Stacked Magnetic Tunnel Junctions With Shared Pinning Layer
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Solution Overview
Problem
Current magnetoresistive random access memory (MRAM) technologies face limitations in achieving high memory densities due to the inability to efficiently configure multiple distinct magnetic states in vertical magnetic tunnel junctions (MTJs) for multi-bit storage and memory cells.
Innovation Solution
The proposed solution involves a magnetic tunnel junction unit with two MTJs stacked vertically, sharing a common pinning layer and having different STT state switching current thresholds, allowing for four distinct magnetic states and enabling multi-bit storage by utilizing a common pinning layer to reduce stack height and processing steps, while the different thicknesses of texture breaking layers facilitate independent tuning of stray fields.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a common pinning layer is shared between multiple MTJs, then device complexity and stack height are reduced, but it becomes difficult to configure multiple distinct magnetic states for multi-bit storage
Solution Approach 1:
The patent applies local quality by making each MTJ's free layer unique through different thicknesses and/or compositions, while sharing the common pinning layer. This allows each MTJ to have distinct magnetic properties and switching characteristics, enabling multi-bit storage states despite the shared reference structure.
Solution Approach 2:
The patent changes physical parameters of the free layers (thickness, composition) to create MTJs with different switching current thresholds. By varying these parameters, multiple stable magnetic states are achieved, allowing multi-bit storage while maintaining a simplified common pinning layer structure.
2Adaptability or versatility
If multiple MTJs are stacked vertically with separate pinning layers, then each MTJ can be independently controlled, but the stack height and number of processing steps increase
Solution Approach 1:
The patent merges multiple reference layers into a single common pinning layer that serves all MTJs. This consolidation reduces the total number of layers in the stack, decreasing stack height and simplifying fabrication while maintaining independent control of each free layer through its unique properties.
Solution Approach 2:
The common pinning layer performs multiple functions simultaneously: it provides the reference magnetization for all MTJs, establishes the antiparallel coupling for each free layer, and reduces overall stack complexity. This multi-functional design eliminates the need for separate pinning layers for each MTJ.
3Ease of manufacture
If texture breaking layers have the same thickness, then fabrication is simplified, but stray fields cannot be independently tuned for each MTJ
Solution Approach 1:
The patent applies local quality by assigning different thicknesses to texture breaking layers positioned between each free layer and the common pinning layer. This allows independent control of stray field effects for each MTJ, enabling precise tuning of magnetic properties while maintaining overall fabrication simplicity through a systematic variation approach.
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 configuration allows for the achievement of four distinct magnetic states, enabling two-bit logical levels and reducing the complexity of layer fabrication, thereby enhancing memory density and readout separation.
Implementation Method 1
The magnetization direction may be switched through the spin-torque transfer (STT) effect, wherein the magnetization direction is changed by passing a relatively high current through the MTJ, perpendicular to the layers forming the MTJ.
Implementation Method 2
a first coupling layer arranged between the first reference layer and the pinning layer, wherein a magnetization of the first reference layer is coupled antiparallel to a magnetization of the pinning layer, through the first coupling layer
Data Source
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AI summary
According to an aspect of the present inventive concept there is provided an MTJ unit, comprising: a first MTJ comprising, in a bottom-up direction, a first free layer, a first tunnel barrier layer and a first reference layer, wherein the first MTJ is switchable between a parallel state and an anti-parallel state through spin-torque transfer, STT, wherein the first MTJ has a first STT state switching current threshold, and the first MTJ has a first resistance when in the parallel state and a second resistance when in the anti-parallel state; a second MTJ arranged above the first MTJ and comprising, in a bottom-up direction, a second reference layer, a second tunnel barrier layer and a second free layer, wherein the second MTJ is switchable between a parallel state and an anti-parallel state through STT, wherein the second MTJ has a second STT state switching current threshold different from the first STT state switching current threshold, and the second MTJ has a third resistance when in the parallel state and a fourth resistance when in the anti-parallel state, wherein the first resistance through fourth resistance are different; and a pinning layer arranged between the first reference layer and the second reference layer and configured to fix a magnetization direction of the first reference layer and the second refence layer.