Self-Reference Read Method for Spin-Transfer Torque Memory

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Solution Overview

Problem

Spin-Torque Transfer Random Access Memory (STRAM) faces challenges due to large magnetic tunnel junction (MTJ) resistance variation, which complicates read operations, especially as the MTJ size shrinks, leading to high power consumption and scaling issues.

Innovation Solution

The method involves applying a first and second read current through a magnetic tunnel junction data cell to store corresponding bit line voltages, allowing for self-reference reading by comparing these voltages to determine the resistance state, thereby overcoming the resistance variation and enabling accurate data retrieval.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Quantity of substance

If the MTJ size is shrunk to increase storage density, then the storage capacity is improved, but the switching magnetic field amplitude increases and the switching variation becomes severer, leading to high power consumption

Engineering Contradiction:

Improvestorage capacityVSAvoidpower consumption

Core Design Contradiction:

Quantity of substanceVSUse of energy by moving object

Solution Approach 1:

The patent replaces the conventional magnetic field-based switching mechanism with a current-induced spin torque transfer mechanism. Instead of using external magnetic fields to switch MTJ magnetization, the invention uses spin-polarized current flowing through the MTJ to induce magnetization switching via spin-transfer torque, thereby reducing the switching field amplitude and power consumption while maintaining small MTJ size for high density

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

2Reliability

If the oxide barrier thickness is increased to improve MTJ reliability, then the MTJ resistance increases, but the MTJ resistance variation becomes larger, creating read operation problems

Engineering Contradiction:

ImproveMTJ reliabilityVSAvoidread operation accuracy

Core Design Contradiction:

ReliabilityVSMeasurement precision

Solution Approach 1:

The patent implements a reference reading mechanism where a reference MTJ with known resistance state is read simultaneously with the data MTJ. The read voltages from both MTJs are compared to determine the data state. This feedback-based comparison compensates for resistance variations caused by oxide barrier thickness variations, enabling accurate read operations even with thicker oxide barriers that improve reliability

Inventive Principle:

Principle #23Feedback

Solution Approach 2:

The patent introduces a reference MTJ as an intermediary element that provides a known resistance state for comparison. The reference MTJ acts as a mediator to eliminate the effect of oxide barrier thickness variations on the data MTJ, allowing accurate determination of the data state through voltage comparison without requiring precise control of oxide barrier thickness

Inventive Principle:

Principle #24Intermediary (Mediator)

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 ensures reliable determination of the MTJ resistance state, independent of variations, facilitating improved scaling and reduced power consumption in STRAM devices.

Implementation Method 1

a new write mechanism, which is based upon spin polarization current induced magnetization switching, was introduced to the MRAM design. This new MRAM design, called Spin-Torque Transfer RAM (STRAM), uses a (bidirectional) current through the MTJ to realize the resistance switching.

Methodology Applied

Scientific EffectSpin-transfer torque:

Implementation Method 2

Data storage is realized by switching the resistance of MTJ between a high-resistance state and a low-resistance state. MRAM switches the MTJ resistance by using a current induced magnetic field to switch the magnetization of MTJ.

Methodology Applied

Scientific EffectElectrical resistance: Electrical Resistance

Data Source

PatentUS8116122B2Spin-transfer torque memory self-reference read method

Publication Date: 2012.02.14 SEAGATE TECH LLC

AI summary

A spin-transfer torque memory apparatus and self-reference read schemes are described. One method of self-reference reading a spin-transfer torque memory unit includes applying a first read current through a magnetic tunnel junction data cell and forming a first bit line read voltage, the magnetic tunnel junction data cell having a first resistance state and storing the first bit line read voltage in a first voltage storage device. Then applying a low resistance state polarized write current through the magnetic tunnel junction data cell, forming a low second resistance state magnetic tunnel junction data cell. A second read current is applied through the low second resistance state magnetic tunnel junction data cell to forming a second bit line read voltage. The second bit line read voltage is stored in a second voltage storage device. The method also includes comparing the first bit line read voltage with the second bit line read voltage to determine whether the first resistance state of the magnetic tunnel junction data cell was a high resistance state or low resistance state.