Abstract:
A method of biasing a magneto resistive sensor element includes providing at least one magneto resistive sensor element having a magnetic sensitivity along a first axis that is parallel to a plane of the at least one sensor element. A magnet is positioned adjacent to the at least one sensor element for biasing the at least one sensor element, wherein the magnet has a magnetization that is non-perpendicular to the plane of the at least one sensor element, and wherein the magnetization includes a component parallel to the plane of the at least one sensor element that increases a sensitive range of the at least one sensor element along the first axis.

Description:
BACKGROUND 
       [0001]    Giant magneto resistive (GMR) sensors were first manufactured in the 1980&#39;s. They are distinguished by their high sensitivity of their electrical resistance to the orientation of an external magnetic field. The GMR effect takes place in a limited range along one axis of the magnetic field. This range is referred to as the anisotropic range. In the anisotropic range, the sensor has a high sensitivity (resistance change versus magnetic field change). In some applications, such as an incremental speed sensor with a back bias magnet for measuring the speed of a magnetic tooth wheel, a small misplacement or inclination of the back bias magnet with respect to a GMR element of the sensor can drive the working point of the GMR element into saturation. The back bias magnet creates an offset in the working point of the sensor from an optimal point near the center of the anisotropic range, to the saturation region. As a result, no signal or a signal of reduced quality is generated, thereby lowering the sensor performance. For gradiometric sensors that operate on a differential principle, the offset problem becomes worse as the sensors are located in two different positions. 
         [0002]    The problem of a GMR element being driven into saturation can be avoided by an increase of the anisotropic range. The anisotropic range where the sensor is sensitive can be extended with technology variations or geometric variations of the sensor, but then different sensors for different applications have to be developed. Furthermore, the extension of the sensitivity range by geometric variations (e.g., lowering the GMR structure width) is restricted due to limitations of state-of-the-art etching processes for the GMR layer. 
         [0003]    One prior approach uses a groove or cavity in the magnet or a shaped metal plate to keep magneto resistive sensors in an optimal working point. Disadvantages of this approach are the higher costs for the additional grooving of the magnet or the additional special shaped metal plate. 
         [0004]    Another solution for reducing the offset induced by the back bias magnet is to reduce the strength of the magnet. However, this approach reduces proportionally the magnetic signal generated by the passing magnetic tooth wheel. 
       SUMMARY 
       [0005]    One embodiment provides a method of biasing a magneto resistive sensor element. The method includes providing at least one magneto resistive sensor element having a magnetic sensitivity along a first axis that is parallel to a plane of the at least one sensor element. A magnet is positioned adjacent to the at least one sensor element for biasing the at least one sensor element, wherein the magnet has a magnetization that is non-perpendicular to the plane of the at least one sensor element, and wherein the magnetization includes a component parallel to the plane of the at least one sensor element that increases a sensitive range of the at least one sensor element along the first axis. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The accompanying drawings are included to provide a further understanding of the present invention and are incorporated in and constitute a part of this specification. The drawings illustrate the embodiments of the present invention and together with the description serve to explain the principles of the invention. Other embodiments of the present invention and many of the intended advantages of the present invention will be readily appreciated as they become better understood by reference to the following detailed description. The elements of the drawings are not necessarily to scale relative to each other. Like reference numerals designate corresponding similar parts. 
           [0007]      FIG. 1  is a graph of resistance versus magnetic field for a GMR sensor. 
           [0008]      FIG. 2  is another graph of resistance versus magnetic field for a GMR sensor. 
           [0009]      FIG. 3  is a diagram illustrating a magneto resistive sensor with a non-perpendicular bias magnetization according to one embodiment. 
           [0010]      FIG. 4  is a graph of resistance versus magnetic field for the magneto resistive sensor shown in  FIG. 3  according to one embodiment. 
           [0011]      FIG. 5  is a graph of the sensitivity range of the magneto resistive sensor shown in  FIG. 3  according to one embodiment. 
           [0012]      FIG. 6  is a diagram illustrating a magneto resistive sensor bridge according to one embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    In the following Detailed Description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology, such as “top,” “bottom,” “front,” “back,” “leading,” “trailing,” etc., is used with reference to the orientation of the Figure(s) being described. Because components of embodiments of the present invention can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. 
         [0014]      FIG. 1  is a graph  100  of resistance versus magnetic field for a GMR sensor based on the so-called spin-valve (SV) concept. A spin-valve GMR sensor structure basically consists of a reference layer with a fixed magnetization direction and a sensor layer which can rotate its magnetization according to an external in-plane magnetic field. The main GMR effect then takes place between the sensor and reference layer. For the case in which the reference magnetization direction points along the x-axis (i.e., parallel to the external magnetic field axis), and the anisotropy axis is along the y-axis,  FIG. 1  shows a typical so-called minor loop behavior with a characteristic linear transition region from a low ohmic state to a high ohmic state. 
         [0015]    The horizontal axis in graph  100  represents the magnitude of a magnetic field (Bx) applied to the sensor in an x-axis parallel to the plane of the sensor, and the vertical axis represents the resistance of the sensor. For relatively small magnetic fields in the x-axis, the sensor operates in the anisotropic range  106 , and the resistance of the sensor changes with the applied magnetic field. In the anisotropic range  106 , the sensor has a high sensitivity (i.e., resistance change versus magnetic field change). For larger magnetic fields in the x-axis, the sensor operates in a saturation region  104  or  108 , and the resistance of the sensor no longer changes with the applied magnetic field (or changes very little). In the saturation region  104 , the resistance of the sensor remains substantially constant at a minimum resistance value (Rmin)  102 . In the saturation region  108 , the resistance of the sensor remains substantially constant at a maximum resistance value (Rmax)  110  as long as the magnetic field range stays within the minor loop magnetic field region (which is dependent on stack design, but is typically less than 100 mT (milli-Tesla)). 
         [0016]      FIG. 2  is another graph  200  of resistance versus magnetic field for a GMR sensor. Again, the horizontal axis represents the magnitude of the magnetic field (Bx) applied to the sensor in an x-axis parallel to the plane of the sensor, and the vertical axis represents the resistance of the sensor. Two working points  206  and  208  of the sensor are shown in  FIG. 2 . Working point  206  represents an optimal working point, and working point  208  represents a working point that is outside of the sensitivity range of the sensor. As mentioned above in the Background section, a small misplacement or inclination of a back bias magnet with respect to the GMR sensor can drive the working point of the GMR element into saturation. Working point  208  represents such a point, which has been offset from the optimal point  206  and into the saturation region. 
         [0017]    As shown in  FIG. 2 , for the optimal working point  206 , when a magnetic field signal  210  is applied to the sensor, a corresponding resistance change signal  204  is generated in the sensor. However, for the working point  208 , when a magnetic field signal  212  is applied to the sensor, the resulting resistance change signal  202  is flat. Since working point  208  is in the saturation region of the sensor, the applied signal  212  does not change the resistance of the sensor, and the resulting signal  202  remains constant at the maximum resistance value (Rmax). 
         [0018]      FIG. 3  is a diagram illustrating a magneto resistive sensor  300  with a non-perpendicular bias magnetization according to one embodiment. Magneto resistive sensor  300  includes a back bias magnet  302  and a sensor element  304 . In the illustrated embodiment, sensor element  304  is configured as a differential sensor, and includes two magnetic field sensitive elements (e.g., magneto resistive elements)  306 A and  306 B. In other embodiments, sensor  300  includes a single magnetic field sensitive element, or more than two such elements. In one embodiment, sensor element  304  also includes an evaluation integrated circuit (not shown) for processing signals generated by the elements  306 A and  306 B. 
         [0019]    In one embodiment, sensor element  304  is a GMR sensor, and elements  306 A and  306 B are GMR elements. In other embodiments, elements  306 A and  306 B are other types of magnetic field sensitive elements, such as Hall sensor elements or xMR elements (e.g., AMR—anisotropic magneto resistance; TMR—tunnel magneto resistance; CMR—colossal magneto resistance). In one embodiment, sensor element  304  is a spin-valve GMR sensor, and includes a reference layer with a fixed magnetization direction and a sensor layer which can rotate its magnetization according to an external in-plane magnetic field. In one embodiment, sensor  300  is configured to measure the speed or position of teeth of a magnetic gear wheel (or magnetic tooth wheel). In another embodiment, sensor  300  is configured as a gradiometric sensor for measuring magnetic field gradients. 
         [0020]    A set of orthogonal x-y-z axes are shown in  FIG. 3 . The x-axis and the y-axis are parallel to the plane of the sensor element  304 . The z-axis is perpendicular to the plane of the sensor element  304 . GMR sensors, such as one embodiment of sensor element  304 , are sensitive to only one component of the magnetic field (here defined as the x-component). In conventional applications, the back bias magnet has a magnetic field that is perpendicular to the sensor plane (i.e., a magnetic field with a z-component, but no x or y component). The field produced by the back bias magnet is modulated (e.g., by a passing magnetic tooth wheel) to generate the x-field signal. Due to the properties of the GMR element, the x-field signal is converted into resistance change. 
         [0021]    In one embodiment, rather than just including a z-component, the magnetic field of the back bias magnet  302  also includes a y-component. The y-component of the field of the back bias magnet  302  is represented in  FIG. 3  by vector, By, and the z-component is represented by vector, Bz. These two components result in a magnetic field B, which is non-perpendicular to the bottom surface of the magnet  302 , and which is non-perpendicular to the sensor plane (i.e., the x-y plane). The y-component of the field generated by the back bias magnet  302  extends proportionally the sensitive range of the sensor element  304  along the x-axis. In one embodiment, back bias magnet  302  is a cubic magnet with a magnetization that is not completely perpendicular to the sensor plane, but also has a y-field component that extends the sensitive range of the sensor element  304  along the x-axis. In another embodiment, back bias magnet  302  is a cylindrically-shaped magnet with a non-perpendicular magnetization direction (i.e., a magnetization with a z-component and a y-component). 
         [0022]      FIG. 4  is a graph  400  of resistance versus magnetic field for the magneto resistive sensor element  304  shown in  FIG. 3  according to one embodiment. The horizontal axis represents the magnitude of the magnetic field (Bx) applied to the sensor element  304  in the x-axis parallel to a plane of the sensor element  304 , and the vertical axis represents the resistance of the magneto resistive elements  306 A and  306 B. Two curves  410  and  412  are shown in  FIG. 4 . Curve  412  (shown with hidden lines) represents the resistance versus magnetic field (Bx) of the magneto resistive sensor elements  306 A and  306 B when a back bias magnet applies a magnetic field that is perpendicular to the sensor plane (i.e., a magnetic field that only includes a Bz component). Curve  410  represents the resistance versus magnetic field (Bx) of the magneto resistive sensor elements  306 A and  306 B using the back bias magnet  302 , which applies a magnetic field that is non-perpendicular to the sensor plane (i.e., the magnetic field includes a Bz component and a By component). By using the back bias magnet  302  with such a non-perpendicular field, the linear range of the sensor  304  is increased by an amount  416  (Bx_ext). 
         [0023]    Two working points  414  and  418  of the sensor element  304  are shown in  FIG. 4 . Working point  414  represents an optimal working point, and working point  418  represents a working point that is outside of the sensitivity range of the sensor  304  when a perpendicular bias magnetic field is applied to the sensor element  304 . As shown in  FIG. 4 , for the optimal working point  414 , when a magnetic field signal  422  is applied to the sensor element  304 , a corresponding resistance change signal  404  or  406  is generated by the sensor element  304 . Signal  404  (shown with hidden lines) represents the resistance change when a perpendicular bias magnetic field is applied to the sensor element  304  (i.e., curve  412 ), and signal  406  represents the resistance change for the sensor  304  when a non-perpendicular bias magnetic field is applied to the sensor element  304  (i.e., curve  410 ). The non-perpendicular bias magnetic field causes curve  410  to be slightly flatter than curve  412 , which results in the signal  406  being slightly smaller than signal  404 . 
         [0024]    For the working point  418 , when a magnetic field signal  424  is applied to the sensor element  304  with a non-perpendicular bias magnetic field (i.e., curve  410 ), a corresponding resistance change signal  402  is generated by the sensor element  304 . When the magnetic field signal  424  is applied to the sensor element  304  with a perpendicular bias magnetic field (i.e., curve  412 ), the resulting resistance change signal is flat. Since working point  418  is in the saturation region of the sensor element  304  (when a perpendicular bias magnetic field is applied to the sensor element  304 ), the applied signal  424  does not change the resistance of the sensor element  304 , and the resulting signal remains constant at the maximum resistance value (Rmax). Thus, as shown in  FIG. 4 , the non-perpendicular bias magnetic field results in an extension of the sensitive range of the sensor element  304  along the x-axis, such that the sensor element  304  still produces a resistance change signal at working points where no such signal would be generated with a perpendicular bias magnetic field. 
         [0025]      FIG. 5  is a graph of the sensitivity range of the magneto resistive sensor element  304  shown in  FIG. 3  according to one embodiment. The horizontal axis in the graph represents the magnitude of the y component (By) of the bias magnetic field applied to sensor element  304  by magnet  302  in mT (milli-Tesla) units, and the vertical axis represents the sensitivity range of sensor  304  in the x-direction in mT units. The sensitive range of sensor element  304  is extended in accordance with the field strength of the y-component of the field produced by the magnet  302 . As shown in  FIG. 5 , the sensitivity range of sensor element  304  increases almost linearly with an increasing magnitude of the y-component (By) of the bias magnetic field applied by magnet  302 . Even if the extension of the linear transition region results in a decrease in sensor sensitivity (e.g., the signal  406  in  FIG. 4  is smaller than the signal  404 ), an advantage of the extension is that the sensor element  304  is more likely to always be operating in a sensitive working point. In one embodiment, magnet  302  has a y-component (By) of greater than 10 mT. In another embodiment, magnet  302  has a y-component (By) of greater than 20 mT. In yet another embodiment, magnet  302  has a y-component (By) of greater than 40 mT. 
         [0026]      FIG. 6  is a diagram illustrating a magneto resistive sensor bridge  600  according to one embodiment. In one embodiment, sensor element  304  ( FIG. 3 ) is configured as a sensor bridge  600 . As shown in  FIG. 6 , sensor bridge  600  includes four GMR elements  602 A- 602 D. A voltage is applied to input terminal  604 , and the output of the bridge  600  is measured at output terminals  606  and  608  (Vsig+ and Vsig−). In response to an external magnetic field, one or more of the GMR elements  602 A- 602 D change in electrical resistance, causing a voltage signal at the bridge output terminals  606  and  608 . 
         [0027]    In back bias magneto resistive sensor applications, a magnet is already present. By using the already present back bias magnet itself to extend the sensitive range of the sensor, embodiments of the present invention provide a simpler and less expensive solution than the previous proposals that involve forming grooves or cavities in the back bias magnet and using a special shaped metal plate. By using the back bias magnet itself to extend the sensitive range via a non-perpendicular magnetic field, the sensitivity can be adapted for each application as desired. Due to the extended range of the sensor, a misalignment or misplacement between sensor and magnet is no longer critical, resulting in sensors that are more tolerant against such misalignment. 
         [0028]    Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.