Dual Magnetic Tunnel Junction Stack for Low-Current Stable MRAM
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Solution Overview
Problem
The integration of dual magnetic tunnel junctions (DMTJs) into CMOS technologies is hindered by high critical switching current density and instability of the upper pinned layer, which affects the net magnetoresistive ratio and resistance-area product, making it challenging to achieve high-density memory arrays with low production costs.
Innovation Solution
A DMTJ cell design with a free layer sandwiched between a lower and upper tunnel barrier layer, where the upper pinned layer has enhanced magnetization stability through an oxide capping layer or magnetic layers with fcc (111) texture, and antiparallel initialization of pinned layers to reduce switching current and improve resistance-area product.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a dual magnetic tunnel junction (DMTJ) structure is used to reduce critical switching current density, then the switching current is reduced, but the upper pinned layer stability deteriorates due to distance from substrate and roughness of underlying layers
Solution Approach 1:
A RuCr alloy layer is introduced as an intermediate layer between the upper pinned layer and the tunnel barrier. This RuCr layer serves as a mediator that provides both magnetic coupling to stabilize the pinned layer magnetization and a smooth crystalline interface to maintain perpendicular magnetic anisotropy, thereby resolving the stability issue while preserving the low switching current benefit of the DMTJ structure
Solution Approach 2:
The upper pinned layer is constructed using composite materials including CoFeB (cobalt ferrite boride) and RuCr (ruthenium chromium) alloys. The CoFeB provides high spin polarization and low damping, while the RuCr layer provides structural stability and magnetic coupling. This composite structure maintains magnetization stability despite the layer's distance from the substrate
2Power
If the upper pinned layer is positioned far from the substrate to enable DMTJ configuration, then the spin torque effect is enhanced, but the film uniformity and smoothness deteriorate with increasing distance from substrate
Solution Approach 1:
The RuCr alloy layer acts as an intermediary that decouples the upper pinned layer from the rough underlying tunnel barrier. This intermediate layer provides a smooth, uniform crystalline interface for the pinned layer while maintaining the necessary magnetic coupling, thereby preserving film quality despite the increased distance from the substrate
Solution Approach 2:
The RuCr layer is specifically engineered with local properties optimized for its position: it has a body-centered cubic crystal structure that provides perpendicular magnetic anisotropy at its interface with the tunnel barrier, while also providing a smooth surface for the overlying pinned layer. This localized optimization of structure and properties compensates for the distance from substrate
3Power
If antiparallel initialization of pinned layers is implemented to reduce switching current, then the critical current is reduced, but the net magnetoresistive ratio becomes more difficult to optimize
Solution Approach 1:
The resistance-area product of the lower tunnel barrier is specifically engineered to be lower than that of the upper tunnel barrier. This parameter change in the lower barrier compensates for the antiparallel magnetization configuration, ensuring that the net magnetoresistive ratio remains high enough for reliable read operations while maintaining the reduced switching current benefit
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 design achieves a lower critical switching current density and enhanced magnetization stability, optimizing the net magnetoresistive ratio and resistance-area product for advanced MRAM and STT-MRAM devices, enabling higher density memory arrays with reduced production costs.
Implementation Method 1
a metal oxide layer or a magnetic layer with (111) texture is formed on a PL2 top surface to improve PL2 magnetization stability
Implementation Method 2
PL1 and PL2 are first and second pinned layers, that adjoin first and second tunnel barrier layers TB1 and TB2, respectively, and create a spin torque effect on the free layer (FL) when a current is passed through the DMTJ
Implementation Method 3
each with their individual tunneling magnetoresistive (TMR) values
Data Source
AI summary
A dual magnetic tunnel junction (DMTJ) is disclosed with a PL1/TB1/free layer/TB2/PL2/capping layer configuration wherein a first tunnel barrier (TB1) has a substantially lower resistance x area (RA1) product than RA2 for an overlying second tunnel barrier (TB2) to provide an acceptable net magnetoresistive ratio (DRR). Moreover, magnetizations in first and second pinned layers, PL1 and PL2, respectively, are aligned antiparallel to enable a lower critical switching current than when in a parallel alignment. An oxide capping layer having a RACAP is formed on PL2 to provide higher PL2 stability. The condition RA1<RA2 and RACAP<RA2 is achieved when TB1 and the oxide capping layer have one or both of a smaller thickness and a lower oxidation state than TB2, are comprised of conductive (metal) channels in a metal oxide or metal oxynitride matrix, or are comprised of a doped metal oxide or doped metal oxynitride layer.