MOS-AK 20058 avril 2005 CMOS compatible integrated magnetometers L. Hébrard 1, J.-B. Kammerer 1, M. Hehn 2, V. Frick 1, A. Schuhl 2, P. Alnot 3, P. French

MOS-AK 2005 8 avril 2005

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) - 3 LPMI (UHP-Nancy) - 4 EIL (TU-Delft -The Netherlands)

MOS-AK - 2005 Page 2/19Strasbourg 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


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)


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


CMOS conventional Hall effect device

• Made of a N-well sensitive to Bz • Based on the Lorentz force : FL = q v x B

P-substrate

Bz

++++++++++++

--------------------

IVH

VH =1

q n tI Bz

1q n t

SA = I = SI IN-well

To increase the sensitivity :

• decrease of t

• increase of I

I

t


Gated Hall effect device

n+n+

P-substrate

DZ

N-well

II

teff

DZ

Vg < Vth

Ibias

VH

Vg

GHD

SI = 120 V/AT against 100 V/AT for a rectangular Hall device with L/W ≥ 3

SA 120 mV/T for Imax 1mA


Short circuit effect

The multi-strips device needs a specific biasing circuit

G 1L/W ≥ 3 Multi-strips device

G 1

Short device

G << 1

VH =G

q n tI BzVH =

1q n t

I Bz


Specific biasing circuit

to preamplification+ + +Vh Vh Vh Vh

VH = 4 x Vh

VH = N x VhAssuming infinite output resistance for the biasing transistors

Yes, but beware of the noise…!!


Excess noise

R R R

2223

2

12 pnTotalNoise IIRNN

V

RI

VNoise 4

2 3 RI

VNoise 4

2 3

I3

I3

4

I3

4

I3

4

I3

4

I3

4

I3

42

I3

4

I3

MOS-AK - 2005Page 10

/19Strasbourg 8 avril 2005

Chopper stabilisation

1/f noise shifted around the chopping frequencyThermal noise is unchangedLow-pass filtering to suppress the 1/f noise



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 Trms on 5Hz-1kHz



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 sensorsto be widely used.



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!



Fluxgate sensor principlesensing (V) excitation (H)

H magnetization (M)

M

V

External field to measure

Miniaturization

• possible (ferro post-

process)• good coupling between the

ferromagnetic core and the

sensing coil is an issue• Core size (Barkausen noise)

We need something to detect the magnetization flipping and saturation



Magnetic Tunnel Junction

Soft layer ±Hcsx

y

z

Hard layer ±Hch

Hcs Hch- Hcs- HchHx

Transverse field Hy = 0

R

Transverse field Hy ≠ 0

Symmetrical response



2D fluxgate sensor using a single MTJ• The soft layer is used as the ferromagnetic core• The junction resistance detects the magnetization changes

• Double excitation

no core-sensing coil coupling problem

• Macroscopic prototype

Triangle : along main axis

Square : perpendicular to main axis



Experimental results

Along the main axis :

1086 V/T

Perpendicular to main axis :

534 V/T

Resolution :

Integrated version

HzT /2

Resolution 1 nT



Integration of the MTJ-Fluxgate

• MTJ above IC (post-processing)

• planar excitation coils

• low noise integrated electronics

• small area MTJ (1m x 1m) 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 ferrolayers of the MTJ

See poster on

Compact modeling ofSpintronic devices inVHDL-AMS



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

Documents

MOS-AK 20058 avril 2005 CMOS compatible integrated magnetometers L. Hébrard 1, J.-B. Kammerer 1, M. Hehn 2, V. Frick 1, A. Schuhl 2, P. Alnot 3, P. French