Unshielded MTJ Sensor Array with Sequential Pulse Reset
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Solution Overview
Problem
Existing magnetic field sensors face challenges such as high cost, large circuit area, and power consumption for mobile applications, inadequate sensitivity, and temperature-dependent issues, particularly when measuring the earth's magnetic field, due to factors like Barkhausen noise and micromagnetic domain fluctuations.
Innovation Solution
A magnetic field sensor configuration using unshielded MTJ sensors in a Wheatstone bridge structure with dynamically stabilized sense elements through shape anisotropy and periodic field pulses, eliminating the need for hard bias layers and reducing manufacturing complexity and cost, while maintaining high signal-to-noise ratio.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If magnetic shields are used in Wheatstone bridge structure, then temperature dependent resistance changes are eliminated and sensitivity is increased, but manufacturing complexity increases due to thick shields requiring carefully tuned NiFe seed and plating steps
Solution Approach 1:
The patent removes the magnetic shield component entirely from the Wheatstone bridge structure, using unshielded MTJ sensors instead. This extraction eliminates the manufacturing complexity associated with thick shields and carefully tuned NiFe seed and plating steps, while maintaining temperature compensation through the bridge structure itself.
Solution Approach 2:
The patent replaces expensive, complex magnetic shields with simpler, thinner unshielded MTJ sensor elements. The unshielded structure uses less material and requires less complex fabrication processes, effectively using a simpler substitute that achieves the same functional goal without the manufacturing burden.
2Measurement precision
If magnetic shields are used to suppress reference elements, then bridge response is controlled, but remnant field impairs low field measuring capabilities
Solution Approach 1:
The patent removes the magnetic shield that causes remnant field problems. By using unshielded MTJ sensors, the system eliminates the source of remnant field interference that impairs low field measurement capabilities, while maintaining bridge response control through the differential configuration.
Solution Approach 2:
The patent converts the potential harm of unshielded sensors (susceptibility to external fields) into a benefit by using differential measurement. The Wheatstone bridge configuration naturally rejects common-mode interference and extracts the differential signal, turning what could be noise into a robust measurement approach.
3Device complexity
If four different pinning directions are used for each sense axis, then magnetic shields are eliminated, but fabrication complexity increases requiring complex and unwieldy magnetization techniques
Solution Approach 1:
The patent uses asymmetric pinning directions in the MTJ sensor structure, where the pinned layer and free layer have different magnetic anisotropy orientations. This asymmetric configuration allows the sensor to achieve the required magnetic field sensitivity without requiring four separate pinning directions, simplifying the fabrication process.
Solution Approach 2:
The patent changes the magnetic anisotropy parameters of the MTJ layers through controlled deposition and annealing processes. By adjusting the crystalline orientation and magnetic properties of the thin films during fabrication, the system achieves the desired pinning behavior without complex post-fabrication magnetization techniques.
4Reliability
If sense elements are pinned using hard magnetic bias layer or anti-ferromagnetic pinning layer, then micromagnetic domain switches are eliminated, but processing cost and complexity increase
Solution Approach 1:
The patent uses the inherent magnetic anisotropy of the MTJ sensor layers themselves to provide the pinning effect, rather than requiring separate hard magnetic bias layers or anti-ferromagnetic pinning layers. The shape anisotropy and magnetocrystalline anisotropy of the thin film structures naturally stabilize the magnetic domains, making the system self-sufficient and eliminating additional processing steps.
5Measurement precision
If AMR sensor configuration is used to achieve required sensitivity, then reasonable resistances are obtained, but circuit area and power consumption are too large for mobile applications
Solution Approach 1:
The patent changes from AMR sensor technology to MTJ sensor technology, which offers higher sensitivity per unit area. The MTJ structure provides greater magnetoresistive effect, allowing smaller sensor elements to achieve the same or better sensitivity, thereby reducing the overall circuit area required for mobile applications.
Solution Approach 2:
The patent replaces large-area AMR sensors with compact MTJ sensor elements that achieve equivalent or superior performance in a smaller footprint. The MTJ structure uses thinner layers and smaller geometric dimensions, effectively using a more space-efficient technology to accomplish the same sensing function.
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 enables efficient, sensitive, and cost-effective magnetic field measurement with reduced power consumption and minimal physical array size, effectively addressing the limitations of existing sensors by stabilizing sense elements before each measurement and minimizing noise impacts.
Implementation Method 1
circuitry configured to apply a current pulse sequentially to and adjacent to each of the i groups
Implementation Method 2
magnetic tunnel junction (MTJ) sensors and giant magnetoresistance (GMR) sensors
Implementation Method 3
dynamically stabilized sense elements through shape anisotropy
Data Source
Figure 1~2
Figure 3~4
Figure 5~9
AI summary
A semiconductor process and apparatus provide a high-performance magnetic field sensor with three differential sensor configurations which require only two distinct pinning axes, where each differential sensor is formed from a Wheatstone bridge structure with four unshielded magnetic tunnel junction sensor arrays, each of which includes a magnetic field pulse generator for selectively applying a field pulse to stabilize or restore the easy axis magnetization of the sense layers to orient the magnetization in the correct configuration. prior to measurements of small magnetic fields. The field pulse is sequentially applied to groups of the sense layers of the Wheatstone bridge structures, thereby allowing for a higher current pulse or larger sensor array size for maximal signal to noise ratio.