunction with various underlayers was studied. MnAl:Ni films with a fixed 3 volume% Ni content deposited on TiN underlayers revealed that PMA was reduced compared to MnAl films, and surface roughness increased dramatically below the ordering temperature. MnAl:C films with 1volume% C showed an increase in PMA, while C in excess of the solubility limit (1.7 atomic %) diffused to the grain edges, degrading PMA. MnAl:SiOx films demonstrated poor PMA. The use of different post-annealing processes was then studied as an alternative to in situ annealing. Rapid thermal annealing (RTA) was found to produce PMA in films at lower annealing temperatures than tube furnace annealing, but tube furnace annealing produced films with higher maximum PMA than RTA. While annealed samples had lower surface roughness than those ordered by high in situ deposition temperatures, relying solely on annealing to achieve L10-ordering resulted drastically reduced PMA. Since the material additions, underlayer systems, and annealing techniques studied either did not reduce film roughness or resulted in reduced PMA of thin films, attention was turned to MTJ stack structures employing heterogeneous material systems for top and bottom electrodes, which might produce film stacks with both high PMA and low surface roughness. As a way to potentially mitigate roughness issues with using MnAl-based thin films as both free and reference layers in an MTJ, exchange coupled heterogeneous structures were studied. Given the high PMA of L10-FePt and low damping of L10-MnAl, L10-FePt/MnAl heterogeneous structures were studied as a way to take advantage of STT potentially being a surface process. Unfortunately, depositing the MnAl at elevated temperatures resulted in interdiffusion between FePt and MnAl, and caused a degradation in PMA. High- and low-anisotropy thin films separated by a thin barrier were then examined in the form of in-plane hard-FePt/barrier layer/inplane soft-FePt film stacks. It was found that significant exchange coupling energy was still observed at barrier thicknesses of around 1 nm. Since scaled MTJs have tunnel barriers below 1 nm, interlayer exchange coupling between the electrodes might thus be used for partially pinning the free layer, thereby increasing effective PMA. It is suggested that pinning a low-damping free layer by a high-PMA reference layer may therefore result in an MTJ with both high effective PMA and low effective damping. Finally, heterogeneous L10-ordered FePt/MgO/MnAl film stacks were explored for pMTJs. Film stacks with MgO barrier layers thinner than 2 nm showed significant interdiffusion between the FePt and MnAl, while film stacks with thicker MgO barrier layers exhibited good ordering and high PMA in both the FePt and MnAl films. It is believed that this limitation is caused by the roughness of the underlying FePt, which was thicker than 2 nm. Unfortunately, MgO barrier layers thinner than 2 nm are needed to make good MTJs. With further study, thin, continuous barriers may be achievable for high-PMA, L10- ordered materials with more materials exploration, deposition optimization, and more advanced thin film processing techniques and fabrication equipment. Use of appropriate underlayers, capping layers, dopant elements, and improved fabrication techniques may help reduce surface roughness while preserving PMA. If smooth electrodes can be developed, the heterogeneous structures discussed have great potential in taking advantage of exchange coupling for developing pMTJs with both high thermal stability and low critical switching current.

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