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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 Strasbourg 8 avril 2005 MOS-AK - 2005 Page 2/19 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) Strasbourg 8 avril 2005 MOS-AK - 2005 Page 3/19 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 Strasbourg 8 avril 2005 MOS-AK - 2005 Page 4/19 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 -------------------- Strasbourg 8 avril 2005 MOS-AK - 2005 Page 5/19 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 Strasbourg 8 avril 2005 MOS-AK - 2005 Page 6/19 Short circuit effect VH = 1 I Bz qnt VH = G I Bz qnt Short device G1 L/W ≥ 3 G << 1 Multi-strips device G1 The multi-strips device needs a specific biasing circuit Strasbourg 8 avril 2005 MOS-AK - 2005 Page 7/19 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…!! Strasbourg 8 avril 2005 MOS-AK - 2005 Page 8/19 Excess noise I3 I3 4 I3 4 2 R 2 VNoise Total Strasbourg 8 avril 2005 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 R VNoise 2 2 3 3 R 44 Page 9/19 Chopper stabilisation 1/f noise shifted around the chopping frequency Thermal noise is unchanged Low-pass filtering to suppress the 1/f noise Strasbourg 8 avril 2005 MOS-AK - 2005 Page 10/19 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 Strasbourg 8 avril 2005 MOS-AK - 2005 Page 11/19 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. Strasbourg 8 avril 2005 MOS-AK - 2005 Page 12/19 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! Strasbourg 8 avril 2005 MOS-AK - 2005 Page 13/19 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 Strasbourg 8 avril 2005 MOS-AK - 2005 Page 14/19 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 Strasbourg 8 avril 2005 Symmetrical response MOS-AK - 2005 Page 15/19 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 Strasbourg 8 avril 2005 MOS-AK - 2005 Page 16/19 Experimental results Along the main axis : 1086 V/T Perpendicular to main axis : 534 V/T Resolution : 2mT / Hz Integrated version Resolution 1 nT Strasbourg 8 avril 2005 MOS-AK - 2005 Page 17/19 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 Strasbourg 8 avril 2005 MOS-AK - 2005 See poster on Compact modeling of Spintronic devices in VHDL-AMS Page 18/19 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 Strasbourg 8 avril 2005 MOS-AK - 2005 Page 19/19