TMR Free Layer Composite Structure with Weak Plasma Etch
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Solution Overview
Problem
Current TMR sensors face challenges in achieving low magnetostriction, low coercivity, and high magnetoresistive ratio while maintaining a low areal resistance, especially with CoFeB-based free layers which often result in high magnetostriction and degraded TMR ratios when combined with NiFe layers.
Innovation Solution
A composite free layer configuration with a first ferromagnetic layer (FL1) undergoing a weak plasma etch treatment before the insertion layer and second ferromagnetic layer (FL2) deposition, utilizing a FL1/INS/FL2 structure where FL1 is modified to enhance surface structure and energy, and INS is an alloy with magnetic and non-magnetic elements, maintaining strong magnetic coupling for improved performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Force
If CoFeB-based free layers are used to achieve low coercivity, then coercivity is reduced, but magnetostriction increases and TMR ratio degrades
Solution Approach 1:
The patent uses a composite free layer structure consisting of CoFeB layer, NiFe layer, and Ru insertion layer. This composite structure combines materials with different magnetic properties to achieve low coercivity while controlling magnetostriction. The Ru insertion layer specifically prevents unwanted magnetic coupling between CoFeB and NiFe, allowing each layer to contribute its advantageous properties.
Solution Approach 2:
The Ru insertion layer acts as an intermediary between the CoFeB and NiFe layers. It prevents direct magnetic coupling that would cause magnetostriction degradation, while still allowing the structure to benefit from the low coercivity of CoFeB and the soft magnetic properties of NiFe.
2Reliability
If NiFe layers are combined with CoFeB to reduce magnetostriction, then magnetostriction is reduced, but TMR ratio degrades
Solution Approach 1:
The Ru insertion layer serves as a mediator that prevents direct contact and strong magnetic coupling between the NiFe and CoFeB layers. This controlled separation maintains the TMR ratio by preventing degradation from direct coupling, while still allowing the composite structure to achieve low magnetostriction through the combined layers.
Solution Approach 2:
The patent applies different materials and properties to different parts of the free layer structure. The CoFeB layer provides low coercivity, the NiFe layer provides soft magnetic properties and low magnetostriction, and the Ru insertion layer provides controlled separation. Each layer has a specific local function that contributes to the overall performance.
3Measurement precision
If weak plasma etch treatment is applied to FL1 surface, then surface structure and energy are modified improving TMR ratio, but manufacturing complexity increases
Solution Approach 1:
The weak plasma etch treatment is applied as a preliminary action to the CoFeB layer surface before depositing the Ru insertion layer and NiFe layer. This pre-treatment modifies the surface structure and energy to optimize subsequent layer formation and achieve high TMR ratio, integrating the complexity into an early manufacturing step.
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 approach achieves a high TMR ratio greater than 60% with low areal resistance and magnetostriction, enhancing signal-to-noise ratio and magnetic stability, while being cost-effective and compatible with existing manufacturing processes.
Implementation Method 1
the FL1 surface structure and surface energy are modified by a weak plasma etch treatment
Implementation Method 2
The tunnel barrier layer is so thin that a current through it can be established by quantum mechanical tunneling of conduction electrons
Implementation Method 3
The electrical resistance through the tunnel barrier layer (insulator layer) varies with the relative orientation of the free layer moment compared with the reference layer moment and thereby converts magnetic signals into electrical signals
Implementation Method 4
The outer pinned layer has a magnetic moment that is fixed in a certain direction by exchange coupling with the adjacent AFM layer
Implementation Method 5
maintaining strong coupling between the FL1 and FL2 magnetic layers
Data Source
AI summary
A composite free layer having a FL1/insertion/FL2 configuration where a top surface of FL1 is treated with a weak plasma etch is disclosed for achieving enhanced dR/R while maintaining low RA, and low λ in TMR or GMR sensors. The weak plasma etch removes less than about 0.2 Angstroms of FL1 and is believed to modify surface structure and possibly increase surface energy. FL1 may be CoFe, CoFe/CoFeB, or alloys thereof having a (+) λ value. FL2 may be CoFe, NiFe, or alloys thereof having a (−) λ value. The thin insertion layer includes at least one magnetic element such as Co, Fe, and Ni, and at least one non-magnetic element. When CoFeBTa is selected as insertion layer, the CoFeB:Ta ratio is from 1:1 to 4:1.