Abstract:
A multi-axis GMR or TGMR based magnetic field sensor system is disclosed. Preferably a three axis sensor system is provided for sensing magnetic flux along three mutually orthogonal axes, which can be used for magnetic compass or other magnetic field sensing applications. The sensing units are operative to sense X and Y axis magnetic flux signals in the device (XY) plane, while Z axis sensitivity is achieved by use of a continuous ring shaped or octagonal magnetic concentrator that is adapted to convert the Z axis magnetic flux signal into magnetic flux signals in the XY plane.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application No. 61/597,368, entitled “PLANAR THREE-AXIS MAGNETOMETER,” filed on Feb. 10, 2012, which is herein incorporated by reference in its entirety for all purposes. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable 
       BACKGROUND OF THE INVENTION 
       [0003]    Multi-axis magnetic sensors or magnetometers, such as three-axis magnetic sensors, are particularly desirable for modern electronic compass applications. However, such devices are usually unable to sense magnetic flux from all three orthogonal axes. For example, conventional magneto-resistive (MR) sensors, such as AMR (anisotropic MR) sensors, GMR (giant MR) sensors, TGMR (tunneling GMR) sensors, and the like, can detect magnetic flux that is parallel to the device plane but cannot detect flux that is perpendicular to the device plane. On the other hand, Hall-effect sensors can sense magnetic flux that is perpendicular to the device plane, i.e., along the Z axis, but cannot sense magnetic flux parallel to the device plane, i.e., in the XY plane. 
         [0004]    There are many known approaches to fabricate a magnetic sensor with three-axis sensitivities. One approach is to package a Z axis sensor of the same technology as the X and Y axis sensors in orthogonal disposition to the two-axis XY sensors. Another approach uses two types of sensor technologies that are disposed on a common die with one constructed to sense vertical magnetic flux signals and the other constructed to sense horizontal magnetic flux signals. Multi-axis sensitivities can also be achieved by building sensors on a sloped surface. A further approach uses a magnetic concentrator that is adapted to convert signals along one axis to an orthogonal direction so that magnetic flux from all three axes can be detected using the same technology. 
         [0005]    However, there are disadvantages associated with each of the known approaches. For example, combining a Z axis magnetic field sensor, whose sensing direction is perpendicular to the device (XY) plane, with an X or Y axis magnetic field sensor(s) requires one or more additional packaging steps in order to install the Z axis magnetic field sensor vertically without significant angle variation. The additional packaging steps add significant cost to the whole product manufacturing process. Furthermore, variation in the positioning angle complicates signal processing since cross-talk signals from the XY plane are introduced if the Z axis magnetic field sensor in not perfectly vertical. 
         [0006]    Hall-effect sensors, which can sense magnetic flux from a direction that is perpendicular to the device (XY) plane, can be built on a common die with two-axis MR sensors; however, the different Hall-effect and MR technologies require different processing steps and resultant fabrication complexity. 
         [0007]    Sensors that are disposed on a sloped surface can detect magnetic flux signals that are parallel and perpendicular to the device (XY) plane, but with the disadvantage of a complicated manufacturing process. For example, typical fabrication steps, including film deposition, photolithographic, etch patterning and the like on sloped surfaces, are much more difficult than on planar surfaces, especially as device dimensions become smaller and smaller. 
         [0008]    A magnetic concentrator can convert magnetic flux signals along one direction to signals along another direction and can also improve sensitivity through flux signal amplification. However, known concentrator configurations complicate signal processing because of cross-talk which can affect the sensing units. 
         [0009]    Therefore, it would be desirable to provide a multiple-axis magnetic sensor or magnetometer having a magnetic concentrator that is less affected by cross-talk. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    A multi-axis GMR or TGMR based magnetic field sensor or magnetometer is disclosed. Preferably a planar three axis sensor is provided for sensing magnetic flux along three mutually orthogonal axes. The sensor or magnetometer can be used for magnetic compass or other magnetic field sensing applications. The GMR or TGMR sensing units are operative to sense X and Y axis magnetic flux signals in the device (XY) plane, while Z axis sensitivity is achieved by use of a continuous ring shaped or octagonal magnetic concentrator that is adapted to convert the Z axis magnetic flux signal into magnetic flux signals in the XY plane. The Z field component is calculated using magnetic flux signals from the X or Y axis sensors through signal processing. The magnetic concentrator functions both as a flux guide (Z to XY plane) and a signal amplifier. Cross talk is minimized by placement of the sensor units symmetrically on both sides of the concentrator. 
         [0011]    Advantageously, GMR and/or TGMR sensors have higher signal amplitudes when compared to AMR or Hall-effect sensors. In instances in which electronic noise is dominating, higher amplitude is especially beneficial to improve the signal to noise ratio. GMR and TGMR sensors also can be much smaller than AMR based sensors, hence, overall device size can be significantly reduced. In addition, GMR and TGMR sensors can have higher field strength over a greater range, which renders such sensors less susceptible to electromagnetic noise in, for example, a smart phone or other handheld electronic devices. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0012]    Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof and from the claims, taken in conjunction with the accompanying drawings, in which: 
           [0013]      FIG. 1A  shows a diagrammatic plan view of a three-axis magnetic field sensor having an octagonal magnetic concentrator in accordance with the present invention; 
           [0014]      FIG. 1B  shows a diagrammatic plan view of a three-axis magnetic field sensor having a ring-shaped magnetic concentrator in accordance with the present invention; 
           [0015]      FIGS. 2A and 2B  show diagrammatic elevation views of a bottom-pinned and a top-pinned GMR and TGMR sensor, respectively, in accordance with the present invention; 
           [0016]      FIG. 3  shows a diagrammatic view of the magnetization structure of the bottom-pinned GMR and TGMR sensor shown in  FIG. 2A ; 
           [0017]      FIG. 4A  shows a diagrammatic plan view of the direction of external magnetic fields used to set the antiferromagnetic layer in the GMR and TGMR sensors shown in  FIGS. 1A and 1B ; 
           [0018]      FIG. 4B  shows a diagrammatic plan view of alternative directions of external magnetic fields used to set the antiferromagnetic layer in the GMR and TGMR sensors shown in  FIGS. 1A and 1B ; 
           [0019]      FIG. 4C  shows a diagrammatic plan view of directions of a “spin flop” transition with a transversely applied external magnetic field to set the antiferromagnetic layer in the GMR and TGMR units shown in  FIGS. 1A and 1B ; 
           [0020]      FIG. 4D  shows the layer magnetization principle associated with  FIG. 4C ; 
           [0021]      FIG. 5  shows a diagrammatic view of the modification of magnetic flux lines near the magnetic concentrator; 
           [0022]      FIGS. 6A and 6B  show the working principle of the magnetic concentrator in a perspective view ( 6 A) and in cross-section ( 6 B); 
           [0023]      FIGS. 7A and 7B  show the working principle of the magnetic concentrator in plan view ( 7 A) and in cross-section ( 7 B); and 
           [0024]      FIG. 8  shows a diagrammatic plan view of a set/reset coil used with the GMR and TGMR sensors. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application No. 61/597,368, entitled “PLANAR THREE-AXIS MAGNETOMETER,” filed on Feb. 10, 2012, which is herein incorporated by reference in its entirety for all purposes. 
         [0026]    Referring to  FIGS. 1A and 1B , magnetometer layouts in accordance with the present invention are shown, respectively, for a system  10  with a continuous octagonal magnetic concentrator  16 , and for a system  10  with a continuous, ring-shaped magnetic concentrator  18 . The devices of  FIG. 1A  and  FIG. 1B  are the same except for the shape of the magnetic concentrator. The magnetic concentrator is formed of a ferromagnetic material having high permeability and low coercive force as per se is known in the art. For illustrative purposes only, each system  10  includes four groups  11 ,  12  of GMR and/or TGMR units  14 ,  15  that are disposed at discrete locations about the octagonal  16  or ring-shaped  18  magnetic concentrator. The units  14  and  15  on each side of the magnetic concentrator  16 ,  18  are structurally identical. Each unit  14 ,  15  has a rectangular or substantially rectangular shape that is disposed so that the longer side  13  of the rectangle is parallel or substantially parallel to a portion  16   a  of the adjacent magnetic concentrator. The units  14  and  15  are disposed on opposite sides of concentrator  16  as shown in  FIG. 1A , or on opposite sides of concentrator  18  as shown in  FIG. 1B . 
         [0027]    Each group  11 ,  12  has four structurally identical units  14 ,  15  that in operation form a Wheatstone bridge. Groups  11  with the longer sides  13  of units  15  parallel to the X axis become Y axis and Z axis sensors, sensing signals in both the Y and Z axes. Groups  12  with the longer sides  13  of units  14  parallel to the Y axis become X axis and Z axis sensors, sensing signals in both the X and Z axes. 
         [0028]    Each of the sensor units  14 ,  15  are constructed as shown diagrammatically in  FIG. 2A  and  FIG. 2B . A structure for a bottom-pinned unit is shown in  FIG. 2A , and for a top-pinned unit is shown in  FIG. 2B . The bottom-pinned unit  20   a    FIG. 2A  has an anti-ferromagnetic layer (AFM)  25   a  at the bottom of the unit  20   a,  closer to the substrate  19  than the pinned layer  24   a,  for pinning the pinned layer  24   a.  The top-pinned unit  20   b  of  FIG. 2B  has an anti-ferromagnetic layer (AFM)  25   b  at the top of the unit  20   b  further from the substrate  19  than the pinned layer  24   b,  for pinning the pinned layer  24   b.    
         [0029]    A ruthenium (Ru) layer  27   a,    27   b  is provided between a reference layer  26   a,    26   b  and the pinned layer  24   a,    24   b.  Adjusting the thickness of the Ru layer  27   a,    27   b  between the reference layer  26   a,    26   b  and the pinned layer  24   a,    24   b  creates a synthetic anti-ferromagnetic (SAF) structure in which the magnetization directions in the reference layer  26   a,    26   b  and the pinned layer  24   a,    24   b  are anti-parallel. 
         [0030]    A non-magnetic (NM) spacer layer  23   a,    23   b  is disposed between the free layer  22   a,    22   b  and the reference layer  26   a,    26   b.  If the spacer layer  23   a,    23   b  is a nonmagnetic (NM) insulator, e.g., MgOx, AlOx, and the like, then the device  20   a,    20   b  is a TGMR sensor. If the spacer layer  23   a,    23   b  is a nonmagnetic (NM) metal, e.g., copper, and the like, then the device  20   a,    20   b  is a GMR based device. 
         [0031]    The magnetization structure of the bottom-pinned GMR or TGMR unit  20   a  shown in  FIG. 2A  will be discussed in conjunction with  FIG. 3 . The magnetization of the pinned layer  24  illustrated by arrow  28  is set to be perpendicular to the longer sides  13  of units  14 ,  15 , while the magnetization of the free layer  22  illustrated by symbols  29  is parallel to the longer sides  13  and perpendicular to the magnetization of the reference layer  26  illustrated by arrow  21 , as defined by its shape anisotropy. The direction of the magnetization in the free layer  22  is defined by a set/reset coil  80  as shown in  FIG. 8  described in greater detail below. The magnetization of the layers of the unit shown in  FIG. 2B  is similar as described for  FIG. 2A . 
         [0032]    In one embodiment, the magnitude of the magnetic moment of the pinned layer  24  is lower than the magnitude of the magnetic moment of the reference layer  26 . As a result, the magnetization  21  of the reference layer  26  will align parallel with an applied external field (H) having a specified field strength, while the magnetization  28  of the pinned layer  24  will be aligned anti-parallel to the applied field (H). 
         [0033]    Referring to  FIG. 4A  and  FIG. 4B , a localized magnetic field  41   a - 41   d  is applied discretely to each sensor group  11 ,  12  to set the pinning direction of the AFM layer  25 . The pinning direction of the AFM layer  25  can be pre-set, for example, by applying a localized magnetic field  41   a - 41   d  to the sensor units in vacuum after the unit has been heated to a pre-established temperature. The field direction of the external magnetic field is oriented either from the center of the device  20  outwardly to the four edges  45  as shown in  FIG. 4A , or inwardly from the four edges  45  towards the center of the device  20  as shown in  FIG. 4B . 
         [0034]    In  FIG. 4C , a localized magnetic field (H)  41  is applied transversely, for example at a 45-degree angle relative to the X direction and/or Y direction, in the XY plane.  FIG. 4D  shows diagrammatically how the AFM pinning directions  42   c  and  42   d  shown in  FIG. 4C  are set by the applied magnetic field (H)  41 . This is referred to as a “spin flop” transition. The effectiveness of the “spin flop” depends on, inter alia, the magnetic field strength of the externally applied magnetic field  41 ; the difference of the magnetic moments between pinned layers  24   c,    24   d  and reference layers  26   c,    26   d,  which preferably is zero or near zero; the annealing temperature during manufacture, which should occur at a temperature above the AFM blocking temperature; the free layer  22   c,    22   d  moment; and the growth field strength and direction for the pinned  22   c,    22   d,  free  24   c,    24   d,  and reference layers  26   c,    26   d.    
         [0035]    The “spin flop” transition causes the magnetization of the free layers  22   c,    22   d  of a nearly balanced SAF structure to align with the direction of the applied magnetic field  41 . Further, the magnetization direction of the reference layers  26   c,    26   d  and of the pinned layers  24   c,    24   d  are anti-parallel and fall along the short axis direction. As is well-known, the AFM layer  25  ( FIG. 3 ) is pinned by the magnetization of the pinned layers  24   c,    24   d.    
         [0036]    As shown in  FIGS. 5-7 , the interaction between the magnetic concentrator  16 ,  18  and units  14 ,  15 , causes an applied magnetic flux  51 , oriented orthogonal or substantially orthogonal (vertical) to substrate  19  to be converted to be substantially parallel to the plane of substrate  19  (horizontal) as shown by the arrows  52  in  FIG. 6A  and  FIG. 6B . The field strength is typically 0.5 Gauss in one embodiment The horizontal flux  52  can then be sensed by units  14 ,  15  that are disposed on both sides of the magnetic concentrator  16 ,  18 . The units  14 ,  15  generate signals of the sensed magnetic field  52  and which signals are directed to a processing device (not shown) for signal processing as known in the art. 
         [0037]      FIG. 7A  illustrates the vertical signal detection (Hz−) using units  14 ,  15  of respective groups  11 ,  12 .  FIG. 7B  illustrates the horizontal signal detection (Hx+) using units  14 ,  15  of respective groups  11 , 12 . As shown in  FIG. 7A , the mathematical difference between the sensed fluxes at the unit  14   a,    15   a  to the left of the magnetic concentrator  16 ,  18  and at the unit  14   b,    15   b  to the right of the concentrator represents the vertical flux (both direction and strength). Similarly, as shown in  FIG. 7B , the mathematical sum of the sensed fluxes represents the horizontal flux (both direction and strength). Thus, the magnetic concentrator  16 ,  18  causes groups  12  having their longer sides  13  parallel to the Y axis to be XZ plane sensors, and groups  11  having their longer sides  13  parallel to the X axis to be YZ plane sensors. 
         [0038]    A set/reset coil arrangement for initiating, setting, and resetting the magnetization directions of the free layer  22  and the magnetic concentrator  16 ,  18  is shown in  FIG. 8 . The solid lines  82   a - 82   d  correspond to set wires that are disposed above the magnetic concentrator  16 ,  18  while the dashed lines  84   a - 84   d  correspond to reset wires that are disposed below the magnetic concentrator  16 ,  18 . Current passing through the wires  82 ,  84  induces magnetic fields which can be used to establish free layer magnetization directions, e.g., two directions in each group  11 ,  12 , as well as to establish the magnetization in the concentrator  16 ,  18 . With multiple groups of sensing units for the XZ plane sensor and multiple groups of sensing units for the YZ plane sensor, it is possible to acquire a differential signal for Z from one of the sensing units and a sum signal for X or Y from another of the sensing units. 
         [0039]    Although preferred embodiments of the invention have been described above, it will be recognized and understood that various modifications may be made in the invention and that the appended claims are intended to cover all such modifications which fall within the spirit and scope of the invention.