SOT-Assisted STT-MRAM Bit Cell Array Design

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Solution Overview

Problem

Perpendicular spin-transfer torque magnetic random access memory (STT-MRAM) is not fast and low-power enough to replace conventional static RAM (SRAM) for applications requiring sub-nanosecond write times, necessitating improvements in switching speed and endurance without degrading read performance or retention.

Innovation Solution

The implementation of spin-orbit torque (SOT) assisted STT-MRAM design, which utilizes a spin-transfer torque current to select the state of bit cells, incorporates a top pinned magnetic tunnel junction (MTJ) and a spin-hall conductive material layer to generate spin-orbit torque, allowing for deterministic switching and reduced switching energy, while maintaining superior retention and resilience to external fields.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If conventional STT-MRAM is used, then non-volatility and unlimited endurance are achieved, but write speed is insufficient to replace conventional SRAM

Engineering Contradiction:

Improvenon-volatility and enduranceVSAvoidwrite speed

Core Design Contradiction:

ReliabilityVSSpeed

Solution Approach 1:

The patent segments the current path into two separate paths: a first current path for write operations that flows through the spin-hall conductive material layer and MTJ, and a second current path for read operations that flows through the access transistor and MTJ. This segmentation allows optimized write speed through spin-orbit torque while maintaining the non-volatility and endurance characteristics of STT-MRAM.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The spin-hall conductive material layer acts as an intermediary that converts charge current into spin current via the spin Hall effect, enabling deterministic magnetization switching without requiring high current densities through the MTJ itself. This intermediary mechanism achieves faster write speeds while preserving the reliability benefits of STT-MRAM.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Reliability

If write current exceeds critical switching current, then magnetization switching is achieved, but power consumption increases

Engineering Contradiction:

Improvemagnetization switchingVSAvoidpower consumption

Core Design Contradiction:

ReliabilityVSUse of energy by moving object

Solution Approach 1:

The patent replaces the direct spin-transfer torque mechanism (which requires high current through the MTJ) with a spin-orbit torque mechanism utilizing the spin Hall effect. The spin-hall conductive material layer converts charge current into spin current that acts on the free layer magnetization, reducing the power required for switching while ensuring reliable magnetization state changes.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

3Device complexity

If same current path is used for read and write, then device simplicity is maintained, but read performance degrades due to high write current

Engineering Contradiction:

Improvecurrent path structureVSAvoidread performance

Core Design Contradiction:

Device complexityVSReliability

Solution Approach 1:

The patent divides the current paths into separate segments: write current flows through the spin-hall conductive material layer and MTJ, while read current flows through the access transistor and MTJ. This segmentation prevents read disturbance during write operations and maintains read performance while only requiring minimal additional structural elements.

Inventive Principle:

Principle #1Segmentation

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 enhances switching speed and endurance, achieves deterministic switching without external magnetic fields, and reduces voltage across the barrier layer, enabling larger bias read speeds and improved endurance, making it suitable for cache replacement memory applications.

Implementation Method 1

incorporates a top pinned magnetic tunnel junction (MTJ) and a spin-hall conductive material layer to generate spin-orbit torque

Methodology Applied

Scientific EffectSpin Hall Effect: Hall Effect

Implementation Method 2

The electrical resistance of an MTJ depends on whether the free layer magnetization and fixed layer magnetization are parallel or anti-parallel to each other

Methodology Applied

Scientific EffectTunneling Magnetoresistance: Magnetoresistance

Implementation Method 3

Spin-transfer torque magnetic random access memory (STT-MRAM)

Methodology Applied

Scientific EffectSpin-Transfer Torque:

Data Source

PatentEP3504711B1High speed, low power spin-orbit torque (SOT) assisted spin-transfer torque magnetic random access memory (STT-MRAM) bit cell array

Publication Date: 2022.03.16 QUALCOMM INC

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AI summary

A magnetic random access memory (MRAM) array including several bit cells is described. Each of the bit cells includes a perpendicular magnetic tunnel junction (pMTJ) -a magnetic tunnel junction with perpendicular anisotropy- including a reference layer, a barrier layer supporting the reference layer, and a free layer supporting the barrier layer. A spin-hall conductive material layer supports the free layer. A driver is operable to set a state of at least one of the plurality of bit cells, the driver being configured to simultaneously drive a current along the spin-Hall conductive material to generate via a Spin Hall effect a Spin Orbit Transfer -SOT- current flowing through the pMTJ and drive a further current through the pMTJ and a portion of the spin- hall conductive material layer, the further current generating with the reference layer a Spin-transfer-torque -STT- current through the pMTJ.