MOS-AK 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 4, F. Braun 1 1 InESS - 2 LPM (UHP-Nancy) - 3 LPMI (UHP-Nancy) - 4 EIL (TU-Delft -The Netherlands)

MOS-AK 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

MOS-AK Page 3/19Strasbourg 8 avril 2005 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)

MOS-AK Page 4/19Strasbourg 8 avril 2005 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

MOS-AK Page 5/19Strasbourg 8 avril 2005 CMOS conventional Hall effect device Made of a N-well  sensitive to B z Based on the Lorentz force : F L = q v x B P-substrate BzBz I VHVH V H = 1 q n t I B z 1 q n t S A = I = S I I N-well To increase the sensitivity : decrease of t increase of I I t

MOS-AK Page 6/19Strasbourg 8 avril 2005 Gated Hall effect device n+ P-substrate DZ N-well II t eff DZ V g < V th I bias VHVH VgVg GHD S I = 120 V/AT against 100 V/AT for a rectangular Hall device with L/W ≥ 3 S A  120 mV/T for I max  1mA

MOS-AK Page 7/19Strasbourg 8 avril 2005 Short circuit effect The multi-strips device needs a specific biasing circuit G  1 L/W ≥ 3 Multi-strips device G  1 Short device G << 1 V H = G q n t I B z V H = 1 q n t I B z

MOS-AK Page 8/19Strasbourg 8 avril 2005 Specific biasing circuit to preamplification VhVh VhVh VhVh VhVh V H = 4 x V h V H = N x V h Assuming infinite output resistance for the biasing transistors Yes, but beware of the noise … !!

MOS-AK Page 9/19Strasbourg 8 avril 2005 Excess noise I3I3 I3I3 4 I3I3 4 I3I3 4 I3I3 4 I3I3 4 I3I3 4 2 I3I3 4 I3I3

MOS-AK Page 10/19Strasbourg 8 avril 2005 Chopper stabilisation 1/f noise shifted around the chopping frequency Thermal noise is unchanged Low-pass filtering to suppress the 1/f noise

MOS-AK Page 11/19Strasbourg 8 avril 2005 Experimental results with 4 and 5-strips devices 4-strips sensor without chopper 5-strips sensor with chopper at 45kHz S A = 375 mV/T for I max = 4.5 mA Resolution of 30  Trms on 5Hz-1kHz

MOS-AK Page 12/19Strasbourg 8 avril 2005 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.

MOS-AK Page 13/19Strasbourg 8 avril 2005 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!

MOS-AK Page 14/19Strasbourg 8 avril 2005 Fluxgate sensor principle sensing (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

MOS-AK Page 15/19Strasbourg 8 avril 2005 Magnetic Tunnel Junction Soft layer ±H cs x y z Hard layer ±H ch H cs H ch - H cs - H ch HxHx Transverse field H y = 0 R Transverse field H y ≠ 0 Symmetrical response

MOS-AK Page 16/19Strasbourg 8 avril D 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

MOS-AK Page 17/19Strasbourg 8 avril 2005 Experimental results Along the main axis : 1086 V/T Perpendicular to main axis : 534 V/T Resolution : Integrated version Resolution  1 nT

MOS-AK Page 18/19Strasbourg 8 avril 2005 Integration of the MTJ-Fluxgate MTJ above IC (post-processing) planar excitation coils low noise integrated electronics small area MTJ (1  m x 1  m)  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 See poster on Compact modeling of Spintronic devices in VHDL-AMS

MOS-AK Page 19/19Strasbourg 8 avril 2005 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