Stacked Multi-Bit MRAM Cell With Independent Free-Layer Switching
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Solution Overview
Problem
Conventional MRAM cells are limited to storing one bit of data, which restricts storage density and thermal stability, especially at small dimensions.
Innovation Solution
A multiple bit MRAM cell design with alternately stacked tunnel barriers and free layers, where each free layer has a unique switch current, allowing independent switching of magnetic moments to achieve multiple resistance states.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If conventional MRAM cell design with single free layer is used, then device structure is simple, but storage density is limited to one bit
Solution Approach 1:
The single free layer is segmented into multiple free layers (first free layer and second free layer), each capable of independent magnetic moment switching. This segmentation enables multi-bit storage by creating distinct resistance states corresponding to different combinations of magnetic moment orientations across the layers.
Solution Approach 2:
Multiple free layers are nested within a single MRAM cell structure, with each free layer contained between tunnel barriers. The nested arrangement allows multiple storage elements to occupy the same spatial footprint, increasing storage density without proportionally increasing device area.
2Quantity of substance
If multiple free layers with different switch currents are implemented, then multiple bits can be stored, but device complexity increases
Solution Approach 1:
Each free layer is engineered with distinct local properties, specifically different switch currents achieved through varying thicknesses or material compositions. This local differentiation enables independent control of each layer's magnetic moment, allowing multi-bit storage capability while maintaining a unified cell structure.
Solution Approach 2:
The switch current parameter is varied across different free layers to enable selective switching. By changing this critical parameter, the patent achieves independent control over each layer's magnetic state transition, facilitating multi-bit storage through sequential or selective activation of layers based on applied current magnitude.
3Quantity of substance
If free layers are made thin to reduce cell size, then storage density increases, but thermal stability deteriorates
Solution Approach 1:
The patent employs composite magnetic layer structures, combining multiple free layers with distinct magnetic properties within the same cell. This composite approach allows optimization of individual layer thicknesses for thermal stability while maintaining overall small cell dimensions, as each layer contributes differently to the total storage capacity and stability profile.
Solution Approach 2:
Instead of increasing single-layer thickness to improve thermal stability (which would increase cell size), the patent transitions to a multi-layer vertical stacking approach. This dimensional reorganization allows thermal stability to be enhanced through cumulative magnetic moment management across layers rather than through increased individual layer dimensions.
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
Enables the storage of multiple bits by distinguishing between different magnetic moment combinations, enhancing storage density and thermal stability while maintaining sharp switching times.
Implementation Method 1
The N tunnel barriers may be formed to meet perpendicular magnetic anisotropy, PMA, conditions at the plurality of magnet-barrier interfaces to set the switch current of each free layer to be different from each other
Implementation Method 2
The state of the MTJ device is sensed by measuring the resistance. The MTJ has low resistance when the magnetic moment of the free layer parallels that of the fixed layer, and has high resistance when the magnetic moment of the free layer is antiparallel to that of the fixed layer
Data Source
Figure 1~2B
Figure 3A~3B
Figure 4A~4B
AI summary
Disclosed are examples of multiple bit magnetoresistive random access memory (MRAM) cells. A multiple bit MRAM cell may comprise a fixed layer, alternately stacked N tunnel barriers and N free layers, and a tunnel cap. N, which may represent number of bits of the MRAM cell, may be greater than or equal to two. Magnetic moment of the fixed layer may be fixed in one perpendicular direction. Magnetic moments of the free layers may be switchable from one to other perpendicular directions upon application of switch currents. The switch currents may be different for different layers. The magnetic moments of the free layers may be switched separately or otherwise independently of other free layers when the switch currents are applied separately.