MTJ Tunnel Barrier Oxidation for Low RA and PMA Preservation

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Solution Overview

Problem

Existing methods for forming tunnel barriers in Spin-Torque MRAM devices fail to maintain perpendicular magnetic anisotropy (PMA) in magnetic layers due to excessive oxidation, leading to degraded reliability and performance, particularly at the 90 nm technology node and beyond.

Innovation Solution

A controlled oxidation process involving passive oxidation steps with low oxygen pressure followed by conventional oxidation steps is used to form a metal oxide tunnel barrier, minimizing oxidation of adjoining magnetic layers and preserving PMA, thereby enhancing the TMR ratio and reducing RA to ≤20 ohm-μm².

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Manufacturing precision

If conventional oxidation steps with high oxygen pressure are used to form the tunnel barrier, then the RA product is reduced (improved), but the perpendicular magnetic anisotropy (PMA) in adjoining magnetic layers is degraded

Engineering Contradiction:

ImproveRA productVSAvoidPMA preservation

Core Design Contradiction:

Manufacturing precisionVSReliability

Solution Approach 1:

The oxidation process is divided into multiple sequential steps with different oxygen pressures. A first oxidation step uses high oxygen pressure (1-100 mtorr) to reduce RA, followed by a second oxidation step using low oxygen pressure (10^-3 to 10^-6 torr) to preserve PMA. This segmentation allows each step to optimize for its specific function without compromising the other.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The oxygen pressure parameter is dynamically changed between oxidation steps. The process transitions from high oxygen pressure (1-100 mtorr) in the first step to low oxygen pressure (10^-3 to 10^-6 torr) in the second step. This parameter change enables the system to achieve low RA while preserving PMA by adjusting the oxidation intensity at different stages.

Inventive Principle:

Principle #35Parameter changes

2Manufacturing precision

If the tunnel barrier RA is reduced for good writability, then the writing voltage is reduced, but the strength of the tunnel barrier against applied voltages is reduced

Engineering Contradiction:

ImproveRA valueVSAvoidtunnel barrier strength

Core Design Contradiction:

Manufacturing precisionVSStrength

Solution Approach 1:

The first oxidation step with high oxygen pressure is performed preliminarily to establish a low RA tunnel barrier foundation. This preliminary action reduces the RA value and writing voltage requirements. Subsequently, the second oxidation step with low oxygen pressure strengthens the tunnel barrier against voltage stress without significantly increasing RA, thus resolving the contradiction between writability and barrier strength.

Inventive Principle:

Principle #10Preliminary action

3Manufacturing precision

If strong oxidation conditions are used during tunnel barrier formation, then the tunnel barrier quality is improved, but the PMA in magnetic layers at the interface is lost

Engineering Contradiction:

Improvetunnel barrier qualityVSAvoidinterface PMA

Core Design Contradiction:

Manufacturing precisionVSReliability

Solution Approach 1:

Different oxidation intensities are applied to different regions of the tunnel barrier formation process. The first oxidation step with high oxygen pressure targets the bulk tunnel barrier quality, while the second oxidation step with low oxygen pressure specifically protects the interface regions where PMA is critical. This local quality approach ensures high tunnel barrier quality without sacrificing interface PMA.

Inventive Principle:

Principle #3Local quality

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 method ensures better preservation of PMA in magnetic layers, resulting in improved writing reliability and higher TMR ratios, suitable for advanced MRAM and Spin-Torque MRAM devices.

Implementation Method 1

The tunnel barrier is typically formed by deposition and oxidation of a thin Mg layer

Methodology Applied

Scientific EffectOxidation: Oxidation

Implementation Method 2

the spin angular moment of electrons incident on a magnetic layer interacts with magnetic moments of the magnetic layer near the interface between the magnetic layer and non-magnetic spacer. Through this interaction, the electrons transfer a portion of their angular momentum to the magnetic layer

Methodology Applied

Scientific EffectSpin transfer torque:

Implementation Method 3

both field-MRAM and Spin-Torque MRAM have a MTJ element based on a tunneling magneto-resistance (TMR) effect

Methodology Applied

Scientific EffectTunneling magneto-resistance (TMR) effect:

Data Source

PatentUS12414476B2Method for forming a perpendicular spin torque oscillator (PSTO) including forming a magneto resistive sensor (MR) over a spin torque oscillator (STO)

Publication Date: 2025.09.09 TAIWAN SEMICONDUCTOR MANUFACTURING CO LTD

AI summary

A method of forming a MTJ with a tunnel barrier having a high tunneling magnetoresistance ratio, and low resistance x area value is disclosed. The method preserves perpendicular magnetic anisotropy in bottom and top magnetic layers that adjoin bottom and top surfaces of the tunnel barrier. A key feature is a passive oxidation step of a first Mg layer that is deposited on the bottom magnetic layer wherein a maximum oxygen pressure is 10-5 torr. A bottom portion of the first Mg layer remains unoxidized thereby protecting the bottom magnetic layer from substantial oxidation during subsequent oxidation and anneal processes that are employed to complete the fabrication of the tunnel barrier and MTJ. An uppermost Mg layer may be formed as the top layer in the tunnel barrier stack before a top magnetic layer is deposited.