MTJ Sensor Array Layout for High SNR Without Magnetic Shields
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Solution Overview
Problem
Existing magnetic field sensors, such as AMR, MTJ, and GMR sensors, face challenges with high cost, large size, inadequate sensitivity, and temperature dependence, while Wheatstone bridge structures with magnetic shields suffer from remnant fields and complex fabrication, limiting their effectiveness in mobile applications.
Innovation Solution
A magnetic field sensor design with unshielded MTJ sensors in a Wheatstone bridge configuration, optimized for high signal-to-noise ratio, uses shape anisotropy to set magnetization directions and incorporates a magnetic field generator for resetting sense elements, reducing complexity and size.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If magnetic shields are used in Wheatstone bridge structures to suppress reference element response, then sensitivity is improved, but device complexity and fabrication difficulty increase due to thick shields and carefully tuned NiFe seed and plating steps
Solution Approach 1:
The patent removes the magnetic shield component entirely from the Wheatstone bridge structure. Instead of using thick magnetic shields to suppress reference element response, the invention uses unshielded MTJ sensors where the reference elements naturally provide the required suppression through their magnetic configuration, eliminating complex NiFe seed and plating steps while maintaining sensitivity
Solution Approach 2:
The patent introduces a soft magnetic underlayer as an intermediary component between the substrate and the MTJ sensors. This underlayer provides magnetic field management and reference element suppression without requiring thick magnetic shields, simplifying the overall structure while maintaining the desired sensitivity performance
2Measurement precision
If magnetic shields are used to suppress reference element response, then sensitivity is improved, but manufacturing cost and fabrication time increase due to carefully tuned NiFe seed and plating steps
Solution Approach 1:
The patent eliminates the magnetic shield component and its associated complex NiFe seed and plating processes. The unshielded MTJ sensor design achieves reference element suppression through magnetic configuration rather than physical shielding, dramatically simplifying manufacturing and reducing costs while maintaining sensitivity
3Measurement precision
If the number of sensor elements in an array is increased to improve signal to noise ratio, then measurement precision is improved, but device area and complexity increase
Solution Approach 1:
The patent combines multiple sensor elements into an array configuration where elements share common reference elements and interconnect structures. This merging approach allows the signal to noise ratio to improve with the number of sense elements while the area increases only linearly rather than quadratically, as shared components reduce the total area requirement
Solution Approach 2:
The patent implements shared reference elements that serve multiple sense elements simultaneously. These universal reference structures provide magnetic field suppression for multiple sensors, reducing the overall device area and complexity while maintaining high signal to noise ratio through the combined output of multiple sense elements
4Measurement precision
If AMR sensor sensing units are made square millimeters in size to achieve required sensitivity, then sensitivity is improved, but device area and cost increase making them unsuitable for mobile applications
Solution Approach 1:
The patent transitions from AMR technology to MTJ technology, changing the fundamental sensing mechanism parameters. MTJ sensors inherently provide higher sensitivity per unit area, allowing the sensor size to be reduced from square millimeters to much smaller dimensions while maintaining or improving sensitivity, making them suitable for mobile applications
Solution Approach 2:
The patent employs composite magnetic tunnel junction structures with multiple functional layers including ferromagnetic layers, tunnel barriers, and soft magnetic underlayers. These composite materials provide enhanced sensitivity and magnetic field management in a compact form factor, eliminating the need for large square millimeter-sized sensing units
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 design achieves improved sensitivity, reduced size, and lower power consumption, while eliminating the need for magnetic shields, thus enhancing performance and manufacturability.
Implementation Method 1
Other types of sensors, such as magnetic tunnel junction (MTJ) sensors and giant magnetoresistance (GMR) sensors
Implementation Method 2
magnetic tunnel junction (MTJ) sensors
Implementation Method 3
two opposite anti-ferromagnetic pinning directions for each sense axis
Implementation Method 4
A reset line may create the resetting magnetic field by providing a reset current pulse
Data Source
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Figure 3~4
Figure 5~6
AI summary
A magnetic sensor includes a plurality of groups, each group comprising a plurality of magnetic tunnel junction (MTJ) devices having a plurality of conductors configured to couple the MTJ devices within one group in parallel and the groups in series enabling independent optimization of the material resistance area (RA) of the MTJ and setting total device resistance so that the total bridge resistance is not so high that Johnson noise becomes a signal limiting concern, and yet not so low that CMOS elements may diminish the read signal. Alternatively, the magnetic tunnel junction devices within each of at least two groups in series and the at least two groups in parallel resulting in the individual configuration of the electrical connection path and the magnetic reference direction of the reference layer, leading to independent optimization of both functions, and more freedom in device design and layout. The X and Y pitch of the sense elements are arranged such that the line segment that stabilizes, for example, the right side of one sense element; also stabilizes the left side of the adjacent sense element.