Low Variability Reference Parameter Generation for MRAM
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Solution Overview
Problem
Current MRAM technologies face challenges in generating a low variability reference resistance due to process and thermal variations, leading to high variability in reference parameters, which can result in read errors during sensing operations.
Innovation Solution
A reference circuit comprising groups of parallel-connected magnetic tunnel junctions (MTJs) is designed, where half of the groups are programmable to have parallel resistance states and the other half to have anti-parallel resistance states, forming a series-parallel resistor network with low variability total resistance, used to generate a stable reference parameter for sensing operations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a reference cell is designed to generate a reference parameter based on a reference resistance midway between RP and RAP, then the reference parameter can be used for sensing operations, but the reference resistance exhibits high variability due to process and thermal variations
Solution Approach 1:
The reference cell is segmented into multiple MTJ elements (e.g., four MTJs) arranged in a specific configuration. Each MTJ element contributes to the total reference resistance, and by segmenting the reference cell into multiple elements rather than using a single MTJ, the patent reduces the impact of process and thermal variations on the overall reference resistance, thereby improving reliability.
Solution Approach 2:
Multiple MTJ elements are merged/combined to form the reference cell structure. The patent combines several MTJ elements in a specific configuration (with selective parallel and series connections) to create a reference resistance that averages out process and thermal variations, reducing variability and improving the stability of the reference parameter.
2Ease of manufacture
If process and thermal variations are present in the reference cell, then manufacturing is simplified, but the reference parameter becomes highly variable leading to read errors
Solution Approach 1:
The reference cell is divided into multiple MTJ elements that can be manufactured using standard MRAM fabrication processes. By segmenting the reference cell into multiple elements rather than requiring a single complex structure, the patent maintains ease of manufacture while the combined effect of multiple elements reduces parameter variability.
Solution Approach 2:
The patent changes the structural parameters of the reference cell by using multiple MTJ elements with specific resistance ratios (e.g., RP:RAP = 1:3 or 1:4) rather than traditional equal resistance MTJs. This parameter change allows the reference resistance to remain midway between RP and RAP while reducing sensitivity to process and thermal variations, thereby improving measurement precision.
3Device complexity
If a single reference cell is used to generate the reference parameter, then the device complexity is low, but the reference parameter exhibits high variability
Solution Approach 1:
The reference cell is segmented into multiple MTJ elements (e.g., four MTJs) with selective parallel and series connections. This segmentation increases reliability by reducing parameter variability while maintaining relatively low device complexity through the use of standard circuit configuration techniques.
Solution Approach 2:
The patent employs dynamic switching of the reference cell configuration using switches that can selectively connect MTJ elements in parallel or series based on the desired resistance state. This dynamic reconfiguration capability allows the same physical structure to provide different resistance values, improving reliability without requiring multiple separate reference cells, thus maintaining low device complexity.
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 solution provides a low variability reference parameter, reducing read errors by ensuring the reference parameter is midway between the resistance states, thus improving the accuracy of data retrieval in MRAM cells.
Implementation Method 1
spin transfer torque-magnetic tunnel junction (STT-MTJ) magnetic random access memory (MRAM) structures
Implementation Method 2
voltage-controlled magnetic anisotropy-magnetic tunnel junction (VCMA-MTJ) MRAM structures
Implementation Method 3
An MTJ is a back end of the line (BEOL) multi-layer structure, which includes a fixed ferromagnetic layer (also referred to as a pinned layer) and a switchable ferromagnetic layer (also referred to as a free layer)
Data Source
AI summary
Disclosed is a reference circuit having an even number m of groups of m parallel-connected magnetic tunnel junctions (MTJs). The MTJs in half of the groups are programmed to have parallel resistances (RP) and the MTJs in the other half are programmed to have anti-parallel resistances (RAP). Switches connect the groups in series, creating a series-parallel resistor network. The total resistance (RT) of the network has low variability and is essentially equal to half the sum of a nominal RP plus a nominal RAP and can be employed as a reference resistance (RREF). Under specific biasing conditions the series-parallel resistor network can generate a low variability reference parameter (XREF) that is dependent on this RREF. Also disclosed are an integrated circuit (IC) that includes the reference circuit and a magnetic random access memory (MRAM) structure, which uses XREF to determine stored data values in MRAM cells and associated methods.