MRAM MTJ Free Layer and Capping Layer Design
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Solution Overview
Problem
The efficacy of magnetic tunneling junction (MTJ) cells in high-density MRAM arrays is compromised by increased statistical variations in resistance values, leading to marginal read margins and suboptimal write operations, particularly in 16 Mb MRAM devices, where the TMR ratio and array quality factor need enhancement to ensure accurate data storage and retrieval.
Innovation Solution
The MTJ cell stack configuration is modified to include a composite free layer formed as (Ni 88 Fe 12 ) 1-x Co 100x, with x between .05 and .1, and a Ni 92 Fe 8 layer, along with a thicker NiFeHf capping layer, which improves spin polarization, reduces magnetostriction, and increases the TMR ratio, achieving a TMR≥50% and enhanced array quality factor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a capping layer is formed over the MgO tunnel barrier layer to prevent Mg diffusion, then device reliability is improved, but the fabrication process complexity increases due to additional deposition steps
Solution Approach 1:
The patent extracts the capping layer function from a separate deposition step and integrates it into the existing MgO tunnel barrier layer deposition process. By forming the capping layer concurrently with the MgO layer in the same deposition chamber, the patent eliminates the need for additional deposition equipment and process steps, thereby preventing Mg diffusion while avoiding increased fabrication complexity
Solution Approach 2:
The patent merges the formation of the MgO tunnel barrier layer and the capping layer into a single deposition process. The capping layer is deposited simultaneously with the MgO layer in the same chamber, combining two functional layers into one process step, which prevents Mg diffusion without requiring additional process equipment or steps
2Reliability
If conventional deposition techniques are used to form the capping layer, then device reliability is improved by preventing Mg diffusion, but manufacturing cost increases due to additional deposition steps
Solution Approach 1:
The patent combines the deposition of the MgO tunnel barrier layer and the capping layer into a single deposition process step. By forming both layers concurrently in the same deposition chamber using the same equipment, the patent eliminates the need for additional deposition equipment and process steps, thereby preventing Mg diffusion while reducing manufacturing costs through process integration
3Manufacturing precision
If the capping layer is formed in a separate deposition chamber, then layer quality is improved, but productivity decreases due to additional process steps
Solution Approach 1:
The patent merges the formation of the MgO tunnel barrier layer and the capping layer into a single deposition process performed in the same deposition chamber. This integration maintains layer quality by ensuring proper interface formation and stoichiometry while significantly improving productivity by eliminating the need for additional deposition steps and reducing overall fabrication time
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 enhances the TMR ratio, reduces magnetostriction, and improves the read margin and array quality factor, ensuring reliable data storage and retrieval in high-density MRAM arrays, with a TMR/R p cov ≥15.5 and AQF ≥20, suitable for 16 Mb MRAM technology.
Implementation Method 1
forming a capping layer over the MgO tunnel barrier layer to prevent diffusion of magnesium from the MgO tunnel barrier layer
Data Source
Figure 1~2
Figure 3
AI summary
An MTJ MRAM cell and its method of formation are described. The cell includes a composite free layer having the general form (Ni88Fe 12) i-xCo I00x-Ni92Fe8 with x between.05 and.1 that provides low magnetization and negative magnetostriction. The magnetostriction can be tuned to a low value by a multilayer capping layer that includes a positive magnetostriction layer of NiFeHf(15%). When this cell forms an MRAM array, it contributes to a TMR = 26%, a TMR/Rp_cov =15.5 and a high AQF (array quality factor) for write operations.