Multi-state MRAM via Layer Segmentation
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Solution Overview
Problem
Conventional magnetic random access memory (MRAM) designs face challenges such as the orange peel effect, magnetic charge biases, limited material choices due to coercivity requirements, scaling difficulties, and the inability to store multiple information states in a single cell, while thermal-assisted switching MRAM designs suffer from high heating current issues and tunneling layer breakdown risks.
Innovation Solution
The solution involves splitting the free layer of a magnetic tunnel junction (MTJ) into a read-sensing layer with little anisotropy and an information storage layer with anisotropy, using thermal-assisted writing and a second bit line to control magnetization direction, allowing for multiple information states and eliminating the need for high voltages across the tunneling barrier.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If thermal-assisted switching is used to address half-select and scaling issues, then the half-select problem is eliminated and scaling becomes feasible, but high heating current is required and tunneling layer breakdown risk increases
Solution Approach 1:
The free layer is segmented into two distinct parts: a read-sensing free layer within the MTJ stack and an information storage free layer with anisotropy. This segmentation allows the heating current to act only on the storage layer during write operations, preventing it from passing through the tunneling barrier and reducing breakdown risk while maintaining thermal-assisted switching effectiveness
Solution Approach 2:
A second bit line is introduced as an intermediary to deliver heating current specifically to the information storage free layer. This intermediary structure enables precise control of thermal assistance without requiring high voltages across the tunneling barrier, solving the contradiction between effective heating and tunneling layer protection
2Device complexity
If the free layer is used for both read and write operations in conventional MRAM, then the structure is simple, but the half-select problem causes unintended cell switching
Solution Approach 1:
The free layer is divided into two functional segments: a read-sensing free layer that remains in the MTJ stack for read operations, and an information storage free layer with anisotropy that receives heating current during write operations. This segmentation eliminates the half-select problem by ensuring that only the selected cell's storage layer is thermally assisted during writing, preventing unintended switching of non-selected cells
Solution Approach 2:
Anisotropy is selectively introduced only in the information storage free layer, not in the read-sensing free layer. This local differentiation in magnetic properties allows the storage layer to maintain stable magnetization directions while the read layer remains responsive to external fields during sensing operations
3Device complexity
If shape anisotropy is used to maintain stored information, then the structure is simple, but switching field scales inversely with cell dimension making scaling difficult
Solution Approach 1:
The patent changes the mechanism from shape anisotropy to exchange anisotropy induced by an antiferromagnetic layer. This parameter change in the anisotropy source allows the switching field to remain relatively constant during scaling, as exchange anisotropy is not inversely proportional to cell dimensions like shape anisotropy, enabling feasible scaling to smaller dimensions
Solution Approach 2:
An antiferromagnetic layer is added to the structure to provide exchange anisotropy to the information storage free layer. This composite material approach replaces the simple shape anisotropy mechanism with a more complex but scalable exchange coupling mechanism, enabling continued scaling while maintaining reliable switching
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 approach enables the storage of multiple information states within a single physical bit, independent optimization of component parts, and a compact cell design without risking tunneling layer breakdown, while maintaining the advantages of thermal-assisted writing.
Implementation Method 1
a second antiferromagnetic layer (AFM), that has a low blocking temperature, is added to exchange bias the free layer
Implementation Method 2
The free layer magnetization is now determined by this second AFM whose direction is determined by sending a heating current through the cell to heat the cell above the second AFM blocking temperature
Implementation Method 3
The field generated by the bit line current provides the aligning field for the second AFM during cooling thereby setting the free layer magnetization parallel or anti-parallel to that of the pinned layer
Implementation Method 4
the MTJ is usually formed so that it exhibits an anisotropy... During the read operation a small current is sent through the MTJ junction to sense its resistance which is low for parallel magnetization and high for anti-parallel magnetization
Data Source
AI summary
A process for manufacturing a random access memory cell, that is capable of storing multiple information states in a single physical bit, is described. The basic structure combines a conventional MTJ with a reference stack that is magnetostatically coupled to the MTJ. The MTJ is read in the usual way but data is written and stored in the reference stack. Through use of two bit lines, the direction of magnetization of the free layer can be changed in small increments each unique direction representing a different information state.