MRAM Crystal Seed Layer for MTJ Diffusion and Texture Control

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Solution Overview

Problem

Conventional magnetic tunnel junction (MTJ) stacks in MRAM devices suffer from imperfections such as grain boundaries, which degrade the quality of the fcc structure and (111) orientation, leading to impaired operating characteristics due to diffusion of species like tantalum or ruthenium from the bottom electrode, and thermal stress.

Innovation Solution

A seed layer with a strong fcc (111) texture, made of crystalline nonmagnetic binary or ternary alloys like CrNi or CrNiFe, is used to separate the bottom electrode from the MTJ stack, reducing grain boundaries and preventing diffusive species from affecting the MTJ stack's performance, thereby enhancing the tunnel magnetoresistance effect.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Manufacturing precision

If conventional MTJ stacks are used without a high-quality seed layer, then the device structure is simpler and easier to manufacture, but grain boundaries form that degrade the fcc structure quality and (111) orientation

Engineering Contradiction:

Improvefcc structure quality and (111) orientationVSAvoidseed layer structure

Core Design Contradiction:

Manufacturing precisionVSDevice complexity

Solution Approach 1:

A high-quality crystalline seed layer with strong fcc (111) texture is deposited beforehand on the bottom electrode before forming the MTJ stack. This preliminary seed layer establishes the desired crystal structure and orientation that guides the growth of subsequent MTJ layers, ensuring low defect density and minimal grain boundaries throughout the stack.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The crystalline seed layer acts as an intermediary between the bottom electrode and the MTJ stack. It mediates the crystal growth process by providing a template with strong (111) orientation that promotes epitaxial growth of the MTJ layers, thereby improving the overall crystalline quality without requiring changes to the MTJ stack composition or structure.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Reliability

If no seed layer is used to separate the bottom electrode from the MTJ stack, then the device structure is simpler, but diffusive species from the bottom electrode degrade MTJ performance

Engineering Contradiction:

ImproveMTJ performance and tunnel magnetoresistanceVSAvoidseed layer insertion

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The crystalline seed layer serves as a diffusion barrier and intermediary layer between the bottom electrode and the MTJ stack. It prevents diffusive species from the bottom electrode from migrating into the MTJ stack while maintaining electrical connectivity and mechanical adhesion, thereby protecting MTJ performance without adding complex multilayer diffusion barriers.

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The seed layer provides localized protection and structural quality enhancement at the critical interface between the bottom electrode and the MTJ stack. By concentrating the functional properties (crystalline quality, diffusion barrier) at this specific location rather than throughout the entire device structure, the solution achieves high reliability with minimal added complexity.

Inventive Principle:

Principle #3Local quality

3Reliability

If conventional seed layers are used, then the manufacturing process is simpler, but thermal stress degrades the MTJ stack operating characteristics

Engineering Contradiction:

Improveoperating characteristics stabilityVSAvoidseed layer material composition

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The seed layer is designed with specific material composition and crystalline structure parameters (strong fcc (111) texture) that match or are compatible with the underlying bottom electrode and overlying MTJ stack. This parameter matching reduces lattice mismatch and thermal expansion differences, thereby minimizing thermal stress and improving the stability of operating characteristics under temperature variations.

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

The use of a high-quality seed layer improves the crystalline structure and reduces diffusion, resulting in improved operating characteristics and increased tunnel magnetoresistance of the MTJ stack, enhancing data storage capabilities.

Implementation Method 1

leading to impaired operating characteristics due to diffusion of species like tantalum or ruthenium from the bottom electrode

Methodology Applied

Scientific EffectDiffusion: Diffusion

Implementation Method 2

A seed layer with a strong fcc (111) texture, made of crystalline nonmagnetic binary or ternary alloys like CrNi or CrNiFe

Methodology Applied

Scientific EffectEpitaxy: Epitaxy

Data Source

PatentUS11842757B2Crystal seed layer for magnetic random access memory (MRAM)

Publication Date: 2023.12.12 TAIWAN SEMICONDUCTOR MANUFACTURING CO LTD

AI summary

Some embodiments relate to a memory device. The memory device includes a magnetoresistive random-access memory (MRAM) cell comprising a magnetic tunnel junction (MTJ). The MTJ device comprises a stack of layers, comprising a bottom electrode disposed over a substrate. A seed layer disposed over the bottom electrode. A buffer layer is disposed between the bottom electrode and the seed layer. The buffer layer prevents diffusion of a diffusive species from the bottom electrode to the seed layer.