Magnetic Tunnel Junctions with Composite Fixed Layers for MRAM
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Solution Overview
Problem
Magnetoresistive random access memory (MRAM) devices with perpendicular magnetic anisotropy (PMA) face challenges in read operations due to low magnetoresistance and increased fixed layer dipolar fields, which affect the bistability and switching mode of magnetic tunnel junctions (MTJs) as they are scaled down.
Innovation Solution
Incorporating a composite fixed layer, synthetic antiferromagnetic (SAF) structure, and dipole layer in the magnetic stack of MTJs to reduce the fixed layer dipolar field and free layer loop offset, thereby enhancing the magnetoresistance and maintaining bistability across the full temperature range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If PMA materials are used in MTJs, then write current density is reduced, but magnetoresistance becomes low due to structural and chemical incompatibility between material layers
Solution Approach 1:
The patent employs composite material structures including CoFeB/MgO/CoFeB tunnel junctions with specific layer compositions and thicknesses. The composite structure of perpendicular magnetization layers combined with tunnel barrier layers creates both the desired PMA for low write current and sufficient magnetoresistance for reliable read operations
Solution Approach 2:
The patent optimizes various parameters including layer thicknesses (e.g., 3-5 nm for CoFeB layers, 1-2 nm for MgO barrier), material compositions, and thermal treatment parameters to achieve the balance between PMA and magnetoresistance. By carefully controlling these parameters, the device achieves both low write current density and high magnetoresistance ratio
2Use of energy by moving object
If MTJ size is reduced to lower write current, then write current decreases, but fixed layer dipolar field increases and affects bistability
Solution Approach 1:
The patent introduces non-magnetic spacer layers (e.g., Ru, Ta, or MgO layers 0.5-2 nm thick) between the fixed layer and adjacent layers. These intermediary layers act as magnetic shields that reduce the dipolar field coupling between the fixed layer and free layer, thereby maintaining bistability in scaled-down devices
Solution Approach 2:
The magnetic stack is segmented into distinct functional layers with non-magnetic spacers separating magnetic layers. This segmentation isolates the fixed layer's dipolar field from affecting the free layer, allowing independent optimization of each layer's properties while maintaining overall device functionality at small dimensions
3Ease of operation
If fixed layer dipolar field is strong, then magnetization switching is affected, but reducing MTJ size increases the dipolar field impact
Solution Approach 1:
Non-magnetic spacer layers are positioned between the fixed layer and other magnetic layers to mediate and reduce the dipolar field interaction. These spacers attenuate the harmful dipolar field while allowing the fixed layer to maintain its magnetization direction for read operations
Solution Approach 2:
The patent utilizes the dipolar field from the fixed layer in a controlled manner by designing the stack structure so that the dipolar field primarily affects the tunnel barrier and free layer magnetization switching in a beneficial way, while the harmful effects are minimized through spacer layers and geometric configuration
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution results in MTJs with high magnetoresistance and reduced fixed layer dipolar fields, ensuring stable resistance states and improved read operations in MRAM devices, even as they are miniaturized.
Implementation Method 1
Magnetic stacks with perpendicular magnetic anisotropy for spin momentum transfer magnetoresistive random access memory
Implementation Method 2
MRAM is a type of solid state memory that uses tunneling magnetoresistance (MR) to store information
Implementation Method 3
The magnetization state of the free layer may be changed by a spin torque switched (STT) write method, in which a write current is applied
Implementation Method 4
Incorporating a composite fixed layer, synthetic antiferromagnetic (SAF) structure, and dipole layer in the magnetic stack of MTJs to reduce the fixed layer dipolar field
Data Source
AI summary
A magnetic tunnel junction (MTJ) includes a magnetic free layer, having a variable magnetization direction; an insulating tunnel barrier located adjacent to the free layer; a magnetic fixed layer having an invariable magnetization direction, the fixed layer disposed adjacent the tunnel barrier such that the tunnel barrier is located between the free layer and the fixed layer, wherein the free layer and the fixed layer have perpendicular magnetic anisotropy; and one or more of: a composite fixed layer, the composite fixed layer comprising a dusting layer, a spacer layer, and a reference layer; a synthetic antiferromagnetic (SAF) fixed layer structure, the SAF fixed layer structure comprising a SAF spacer located between the fixed layer and a second fixed magnetic layer; and a dipole layer, wherein the free layer is located between the dipole layer and the tunnel barrier.