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Process control of spin-orbit torques and Tungsten Beta-phase window for SOT-MRAM applications
KTH, School of Electrical Engineering and Computer Science (EECS).
2018 (English)Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

The microelectronics industry is facing major challenges related to the volatility of CMOS cache memory elements (SRAM and eDRAM). Due to decreasing device size, leakage current in standby mode are now dominating the power dissipation of CMOS circuits. There is considerable interest in integrating non-volatile memories (NVM) in memory hierarchy to solve these issues. Magnetic Random Access Memories(MRAM) technology is foreseen as a credible candidate to replace the embedded cache memory due to its non-volatility, high speed, high endurance and high density capabilities. Most of the research done is on Spin-Transfer-Torque (STT) MRAM. Even though this memory is performing extremely good, it has intrinsic limitations for ultrafast memories (L1-L2) cache. To overcome these drawbacks, a new emerging magnetic memory class has been proposed: the Spin-Orbit-Torque (SOT) MRAM. The SOT mechanism allows for decoupling the read and write operations using a novel 3-terminal geometry enabling for reliable deterministic magnetization reversal at sub-ns scale.For generation of SOTs, materials with high Spin-Orbit-Coupling (SOC) are used, they are generally heavy metals (Pt, Ta, W...). Tungsten (W) has been shown to have the highest efficiency in generating SOTs (-30%) and is CMOS technology compatible. However, W presents two issues: i) it has poor adhesion to oxide, ii) it has the particularity to posses two distinct phases: a β phase (Wβ) that is highly resistive and generates high SOT, and an α phase that has a much lower resistivity and very poor efficiency. In addition, β to α-phase transition occurs at ∼4nm, which is problematic to control the fabrication of SOT-MRAM since any process variation can impact SOT.This thesis work concerns the extension of Wβ phase window of Tungsten by doping it and and studying the impact on SOT and magnetic properties. By playing with different combination, we prove the possibility to extend Wβ phase to more than 10nm while improving adhesion to substrate and maintaining SOT efficiency at the same level as pure W. When analysing the magnetic properties, we further show that saturation magnetization is not affected and that we can control the perpendicular magnetic anisotropy value which are keys to the success of SOT-MRAM. Finally, switching studies have been performed in simplified devices and we show that these new W processes enable very low critical switching current density and require very low in-plane external field required for switching deterministically, making these developments a promising solution for inserting W in SOT-MRAM development.

Place, publisher, year, edition, pages
2018. , p. 44
Series
TRITA-EECS-EX ; 2018:511
National Category
Computer and Information Sciences
Identifiers
URN: urn:nbn:se:kth:diva-239392OAI: oai:DiVA.org:kth-239392DiVA, id: diva2:1264914
External cooperation
Imec, Leuven
Examiners
Available from: 2018-11-21 Created: 2018-11-21 Last updated: 2018-11-21Bibliographically approved

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