STT-RAM MTJ Free Layer Nanocurrent Channel Design

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Solution Overview

Problem

Current Magnetic Tunneling Junction (MTJ) structures for Spin-Transfer Torque RAM (STT-RAM) devices face challenges in achieving low switching current density and high thermal stability while maintaining acceptable read and write voltages, thermal stability, and resistance uniformity for advanced memory applications such as 64 Mb STT-RAMs.

Innovation Solution

A MTJ element with a free layer configuration of FL1/NCC/FL2, where the NCC layer is a nanocurrent channel layer made of conductive RM grains in an insulator matrix, and a MgO tunnel barrier formed using a natural oxidation method, along with a composite bottom electrode structure, to reduce switching current density and enhance thermal stability and resistance uniformity.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If conventional MTJ structures are used, then manufacturing simplicity is maintained, but switching current density is high and thermal stability is insufficient

Engineering Contradiction:

Improvethermal stabilityVSAvoidfree layer structure complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The free layer is segmented into two distinct ferromagnetic layers (FL1 and FL2) separated by a nanocurrent channel (NCC) layer. This segmentation allows independent optimization of each layer's magnetic properties and thickness to achieve both low switching current and high thermal stability simultaneously.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The NCC layer is introduced with specific local properties (high resistance, narrow conductivity channels) to locally control current distribution and magnetic coupling. This local quality enhancement enables precise control over switching characteristics without affecting the entire device uniformly.

Inventive Principle:

Principle #3Local quality

2Use of energy by moving object

If switching current density is reduced, then energy consumption decreases, but read and write voltages become unacceptable

Engineering Contradiction:

Improveswitching current densityVSAvoidread and write voltages

Core Design Contradiction:

Use of energy by moving objectVSEase of operation

Solution Approach 1:

Multiple parameters including the thicknesses of FL1, FL2, and NCC layers, as well as the resistance and magnetic properties of each layer, are optimized to achieve the desired balance. By changing these parameters, the switching current density is reduced while maintaining acceptable voltage levels for operation.

Inventive Principle:

Principle #35Parameter changes

3Manufacturing precision

If resistance uniformity is improved, then device performance increases, but manufacturing complexity increases

Engineering Contradiction:

Improveresistance uniformityVSAvoidfabrication process complexity

Core Design Contradiction:

Manufacturing precisionVSDevice complexity

Solution Approach 1:

The NCC layer is deposited and processed in advance with controlled thickness and resistance properties before the final MTJ structure is completed. This preliminary action ensures that subsequent processing steps maintain uniformity without requiring complex in-situ adjustments during final fabrication.

Inventive Principle:

Principle #10Preliminary action

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 achieves a critical current density less than 2×10^6 A/cm², high thermal stability, and improved resistance uniformity, enabling enhanced performance for advanced STT-RAM devices with reduced error rates and increased storage reliability.

Implementation Method 1

spin-transfer torque (STT)-RAM application... the nano-conducting channels in the middle NCC layer magnetically and electrically couple the two portions FL1 and FL2 of the free layer

Methodology Applied

Scientific EffectSpin transfer torque:

Implementation Method 2

The tunnel barrier layer is thin enough that a current through it can be established by quantum mechanical tunneling of conduction electrons

Methodology Applied

Scientific EffectQuantum mechanical tunneling:

Implementation Method 3

Both field-MRAM and STT-RAM have a MTJ element based on a tunneling magneto-resistance (TMR) effect... the relative orientation of the magnetic moments between the free and pinned layers that determines the tunneling current and therefore the resistance of the tunneling junction

Methodology Applied

Scientific EffectTunneling magnetoresistance: Magnetoresistance

Data Source

PatentUS9331271B2Structure and method to fabricate high performance MTJ devices for spin-transfer torque (STT)-RAM application

Publication Date: 2016.05.03 TAIWAN SEMICONDUCTOR MANUFACTURING CO LTD

AI summary

A STT-RAM MTJ is disclosed with a MgO tunnel barrier formed by natural oxidation and containing an oxygen surfactant layer to form a more uniform MgO layer and lower breakdown distribution percent. A CoFeB/NCC/CoFeB composite free layer with a middle nanocurrent channel layer minimizes Jc0 while enabling thermal stability, write voltage, read voltage, and Hc values that satisfy 64 Mb design requirements. The NCC layer has RM grains in an insulator matrix where R is Co, Fe, or Ni, and M is a metal such as Si or Al. NCC thickness is maintained around the minimum RM grain size to avoid RM granules not having sufficient diameter to bridge the distance between upper and lower CoFeB layers. A second NCC layer and third CoFeB layer may be included in the free layer or a second NCC layer may be inserted below the Ru capping layer.