Perpendicular MRAM Initialization via Magnetic Field Sequences
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Solution Overview
Problem
Perpendicular MRAM devices face challenges with thermal stability and require methods to initialize magnetization directions of magnetic layers effectively, especially for reference bits, which are identical to data bits and suffer from low thermal stability, necessitating costly refresh mechanisms and potential operational slowdowns.
Innovation Solution
A sequence of externally generated magnetic fields is applied to initialize the magnetization directions of the pinned and reference layers in perpendicular MTJ MRAM elements, with varying coercivity fields used to selectively switch the magnetization directions, ensuring the pinned and reference layers are anti-parallel, while the free layer has the lowest coercivity, allowing for reliable initialization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If reference bits are made of identical MRAM cell structure as data bits to simplify fabrication and circuit design, then manufacturing complexity is reduced, but thermal stability of reference bits deteriorates requiring costly refresh mechanisms
Solution Approach 1:
The patent applies local quality by giving the reference layer in the reference bit a different magnetic anisotropy (higher coercivity) compared to the reference layer in data bits. This is achieved through different material composition or thickness in the reference layer, making it more resistant to thermal fluctuations while maintaining the overall MRAM cell structure similarity for ease of fabrication.
Solution Approach 2:
The patent implements preliminary action by initializing the magnetization directions of all magnetic layers (pinned layer, reference layer, and free layer) to specific predetermined directions during or after fabrication. This initialization ensures that reference bits start in a known stable state without requiring continuous refresh mechanisms during operation.
2Reliability
If external magnetic fields are applied to initialize magnetization directions of pinned and reference layers, then thermal stability is improved, but device complexity increases due to additional initialization circuitry
Solution Approach 1:
The patent replaces the conventional approach of using complex initialization circuitry with a simplified method that utilizes externally applied magnetic fields during fabrication or initialization. This substitution reduces device complexity by eliminating the need for sophisticated on-chip initialization circuits while still achieving reliable magnetization alignment.
Solution Approach 2:
The patent employs parameter changes by varying the coercivity of the reference layer through material composition or thickness adjustments. This allows the reference layer to maintain stable magnetization at operating temperatures without requiring complex initialization mechanisms, as the physical parameters of the layer itself provide the necessary stability.
3Reliability
If refresh mechanisms are implemented to ensure reference bit accuracy, then reliability is improved, but operational speed deteriorates
Solution Approach 1:
The patent applies preliminary action by initializing the magnetization directions of all magnetic layers to specific predetermined directions during or after fabrication. This one-time initialization ensures reference bits start in a known stable state without requiring continuous refresh mechanisms during operation, thereby maintaining high operational speed while ensuring reliability.
4Quantity of substance
If perpendicular MRAM devices are scaled to sub-30 nm dimensions to increase data density, then storage capacity is improved, but thermal stability deteriorates
Solution Approach 1:
The patent applies local quality by enhancing the magnetic anisotropy of the reference layer through specific material composition or thickness adjustments. This localized modification ensures that even at sub-30 nm dimensions, the reference layer maintains sufficient thermal stability while the overall device achieves high data density through scaling.
Solution Approach 2:
The patent employs composite materials by using specific material combinations in the magnetic layers, particularly in the reference layer, to achieve both high coercivity for thermal stability and compatibility with sub-30 nm fabrication processes. This allows simultaneous achievement of high data density and maintained thermal stability.
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
This method enhances the thermal stability of MRAM devices by ensuring accurate initialization of magnetization directions, reducing the need for costly refresh mechanisms and maintaining operational speed, making them suitable for high data rate applications.
Implementation Method 1
A sequence of externally generated magnetic fields is applied to initialize the magnetization directions of the pinned and reference layers in perpendicular MTJ MRAM elements
Implementation Method 2
varying coercivity fields used to selectively switch the magnetization directions
Data Source
AI summary
Methods using a sequence of externally generated magnetic fields to initialize the magnetization directions of each of the layers in perpendicular MTJ MRAM elements for data and reference bits when the required magnetization directions are anti-parallel are described. The coercivity of the fixed pinned and reference layers can be made unequal so that one of them can be switched by a magnetic field that will reliably leave the other one unswitched. Embodiments of the invention utilize the different effective coercivity fields of the pinned, reference and free layers to selectively switch the magnetization directions using a sequence of magnetic fields of decreasing strength. Optionally the chip or wafer can be heated to reduce the required field magnitude. It is possible that the first magnetic field in the sequence can be applied during an annealing step in the MRAM manufacture process.