Multi-bit Perpendicular Magnetic Tunnel Junction for High-Density Memory
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Solution Overview
Problem
Conventional perpendicular magnetic tunnel junctions (p-MTJ) with a full spin valve structure support only single-bit operation, making it difficult to replace NAND flash memory, and the process complexity increases due to the use of rare-earth elements in synthetic antiferromagnetic layers, which leads to severe diffusion issues.
Innovation Solution
A multi-bit perpendicular magnetic tunnel junction is designed with a structure comprising an upper synthetic antiferromagnetic layer, a pinned layer, a lower dual free layer, and an upper free layer, allowing for four resistance states based on magnetization directions, enabling multi-bit operation and high integration through a simpler structure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a conventional full p-MTJ spin valve structure is used, then single-bit operation is supported, but multi-bit operation capability is lost
Solution Approach 1:
The free layer is segmented into multiple sub-layers (first free layer, second free layer, third free layer) with different magnetization directions. Each sub-layer can be independently controlled to achieve different resistance states, enabling multi-bit operation while maintaining a manageable structural complexity through systematic division of functional layers.
Solution Approach 2:
The patent introduces a vertical stacking dimension by laminating multiple free layers and tunnel barrier layers in the thickness direction. This vertical arrangement creates additional states along the depth dimension, allowing the device to encode multiple bits of information through the combination of magnetization directions across different vertical layers.
2Reliability
If rare-earth elements are used in synthetic antiferromagnetic layers, then perpendicular magnetism is fixed, but diffusion issues worsen
Solution Approach 1:
The patent extracts and removes rare-earth elements from the synthetic antiferromagnetic layer structure. Instead, it uses conventional ferromagnetic materials with controlled magnetization directions, thereby eliminating the harmful diffusion effects of rare-earth elements while maintaining the necessary magnetic stability through alternative design approaches.
Solution Approach 2:
The patent changes the material composition parameters by substituting rare-earth elements with conventional ferromagnetic materials. It adjusts the thickness and magnetization characteristics of the free layers to achieve the desired perpendicular magnetism stability without relying on rare-earth elements, thereby resolving the diffusion issue while maintaining reliability.
3Adaptability or versatility
If the number of layers is increased, then multi-bit operation is enabled, but process cost and process time increase
Solution Approach 1:
The patent merges multiple functional layers (free layers, tunnel barrier layers, and antiferromagnetic layers) into a vertically stacked configuration where each layer serves multiple purposes. For example, the synthetic antiferromagnetic layers provide both magnetic coupling and structural support, reducing the need for additional dedicated layers and thereby controlling process complexity while enabling multi-bit operation.
Solution Approach 2:
The patent designs layers with multi-functional characteristics where the same layer structure performs multiple functions. The tunnel barrier layers serve both as electrical insulation and as templates for magnetization direction control, while the free layers simultaneously store information and provide magnetic coupling. This multi-functionality reduces the total number of processing steps required.
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 multi-bit perpendicular magnetic tunnel junction achieves four distinct resistance states by varying the magnetization directions of the dual free layers, enabling efficient multi-bit operation and reducing process complexity, thus potentially replacing NAND flash memory while improving integration and reliability.
Implementation Method 1
when the magnetization direction of a pinned layer and the magnetization direction of a free layer are parallel, the state of the full p-MTJ spin valve transitions to a parallel state, and when the directions are anti-parallel, the state of full p-MTJ spin valve transitions to an anti-parallel state
Implementation Method 2
a synthetic antiferromagnetic layer for fixing the perpendicular magnetism of a CoFeB pinned layer in a spin valve
Data Source
AI summary
Disclosed is a memory device including a multi-bit perpendicular magnetic tunnel junction, wherein the multi-bit perpendicular magnetic tunnel junction includes an upper synthetic antiferromagnetic layer, pinned layer, lower dual free layer, and upper free layer formed in a laminated manner between a top electrode and a bottom electrode.