Spin Transfer MRAM Free Layer with Ta or Hf Spacer

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Solution Overview

Problem

Conventional MRAM devices face challenges with high power consumption due to the need for large currents to switch magnetic moments as device sizes decrease, and the use of certain materials degrades the magnetoresistance ratio and spin torque efficiency.

Innovation Solution

A CPP-MTJ MRAM device is designed with a free layer formed as an exchange-coupled lamination of two CoFeB layers separated by a thin layer of Ta or Hf, using MgO as the tunneling barrier and Cr or Cu as the spacer layer, to enhance the spin torque effect and reduce critical current density.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Area of moving object

If device size is decreased to improve integration density, then more devices can be packed in a given area, but the current carrying lines become narrower requiring greater currents to produce necessary switching fields, greatly increasing power consumption

Engineering Contradiction:

Improvedevice areaVSAvoidpower consumption

Core Design Contradiction:

Area of moving objectVSUse of energy by moving object

Solution Approach 1:

The patent replaces the conventional external magnetic field switching mechanism (which requires large currents in adjacent lines) with a spin transfer torque mechanism. In this mechanism, spin-polarized current flowing through the magnetic tunneling junction directly exerts torque on the free layer magnetization to cause switching, eliminating the need for separate external magnetic field generation and significantly reducing the current required for switching operations

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Solution Approach 2:

The patent modifies the magnetic layer structure by using CoFeB alloy with specific thickness (1.5-3 nm) and combining it with MgO tunnel barrier and Ta/Hf spacer layers. This changes the magnetic properties (anisotropy energy, spin torque efficiency) to enable switching at lower current densities while maintaining thermal stability, directly addressing the power consumption issue

Inventive Principle:

Principle #35Parameter changes

2Ease of manufacture

If conventional MTJ structure is used with standard materials, then device fabrication is straightforward, but the magnetoresistance ratio and spin torque efficiency are degraded

Engineering Contradiction:

Improvefabrication simplicityVSAvoidmagnetoresistance ratio

Core Design Contradiction:

Ease of manufactureVSReliability

Solution Approach 1:

The patent employs a composite magnetic layer structure consisting of CoFeB ferromagnetic layer combined with MgO tunnel barrier layer and Ta or Hf spacer layer. This composite structure leverages the high spin polarization of CoFeB, the excellent tunneling characteristics of MgO, and the magnetic anisotropy of Ta/Hf to achieve simultaneously high magnetoresistance ratio and efficient spin torque transfer, overcoming the limitations of conventional single-material or simple layered structures

Inventive Principle:

Principle #40Composite materials

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 configuration significantly improves the magnetoresistance ratio and spin angular momentum deposition rate, reducing the required spin transfer current density and enhancing device efficiency.

Implementation Method 1

The switching of the free layer moment direction (writing) is accomplished by external magnetic fields that are the result of currents passing through conducting lines adjacent to the cell. The spin transfer device shares some of the operational features of the conventional MTJ cell described above, except that the switching of the free layer magnetic moment is produced by the spin polarized current itself.

Methodology Applied

Scientific EffectSpin transfer torque:

Implementation Method 2

unpolarized conduction electrons passing through a first magnetic layer having its magnetic moment oriented in a given direction (such as the pinned layer) are preferentially polarized by their passage through that layer by a quantum mechanical exchange interaction with the polarized bound electrons in the layer

Methodology Applied

Scientific EffectSpin polarization:

Implementation Method 3

The probability of such a polarized electron then tunneling through the intervening tunneling barrier layer into the lower layer then depends on the availability of states within the lower layer that the tunneling electron can occupy

Methodology Applied

Scientific EffectQuantum tunneling:

Implementation Method 4

one of the magnetic layers has its magnetic moment fixed in direction (pinned) by exchange coupling to an antiferromagnetic layer

Methodology Applied

Scientific EffectExchange coupling:

Implementation Method 5

such a shape produces a magnetic anisotropy within the free layer that assists its magnetic moment in retaining a thermally stable fixed position after switching fields have been turned off

Methodology Applied

Scientific EffectMagnetic anisotropy: Anisotropy

Data Source

PatentUS7480173B2Spin transfer MRAM device with novel magnetic free layer

Publication Date: 2009.01.20 TAIWAN SEMICONDUCTOR MANUFACTURING CO LTD

AI summary

A CPP MTJ MRAM element utilizes transfer of spin angular momentum as a mechanism for changing the magnetic moment direction of a free layer. The device includes a tunneling barrier layer of MgO and a non-magnetic CPP layer of Cu or Cr and utilizes a novel free layer comprising a thin layer of Ta or Hf sandwiched by layers of CoFeB. The device is characterized by values of DR/R between approximately 95% and 105%.