MRAM Midpoint Reference Circuit for MTJ Lifetime Extension

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Solution Overview

Problem

Existing Magnetic Random Access Memory (MRAM) technologies face challenges in reliably setting the state of Magnetic Tunnel Junction (MTJ) elements, particularly in 'Toggle' MRAM, due to the lack of a direct write method and susceptibility to time-dependent dielectric breakdown, which affects the longevity and accuracy of midpoint references used in sense amplifiers.

Innovation Solution

The implementation of a midpoint reference circuit with multiple MTJ devices and transistor circuitry that allows independent biasing and reduced voltage across each MTJ element, enabling accurate setting of MTJ states and extending the lifetime of MTJ elements by preventing significant resistance addition from access transistors, ensuring consistent current flow direction and minimizing capacitive imbalances.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Duration of action of stationary object

If MTJ elements are used in midpoint generator with reduced voltage applied across insulating layer, then time-dependent dielectric breakdown lifetime is extended, but device complexity increases due to multiple MTJ elements and transistor circuitry

Engineering Contradiction:

ImproveMTJ element lifetimeVSAvoidmidpoint generator structure

Core Design Contradiction:

Duration of action of stationary objectVSDevice complexity

Solution Approach 1:

The midpoint generator is divided into multiple MTJ elements (first and second MTJ elements) with separate control, allowing independent voltage reduction across each insulating layer. This segmentation enables extended lifetime by distributing the voltage stress and allowing separate biasing control for each element.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

Transistor circuitry is introduced as an intermediary component to control and reduce the voltage applied across each MTJ element's insulating layer. The transistors act as mediators that can independently bias each MTJ element, reducing voltage stress and extending lifetime while managing the increased device complexity through controlled interfaces.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Stability of the object's composition

If multiple MTJ elements are configured in midpoint generator with independent biasing, then capacitive imbalances are minimized, but manufacturing precision requirements increase

Engineering Contradiction:

Improvecapacitive balanceVSAvoidMTJ element matching

Core Design Contradiction:

Stability of the object's compositionVSManufacturing precision

Solution Approach 1:

Each MTJ element is provided with independent local control through separate transistor circuitry, allowing the voltage and bias conditions to be locally optimized for each element. This local quality control enables precise matching of capacitive characteristics by adjusting each element's operating conditions independently, achieving better capacitive balance while managing manufacturing variations.

Inventive Principle:

Principle #3Local quality

3Reliability

If voltage is reduced across MTJ elements in reference column, then reliability is improved, but power consumption decreases which may affect readout performance

Engineering Contradiction:

ImproveTDDB resistanceVSAvoidpower consumption

Core Design Contradiction:

ReliabilityVSUse of energy by moving object

Solution Approach 1:

The voltage parameter across the MTJ elements in the reference column is changed and reduced compared to the data column MTJ elements. This parameter change improves reliability by reducing voltage stress and resistance to time-dependent dielectric breakdown. The transistor circuitry enables independent control of this voltage parameter, allowing optimization of the reliability-power tradeoff by adjusting the bias conditions of each MTJ element separately.

Inventive Principle:

Principle #35Parameter changes

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 solution enhances the reliability and speed of MRAM operations by allowing precise setting of MTJ states, reducing capacitive imbalances, and extending the time-dependent dielectric breakdown lifetime of MTJ elements, thereby improving overall memory access efficiency and longevity.

Implementation Method 1

When the magnetic vectors are aligned, the resistance of the MTJ cell, i.e. the resistance to current flow between the magnetic layers, is a minimum, Rmin, and when the magnetic vectors are opposed or misaligned the resistance of the MTJ cell is a maximum, Rmax.

Methodology Applied

Scientific EffectMagnetoresistance: Magnetoresistance

Data Source

PatentUS8184476B2Random access memory architecture including midpoint reference

Publication Date: 2012.05.22 EVERSPIN TECHNOLOGIES INC

AI summary

A random access memory architecture includes a first series connected pair of memory elements (202, 206, 302, 306, 402, 404) having a first resistance and a second series connected pair of memory elements (204, 208, 304, 308, 406, 408) having a second resistance coupled in parallel with the first series connected pair of memory elements, wherein a current flows in the first direction through both of the first and second series connected pair of memory elements. A sense amplifier (14) is coupled to an array (16) of MRAM cells (77), each including a memory element, and includes a voltage bias portion (12), the voltage bias portion including the first and second series connected pair of memory elements. The memory elements may be, for example, magnetic tunnel junctions.