Transcript CMOS compatible integrated magnetometers - Mos-AK

CMOS compatible integrated
magnetometers
L. Hébrard1, J.-B. Kammerer1, M. Hehn2, V. Frick1,
A. Schuhl2, P. Alnot3, P. French4, F. Braun1
1
InESS -
2
LPM (UHP-Nancy) -
MOS-AK 2005
3
LPMI (UHP-Nancy) -
4
EIL (TU-Delft -The Netherlands)
8 avril 2005
Outline
• Magnetic measurement techniques
• Hall effect magnetic sensors
– Potential applications
– Conventional Hall effect sensors
– Multi-strip Hall device
– Need for accurate compact models
• High resolution integrated magnetometers
– Conventional approaches
– Fluxgate like technique using a MTJ
– Need for a good compact model of the MTJ
• Conclusion
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CMOS compatible Magnetic
Measurement Techniques
• Without post-processing
– Hall effect sensors, 1D and 2D/3D
• With post-processing for ferromagnetic layer
– Fluxgate
– Spintronic devices (MTJ, GMR)
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Hall effect sensor applications
• Mainly for low cost applications :
• Automotive field – contactless displacement sensor,…
• Energy metrology – contactless current sensing
• Medical instrumentation :
• Magnetic Resonance Imaging
• Magnetic tracking for endovascular intervention
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CMOS conventional Hall effect device
• Made of a N-well  sensitive to Bz
• Based on the Lorentz force : FL = q v x B
I
Bz
N-well
t
P-substrate
1
VH =
I Bz
qnt
1
SA =
I = SI I
qnt
To increase the sensitivity :
• decrease of t
++++++++++++
I
VH
• increase of I
--------------------
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Gated Hall effect device
Vg < Vth
I
n+
I
n+
DZ
N-well
GHD
teff
VH
Vg
DZ
Ibias
P-substrate
SI = 120 V/AT against 100 V/AT for a rectangular Hall device with L/W ≥ 3
SA  120 mV/T for Imax  1mA
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Short circuit effect
VH =
1
I Bz
qnt
VH =
G
I Bz
qnt
Short device
G1
L/W ≥ 3
G << 1
Multi-strips device
G1
The multi-strips device needs a specific biasing circuit
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Specific biasing circuit
VH = 4 x Vh
Vh
+
Vh
+
Vh
+
Vh
Assuming infinite output resistance for the biasing transistors
to preamplification
VH = N x Vh
Yes, but beware of the noise…!!
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Excess noise
I3
I3
4
I3
4
2
R
2
VNoise
Total
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I3
4
I3
I3
4
R
I3
4
I3
4

R
I3
4
N3  N 2

 R  I n2  I p2
12
MOS-AK - 2005
II
VNoise  22  3 3 R R
44

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Chopper stabilisation
1/f noise shifted around the chopping frequency
Thermal noise is unchanged
Low-pass filtering to suppress the 1/f noise
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Experimental results with 4 and 5-strips devices
4-strips sensor without chopper
5-strips sensor with chopper at 45kHz
• SA = 375 mV/T for Imax = 4.5 mA
• Resolution of 30 mTrms on 5Hz-1kHz
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Need for accurate models
• Hall effect sensors are easy to integrate in CMOS
• Smart biasing and signal conditioning
• Noise level depends on the material properties and on the electrical
resistance R between adjacent strips
• Effective sensitivity depends on the ratio R/r where r is the output
resistance of the biasing transistors
• Non-linearity depends on the extension of the depleted zones
• Temperature,…
Accurate compact models are required for these sensors
to be widely used.
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Conventional approaches for high resolution
magnetometer integrated in CMOS
• Flux concentrators above IC + Hall effect sensors :
• Hysteresis
• High area
• Fluxgate : technique known since 1930
• Commercially available as macroscopic sensors
• No hysteresis
• Compatible with CMOS
• Size reduction is still a problem!
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Fluxgate sensor principle
sensing (V)
H
excitation (H)
External field to measure
magnetization (M)
Miniaturization
• possible (ferro post-process)
• good coupling between the
ferromagnetic core and the
sensing coil is an issue
• Core size (Barkausen noise)
M
V
We need something to detect the magnetization flipping and saturation
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Magnetic Tunnel Junction
R
z
y
x
Soft layer ±Hcs
- Hch
- Hcs
Hcs
Hch
Transverse field Hy = 0
Transverse field Hy ≠ 0
Hard layer ±Hch
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Symmetrical response
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Hx
2D fluxgate sensor using a single MTJ
• The soft layer is used as the ferromagnetic core
• The junction resistance detects the magnetization changes
no core-sensing coil coupling problem
• Double excitation
• Macroscopic prototype
Triangle : along main axis
Square : perpendicular to main
axis
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Experimental results
Along the main axis :
1086 V/T
Perpendicular to main axis :
534 V/T
Resolution : 2 mT / Hz
Integrated version
Resolution  1 nT
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Integration of the MTJ-Fluxgate
• MTJ above IC (post-processing)
• planar excitation coils
• low noise integrated electronics
• small area MTJ (1mm x 1mm)  no Barkhausen noise
Compact model of the MTJ is required to simulate the fluxgate system!
A first model has been developped :
• magnetization vector
• demagnetizing field (junction shape)
• coupling factor between both ferro
layers of the MTJ
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See poster on
Compact modeling of
Spintronic devices in
VHDL-AMS
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Conclusion
• Not only MOS transistors in CMOS chip
• Hall effect sensors can find wide applications
• Fully compatible with CMOS
• On-chip circuitry advantage
• Need for accurate compact models
• High resolution magnetometers
• Resolution below 1nT
• Post-process cost justified by high resolution
• Need for compact models for spintronic devices
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