Abstract:
A three-axis magnetic sensor or magnetometer is provided. Two magnetic sensor Wheatstone bridges using barber pole AMR structures are fabricated on opposite sides of a bump structure formed on a substrate to provide surfaces that are at a predetermined angle with respect to the flat surface of the substrate. The bridge assembly is oriented along the Y axis and the bridges are interconnected such that Y and Z channel signals can be produced by processing of the bridge signals. The X channel signals are provided by an X axis sensor provided on the level surface of the substrate.

Description:
BACKGROUND OF THE INVENTION 
     Multi-axis magnetic sensors or magnetometers, such as three-axis magnetic sensors, are particularly desirable for modern electronic compass applications. Known magnetoresistive (MR) sensors, such as AMR (anisotropic MR) sensors, GMR (giant MR) sensors, TGMR (tunneling GMR) sensors, and the like, however, can only detect magnetic flux that is parallel to the device plane and 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. Thus complex geometric arrangements of these sensors are required in order to measure all three axes in a single device. 
     One of the most common types of magnetic field sensor is the well-known magnetoresistive (MR) sensor where, generally, the resistivity of the sensor varies according to a local magnetic field oriented in the same plane as the magnetoresistance. “Barber-pole” structures are added to allow a sensing of the magnetic field along one axis to include direction, or vector, information. Magnetoresistive sensors have been used successfully in electronic compass applications, using two sensors to detect the magnetic field in the same plane as the surface they are mounted on, (X, Y), with an additional sensor mounted in a particular way so that the sensitive element is properly aligned to sense the component of the magnetic field orthogonal (Z) to the plane of the system. 
     There are many known approaches to fabricating 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. For example, three sensors are encapsulated separately before being soldered on a PCB as a module. In this case, the orthogonal (Z) axis sensor is mounted along the axis orthogonal to the PCB directly rather than along the plane, as in, for example, U.S. Pat. No. 7,271,586. This particular orthogonal axis sensor mounting, however, can be technically challenging, and significantly increases the cost of manufacturing, as well as results in an increase in the thickness of the final product. 
     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. For example, U.S. Pat. No. 7,126,330 describes a device where two magnetic field sensing devices are provided on a first surface to detect co-planar orthogonal X, Y axes and a third magnetic field sensing unit is disposed in a trench that is created in the first surface in order to detect the magnetic field in the Z axis. The &#39;330 patent, however, is limited by the accuracy with which the inclined walls of the trench can be made so that they are at the same inclined angle. 
     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. 
     There is a need, therefore, for a low profile, inexpensive, but high performance, three-axis magnetic field sensor that can be produced in large volume using a simple manufacturing process. 
     BRIEF SUMMARY OF THE INVENTION 
     A three-axis magnetic sensor or magnetometer is provided. Two magnetic sensor Wheatstone bridges using barber pole AMR structures are fabricated on opposite sides of a bump structure formed on a silicon or other substrate or wafer, i.e., on surfaces that are at a predetermined angle with respect to the flat surface of the substrate. In one embodiment, the bump structure is S i O 2  formed on a silicon substrate using known photolithographic techniques. Alternatively, the bump structure can be Al 2 O 3 , Si 3 N 4 , polyimide, hard baked photoresist or other materials on which the magnetic sensor can be fabricated. The slope angle of the bump structure can vary and is only limited by the photolithography process. 
     In one embodiment of the present invention a bridge assembly is oriented along the Y axis and the bridges are interconnected such that Y and Z channel signals can be produced by processing of the bridge signals. The X channel signals can be provided by an X axis sensor provided on the level surface of the substrate or wafer. 
     In another aspect of the invention, the bridge assembly can be oriented along the X axis to produce X and Z channel signals. In this case, the Y channel signals can be provided by a Y axis sensor on the level surface of the substrate or wafer. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Various aspects of at least one embodiment of the present invention are discussed below with reference to the accompanying figures. It will be appreciated that for simplicity and clarity of illustration, elements shown in the drawings have not necessarily been drawn accurately or to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity or several physical components may be included in one functional block or element. Further, where considered appropriate, reference numerals may be repeated among the drawings to indicate corresponding or analogous elements. For purposes of clarity, not every component may be labeled in every drawing. The figures are provided for the purposes of illustration and explanation and are not intended as a definition of the limits of the invention. In the figures: 
         FIGS. 1A and 1B  are schematic diagrams of a conceptual rendering of an embodiment of the present invention; 
         FIGS. 2A and 2B  are circuit schematic diagrams of an embodiment of the present invention; 
         FIGS. 3A and 3B  are schematic diagrams of a layout of an embodiment of the present invention; 
         FIG. 4  is a schematic diagram of a barber pole AMR component provided on a bump structure in accordance with an embodiment of the present invention; 
         FIGS. 5A and 5B  are circuit schematic diagrams in accordance with an embodiment of the present invention; 
         FIG. 6  is a schematic diagram of a physical layout of an embodiment of the present invention; 
         FIG. 7  is a circuit schematic diagram in accordance with an embodiment of the present invention; 
         FIG. 8  is a schematic diagram of a layout of an embodiment of the present invention; and 
         FIG. 9  is a circuit schematic diagram in accordance with the embodiment of the present invention shown in  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the embodiments of the present invention. It will be understood by those of ordinary skill in the art that these embodiments of the present invention may be practiced without some of these specific details. In other instances, well-known methods, procedures, components and structures may not have been described in detail so as not to obscure the embodiments of the present invention. 
     Prior to explaining at least one embodiment of the present invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting. 
     It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination. 
     Generally, and as an overview, referring now to  FIG. 1A , a three-axis magnetometer  100  includes a flat substrate  104 , for example, a silicon substrate. A bump  108  is created on the surface  104  by operation of any of the known wafer processes, for example, photolithography, where the substrate  104  is first covered with SiO 2  and then photoresist is spun and patterned on the SiO 2  covered substrate  104 . Through a thermal reflow step, a photoresist sidewall becomes rounded up. A subsequent dry etch process will remove the photoresist as well as the SiO 2  at the same time. As a result, a pattern and sidewall profile will be transferred to the SiO 2 . The bump structure  108  can be made of SiO 2  or other materials, including Al 2 O 3 , Si 3 N 4 , Polyimide, Hard baked photoresist, silicon, etc. The slope angle can vary, as long as it is not too steep for the photolithography process. 
     The bump  108  includes an Up inclined surface  112  and a Down inclined surface  116 . The use of “Up” and “Down” is merely for explanatory purposes to provide labels for the two surfaces  112 ,  116  to aid in explaining the invention. A first Wheatstone bridge WBy is provided on the Up surface  112  and a second Wheatstone bridge WBz is provided on the Down surface  116 . A third Wheatstone bridge WBx is provided on the flat substrate  104 . Each of these bridges comprises barber pole (BBP) resistors as is known in the art. 
     As a convention, WBx is oriented to detect a magnetic field along an X axis, as shown. The Z axis is defined as being perpendicular to the flat surface  104 , as shown in  FIG. 1B , which is a side view of  FIG. 1A , while the Y axis is co-planar with the X axis but perpendicular thereto. As can be seen from  FIG. 1B , therefore, the two bridges WBy and WBz on the sloped surfaces  112 ,  116  will detect a magnetic field that has components from each of the Y and Z axes. As the slope angles are the same, and as will be described below in more detail, the signals coming from the two bridges WBy, WBz can be processed to obtain the values in the respective Y and Z axis. The arrangement of the two bridge WBy and WBz may be referred to as a Y/Z detector herein. 
     Referring now to  FIG. 2A , the bridge WBy comprises four BBP resistors R 1 -R 4  arranged in the known Wheatstone bridge configuration. Two signals Y+, Y− represent the taps used to determine the magnetic field strength along the axis coplanar and perpendicular to the resistors R 1 -R 4 . As the bridge WBy is at an angle on the Up surface  112  the magnetic field measured by the bridge WBy will have components of the Y and Z axes. Similarly, the bridge WBz comprises four BBP resistors R 5 -R 8  also arranged in the known Wheatstone bridge configuration. Two signals Z+, Z− represent the taps used to determine the magnetic field strength along the axis coplanar and perpendicular to the resistors R 5 -R 8 . As the bridge WBz is at an angle on the Down surface  116  the magnetic field measured by the bridge WBz will also have components of the Y and Z axes although the taps on the bridge WBz are arranged to indicate the Y axis component opposite to that of the bridge WBy. 
     The X and Y axes, in  FIG. 2A , are in the plane of the drawing while the Z axis is coming up out of the drawing plane as represented by the dot at the intersection of the X and Y axes. 
     In operation, a first differential amplifier  204  is used to determine a difference ΔVy between Y+ and Y− as indicative of the magnetic field detected by WBy and a second differential amplifier  208  determines a difference ΔVz between Z+ and Z− as indicative of the magnetic field detected by WBz. In order to determine the magnetic field Vz along the Z axis, ΔVz and ΔVy are added together by operation of a first adder  212  to “cancel out” the opposite Y axis components in each of the signals ΔVz and ΔVy. The magnetic field Vy along the Y axis is determined by subtracting ΔVz from ΔVy using a subtractor  216  to “cancel out” the Z axis component. 
     As shown in  FIG. 2A , the bridge WBx operates according to known principles and a third differential amplifier  220  determines Vx by taking the difference between X+ and X−. 
     In an embodiment of the present invention, a plurality of bumps B 1 -B 8  are provided as shown in  FIGS. 3A and 3B . Each bump B 1 -B 8  comprises an Up and Down surface  112 ,  116  as defined above. This is shown from the direction  3 A- 3 A in  FIG. 3B . As will be described below, the bridges WBy and WBz are distributed across the bumps B 1 -B 8  in order to detect the magnetic field along the Y, Z axes. 
     As has been discussed above, each resistor R 1 -R 8  in the bridges WBy and WBz comprises a BBP structure  404 . These BBP structures  404 , comprising include an AMR material strip  408  and conductive straps  412 , are arranged on the Up and Down surfaces  112 ,  116  as shown in  FIG. 4 . A good description of AMR-type sensor units can be found in U.S. Pat. No. 7,126,330 discussed above. As a convention followed in this specification, the BBP structure  404  on the Up surface  112  is referred to as a Forward BBP, i.e., the conductive straps are in the //////// direction, while the BBP structure  404  on the Down surface  116  is referred to as a Back BBP as the conductive straps are in the \\\\\\\\ direction, each with respect to the magnetic field H represented by the arrow H. 
     As there are eight bumps B 1 -B 8 , each of the eight resistors R 1 -R 8  of the two bridges WBy, WBz is divided into an A and B resistive element as schematically shown in  FIGS. 5A and 5B . 
     As above, the resistors R 1 -R 4  of bridge WBy are distributed on the Up surfaces  112  of bumps B 1 -B 8  where resistors R 1 -A, R 1 -B, R 3 -A and R 3 -B are forward BBPs and the resistors R 2 -A, R 2 -B, R 4 -A and R 4 -B are back BBPs. The resistors R 5 -R 8  of bridge WBz are distributed on the Down surfaces  116  of bumps B 1 -B 8  where resistors R 5 -A, R 5 -B, R 7 -A and R 7 -B are back BBPs and the resistors R 6 -A, R 6 -B, R 8 -A and R 8 -B are forward BBPs. 
     Referring to  FIG. 6 , the resistive elements R 1 -A, R 1 -B, . . . R 8 -A and R 8 -B are distributed across the bumps B 1 -B 8 . Advantageously, any process deviations or slight differences in the angles of the sloped surfaces of the bumps will be averaged out by the interdigitation of the resistive elements of the two bridges WBy and WBz. 
     An electrical schematic of the two bridges WBy, WBz and the respective bumps B 1 -B 8  is presented in  FIG. 7 . As shown, the two bridges are interdigitated with one another by positioning corresponding resistors on opposite faces of the same bump. As an example, bump B 3  includes forward resistor element R 1 -A on the Up surface  112  with back resistor element R 5 -A on the Down surface  116  of the bump B 3 . 
     In the foregoing embodiment, the Y/Z detector is used in conjunction with an X axis detector to obtain magnetic field measurements in all three axes using an integrated device. In an alternate embodiment, the X axis detector is replaced with two additional Wheatstone bridges on a second set of bumps that is oriented orthogonal to the bumps that make up the Y/Z detector. 
     Thus, as shown in  FIG. 8 , an X/Z detector having the same construction as the Y/Z detector above is provided in the substrate and orthogonal to the Y/Z detector. As shown, the X/Z detector includes bumps B 9 -B 16  with corresponding Up and Down surfaces and BBP resistor elements constructed the same as the Y/Z detector already described. Thus, a bump axis Bay of the Up/Down surfaces of bumps B 1 -B 8  is aligned with the Y axis while a bump axis BAx of the bumps B 9 -B 16  is aligned with the X axis. Two Wheatstone bridges WBx and WBz 1  are provided thereon and sense magnetic fields in the X and Z axes, respectively. 
     In operation, referring to the schematic in  FIG. 9 , the measurement of the field along the Y axis is obtained as was previously described. Similarly, the measurement along the X axis is obtained by taking the difference between ΔVz1 and ΔVx by operation of subtractor  906 . 
     Advantageously, a more accurate measurement of the magnetic field along the Y axis is obtained by summing together ΔVz0, ΔVy, ΔVz1 and ΔVx by adders  212 ,  908  and  910  such that the opposing measurements in the X axis cancel out each other, as do the opposing measurements in the Y axis therefore leaving only the measurements due to the magnetic field in the Z axis. 
     The BBP structures  404  may be provided on the UP and Down sloped surfaces  112 ,  116  by known photolithography deposition processes. Accordingly, the angle of the bump slopes will affect the deposition process resulting in the AMR strip and conductive straps being slightly thicker on the upper portion of the slope relative to the deposition on the lower portion of the slope. It has been determined that this slight difference in thickness provides a functional advantage although certainly the deposition process could also be configured to deposit the bridges to have a consistent thickness on all portions of the slopes. 
     It should be noted that the number of bumps could be chosen to be greater than eight and one of ordinary skill in the art would understand how to distribute the bridge resistor elements across the bumps. 
     In addition, while the bumps are shown as having a flat surface between the Up and Down surfaces,  112 ,  116 , i.e., a trapezoidal cross-section, the bumps could be more triangular in cross-section and come to a point rather than have a flat section at the top. Thus, a “bump” is a structure that provides a pair of adjacent, symmetric inclined surfaces. 
     Further, the differential amplifiers, adders and subtractors may be incorporated or integrated into the substrate or provided “off” the substrate. In addition, the functions of the differential amplifiers, the adders and subtractors may be implemented within, for example, an ASIC in a digital, analog or hybrid implementations and such implementations are considered to be within the scope of this disclosure. 
     Having thus described several features of at least one embodiment of the present invention, it is to be appreciated that various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure and are intended to be within the scope of the invention. Accordingly, the foregoing description and drawings are by way of example only, and the scope of the invention should be determined from proper construction of the appended claims, and their equivalents.