CMOS compatible integrated magnetometers

1. 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)

2. 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 - 2005

3. 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 - 2005

4. 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 - 2005

5. 1 1 VH = I Bz q n t q n t SA = I = SI I CMOS conventional Hall effect device • Made of a N-well  sensitive to Bz • Based on the Lorentz force : FL = q vxB I Bz t N-well P-substrate • To increase the sensitivity : • decrease of t • increase of I ++++++++++++ I VH -------------------- MOS-AK - 2005

6. Gated Hall effect device Vg < Vth I I n+ n+ VH DZ GHD N-well teff Vg DZ Ibias P-substrate SI = 120 V/AT against 100 V/AT for a rectangular Hall device with L/W ≥ 3 for Imax 1mA SA 120 mV/T MOS-AK - 2005

7. VH = I Bz G 1 VH = I Bz q n t q n t Short device G  1 L/W ≥ 3 G << 1 Multi-strips device G  1 The multi-strips device needs a specific biasing circuit Short circuit effect MOS-AK - 2005

8. Specific biasing circuit VH = 4 x Vh Vh Vh Vh Vh to preamplification + + + VH = N x Vh Assuming infinite output resistance for the biasing transistors Yes, but beware of the noise…!! MOS-AK - 2005

9. 2 I3 I3 I3 I3 I3 I3 I3 I3 I3 4 4 4 4 4 4 4 Excess noise MOS-AK - 2005

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

11. 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 MOS-AK - 2005

12. 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 - 2005

13. 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 - 2005

14. sensing (V) excitation (H) H External field to measure Miniaturization magnetization (M) • 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 Fluxgate sensor principle MOS-AK - 2005

15. Transverse field Hy ≠ 0 Magnetic Tunnel Junction R z y Soft layer ±Hcs x Hx Hcs - Hch - Hcs Hch Transverse field Hy = 0 Hard layer ±Hch Symmetrical response MOS-AK - 2005

16. 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 MOS-AK - 2005

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

18. 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 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 MOS-AK - 2005

19. 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 MOS-AK - 2005