Abstract:
A multi-chip push-pull magnetoresistive bridge sensor utilizing magnetic tunnel junctions is disclosed. The magnetoresistive bridge sensor is composed of a two or more magnetic tunnel junction sensor chips placed in a semiconductor package. For each sensing axis parallel to the surface of the semiconductor package, the sensor chips are aligned with their reference directions in opposition to each other. The sensor chips are then interconnected as a push-pull half-bridge or Wheatstone bridge using wire bonding. The chips are wire-bonded to any of various standard semiconductor lead frames and packaged in inexpensive standard semiconductor packages.

Description:
CROSS-REFERENCE TO A RELATED APPLICATION 
       [0001]    This application is a 35 U.S.C. §371 national phase application of PCT/CN2012/071854, filed on Mar. 2, 2012, which claims priority to a Chinese Patent Application No. 201110050705, filed on Mar. 3, 2011, incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to the general field of magnetic field detection by MTJ or GMR devices with particular reference to methods for integrating magnetic sensors into push-pull bridges using standard semiconductor packaging techniques. 
       BACKGROUND ART 
       [0003]    Magnetic sensors are widely used in modern systems to measure or detect physical parameters including but not limited to magnetic field strength, current, position, motion, orientation, and so forth. There are many different types of sensors in the prior art for measuring magnetic field and other parameters. However, they all suffer from various limitations well known in the art, for example, excessive size, inadequate sensitivity and/or dynamic range, cost, reliability and other factors. Accordingly a need exists for improved magnetic sensors, especially sensors that can be easily integrated with semiconductor devices and integrated circuits and manufacturing methods thereof. 
         [0004]    Magnetic tunnel junction (MTJ) sensors have the advantages of high sensitivity, small size, low cost, and low power consumption. Although MTJ devices are compatible with standard semiconductor fabrication processes, methods for building high sensitivity devices with sufficient yield for low cost mass production have not been adequately developed. In particular, yield issues due to offset in the magnetoresistive response of MTJ sensors, and difficulty in matching the magnetoresistive response of MTJ elements when combined to form bridge sensors have proven difficult. 
       SUMMARY OF THE INVENTION 
       [0005]    In order to overcome the above-mentioned problems in the prior art, the present invention describes a magneto-resistive sensor using MTJ or GMR sensors in a multi-chip configuration to form push-pull bridge sensors. 
         [0006]    A first possible implementation of the present invention is a push-pull half-bridge magnetic field sensor. The sensor includes one or more pairs of MTJ or GMR magnetoresistive sensor chips, wherein one of the sensor chips in each pair is rotated 180 degrees with respect to the other, and the sensor chips are adhered to a standard semiconductor package lead frame. Each sensor chip contains one or more MTJ or GMR sensor elements interconnected as a single magnetoresistive element or a plurality of MTJ or GMR magnetoresistive elements. The MTJ or GMR elements have a resistance that is linearly proportional to an applied magnetic field over some portion of their magnetoresistive transfer curve. Each of the magnetoresistive sensor chips have substantially the same R H  and R L  values. Note here and other places in this article the terms “substantially equal” or “of roughly equal size”, refers to the difference is very small, generally within 5%. The bond pads of the sensor chip are designed such that more than one wire bond may be attached to each side of the string of magnetoresistive elements. 
         [0007]    The sensor chips are designed such that the intrinsic saturation field of each magnetoresistive sensor chip minus the offset field of the sensor chip&#39;s transfer curve is greater than the desired maximum magnetic field the sensor bridge is intended to measure. 
         [0008]    Furthermore, sensor chips are tested and sorted before assembly in order to better match their transfer curve characteristics. 
         [0009]    Moreover, the two half bridges maybe are oriented at 90 degrees with respect to each other in order to produce a two-axis magnetic field sensor. 
         [0010]    Finally, the lead frame and sensor chips are encapsulated in plastic to form a standard semiconductor package. 
         [0011]    Another possible implementation of the present invention is a full bridge push-pull magnetic field sensor, comprised of one or more pairs of identical MTJ or GMR magnetoresistive sensor chips, wherein one of the sensor chips in each pair is rotated 180 degrees with respect to the other, and the sensor chips are adhered to a standard semiconductor package lead frame. Each sensor chip is configured as a pair of magnetoresistive elements, and each of the magnetoresistive elements in the pair is composed of a string of one or more GMR or MTJ magnetoresistive sensor elements. The MTJ or GMR elements have a resistance that is linearly proportional to an applied magnetic field over some portion of their magnetoresistive transfer curve. Each of the magnetoresistive sensor chips have substantially the same R H  and R L  values. The bond pads of each sensor chip are designed such that more than one wire bond may be attached to each side of each string of magnetoresistive elements. Each magnetoresistive sensor chip has a crossover in the top and bottom conductors, such that the bond pads on one side of the sensor chip are swapped in position with respect to the magnetoresistive elements, in order to permit wirebonding of the two identical chips in order to form a push-pull full-bridge sensor without crossing the bond wires. The input and output connections of the bridge composed of the magnetoresistive sensor chips are wire bonded to the lead frame. 
         [0012]    Furthermore, the sensor chips are designed such that the intrinsic saturation field of each magnetoresistive sensor chip minus the offset field of the sensor chip&#39;s transfer curve is greater than the desired maximum magnetic field the sensor bridge is intended to measure. The sensor chips may be tested and sorted before assembly in order to better match their transfer curve characteristics. Finally, the lead frame and sensor chips are encapsulated in plastic to form a standard semiconductor package. 
         [0013]    A two-axis version of the second implementation may be fabricated within a single package sensor wherein two full bridges as described in claim  6  are oriented at 90 degrees with respect to each other. Compared with the prior art, the present invention has the following advantages; it provides a push-pull bridge easily manufactured in a standard semiconductor package, comprising at least one pair of MTJ sensor chip, wherein a chip relative to the other chip is rotated 180 degrees. In this arrangement, the non-ideal offset in magnetoresistive response that occurs during manufacturing of the sensors cancels, thus providing linear sensor with nearly ideal response. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0014]    FIG.  1 —Schematic drawing of the magnetoresistive response of a spin-valve sensing element with the reference layer magnetization pointing in the negative H direction. 
           [0015]    FIG.  2 —Schematic drawing of the magnetoresistive response of a spin-valve sensing element with the reference layer magnetization pointing in the positive H direction. 
           [0016]    FIG.  3 —Schematic drawing of a half-bridge composed of magnetoresistive sensors. 
           [0017]    FIG.  4 .—Output of a push-pull half-bridge in which R +  is connected to V bias  and R −  is connected to ground. 
           [0018]    FIG.  5 —Output of a push-pull half-bridge in which R −  is connected to V bias  and R +  is connected to ground. 
           [0019]    FIG.  6 —A drawing showing how two magnetoresistive sensor chips may be oriented with respect to each other and interconnected in order to form a half-bridge sensor. 
           [0020]    FIG.  7 —A drawing showing how two magnetoresistive sensor chips may be placed within a standard semiconductor package in order to form a push-pull half-bridge. 
           [0021]    FIG.  8 —A drawing of a completed half-bridge sensor in a standard semiconductor package. 
           [0022]    FIG.  9 —Schematic drawing of a full-bridge sensor composed of magnetoresistive elements. 
           [0023]    FIG.  10 —Field dependence of the output of a push-pull full-bridge magnetoresistive sensor. 
           [0024]    FIG.  11 —A schematic drawing showing how two magnetoresistive chips may be located with respect to each other and interconnected in order to form a push-pull full bridge. 
           [0025]    FIG.  12 —A drawing showing how two magnetoresistive sensor chips may be placed within a standard semiconductor package in order to form a push-pull full-bridge. 
           [0026]    FIG.  13 —A drawing of a completed full-bridge sensor in a standard semiconductor package. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    The combination of the preferred embodiments of the invention are described in detail, in order to make the advantages and features of the invention more clear to those skilled in the art and also to define the scope of protection of the invention. 
         [0028]    The general form of the magnetoresistive transfer curve of a GMR or MTJ magnetic sensor element suitable for linear magnetic field measurement is shown schematically in  FIGS. 1 and 2 . The transfer curves depicted in the figures, saturate at low  1  and high 2 resistance values, R L  and R H , respectively. In the region between saturation, the transfer curves are linearly dependent on the applied magnetic field, H. The transfer curves need not be symmetric about the H=0 point in the plots. The saturation fields  4 ,  5 ,  14 , and  15  are typically offset by an amount H o  such that the R L  saturation region is closer to the H=0 point. The value of H o ′ which is often referred to as “orange peel” or Neel coupling typically ranges from 1 to 25 Oe. It is a related to roughness of the ferromagnetic films within the GMR and MTJ structures, and it is dependent on materials and manufacturing processes. 
         [0029]    The transfer curves in  FIGS. 1 and 2  are minor images of each other, and they represent two different orientations of the applied magnetic field with respect to the sensor element. Here +/− designates the direction of the pinned layer magnetization with respect to the measurement directions, the resistance of each sensing arm may be written: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       R 
                       + 
                     
                      
                     
                       ( 
                       H 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         
                           
                             R 
                             H 
                           
                           - 
                           
                             R 
                             L 
                           
                         
                         
                           2 
                            
                           
                               
                           
                            
                           
                             H 
                             s 
                           
                         
                       
                        
                       
                         ( 
                         
                           H 
                           - 
                           
                             H 
                             o 
                           
                         
                         ) 
                       
                     
                     + 
                     
                       
                         
                           R 
                           H 
                         
                         + 
                         
                           R 
                           L 
                         
                       
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       R 
                       - 
                     
                      
                     
                       ( 
                       H 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         
                           
                             R 
                             L 
                           
                           - 
                           
                             R 
                             H 
                           
                         
                         
                           2 
                            
                           
                               
                           
                            
                           
                             H 
                             s 
                           
                         
                       
                        
                       
                         ( 
                         
                           H 
                           + 
                           
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                             o 
                           
                         
                         ) 
                       
                     
                     + 
                     
                       
                         
                           R 
                           H 
                         
                         + 
                         
                           R 
                           L 
                         
                       
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0030]    Assuming these magnetoresistive elements  23  and  24  are connected in series in a half-bridge configuration as shown in  FIG. 3 , and biased using voltage V bias    20 , the output of a half bridge composed of the two sensing arms with oppositely oriented pinned layers may be written in one of two different ways, depending on which sensor element  23  or  24  is connected to ground  21  and to V bias    20 . For  FIG. 3 , the half-bridge response may be written as 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       V 
                       A 
                     
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                       H 
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                           2 
                         
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                   ( 
                   3 
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         [0031]    And in the case where V bias  and GND are swapped, it may further be written as 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       V 
                       B 
                     
                      
                     
                       ( 
                       H 
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                        
                       
                         ( 
                         
                           
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                             bias 
                           
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                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0032]    The different half-bridge transfer curves  30  and  40  are illustrated in  FIGS. 4 and 5 . Note there are regions  35  and  45  in which the response of the half-bridge is linear in the applied field, and this linear region is symmetric about the H=0, V=Vbias/2 point  36 ,  46 , in spite of the fact that the individual sensor elements are not symmetric about H=0. Furthermore, note that the extent of the linear region  30 ,  40  is less than the linear region of each of the individual sensor element transfer curves R +    3  and R −    13 . 
         [0033]    The equations can be expressed in terms of magnetoresistance. Assuming the magnetoresistance is expressed as MR=(R H −R L )/R L , then 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       V 
                       A 
                     
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                       ( 
                       H 
                       ) 
                     
                   
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                                   s 
                                 
                                 - 
                                 
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                                   o 
                                 
                               
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                             + 
                             
                               
                                 ( 
                                 
                                   2 
                                   / 
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                                 ) 
                               
                                
                               
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                                 s 
                               
                             
                           
                         
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                         1 
                       
                       ) 
                     
                      
                     
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                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
             
               
                 
                   
                     
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                       B 
                     
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                       ( 
                       H 
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                                   / 
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                                 ) 
                               
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                                 s 
                               
                             
                           
                         
                         + 
                         1 
                       
                       ) 
                     
                      
                     
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                           bias 
                         
                         2 
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
         [0034]    These equations hold in the linear region  35 ,  45 . Note the response is unipolar in voltage response, offset from zero voltage by an amount V bias /2 indicated at points  36 ,  46 . Equations 5 and 6 predict the sensitivity will increase as MR increases, and decrease as H s  increases. 
         [0035]    The extent of the linear region in the half bridge is reduced from 2H s  to 
         [0000]        H   Linear =2( H   s   −H   o )  (7)
 
         [0036]    The device will therefore function as a linear sensor provided H o  is less than H s , but the linear field range is reduced. This behavior is common to all push-pull bridges if the offset in the and R −  sensor elements is in the opposite direction. 
         [0037]    In order to guarantee the bridge is linear over the desired field range a linear sensor should thus be designed with the H s  of each magnetoresistive elements in the bridge greater than the maximum field that the sensor bridge is intended to measure and given by 
         [0000]        H   s   =H   max   +H   o ,  (8)
 
         [0038]    Where H max  is the maximum field the bridge sensor is intended to measure. 
         [0039]      FIG. 6  shows one possible arrangement and design of magnetoresistive sensor chips for a half-bridge magnetic field sensor that is sensitive to fields along axis  50 . Here two sensor chips  51 , 52  are rotated 180 degrees with respect to each other, such that their reference layers are pointing in opposite directions, as depicted by the arrows in  FIG. 6 . The half bridge has three terminals  55 ,  56 ,  57  used for electrical connections V bias , V A /V B , and GND respectively. The sensor chips may contain arrays or series connected strings  54  of MTJ or GMR elements in order to increase the resistance of the sensor chip. The sensor chips are interconnected using wire bonds  53 . The bond pads are designed such that each side of the magnetoresistive elements  54  can have more than one wire bond. 
         [0040]      FIG. 7  is a schematic illustration of one possible bonding diagram for a standard semiconductor package for the push-pull magnetoresistive sensor. The oppositely oriented sensor chips  51 ,  52  are adhered to the lead frame paddle  67 , and wirebonded to lead frame terminals  65 ,  66 , and  67 . The lead frame is then encapsulated in plastic as illustrated in  FIG. 8 , and lead frame terminals  65 ,  66 , and  67  form the connection pins of the plastic package  68 .  FIGS. 7 and 8  only represent one of many possible package configurations. 
         [0041]    A full bridge push-pull sensor is shown schematically in  FIG. 9 . The sensor is essentially two half bridges, one of type V A (H) and the other of type V B (H) connected n parallel between V bias    70  and ground  71 . The sensor is thus composed of four magnetoresistive elements, two 75 and 77 of type R −  and two 74 and 76 of type R + . 
         [0042]    For the full bridge, the output is given as 
         [0000]        V ( H )= V   A ( H )− V   B ( H )  (9)
 
         [0043]    This response  80  is plotted in  FIG. 10 . Over a limited field range around the H=0 between 80 and 82, the output is linear and is related to the magnetic field as follows: 
         [0000]    
       
         
           
             
               
                 
                   
                     V 
                      
                     
                       ( 
                       H 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         
                           ( 
                           
                             
                               R 
                               H 
                             
                             - 
                             
                               R 
                               L 
                             
                           
                           ) 
                         
                          
                         
                           V 
                           bias 
                         
                       
                       
                         
                           
                             H 
                             s 
                           
                            
                           
                             ( 
                             
                               
                                 R 
                                 H 
                               
                               + 
                               
                                 R 
                                 L 
                               
                             
                             ) 
                           
                         
                         - 
                         
                           
                             H 
                             o 
                           
                            
                           
                             ( 
                             
                               
                                 R 
                                 H 
                               
                               - 
                               
                                 R 
                                 L 
                               
                             
                             ) 
                           
                         
                       
                     
                      
                     H 
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
         [0044]    Unlike the half-bridge response  30  and  40 , the full-bridge response V(H)  80  is bipolar in voltage response and the response to magnetic field H is twice as strong. It may be expressed in terms of magnetoresistance as 
         [0000]    
       
         
           
             
               
                 
                   
                     V 
                      
                     
                       ( 
                       H 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         V 
                         bias 
                       
                       
                         
                           ( 
                           
                             
                               H 
                               s 
                             
                             - 
                             
                               H 
                               o 
                             
                           
                           ) 
                         
                         + 
                         
                           
                             ( 
                             
                               2 
                               / 
                               MR 
                             
                             ) 
                           
                            
                           
                             H 
                             s 
                           
                         
                       
                     
                      
                     H 
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
         [0045]    Like the half-bridge sensors, the full bridge sensitivity increases as MR is increased, and the sensitivity decreases as H s  increases. For MR&gt;&gt;(H s +H o )/(2H s ) the response does not increase much. The point of no return is about MR&gt;500%. 
         [0046]      FIG. 11  shows two sensor chips  91  and  92  rotated by 180 degrees with respect to each other forming a full-bridge layout. Each sensor chip  94  contains a pair of magnetoresistive composition. Each magnetoresistance element consists of a string of one or more MTJ or GMR sensing units. Said sensor ships have a means for swapping bond pad positions on opposite side of the chip such as the intersection  95 . This permits the bond pads on each side of the chip  95  to be exchanged. The purpose is to permit wireboding to external leads  93  to form push-pull full bridge sensor without crossing bond wires. The full-bridge sensors are used to detect the magnetic field along the direction of the reference layer collinear axis  90  direction of the magnetic field. 
         [0047]      FIG. 12  shows the two sensor chips  91  and  92  fixed to the package lead frame  100  in one possible arrangement. Here, two sensor chips  103  are interconnected using bonding wires to pins V A    101 , V B    102 , V Bias    104  and GND  105 . 
         [0048]      FIG. 13  shows the sensor chips and lead frame encapsulated in plastic  103  to form a standard semiconductor package  110 . If necessary, the sensor chips can be tested before packaging and sorting in order to substantially match their response and provide better performance. This testing can be accomplished at silicon wafer level test, and using a binning and marking method to classify the chips. 
         [0049]    It will be apparent to those skilled in the art that various modifications can be made to the proposed invention without departing from the scope or spirit of the invention. Further, it is intended that the present invention cover modifications and variations of the present invention provided that such modifications and variations come within the scope of the appended claims and their equivalence.