Composite Free Layer for MRAM Tunnel Junctions
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Current Magnetic Tunneling Junction (MTJ) designs for MRAM devices face challenges in optimizing high tunneling magnetoresistive ratio, low bit-to-bit resistance variation, and reduced bit line shorting while maintaining excellent switching properties, as improvements in one parameter often lead to degradation in others.
Innovation Solution
A composite free layer configuration with a trilayer stack of CoFe/Fe/NiFeHf, where CoFe provides a high MR ratio and NiFeHf reduces shorting, is introduced, allowing for improved switching performance without compromising other MTJ properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a traditional NiFe free layer is used in MTJ, then reliable switching characteristics are achieved, but tunneling magnetoresistive ratio is limited
Solution Approach 1:
The patent employs a composite free layer structure consisting of multiple layers (CoFeB, NiFe, and amorphous layer) rather than a single material. The CoFeB layer provides high TMR ratio, the NiFe layer ensures reliable switching characteristics, and the amorphous layer reduces shorting. This composite approach allows simultaneous achievement of high TMR ratio and reliable switching that cannot be obtained with traditional single-material NiFe free layers.
Solution Approach 2:
Different layers within the composite free layer are assigned different local functions: the CoFeB layer specifically targets high TMR ratio at the MgO interface, the NiFe layer provides switching reliability, and the amorphous layer addresses shorting issues. This localized functional assignment allows each layer to optimize its specific contribution without compromising overall performance.
2Speed
If free layer thickness is reduced to improve switching speed, then switching field decreases, but bit line shorting increases
Solution Approach 1:
The amorphous layer acts as an intermediary between the crystalline free layers and the underlying structures. It specifically addresses the shorting problem that arises when free layer thickness is reduced for faster switching. The amorphous layer provides electrical isolation and prevents direct contact that would cause shorting, enabling thin free layer designs without sacrificing reliability.
Solution Approach 2:
The patent changes the physical state parameter of one layer from crystalline to amorphous to achieve different functional properties. The amorphous layer, being non-crystalline, provides different electrical and magnetic characteristics compared to crystalline layers, specifically reducing shorting while allowing the crystalline layers to maintain their switching performance.
3Measurement precision
If CoFeB is used to achieve high TMR ratio, then reading margin improves, but writing margin deteriorates due to high anisotropy energy
Solution Approach 1:
The free layer is segmented into multiple layers with different thicknesses and compositions. The CoFeB layer is made thin (4-8 nm) to provide high TMR ratio for reading margin, while the additional NiFe layer and amorphous layer compensate for the high anisotropy energy issue, making the overall structure more writable. This segmentation allows independent optimization of reading and writing characteristics.
Solution Approach 2:
The composite structure combines CoFeB (high TMR), NiFe (good switching), and amorphous material (reduced anisotropy) to achieve a balance between reading and writing margins. The combination allows the system to benefit from the high TMR of CoFeB while mitigating its high anisotropy through the other materials.
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 achieves a higher MR ratio, lower switching current, and reduced number of shorted bits, enhancing MRAM performance with improved reading and writing margins and reduced error counts.
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
The spin-transfer effect arises from the spin dependent electron transport properties of ferromagnetic-spacer-ferromagnetic multilayers. 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. The magnetic moment of the free layer may change in response to external magnetic fields and it is the relative orientation of the magnetic moments between the free and pinned layers that determines the tunneling current and therefore the resistance of the tunneling junction.
Data Source
AI summary
A magnetic tunneling junction (MTJ) in an MRAM array is disclosed with a composite free layer having a FL1/FL2/FL3 configuration where FL1 and FL2 are crystalline magnetic layers and FL3 is an amorphous NiFeX layer for improved bit switching performance. FL1 layer is CoFe which affords a high magnetoresistive (MR) ratio when forming an interface with a MgO tunnel barrier. FL2 is Fe to improve switching performance. NiFeX thickness where X is Hf is preferably between 20 to 40 Angstroms to substantially reduce bit line switching current and number of shorted bits. Annealing at 330° C. to 360° C. provides a high MR ratio of 190%. Furthermore, low Hc and Hk are simultaneously achieved with improved bit switching performance and fewer shorts without compromising other MTJ properties such as MR ratio. As a result of high MR ratio and lower bit-to-bit resistance variation, higher reading margin is realized.