MRAM Cell Strap Design for Thermally-Assisted Writing

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Solution Overview

Problem

Conventional thermally-assisted switching magnetic tunnel junction (TAS-MTJ) based MRAM cells face limitations in achieving high resistance-area (RA) values while maintaining low power density and avoiding voltage breakdown, requiring high current densities and thin insulating layers that compromise heating efficiency.

Innovation Solution

The MRAM cell design incorporates a magnetic tunnel junction with a first and second strap portion connecting to selection transistors, allowing for independent control of heating and field currents, enabling effective heating and magnetization switching with lower power consumption and higher RA values, and optionally using spin-polarized write currents.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Temperature

If a heating current is passed through the magnetic tunnel junction to raise the temperature above the blocking temperature, then the switching capability is improved, but the power density increases and voltage breakdown risk increases

Engineering Contradiction:

Improvejunction temperatureVSAvoidpower density

Core Design Contradiction:

TemperatureVSPower

Solution Approach 1:

The patent introduces a dedicated heating line as an intermediary element that carries the heating current to the magnetic tunnel junction. This separates the heating function from the bit line, allowing independent control of heating current without requiring high current density through the junction itself, thereby reducing power density and voltage breakdown risk while achieving the necessary temperature increase for switching.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Use of energy by moving object

If the resistance-area product (RA) of the junction is made small to allow sufficient heating current, then heating capability is improved, but read performance deteriorates

Engineering Contradiction:

Improveheating current capabilityVSAvoidread performance

Core Design Contradiction:

Use of energy by moving objectVSMeasurement precision

Solution Approach 1:

The patent segments the current paths by introducing a dedicated heating line separate from the bit line. This allows the junction to have high RA for good read performance while the heating line provides the necessary heating current capability. The heating function and read function are separated into different current paths, resolving the contradiction between heating capability and read performance.

Inventive Principle:

Principle #1Segmentation

3Ease of operation

If a dedicated field line is used for writing, then writing capability is improved, but device complexity increases

Engineering Contradiction:

Improvewriting capabilityVSAvoidnumber of conductors

Core Design Contradiction:

Ease of operationVSDevice complexity

Solution Approach 1:

The patent makes the heating line multi-functional by using it for both heating during thermally-assisted switching and for writing during spin-transfer torque switching. This eliminates the need for a separate dedicated field line, reducing device complexity while maintaining full writing capability through both TAS and STT modes.

Inventive Principle:

Principle #6Universality (Multi-functionality)

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 design enhances heating efficiency, reduces power consumption, and allows the use of high RA values, improving switching speed and read performance while minimizing electromigration and voltage breakdown risks.

Implementation Method 1

passing a heating current in said first and second strap portions via the first and second selection transistors for heating said magnetic tunnel junction to a high temperature threshold

Methodology Applied

Scientific EffectJoule heating: Joule Heating

Implementation Method 2

a field current adapted to generate a magnetic field for aligning the magnetization of the storage layer in accordance with the polarity of the field current

Methodology Applied

Scientific EffectMagnetic field generation: Magnetic Field

Implementation Method 3

pass a spin-polarized write current across the magnetic tunnel junction, via at least one of the selection transistors, for aligning the magnetization of the storage layer in accordance with the flow direction of the spin polarized write current

Methodology Applied

Scientific EffectSpin transfer torque:

Data Source

PatentUS8289765B2Active strap magnetic random access memory cells configured to perform thermally-assisted writing

Publication Date: 2012.10.16 ALLEGRO MICROSYSTEMS LLC

AI summary

A magnetic random access memory (MRAM) cell with a thermally assisted writing procedure comprising a magnetic tunnel junction formed from a magnetic storage layer, a reference layer, and an insulating layer inserted between the reference layer and the storage layer; and a first strap portion laterally connecting one end of the magnetic tunnel junction to a first selection transistor; wherein the cell further comprises a second strap portion extending opposite to the first strap portion and connecting laterally said one end of the magnetic tunnel junction to a second selection transistor, and in that said first and second strap portions being adapted for passing a portion of current via the first and second selection transistors. The disclosed cell has lower power consumption than conventional MRAM cells.