Abstract:
A magnetoresistive sensor bridge utilizing magnetic tunnel junctions is disclosed. The magnetoresistive sensor bridge is composed of one or more magnetic tunnel junction sensor chips to provide a half-bridge or full bridge sensor in a standard semiconductor package. The sensor chips may be arranged such that the pinned layers of the different chips are mutually anti-parallel to each other in order to form a push-pull bridge structure. The sensor chips are then interconnected using wire bonding. The chips can be wire-bonded to various standard semiconductor leadframes and packaged in inexpensive standard semiconductor packages. The bridge design may be push-pull or referenced. In the referenced case, the on-chip reference resistors may be implemented without magnetic shielding.

Description:
CROSS-REFERENCE TO A RELATED APPLICATION 
       [0001]    This application is a 35 U.S.C. §371 national phase application of PCT/CN2011/085124, filed on Dec. 31, 2011, which claims priority to a Chinese Patent Application No. 201110141214.7, filed on Jan. 17, 2011, incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to the field of magnetic field measurement using a magnetic tunnel junction (MTJ, Magnetic Tunnel Junction) or giant magnetoresistance (GMR Giant Magnetoresistance) device, in particular, relates to the integration of MTJ or GMR chips into a standard semiconductor package. 
       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, inadequate sensitivity and/or dynamic range, cost, reliability and other factors. Thus, there continues to be a need 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 difficulty in MTJ process and backend packaging process, and difficulty in matching the magnetoresistive response of MTJ elements when combined to form bridge sensors have proven difficult. 
       SUMMARY OF THE INVENTION 
       [0005]    The purpose of the present invention is to provide a process for preparing a high-performance multi-chip, push-pull bridge linear magnetoresistive sensor in a standard semiconductor package utilizing MTJ or GMR sensor chips and standard semiconductor fabrication processes. In order to achieve the above stated goals, one aspect of the present invention is to provide an single package bridge-type magnetic field sensor, comprising one or more pairs MTJ or GMR magnetoresistive sensor chips, wherein the sensor chips are fixed on a die attach area of a standard semiconductor package, and each sensor chip includes a fixed resistance value reference resistor and a sensing resistor that changes in response to an external magnetic field. Both the reference resistor and the sensing resistor include a plurality of interconnected MTJ or GMR sensor elements in the form of an array. 
         [0006]    Additionally, each of the reference resistor and the sensing resistor is magnetically biased using a strip-shaped permanent magnet, sitting between the columns in the sensor arrays. The resistance value of the sensing resistor depends linearly on the external magnetic field. The sensor chips include bond pad used to electrically connect the sensor chips to the die attach area and adjacent sensor chip using a plurality of bonding lines in order to constitute a bridge sensor. The sensor chips and a leadframe are encapsulated to form a standard semiconductor package. 
         [0007]    Another aspect of the present invention is to provide a single package bridge-type magnetic field sensor, the sensor comprises one or more pairs of MTJ or GMR magnetoresistive sensor chips, wherein the sensor chips are fixed onto a die attach area of a standard semiconductor package; each sensor chip includes a fixed reference resistor and a sensing resistor that responds to an external magnetic field; each of the reference and the sensing resistors comprises a plurality of MTJ or GMR magnetoresistive elements, wherein the MTJ or GMR magnetoresistive elements are arranged in a matrix and interconnected to form a single magnetoresistive element; the resistance of the sensing resistor is linearly proportional to the external magnetic field; the sensor chips include bond pads so that each pin of the magnetoresistance elements can be connected to a bonding wire; the bonding wires are used to interconnect the sensor chips and to connect the sensor chips to the die attach area in order to enable the fabrication of a bridge sensor. A leadframe and the sensor chips are encapsulated in plastic to form a standard semiconductor package. 
         [0008]    Compared with the prior art, the present invention is advantageous: it describes a method for manufacturing a linear high-performance magnetoresistive sensor that is easy to manufacture, low cost, and suitable for mass production. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    FIG.  1 —A schematic drawing of the magnetoresistive response of a spin-valve sensing element with the reference layer magnetization pointing in the negative H direction. 
           [0010]    FIG.  2 —A schematic drawing of a half-bridge with a fixed reference resistor and a sensing resistor. 
           [0011]    FIG.  3 —An embodiment of a half-bridge in a magnetoresistive sensor chip where both reference resistor and sensing resistor made of plural MTJ elements arranged in row arrays and bar-shape permanent magnets are used to bias the MTJ elements. 
           [0012]    FIG.  4 —An embodiment of a half-bridge in a magnetoresistive sensor chip where both reference resistor and sensing resistor made of plural MTJ elements arranged in matrices. 
           [0013]    FIG.  5 —A drawing of half-bridge magnetoresistive sensor chip placed within a standard semiconductor package. 
           [0014]    FIG.  6 —A schematic drawing of a full-bridge sensor. 
           [0015]    FIG.  7 —A drawing of a full-bridge sensor with two half-bridge magnetoresistive sensor chips placed within a standard semiconductor package. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]    These sensor elements are configured as spin valves, where one of the magnetic layers has a magnetization with an orientation that is fixed in order to serve as a reference. This fixed layer can be a single magnetic layer or a synthetic antiferromagnetic structure composed of a pinned ferromagnetic layer exchange coupled to a reference ferromagnetic layer, wherein the pinned ferromagnetic layer is made magnetically insensitive by exchange coupling it to an antiferromagnetic layer. The other magnetic layer, the so called free layer, rotates in response to an applied magnetic field. The resistance of the spin valve varies in proportion to the relative difference in the orientation of the magnetization of the free and pinned ferromagnetic layers. Because the free layer rotates in response to the applied magnetic field, the sensor is sensitive to the applied magnetic field. In a MTJ element, the free layer and fixed layer are separated by a tunnel barrier. Electrical current flows through the tunnel barrier. In a GMR element, the free layer and the pinned layer are separated by a non-magnetic metallic layer. Electrical current can flow either in the plane of the multilayer thin film or perpendicular to the plane. 
         [0017]    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  FIG. 1 . The transfer curve depicted in the figures saturates at low  1  and high  2 , resistance values, R L  and R H , respectively. In the region between saturation, the transfer curve is linearly dependent on the applied magnetic field, H. In non-ideal cases, the transfer curves is not symmetric about the H=0 point in the plot. The saturation fields  4  and  5  are typically offset by an amount that is determined by the interlayer coupling between the free layer and the pinned layer. A major contributor to the interlayer coupling, so called Neel coupling or “orange-peel” coupling, is related to roughness of the ferromagnetic films within the GMR and MTJ structures, and it is dependent on materials and manufacturing processes. 
         [0018]    Between the saturation fields,  4  and  5 , is the operation field region where ideally the response of the MTJ or GMR resistance is linear. Sensitivity of the MTJ element, the slope  3  of the transfer curve in  FIG. 1 , depends upon the stiffness of the free layer in response to the applied magnetic field. The slope  3  can be tuned by the shape of the MTJ element, to achieve the field sensitivity for specific designs and purposes. Usually MTJ element is pattern into an elongated shape such as but not limited to ellipse, rectangle, and diamond, oriented orthogonally with respect to the pinned layer. In some cases, the free layer can be biased or stabilized by a permanent magnet in the direction perpendicular to the pinning layer. In some cases for high field sensitivity, magnetic field concentrators, or flux guides, can be integrated in the magnetic field sensor to amplify the magnetic field on the free layer of the MTJ elements. 
         [0019]      FIG. 2  shows the schematic of a half-bridge configuration  10  with a bias voltage  15  on a series of a reference resistor  13  with a fixed resistance and a sensing resistor  14  of which the resistance responds to the applied magnetic field. The output voltage  12  is then the voltage difference across the sensing resistor. 
         [0020]      FIG. 3  shows a design of a half-bridge in a magnetoresistive chip  20 . Both reference resistor  23  and sensing resistor  24  are composed of a plurality of MTJ elements,  231 , and  241 , respectively, which are arranged several columns. MTJ elements are connected in series to form the reference resistor and sensing resistor. In between the MTJ element columns there are bar-shaped permanent magnets  26  (PM) to bias the MTJ free layers in the direction perpendicular to the pinned layer. In this case, the PM bars are oriented in the pinned layer magnetization direction. In chip fabrication, the PM&#39;s must be magnetized in the direction perpendicular to the pinned layer in order to provide stabilization field for the free layers. The PM&#39;s are not necessarily fabricated in the same plane of the MTJs. However, they should be close to provide sufficient bias field strength. Since the reference resistor should not be sensitive to the applied magnetic field, the reference MTJ elements  231  can be with different shape and/or different shape aspect ratio from the sensing MTJ element  241  in order to obtain shape anisotropy and magnetic stiffness against applied field. Alternatively, a magnetic shield  27  can be integrated in the chip to screen magnetic field/flux for the reference MTJ elements. In general, the magnetic shield is a piece of soft magnet placed on top of the reference MTJ elements, covering all the elements so that it shields the magnetic field from the elements and the fringe field of the shield at the edges will not affect the MTJ elements. 
         [0021]      FIG. 4  shows a second design of a half-bridge in a magnetoresistive chip  30 . Both reference resistor  33  and sensing resistor  34  comprise a plurality of MTJ elements,  331 , and  341 , respectively, which are arranged in matrix configuration to achieve large area utilization. MTJ elements are connected in series for both the reference resistor and sensing resistor. Since the reference resistor should not be sensitive to the applied magnetic field, the reference MTJ elements  331  can be with different shape and/or difference shape aspect ratio from the sensing MTJ element  341  to obtain shape anisotropy and magnetic stiffness against applied field. Alternatively, a magnetic shield  37  can be integrated in the chip to screen magnetic field/flux for the reference MTJ elements. In general, the shield is a piece of soft magnet placed on top of the reference MTJ elements, covering all the elements so that it shield the magnetic field from the elements and the fringe field of the shield at the edges will not affect the MTJ elements. 
         [0022]      FIG. 5  is a drawing of a half-bridge magnetoresistive chip  43  placed and connected in a standard semiconductor package. Wire bonding technique is used for the connection. The magnetoresistive sensor chips are wire bonded to each other and the leadframe. The half-bridge chip can be one of the above embodiments in  FIGS. 3 and 4 . The field sensing direction  46  is also shown with respect to the package orientation  47 . 
         [0023]      FIG. 6  shows the schematic of a full-bridge  50  that is essentially two half bridges, one of includes reference resistor R ref1    531  and sense resistor R s1    541 , and the other includes reference resistor R ref2    532 , and sense resistor R s2    542 , connected in parallel between V bias    55  and GND  51 . The output is the voltage difference between V +  and V_. 
         [0024]      FIG. 7  is a drawing of a full-bridge sensor composed of two electrically interconnected magnetoresistive chips  631  and  632  placed in a standard semiconductor package. Wire bonding technique is used to make the electrical connections. The magnetoresistive sensor chips are wire bonded to each other and a die attach area. The type of the two magnetoresistive chips can be one of the above embodiments in  FIGS. 3 and 4 . The field sensing direction  68  is also shown with respect to the package orientation  67 . In this full-bridge sensor embodiment, the two magnetoresistive chips are oriented opposite to each other so that the response of the sensing resistors to applied magnetic field is opposite in polarity. Since the resistance of the reference resistors and of the sensing resistors in zero field should match each other well, all the MTJ elements will be completed in the same fabrication process. Furthermore, the shape and/or shape aspect ratio of the MTJ elements for reference resistor and for sensing resistor can only be adjusted under the constraint for resistance matching. 
         [0025]    A push-pull full bridge sensor can provide higher sensitivity and larger output voltage than a conventional full bridge sensor. Instead of having two reference resistors with fixed resistance, the push-pull full bridge is configured in the way that all the four resistors respond to the applied magnetic field. 
         [0026]    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.