Magnetic Sensor Zero Offset via Orthogonal Pinning
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Solution Overview
Problem
Existing magnetic field sensors face challenges in achieving zero offset with minimal compensation angle and sensor-to-sensor variation, particularly in mobile applications, due to complexities in setting orthogonal pinning directions and temperature-dependent sensitivity issues.
Innovation Solution
A method and structure for forming reference layers with orthogonal pinning directions using a synthetic antiferromagnet (SAF) structure, where the pinned layer includes layers of CoFe and CoFeB materials, separated by a coupling layer, to reduce offset deviation and simplify the fabrication process, allowing for integrated circuit sensors with reduced compensation angles and variations across a wafer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional AMR sensor configurations are used to achieve required sensitivity and reasonable resistances, then sensitivity and resistance matching are improved, but sensor size and cost increase
Solution Approach 1:
The patent transitions from AMR (anisotropic magnetoresistance) to TMR (tunnel magnetoresistance) effect, fundamentally changing the measurement mechanism. This parameter change enables achieving higher sensitivity (over 100 mV/Oe) with much smaller sensor area (100 micrometers squared), directly resolving the contradiction between sensitivity and size
Solution Approach 2:
The patent employs composite magnetic layer structures including CoFeB (cobalt iron boron), CoFe (cobalt iron), and MgO (magnesium oxide) tunnel barriers. These composite materials provide both the necessary magnetic properties for high sensitivity and the structural characteristics for miniaturization, enabling compact high-performance sensors
2Manufacturing precision
If multiple pinning directions are set individually for each sense axis, then orthogonal pinning directions are achieved, but fabrication complexity increases
Solution Approach 1:
The patent merges the setting of orthogonal pinning directions into a single annealing process. By applying a magnetic field during annealing, both the first and second pinning layers are simultaneously oriented at 45 degrees relative to crystallographic directions, achieving precise orthogonal alignment without separate fabrication steps for each axis
Solution Approach 2:
The patent performs preliminary magnetic field application during the annealing process to pre-establish the orthogonal pinning directions before final sensor assembly. This preliminary action ensures that both pinning layers are correctly oriented during manufacturing, eliminating the need for complex post-fabrication alignment procedures
3Measurement precision
If compensation angle is increased to achieve zero offset, then average offset is corrected, but offset variation and standard deviation increase
Solution Approach 1:
The patent intentionally introduces asymmetry in the magnetic layer structure by using different thicknesses of CoFeB and CoFe layers in the pinned and reference layers. This asymmetric design, combined with the applied magnetic field during annealing, achieves precise zero offset with minimal compensation angle (1 degree), thereby reducing offset variation and improving reliability
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 enables the creation of magnetic field sensors with zero average offset and reduced sensor-to-sensor variation, improving sensitivity and temperature stability while minimizing manufacturing complexity and cost, suitable for mobile applications.
Implementation Method 1
the pinned layer includes layers of CoFe and CoFeB materials, separated by a coupling layer
Implementation Method 2
A method and structure for forming reference layers with orthogonal pinning directions using a synthetic antiferromagnet (SAF) structure
Implementation Method 3
CMOS-compatible magnetoelectronic field sensors used to sense magnetic fields
Implementation Method 4
The different pinning directions are typically set during an anneal process
Data Source
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AI summary
A sensor and fabrication process are provided for forming reference layers with substantially orthogonal magnetization directions having zero offset with a small compensation angle. An exemplary embodiment includes a sensor layer stack of as magnetoresistive thin-film based magnetic field sensor, the sensor layer stack comprising a pinning layer; a pinned layer including a layer of amorphous material over the pinning layer, and a first layer of crystalline material over the layer of amorphous material; a nonmagnetic coupling layer over the pinned layer; a fixed layer over the nonmagnetic coupling layer; a tunnel barrier over the fixed layer; and a sense layer over the nonmagnetic intermediate layer. Another embodiment includes a sensor layer stack where a pinned layer including two crystalline layers separated by a amorphous layer.