TMR Sensor Composite Inner Pinned Layer for Low RA and High MR Ratio
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Solution Overview
Problem
Conventional TMR sensors with MgO barrier layers face challenges in achieving a high MR ratio while maintaining a low RA value and low magnetostriction, particularly due to difficulties in controlling annealing temperature and uniformity, and degradation of MR ratio when using CoFe/NiFe free layers with MgO barriers.
Innovation Solution
A composite inner pinned layer with a CoFeB/Fe/Co configuration, combined with a Mg/MgO/Mg tunnel barrier layer formed using DC sputtering and natural oxidation, enhances crystallization and pinning strength, allowing for a high MR ratio and low RA value while reducing noise and maintaining low magnetostriction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a MgO barrier layer is used in TMR sensors, then the MR ratio is improved, but the RA value increases and manufacturing complexity increases due to annealing temperature control difficulties
Solution Approach 1:
A CoFeB layer is introduced as an intermediary between the MgO barrier layer and the free layer. This intermediate layer serves multiple functions: it improves the crystallization of the MgO barrier layer, enhances spin polarization, and reduces the RA value while maintaining a high MR ratio, thereby resolving the contradiction between achieving high MR ratio and low RA value
Solution Approach 2:
The patent changes the material composition parameter by using CoFeB (cobalt-iron-boron) instead of conventional CoFe or CoFeNi alloys in the pinned layer. This material substitution enables better control of annealing temperature and improves the overall performance of the TMR sensor, allowing simultaneous achievement of high MR ratio and low RA value
2Manufacturing precision
If a CoFe/NiFe free layer is used with MgO barrier, then the RA value is reduced, but the MR ratio degrades
Solution Approach 1:
The patent employs a composite structure combining CoFeB pinned layer, MgO barrier layer, and CoFe/NiFe free layer. The CoFeB layer provides high spin polarization and controls crystallization, while the CoFe/NiFe free layer maintains low magnetostriction. This composite material approach enables simultaneous achievement of low RA value and high MR ratio that cannot be achieved with single material systems
3Measurement precision
If annealing temperature is increased to improve crystallization, then the MR ratio is improved, but the manufacturing complexity and difficulty increase
Solution Approach 1:
The CoFeB layer is designed to promote preliminary crystallization of the MgO barrier layer during the annealing process. This preliminary action by the CoFeB layer reduces the required annealing temperature and simplifies the manufacturing process, while still achieving the necessary crystallization quality for high MR ratio performance
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 solution achieves a high MR ratio of over 40% with a low RA value (<2 ohm-um2) and low magnetostriction, improving signal-to-noise ratio and reducing noise in TMR sensors, while being cost-effective and compatible with existing manufacturing processes.
Implementation Method 1
The tunnel barrier layer is so thin that a current through it can be established by quantum mechanical tunneling of conduction electrons
Implementation Method 2
a Mg/MgO/Mg tunnel barrier layer formed using DC sputtering and natural oxidation
Implementation Method 3
a Mg/MgO/Mg tunnel barrier layer formed using DC sputtering and natural oxidation
Implementation Method 4
The electrical resistance through the 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
Data Source
AI summary
A high performance TMR sensor is fabricated by employing a composite inner pinned (AP1) layer in an AP2/Ru/AP1 pinned layer configuration. In one embodiment, there is a 10 to 80 Angstrom thick lower CoFeB or CoFeB alloy layer on the Ru coupling layer, a and 5 to 50 Angstrom thick Fe or Fe alloy layer on the CoFeB or CoFeB alloy, and a 5 to 30 Angstrom thick Co or Co rich alloy layer formed on the Fe or Fe alloy. A MR ratio of about 48% with a RA of <2 ohm-um2 is achieved when a CoFe AP2 layer, MgO (NOX) tunnel barrier, and CoFe/NiFe free layer are used in the TMR stack. Improved RA uniformity and less head noise are observed. Optionally, a CoFe layer may be inserted between the coupling layer and CoFeB or CoFeB alloy layer to improve pinning strength and enhance crystallization.