PATENT DOCUMENT

Publication Number: US-9778324-B2
Application Number: US-201514856302-A
Country: US
Kind Code: B2

Title: Yoke configuration to reduce high offset in X-, Y-, and Z-magnetic sensors

Abstract:
An electronic device may be provided with an electronic compass. The electronic compass may include magnetic sensors. The magnetic sensors may include thin-film magnetic sensor elements such as giant magnetoresistance sensor elements. Magnetic flux concentrators may be used to guide magnetic fields through the sensor elements. The magnetic flux concentrators may be configured to reduce the angular sensitivity of the magnetic sensors. A magnetic flux concentrator may be formed from multiple stacked layers of soft magnetic material separated by non-magnetic material. The non-magnetic material may have a thickness allows the magnetic layers to magnetically couple through the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction.

Claims:
What is claimed is: 
     
       1. A magnetic sensor, comprising:
 a substrate; and 
 a thin-film magnetic sensor element on the substrate; and 
 a multilayer magnetic flux concentrator on the substrate that directs magnetic flux through the thin-film magnetic sensor, wherein the multilayer magnetic flux concentrator includes at least first and second magnetic layers separated by a non-magnetic layer that causes the first and second magnetic layers to exhibit a Ruderman-Kittel-Kasuya-Yosida interaction. 
 
     
     
       2. The magnetic sensor defined in  claim 1  wherein the non-magnetic layer has a thickness of 6-11 angstroms. 
     
     
       3. The magnetic sensor defined in  claim 2  wherein the multilayer magnetic flux concentrator comprises at least a third magnetic layer stacked above the first and second magnetic layers. 
     
     
       4. The magnetic sensor defined in  claim 3  further comprising a non-magnetic layer between the second and third magnetic layers. 
     
     
       5. The magnetic sensor defined in  claim 4  wherein the non-magnetic layer between the second and third magnetic layers has a thickness of 6-11 angstroms. 
     
     
       6. The magnetic sensor defined in  claim 5  wherein the thin-film magnetic sensor element comprises a giant magnetoresistance sensor element. 
     
     
       7. The magnetic sensor defined in  claim 2  wherein the non-magnetic layer comprises ruthenium. 
     
     
       8. The magnetic sensor defined in  claim 7  wherein the first and second magnetic layers each include a layer of CoFe adjacent to the non-magnetic layer. 
     
     
       9. The magnetic sensor defined in  claim 1  wherein the thin-film magnetic sensor element comprises a giant magnetoresistance sensor element. 
     
     
       10. A magnetic sensor, comprising:
 a substrate; 
 first and second elongated thin-film magnetic sensor elements on the substrate; and 
 an elongated magnetic flux concentrator interposed between the first and second elongated thin-film magnetic sensor elements, wherein the elongated magnetic flux concentrator has at least a first magnetic layer, a second magnetic layer, and a non-magnetic layer that is interposed between the first and second magnetic layers, wherein the non-magnetic layer has a thickness that allows the first and second magnetic layers to magnetically couple. 
 
     
     
       11. The magnetic sensor defined in  claim 10  wherein the elongated magnetic flux concentrator has first and second opposing parallel edges and wherein the first and second elongated thin-film magnetic sensor elements run respectively along the first and second edges. 
     
     
       12. The magnetic sensor defined in  claim 10  wherein the non-magnetic layer has a thickness of 6-11 angstroms so that the first and second magnetic layers are magnetically coupled by a Ruderman-Kittel-Kasuya-Yosida interaction. 
     
     
       13. The magnetic sensor defined in  claim 10  wherein the first and second elongated thin-film magnetic sensor elements comprises giant magnetoresistance sensor elements. 
     
     
       14. The magnetic sensor defined in  claim 13  wherein the elongated magnetic flux concentrator further comprises third and fourth magnetic layers, a non-magnetic layer interposed between the third and fourth magnetic layers, and a non-magnetic layer interposed between the second and third magnetic layers. 
     
     
       15. An electronic compass, comprising:
 thin-film magnetic sensor elements; 
 a plurality of Z-shaped magnetic flux concentrators and reversed-Z-shaped magnetic flux concentrators that direct magnetic flux through the magnetic sensor elements; and 
 a resistive bridge circuit having first, second, third, and fourth arms, wherein the first and second arms contain an equal number of Z-shaped magnetic flux concentrators and wherein the first and second arms contain an equal number of reversed-Z-shaped magnetic flux concentrators. 
 
     
     
       16. The electronic compass defined in  claim 15  wherein the thin-film magnetic sensor elements comprise giant magnetoresistance sensor elements. 
     
     
       17. The electronic device defined in  claim 16  wherein the Z-shaped magnetic flux concentrators and the reversed-Z-shaped magnetic flux concentrators each have multiple magnetic layers separated by an interposed non-magnetic layer. 
     
     
       18. The electronic device defined in  claim 17  wherein the non-magnetic layer has a thickness of 6-11 angstroms. 
     
     
       19. The electronic device defined in  claim 18  wherein the multiple magnetic layers include a first magnetic layer having a chemically mechanically polished surface and include a second magnetic layer that is separated from the first magnetic layer by the non-magnetic layer. 
     
     
       20. The electronic device defined in  claim 19  wherein the non-magnetic layer comprises a material that is selected from the group consisting of: copper and ruthenium.

Description:
This application claims the benefit of and claims priority to provisional patent application No. 62/149,273 filed Apr. 17, 2015, which is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     This relates generally to electronic devices, and, more particularly, to electronic devices with electronic compasses. 
     Electronic devices such as cellular telephones may contain electronic compasses. An electronic compass includes magnetic sensors that detect the Earth&#39;s magnetic field. Compass readings may be used to provide orientation information to a navigation application or to other programs that use magnetic sensor data. 
     The magnetic sensors in electronic compasses may be formed from thin-film sensor structures. Magnetic flux concentrators are used to guide and amplify ambient magnetic fields, thereby enhancing the ability of thin-film sensors to detect weak fields such as the Earth&#39;s magnetic field. The magnetic flux concentrators are formed from soft magnetic materials. 
     Magnetic structures in a magnetic sensor such as the magnetic materials in a magnetic flux concentrator can become magnetized upon exposure to magnetic fields. For example, a magnetic flux concentrator may become magnetized when an external magnet or other source of a large external magnetic field is brought into the vicinity of the magnetic flux concentrator. The magnetization of a flux concentrator that has been exposed to magnetic fields in this way will relax to a remnant state upon removal of the external magnetic field. A remnant state will typically be characterized by a complex pattern of magnetic domains. This pattern of magnetic domains can give rise to a leakage flux that creates an undesired offset in the electronic compass. The offset can introduce inaccuracies in magnetic field readings and can limit the dynamic range of the electronic compass. 
     In some compass designs, magnetic flux concentrators have shapes that cause the compass to be more sensitive to magnetic fields with one angular orientation than another. If care is not taken, this angular sensitivity can give rise to inaccuracies in magnetic field readings. 
     It would therefore be desirable to be able to provide improved flux concentrators for magnetic compasses. 
     SUMMARY 
     An electronic device may be provided with an electronic compass. The electronic compass may include magnetic sensors. The magnetic sensors may include an X-axis sensor, a Y-axis sensor, and a Z-axis sensor. 
     The magnetic sensors may include thin-film magnetic sensor elements such as giant magnetoresistance sensor elements. Magnetic flux concentrators may be used to guide magnetic fields through the sensor elements. The magnetic flux concentrators may have a balanced set of shapes to reduce the angular sensitivity of the magnetic sensors. 
     Magnetic flux concentrators may be formed from multiple thin stacked layers of soft magnetic material separated by non-magnetic material. The non-magnetic material may have a thickness that is sufficiently small to allow the magnetic layers to magnetically couple through the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an illustrative electronic device having a magnetic sensor such as an electronic compass in accordance with an embodiment. 
         FIG. 2  is a cross-sectional side view of an illustrative thin-film magnetic sensor element in accordance with an embodiment. 
         FIG. 3  is a perspective view of an illustrative Z-axis magnetic sensor in accordance with an embodiment. 
         FIG. 4  is a top view of an illustrative X-axis or Y-axis magnetic sensor in accordance with an embodiment. 
         FIG. 5  is a schematic diagram of illustrative circuitry for measuring the resistance of magnetic sensor elements in accordance with an embodiment. 
         FIG. 6  is a top view of an illustrative magnetic sensor configuration of the type that may be used to form a positive sensor for the positive arm of a bridge circuit in accordance with an embodiment. 
         FIG. 7  is a top view of an illustrative magnetic sensor configuration of the type that may be used to form a negative sensor for the negative arm of a bridge circuit in accordance with an embodiment. 
         FIG. 8  is a circuit diagram of an illustrative bridge circuit for a magnetic sensor that may include positive and negative sensor arms of the types shown in  FIGS. 6 and 7  in accordance with an embodiment. 
         FIG. 9  is a cross-sectional side view of an illustrative magnetic flux concentrator with stacked magnetic layers separated by a non-magnetic layer in accordance with an embodiment. 
         FIGS. 10, 11, 12, 13, and 14  are cross-sectional side views of a multilayer magnetic flux concentrator during a series of illustrative fabrication steps in accordance with an embodiment. 
         FIG. 15  is a cross-sectional side view of an illustrative magnetic flux concentrator having a stack of magnetic layers separated by interposed non-magnetic layers in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     An illustrative electronic device of the type that may be provided with magnetic sensor circuitry such as an electronic compass is shown in  FIG. 1 . As shown in  FIG. 1 , electronic device  10  may have control circuitry  16 . Control circuitry  16  may include storage and processing circuitry for supporting the operation of device  10 . The storage and processing circuitry may include storage such as hard disk drive storage, nonvolatile memory (e.g., flash memory or other electrically-programmable-read-only memory configured to form a solid state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Processing circuitry in control circuitry  16  may be used to control the operation of device  10 . The processing circuitry may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors, power management units, audio chips, application specific integrated circuits, etc. 
     Input-output circuitry in device  10  such as input-output devices  12  may be used to allow data to be supplied to device  10  and to allow data to be provided from device  10  to external devices. Input-output devices  12  may include buttons, joysticks, scrolling wheels, touch pads, key pads, keyboards, microphones, speakers, tone generators, vibrators, cameras, sensors, light-emitting diodes and other status indicators, data ports, displays, etc. A user can control the operation of device  10  by supplying commands through input-output devices  12  and may receive status information and other output from device  10  using the output resources of input-output devices  12 . 
     Control circuitry  16  may be used to run software on device  10  such as operating system code and applications. During operation of device  10 , the software running on control circuitry  16  may control device  10  using information from sensors and other input-output devices. 
     Device  10  may be a tablet computer, a laptop computer, a desktop computer, a display, a cellular telephone or other portable device, a media player, a wristwatch device or other wearable electronic equipment, part of an embedded system that includes a display and/or other components, or other suitable electronic device. 
     Input-output devices  12  may include one or more magnetic sensors. The magnetic sensors may be used to measure the Earth&#39;s magnetic field or other magnetic fields. With one suitable arrangement, which is sometimes described herein as an example, devices  12  include an electronic compass such as electronic compass  14  for measuring the Earth&#39;s magnetic field (and, if desired, other external magnetic fields). Electronic compass  14  may be, for example, a three-axis magnetic sensor having magnetic sensors  18  for three orthogonal directions (e.g., X-axis and Y-axis magnetic sensors for making magnetic field measurements along lateral X and Y dimensions and a Z-axis magnetic sensor for making magnetic field measurements along vertical dimension Z). 
     Magnetic sensors  18  may include thin-film magnetic sensor elements such as thin-film magnetoresistance sensor elements. Thin-film magnetoresistance sensor elements may be based on anisotropic magnetoresistance (AMR) effects, may be based on giant magnetoresistance (GMR) effects, or may be based on tunneling magnetoresistance (TMR) effects. Other types of sensors  18  may be used, if desired. Configurations in which the magnetic sensor elements for compass  14  are based on giant magnetoresistance effects are sometimes described herein as an example. This is, however, merely illustrative. Compass  14  may sense magnetic fields using any suitable type of magnetic sensor. 
     A cross-sectional side view of an illustrative giant magnetoresistance magnetic sensor element is shown in  FIG. 2 . As shown in  FIG. 2 , thin-film magnetic sensor element  20  (e.g., a giant magnetoresistance sensor element) may include a stack of thin-film structures formed on substrate  22 . Pinning layer  24  may be formed from a material such as FeMn, CrMn, IrMn, or PtMn. Pinned layer  26  may be formed from a magnetic material such as NiFe, CoFe, or NiCo. Magnetic field  28  in pinned layer  26  has a fixed orientation that is established by pinning layer  24 . Magnetic layer  34  may be formed on top of layer  26 . A non-magnetic layer  32  such as a layer of Cu or Al—Cu may be formed between layers  34  and  26 . The resistance of layer  32  may be monitored at terminals  30 . Magnetic layer  34  is a free layer (sometimes referred to as a sense layer) having a magnetic field that reflects the state of external magnetic field  36 . When, for example, magnetic field  36  is oriented in direction  40 , the magnetic field in layer  34  will be oriented in direction  40  and will be parallel to magnetic field  28 . In this situation, the electrical resistance in layer  32  will have a first value. When external magnetic field  36  is oriented in direction  42 , the magnetic field in layer  34  will be oriented in direction  42  and will be antiparallel to magnetic field  28 . When the magnetic fields in layers  34  and  26  are antiparallel, the electrical resistance in layer  32  will have a second value that is greater than the first value. Changes in resistance in layer  32  may therefore be used to measure external magnetic field  36 . If desired, other configurations may be used for thin-film magnetic sensor elements in sensors  18 . The illustrative configuration of  FIG. 2  is presented as an example. 
     Magnetic sensor elements such as thin-film giant magnetoresistance magnetic sensor element  20  of  FIG. 2  may exhibit desirable attributes such as low power consumption. Magnetic flux concentrators that are formed from soft magnetic materials may be used to amplify ambient magnetic fields and to direct ambient magnetic fields through the thin-film sensor elements. The shape of the flux concentrators (which may sometimes be referred to a flux guides, flux directing structures, magnetic flux concentrating yokes, etc.) may be different for the different axes in compass  14 . Thin-film magnetic sensors that lie in the horizontal (X-Y) plane may use flux concentrators that redirect fields within the X-Y plane. Magnetic field measurements that are made on vertically oriented magnetic fields (i.e., magnetic fields running along out-of-plane axis Z) may be made using a flux concentrator of the type shown in  FIG. 3 . 
     As shown in  FIG. 3 , magnetic sensor  18  may include sensor elements  20  and magnetic flux concentrator  38 . Magnetic flux concentrator  38  may redirect out-of-plane magnetic fields  36  that are parallel to the Z-axis so that they pass horizontally through sensor elements  20  (i.e., parallel to dimension X in the example of  FIG. 3 ). Flux concentrator  38  may have an elongated rectangular box shape or other suitable shape. The length of flux concentrator  38  along axis Y may, for example, be tens or hundreds of microns or other suitable length. The width of flux concentrator  38  along axis X may be 1-10 microns, 2-20 microns, 3-8 microns, less than 9, more than 1 micron, less than 20 microns, 5-15 microns, or other suitable width. The height of flux concentrator  38  along dimension Z may be 1-20 microns, 5-15 microns, less than 13 microns, more than 7 microns, or other suitable height. 
     Magnetic sensor elements  20  may include first and second elongated thin-film magnetic sensor elements  20 A and  20 B that extend along the opposing edges of flux concentrator  38 . When oriented as shown in  FIG. 3 , sensor element  20 A may register an increase in resistance whenever sensor element  20 B registers a decrease in resistance. Sensor elements  20 A and  20 B may therefore sometimes be referred to as positive and negative sensor elements (positive and negative sensors) and may be placed in respective positive and negative arms of a resistive bridge circuit or other circuit to facilitate resistance measurements. 
     A top view of an illustrative flux concentrator of the type that may be used to direct and amplify magnetic field  36  when making magnetic field measurements on magnetic field  36  that is in the X-Y plane is shown in  FIG. 4 . In the example of  FIG. 4 , sensor  18  is an X-axis magnetic sensor. Incoming magnetic field  36  along axis X is directed through active region  20  of a strip of thin-film magnetoresistance sensor structures (strip  20 ′) by magnetic flux concentrators  38  (see, e.g., magnetic field  36 ′, which is measured by active region  20 ). Flux concentrators  38  may have a Z-shape or other suitable shape. Flux concentrators such as flux concentrators  38  of  FIG. 4  may have lengths of 10-50 microns, less than 50 microns, more than 10 microns, or other suitable length, may have widths of 1-20 microns, more than 1 micron, less than 15 microns, less than 20, more than 4 microns, 3-15 microns, or other suitable width, and may have heights (i.e., thicknesses in dimension Z) that are 1-20 microns, less than 4 microns, 1-3 microns, less than 2.5 microns, 0.1-5 microns, 0.2-4 microns, 0.5-5 microns, more than 0.5 microns, more than 1 micron, less than 20 microns, or other suitable thickness. 
     If desired, other flux concentrator designs may be used for forming the flux concentrator structures in sensors  18  of compass  14 . The configurations shown  FIGS. 3 and 4  are merely illustrative. 
       FIG. 5  is an illustrative resistive bridge circuit (i.e., a Wheatstone bridge) of the type that may be used to measure the resistance(s) of one or more magnetic sensor elements  20 . In the example of  FIG. 5 , resistance R 1  corresponds to a first magnetic sensor element (e.g., positive element  20 A of  FIG. 3 ) and resistance R 2  corresponds to a second magnetic sensor element (e.g., negative element  20 B of  FIG. 3 ). Reference voltages are applied to terminals  42  of bridge circuit  40 . Paths  44  may be used to convey signals from nodes  54  to measurement circuit  46  (e.g., part of control circuitry  16  of  FIG. 1 ). Circuit  46  may contain components such as differential amplifier  48  for producing an output that is proportional to the voltage difference across nodes  54 . Analog-to-digital converter  50  may convert this output to a digital output on path  52 . The output on path  52  will be proportional to the resistance of sensor elements  20  and will therefore reflect the strength of external magnetic field  36  that is being measured by the magnetic sensor elements in the bridge circuit. 
     When external magnetic fields of sufficient strength are applied to compass  14 , the soft magnetic material of the flux concentrators can become magnetized. The patterns of magnetic domains that are established in magnetized flux concentrators can lead to offsets in the readings of magnetic sensors  18  and therefore compass  14 . Magnetic field inaccuracies may also be caused by the shapes of the flux concentrators. For example, a sensor that has Z-shaped magnetic flux concentrators may exhibit excessive sensitivity when a magnetic field that is being measured is aligned with the middle segments of the Z-shaped concentrators. Offsets and uneven angular sensitivity can lead to undesired measurement inaccuracies when measuring magnetic fields. 
     With one suitable arrangement, undesired offsets may be minimized by forming magnetic flux concentrators with magnetically stable multilayer configurations. Flux concentrators  38  may also be configured so that structures that tend to enhance sensitivity at a given magnetic field angle are balanced by structures that tend to reduce sensitivity at the same magnetic field angle. 
     An illustrative arrangement that may be used to reduce angular sensitivity is shown in  FIGS. 6 and 7 . In this illustrative arrangement, the structures of  FIG. 6  are used to form a positive sensor arm in a bridge circuit and the structures of  FIG. 7  are used to form a negative sensor arm in the bridge circuit. Sensor  18  of  FIG. 6  may therefore be referred to as a positive magnetoresistance sensor and sensor  18  of  FIG. 7  may be referred to as a negative magnetoresistance sensor. The positive and negative sensors may each include multiple sensor elements  20 . Resistance measurements from these elements  20  can be added to each other to increase signal strength (e.g., the resistances of the elements  20  in the positive sensor can be added to each other and the resistances of the elements  20  in the negative sensor can be added to each other. 
     Due to the orientation of concentrators  38  and elements  20  in the positive and negative sensors, the positive sensor exhibits a positive resistance change in its elements  20  whenever the negative sensor exhibits a negative resistance change in its elements  20  in the presence of external field  36 . If desired, a pair of positive sensors may be located in diagonally opposing arms of a Wheatstone bridge and a pair of negative sensors may be located in diagonally opposing arms of the same Wheatstone bridge. Configurations in which the positive and negative sensors are located in first and second Wheatstone bridge arms may also be used. Moreover, resistance measurement circuits other than Wheatstone bridge circuits may also be used in measuring sensor resistances. The use of positive and negative sensors of the types shown in  FIGS. 6 and 7  in a bridge circuit to form a magnetic sensor  18  in compass  14  is merely illustrative. 
     Sensors  18  of  FIGS. 6 and 7  have magnetic flux concentrators that extend along dimension X for making X-axis magnetic field measurements (i.e., positive sensor  18  of  FIG. 6  and negative sensor  18  of  FIG. 7  are collectively used in forming an X-axis magnetoresistance sensor  18 ). A Y-axis sensor may use the same type of structures when the magnetic sensors and sensor elements run along the Y dimension. The use of the structures of  FIGS. 6 and 7  to form an X-axis magnetic sensor for compass  14  is presented as an example. 
     In the example of  FIG. 6 , sensor  18  has four thin-film magnetoresistance sensor elements  20 . Magnetic flux concentrators  38  include two Z-shaped magnetic flux concentrators  38 - 2  and  38 - 4 . Magnetic flux concentrators  38  also include two reversed-Z-shaped magnetic flux concentrators  38 - 1  and  38 - 3 . Magnetic flux concentrator  38 - 5  has a straight bar shape. Magnetic field  36 ′ that has been directed through sensor elements  20  results in changes in the resistances of these elements. These resistances (RP- 1 , RP- 2 , RP- 3 , and RP- 4 ) may be added together to enhance the signal-to-noise ratio of sensor  18 . 
     Each of flux concentrators  38  (Z-shaped and reversed-Z-shaped) has first and second parallel segments  60  and  64  that are joined by an angled intermediate segment  62 . In the Z-shaped concentrators, intermediate segments  62  are oriented at an angle A of 45° with respect to parallel segments  60  and  64  (i.e., segments  60  and  64  run parallel to axis X). In the reversed-Z-shaped concentrators, intermediate segments  62  are oriented at an angle A of −45° with respect to parallel segments  60  and  64 . 
     The angular orientation of the intermediate segments in the flux concentrators tends to enhance magnetic field sensitivity in directions that are aligned with the intermediate segments. Consider, as an example, a scenario in which it is desired to measure an external magnetic field Bex that is oriented at an angle B of 45° with respect to axis X. Sensor  18  of  FIG. 6  is primarily sensitive to X-axis fields, but because illustrative field Bex is aligned with the intermediate segments  62  of reversed-Z-shaped magnetic flux concentrators  38 - 1  and  38 - 3 , these portions of the flux concentrators will tend to gather more magnetic flux than segments that are oriented in other directions. Positive magnetic sensor  18  of  FIG. 6  may therefore exhibit locally enhanced sensitivity from the reversed-Z concentrators at magnetic field orientations of 45°. 
     Due to the presence of segments  62  of concentrators  38 - 2  and  38 - 4  sensor  18  of  FIG. 6  will also exhibit enhanced response for magnetic fields that are oriented at −45°. 
     Sensor  18  of  FIG. 7  can be implemented using a mirror image layout of the flux concentrators of  FIG. 6 . As with positive sensor  18  of  FIG. 6 , negative X-axis sensor  18  of  FIG. 7  contains two Z-shaped flux concentrators (concentrators  38 - 6  and  38 - 8 ) and two reversed-Z-shaped flux concentrators ( 38 - 7  and  38 - 9 ). Flux concentrator  38 - 10  may have a straight bar shape. When exposed to illustrative magnetic field Bex, which is oriented at an angle B of 45° with respect to axis X, segments  62  of reversed-Z-shaped concentrators  38 - 7  and  38 - 9  will tend to gather more magnetic flux than Z-shaped concentrators  38 - 6  and  38 - 8 . When exposed to a field at −45°, the reversed-Z-shaped concentrators will gather less flux than the Z-shaped concentrators. 
     The complementary layouts of the positive and negative sensors  18  allows off-axis sensing errors in the positive sensor to be cancelled by identical off-axis sensing errors in the negative sensor when these sensors are placed in a bridge circuit. The numbers of Z-shaped and reversed-Z-shaped concentrators in positive sensor  18  are matched by the numbers of Z-shaped and reversed-Z-shaped concentrators in negative sensor  18 , so the angular responses of the positive and negative sensors are balanced with respect to each other. 
     An illustrative bridge circuit into which the sensor elements of  FIGS. 6 and 7  may be incorporated is shown in  FIG. 8 . As shown in  FIG. 8 , bridge circuit  40  may have arms  70 ,  72 ,  74 , and  76 . The positive sensor of  FIG. 6  may be located in arm  70  and the negative sensor of  FIG. 7  may be located in arm  72 . Arms  74  and  76  may be provided with reference resistors or arm a duplicate of positive sensor  70  may be incorporated into arm  76  and a duplicate of negative sensor  72  may be placed in arm  74  to enhance the signal-to-noise ratio of the output signal across terminals  54 . 
     If desired, the stability of the magnetic domain pattern in flux concentrators  38  may be enhanced by using multiple layers of magnetically coupled soft magnetic material in forming flux concentrators  38 . The magnetic layers may be sufficiently thin to encourage magnetic domains to remain oriented within the plane of the flux concentrator. A non-magnetic coupling layer may be located between the magnetic layers and may have a configuration that encourages magnetic coupling between the magnetic layers. Magnetic flux concentrators with this type of configuration may be characterized by stable magnetic domain patterns (e.g., all magnetic domains may be oriented in alternating directions in alternating magnetic layers due to the magnetic coupling between layers). A multilayer flux concentrator will therefore be less likely to acquire a magnetic domain pattern that produces an unexpected and undesired magnetic sensor offset in compass  14 . 
     A cross-sectional side view of an illustrative flux concentrator with a multilayer configuration is shown in  FIG. 9 . As shown in  FIG. 9 , flux concentrator  38  may have a first magnetic layer such as magnetic layer  38 A, a non-magnetic layer such as layer  90 , and a second magnetic layer such as magnetic layer  38 B. Additional magnetic layers and non-magnetic layers may be included in the stack of magnetic layers for concentrator  38  if desired (e.g., flux concentrator  38  may have two or more layers of magnetic material, three or more layers of magnetic material, four or more layers of magnetic material, etc.). 
     Magnetic layers  38 A and  38 B may be formed from a magnetic material such as NiFe (permalloy), NiP, CoFe, or other soft magnetic materials. Layers  38 A and  38 B may each contain a single non-magnetic material or may be formed from stacks of two or more layers of soft magnetic materials. As an example, layer  38 B may include an upper layer of NiFe (e.g., an electroplated layer of about 0.5-5 microns on a sputter-deposited layer of about 5 nm) and a lower layer of CoFe (e.g., a 1 nm layer), whereas layer  38 A may include an upper layer of CoFe (e.g., a layer that is 1 nm thick) and a lower layer of NiFe (e.g., a sputtered NiFe layer that is 5 nm thick on an electroplated NiFe layer of about 0.5-5 microns). Other layer thicknesses may be used, if desired. Non-magnetic layer  90  may be formed from a layer of copper, ruthenium, or other non-magnetic material. The thickness T 3  of non-magnetic layer  90  may be less than 0.2 microns, less than 0.1 microns, more than 0.01 microns, or other suitable thickness that allows layers  38 A and  38 B to magnetically couple. With one suitable configuration, the thickness T 3  of layer  90  may be about 8 angstroms (e.g., 6-11 angstroms) or other suitable thickness that promotes magnetic coupling due to the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. The Ruderman-Kittel-Kasuya-Yosida interaction is a magnetic coupling mechanism that can strongly couple the magnetic fields of layers  38 A and  38 B (e.g., so that the north-south alignment of the field in layer  38 A and  38 B are opposite to each other) and thereby enhance the stability of the magnetic domain pattern in flux concentrator  38 . 
     The thicknesses T 1  and T 2  of magnetic layers  38 A and  38 B may be less than 1 micron, or larger thicknesses may be used if desired (e.g., thicknesses T 1  and T 2  may be less than 2 microns, may be more than 0.2 microns, etc.). Thickness T 1  may be equal to thickness T 2  or thicknesses T 1  and T 2  may differ. 
     An illustrative process for forming a magnetic flux concentrator from a stack of thin magnetic layers is shown in  FIGS. 10, 11, 12, 13, and 14 . 
     Initially, a first magnetic layer such as magnetic layer  38 A may be deposited and patterned on substrate  22 , as shown in  FIG. 10 . Magnetic layer  38 A may be formed on substrate  22  using electroplating or other suitable deposition techniques. For example, layer  38 A may be formed by plating a layer of NiFe about 0.5-5 microns thick followed by sputter deposition of a NiFe layer of 5 nm in thickness and sputter deposition of a CoFe layer of 1 nm in thickness. The plating process or other deposition process used to form layer  38 A may cause upper surface  100  to exhibit surface roughness in excess of what is desired for forming a thin (e.g., 8 angstrom) Ruderman-Kittel-Kasuya-Yosida coupling layer on layer  38 A. Accordingly, chemical mechanical polishing (CMP) or other smoothing techniques may be used to smooth surface  100 . 
     In the example of  FIGS. 11 and 12 , surface  100  is smoothed using chemical mechanical polishing techniques. Initially, aluminum oxide layer  102  is deposited over layer  38 A, as shown in  FIG. 11 . Layer  102  is hard and promotes the formation of planar polished surfaces. Following CMP polishing of the structures of  FIG. 11 , surface  100  of layer  38 A is smooth as shown in  FIG. 12  and is ready to accept a layer of non-magnetic material. Non-magnetic layer  90  may therefore be deposited on polished surface  100 , as shown in  FIG. 13 . Layer  90  may be a layer of copper, ruthenium, or other non-magnetic material with a thickness that helps promote Ruderman-Kittel-Kasuya-Yosida coupling between the magnetic layers of concentrator  38 . Layer  90  may be formed by depositing a 5 nm magnetic layer of a material such as NiFe layer, an eight angstrom (or 6-11 angstrom) layer of copper, ruthenium, or other non-magnetic material (e.g., a sputtered layer), and another 5 nm magnetic layer such as a layer of NiFe. The NiFe that is deposited in this way may serve as a seed layer for subsequent plating operations. As shown in  FIG. 14 , for example, second magnetic layer  38 B may be formed on top of layer  90  by electroplating, sputter deposition, and patterning layer  38 B. For example, layer  38 B may be formed by sputter deposition of a 1 nm CoFe layer, sputter deposition of a 5 nm NiFe layer, and electroplating of a thicker NiFe layer (e.g., a layer of about 0.5-5 microns in thickness). Layer  38 B overlaps layer  38 A to form stacked flux concentrator  38 . Because layers  38 B and  38 A are magnetically coupled (e.g., through the Ruderman-Kittel-Kasuya-Yosida coupling mechanism), the orientation of the remnant magnetic field in layer  38 A (south to north in the  FIG. 14  example) is opposite to the orientation of the remnant magnetic field in layer  38 B (north to south in the  FIG. 14  example). The magnetic domains in layers  38 A and  38 B are therefore organized in a stable pattern that will resist unwanted disorderly magnetization patterns when exposed to external magnetic fields. As a result, use of magnetic flux concentrator  38  of  FIG. 14  will help reduce offset in electronic compass  14 . 
     If desired, more than two magnetic layers may be incorporated into a magnetic flux concentrator (e.g., three or more, four or more layers, five or more layers, six or more layers, etc.). As shown in  FIG. 15 , for example, flux concentrator  38  may have four magnetic layers  38 A,  38 B,  38 C, and  38 D separated by respective non-magnetic layers  90 . Layers  90  may have thicknesses that allow layers  38 A,  38 B,  38 C, and  38 D to magnetically couple through the Ruderman-Kittel-Kasuya-Yosida interaction (e.g., layers  90  may each be about 8 angstroms thick, 6-11 angstroms thick, etc.). Flux concentrators formed from stacked magnetic layers may be used in sensors of the types shown in  FIGS. 3, 4, 6, 7 , and other suitable sensors. 
     The foregoing is merely illustrative and various modifications can be made by those skilled in the art without departing from the scope and spirit of the described embodiments. The foregoing embodiments may be implemented individually or in any combination.

Metadata:
Filing Date: 20150916
Publication Date: 20171003
Grant Date: 20171003
Priority Date: 20150417
Inventors: YANG HENRY H.
Choi Hyuk J.
Assignee: APPLE INC
CPC Classifications: [{"code": "G01R33/093", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01R33/0011", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01R33/0011", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01R33/093", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 57129729