Composite Free Layer MTJ for MRAM Bit Switching
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Solution Overview
Problem
Existing Magnetic Tunneling Junction (MTJ) designs face challenges in optimizing bit switching characteristics while maintaining a high magnetoresistive (MR) ratio and minimizing bit line shorting and switching variations.
Innovation Solution
A composite free layer configuration is introduced, comprising a crystalline ferromagnetic layer with a (001) bcc structure adjoining a MgO tunnel barrier layer and an amorphous layer, specifically a Fe/NiFeHf layer, to enhance bit switching and reduce shorted bits without degrading the MR ratio.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a traditional NiFe free layer is used in MTJ, then reproducible and reliable switching characteristics are achieved, but the TMR ratio is limited and bit line shorting occurs
Solution Approach 1:
The patent employs a composite free layer structure consisting of multiple ferromagnetic layers with different compositions and crystal structures. Specifically, it combines a bcc (001) crystalline layer (CoFeB) that provides high TMR ratio with an fcc amorphous layer (NiFe) that ensures reliable switching characteristics. This composite structure allows simultaneous achievement of high TMR ratio and reproducible switching behavior that neither material could achieve alone.
Solution Approach 2:
The patent applies different material properties to different regions of the free layer. The lower interface with the tunnel barrier uses a bcc (001) crystalline structure optimized for high spin polarization and TMR ratio, while the upper portion uses an fcc amorphous structure optimized for switching reliability. This spatial differentiation of material quality allows each region to contribute its optimal properties to the overall MTJ performance.
2Quantity of substance
If the free layer composition is optimized for high TMR ratio, then magnetoresistive performance improves, but switching field uniformity and thermal stability deteriorate
Solution Approach 1:
The patent uses a composite free layer where the bcc (001) crystalline layer (CoFeB) provides high spin polarization for elevated TMR ratio, while the fcc amorphous layer (NiFe) contributes to uniform switching field and thermal stability. The combination allows the MTJ to achieve high magnetoresistive performance while maintaining consistent switching characteristics across the device array.
Solution Approach 2:
The patent systematically varies the thickness and composition parameters of each layer in the composite free layer. By adjusting the thickness of the bcc crystalline layer to optimize TMR ratio and the thickness of the fcc amorphous layer to ensure switching uniformity, the patent achieves a balanced optimization of both TMR ratio and switching field uniformity through parameter tuning.
3Quantity of substance
If a single crystalline ferromagnetic layer is used, then high TMR ratio is achieved, but bit line shorting and switching variations increase
Solution Approach 1:
The patent employs a composite free layer structure where the bcc (001) crystalline layer provides high TMR ratio while the fcc amorphous layer reduces bit line shorting and switching variations. The amorphous layer's lack of long-range order and different magnetic properties help suppress unwanted magnetic interactions that cause shorting, while the crystalline layer maintains high spin polarization for elevated TMR ratio.
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 reduces the number of shorted bits and lowers the bit switching current while maintaining a high TMR ratio, improving the overall performance of MRAM devices by optimizing bit switching characteristics and thermal stability.
Implementation Method 1
The tunnel barrier layer is thin enough that a current through it can be established by quantum mechanical tunneling of conduction electrons.
Implementation Method 2
When a spin-polarized current transverses a magnetic multilayer in a current perpendicular to plane (CPP) configuration, the spin angular moment of electrons incident on a ferromagnetic layer interacts with magnetic moments of the ferromagnetic layer near the interface between the ferromagnetic and non-magnetic spacer. Through this interaction, the electrons transfer a portion of their angular momentum to the ferromagnetic layer.
Implementation Method 3
A MTJ element may be based on a tunneling magneto-resistance (TMR) effect wherein a stack of layers has a configuration in which two ferromagnetic layers are separated by a thin non-magnetic dielectric layer.
Data Source
AI summary
A MTJ in an MRAM array is disclosed with a composite free layer having a lower crystalline layer contacting a tunnel barrier and an upper amorphous layer for improved bit switching performance. According to one embodiment, the amorphous layer has a NiFeM1/NiFeM2 configuration where M1 and M2 are Mg, Hf, Zr, Nb, or Ta, and M1 is unequal to M2. The crystalline layer is Fe, Ni, or FeB with a thickness of at least 6 Angstroms that affords a high magnetoresistive ratio. The M1 and M2 elements in the NiFeM1 and NiFeM2 layers each have a content of 5 to 30 atomic %. The NiFeM1/NiFeM2 configuration substantially reduces bit line switching current and number of shorted bits. In an alternative embodiment, the crystalline layer may be a Fe/NiFe bilayer. Annealing at 300° C. to 360° C. provides a high magnetoresistive ratio of about 150%.