Abstract:
The present invention discloses a design of a single-chip push-pull bridge sensor, composed of magnetoresistive elements, utilizing on-chip permanent magnets. The permanent magnets are oriented to preset magnetization directions of free layers of adjacent sensor bridge arms so that they point to different directions with respect the same sensing direction, enabling push-pull operation. The push-pull bridge sensor of the present invention is integrated on a single chip. Additionally, an on-chip coil is disclosed to reset or calibrate the magnetization directions of the free layers of the magnetoresistive elements.

Description:
CROSS-REFERENCE TO A RELATED APPLICATION 
     This application is a 35 U.S.C. §371 national phase application of PCT/CN2012/073495, filed on Apr. 1, 2012, which claims priority to a Chinese Patent Application No. CN 201120097042, filed on Apr. 16, 2011, and a Chinese Patent Application No. CN201110326725, filed on Oct. 25, 2011, incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The invention relates to a magnetic field sensors, in particular single-chip push-pull magnetic field sensor bridges. 
     BACKGROUND ART 
     Magnetic field sensors can be used to detect magnetic field direction, intensity, and the location of magnetic objects. Magnetoresistive sensors can be used to produce a high performance push-pull sensor bridge with the advantages of low offset, high sensitivity, and good temperature stability. Magnetic tunnel junction (MTJ) sensors are a new type of magnetoresistive sensor which has been used recently in various applications It utilizes a magnetic multilayer and the magnetoresistance effect is called tunneling magnetoresistance (TMR), The TMR effect is related to the magnetic orientation of magnetization of the ferromagnetic layers making up the magnetic multilayer, TMR is advantageous since it is a larger magnetoresistance effect than that of commonly used technologies such as anisotropic magnetoresistance (AMR) effect, giant magnetoresistance (GMR) effect, or the Hall Effect, thereby producing a larger signal. When compared to the Hall Effect TMR has better temperature stability. TMR has a further advantage of high resistivity and therefore can have very low power consumption. To summarize, MTJ devices have higher sensitivity, lower power consumption, better linearity, wider dynamic range, and better thermal compensation characteristics than AMR, GMR, or Hall devices. Due to its high sensitivity, the resulting noise performance of TMR sensors is better. In addition MTJ materials can be fabricated using existing semiconductor processes thereby providing the capability to produce MTJ sensors with very small size. 
     It is common to use a push-pull sensor bridge rather than a single sensor element in order to produce a magnetic field sensor, since bridge sensors have higher sensitivity, and because a bridge configuration removes offset and suppresses thermal effects magnetoresistive This however is not ideal for manufacturing, since there are no standard methods for setting the magnetization direction of the adjacent arms. Present techniques for manufacturing magnetoresistive sensor push-pull bridges are as follows:
     (1) Double Deposition: Here, two deposition processes are used to deposit GMR or TMR films which have differently oriented pinned layers onto the same wafer. This method is complex production process, depending on the specific nature of the films, if a second anneal is needed; it can significantly affect the deposition of the first film. Because two separate deposition steps are used, it is difficult to match the resistance and performance of bridge legs built form the different films, degrading the overall performance of the sensor;   (2) Multi-Chip Packaging Technology: In this process, two or more sensor chips diced from the same wafer are packed together so that their resistance and performance characteristics are well-matched, but during packaging one is rotated relative to the other by 180° in a multi-chip package, in order to produce a push-pull half bridge. This method produces reasonably well-behaved push-pull bridges with good sensitivity and fair temperature compensation. However, because of the multi-chip packaging technology there are performance and cost disadvantages. The package size large; production costs are high; alignment is difficult and it is not always possible to achieve an accurate 180° flip; If the sensor chips are not properly aligned at 180°, then the performance characteristics of the two chips may not match well. In short, although the Multi-Chip Packaging process is standard and capable of producing good push-pull sensor bridges, it brings about cost and potential performance problems;   (3) Laser Assisted Local Magnetic Annealing: In this method, GMR or MTJ wafers are initially annealed at high temperature in a strong magnetic field, which sets the magnetization of the different bridge arms in the same direction. At a later step in the process, a scanning laser beam plus reversed magnetic field is used to locally heat the wafer in the regions where the pinned layer needs to be reversed. Although it sounds easy in concept, the local laser heating method requires special equipment that is not commercially available, and development of the equipment is expensive. The process is expensive to utilize, since it requires a long time to process an entire wafer. Performance is also an issue, since it can be difficult to properly match other performance of the push and pull sensor arms that result in the process, and the consistency of the different bridge arms cannot be guaranteed.   

     As illustrated above, there are few good options for producing low-cost MTJ or GMR sensor bridges with good performance using standard semiconductor processes. 
     SUMMARY OF THE INVENTION 
     The present invention provides a design and mass production method for single-chip push-pull magnetic field sensor bridges, which comprise a plurality of sensor arms composed of electrically interconnected magnetoresistive elements, which may be MTJ, AMR, or GMR elements. The magnetoresistive elements are sensitive to a magnetic field applied along an axis. Permanent magnets, which may be oriented at an angle with respect to the sensing axis, are provided to bias the magnetoresistive elements. 
     Preferably, the length of each permanent magnet is larger than the gap between adjacent permanent magnets in order to make the magnetic field between the permanent magnets uniform. 
     Preferably, each of the pair of the permanent magnets disposed on two sides of a magnetoresistive element has an edge closest to the magnetoresistive element, wherein the permanent magnet edge is set to form an angle with respect to the sensing direction of the magnetoresistive element, and said angle is either a sharp angle or an obtuse angle 
     Preferably, the pair of the permanent magnets on the sides of the magnetoresistive element generate a magnetic bias field along a biasing direction. 
     Preferably, the thickness of the permanent magnets can be changed to change the bias field. 
     Preferably, each of the pair of the permanent magnets disposed on two sides of a magnetoresistive element has an edge closest to the magnetoresistive element, and the strength of the permanent magnet bias field can be adjusted by varying the orientation of the magnetization direction of the permanent magnet material or the angle of the edge boundary with respect to the sensing direction. 
     Preferably, the bias field provided by the permanent magnets is uniform. 
     Preferably, the magnetoresistive elements are parallel to each other. 
     Preferably, an electric coil is provided in the vicinity of the magnetoresistive elements in order to calibrate the sensor and compensate offset, and there is an insulation layer between the magnetoresistive elements and the electric coil. 
    
    
     
       FIGURES 
         FIG. 1  is a schematic drawing of a magnetic tunnel junction. 
         FIG. 2  is an ideal resistance as a function of magnetic field output response of a magnetic tunnel junction. 
         FIG. 3  shows how MTJs may be interconnected in series to form a bridge arm. 
         FIG. 4  shows the change in resistance of a MTJ structure in response to a magnetic field applied perpendicular to the pinned layer magnetization direction. 
         FIG. 5  shows two identical MTJs integrated on the same substrate and using permanent magnets in order to bias the free layers. 
         FIG. 6  shows the use of permanent magnets and shape anisotropy in order to bias the free layer of the different bridge arms. 
         FIG. 7  shows the design concept for a half-bridge push-pull magnetic field sensor. 
         FIG. 8  shows the design concept for a full-bridge push-pull magnetic field sensor. 
         FIG. 9  shows one possible layout for a full-bridge push-pull sensor. 
         FIG. 10  illustrates a full-bridge push-pull sensor with the sensitive direction perpendicular to the easy-axis of the sensor element. 
         FIG. 11  illustrates a full-bridge push-pull sensor with the sensitive direction parallel to the easy-axis of the sensor element. 
         FIG. 12  shows a plan for integrating a coil that can be used for offset compensation and calibration. 
         FIG. 13  shows the layout of a coil that may be used for offset compensation and calibration. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to  FIGS. 1-13  below, preferred embodiments of the present invention will be described in detail so that the advantages and features of the present invention can be more easily understood by a person skilled in the art, and thus more clearly defining the scope of protection of the present invention. 
       FIG. 1  illustrates a standard MTJ element. A standard MTJ element  1  comprises a magnetic free layer  6  and a magnetic pinned layer  2 , which are separated by a tunnel barrier layer  5 . The free layer  6  is made from a ferromagnetic material, and the magnetization direction of the magnetic free layer  7  can change in response to a change of the external magnetic field. The pinned layer  2  magnetization direction does not change, and it is set into direction  8 . A magnetic pinned layer is often comprised of antiferromagnetic layer  3  above or below a ferromagnetic layer  4 . The MTJ structure is usually deposited on the conductive seed layer  16  and it is capped with an electrode layer on top of  17 . The resistance of the MTJ element  16  is measured between the seed layer  17  and the upper electrode layer. The measured resistance value  18  of the MTJ represents the relative orientation between the free layer  6  and pinned layer  8  magnetization directions. 
       FIG. 2  illustrates the ideal output response of a MTJ element. In saturation, R L    20  and R H    21  represent low resistance and high resistance states. When the free layer  7  and pinned layer  8  magnetization are parallel, the measured resistance value  8  of the MTJ element is the low resistance state  20 ; when the magnetization direction of the free layer  7  and the pinned layer  8  are anti-parallel, the resistance value  18  of the MTJ element is the high resistance state  21 . By known techniques, MTJ resistor element  1  may be made to show a resistance that varies linearly with applied magnetic field. The linear operating magnetic field range of the MTJ is between H s  and H s . 
       FIG. 3  illustrates the method for interconnecting MTJ elements in order to form a bridge arm. A long string of MTJ elements reduces sensor noise and improves stability of the device. This results because the bias voltage on each MTJ  1  in the chain of MTJs decreases with increasing number of MTJs. Additionally, the current required to produce a large output voltage is decreased as the number of MTJs increases, which decreases shot noise and improves ESD immunity. In addition, with the increasing number of MTJ elements  1 , MTJ noise is further reduced, because the uncorrelated random fluctuations of each MTJ element average out. 
       FIG. 4  shows the relative directions of the free layer and pinned layer magnetization. As shown, the magnetization direction of the free layers  7  and the pinned layer  8 , are related by an angle α. A magnetic field applied along sensitive direction  9  produces a different response for the MTJ elements oriented at positive and negative α. By setting different permanent magnet bias field directions, the different free layers can be set at the correct angle with respect to the pinning layer magnetization direction  8 . The magnetization direction  7  of the different free layers produced by the different magnetic bias fields causes the free layers of the MTJ elements to respond oppositely to applied magnetic field  9 . The magnetization direction  7  (solid arrow in  FIG. 4 ) rotates closer to parallel with the pinned layer magnetization direction  8 , reducing the resistance of the corresponding MTJ element. The magnetization direction ( 7 ) (dashed arrow in  FIG. 4 ) rotates closer to anti-parallel with the pinned layer magnetization direction  8 , increasing the resistance of the corresponding MTJ element. It is therefore apparent; this design produces elements that have opposing response to an applied magnetic field. 
       FIG. 5  illustrates the use of two strip-shaped permanent magnets to bias the magnetization direction of the free layers. Here, each permanent magnet  13  has a length that is large relative to the gap  12  between the magnets in order to avoid nonuniformity in fields which occurs at the ends of the gap. Although the permanent magnets are magnetized in the same direction, the bias fields in the gap  10  are perpendicular to the surface of the permanent magnets. 
       FIG. 6  illustrates a design that uses permanent magnets and the shape anisotropy to control the magnetization direction of the free layers. Note the magnetization direction of the magnetic free layer  7  depends on both the shape anisotropy and the magnetic bias field  10 . The shape of the magnetoresistive elements may be generally rectangular, diamond-like or elliptical. These oblong shapes produce anisotropy that tends to make the magnetization direction of the free layer  7  align along the long direction of the magnetoresistive element. The strength of the shape anisotropy may be controlled by changing the ratio of the major axis and the minor axis. The orientation of the magnetization is determined by the competing results of shape anisotropy and the magnetic bias field  10 . The magnetic bias field  10  strength of the permanent magnets magnet depends on the surface magnetic pole density due to the alignment of the magnetization  11  of the permanent magnet, the closer the magnetization is to perpendicular to the interface  14 , the surface magnetic charge density of greater accumulation, and the greater magnetic bias field produced at the MTJ element. Surface magnetic charge density is proportional to sin θ, where θ is the angle of the interface  14  and the permanent magnets  11  of the angle between the magnetization directions. By adjusting the magnetic bias field  10  and the shape anisotropy of the magnetoresistive element, the magnetization of the magnetoresistive element can be preset angle α. In this design, the sensitivity direction  9  and the magnetic pinned layer  8  perpendicular to the magnetization direction. 
       FIG. 7  shows the design concept for a half-bridge push-pull magnetic field sensor. As shown, the magnetic resistors R 11  and R 12  constitute a half-bridge. Both bridge arms share the same pinned layer direction  8 . The two magnetoresistive elements comprising the bridge arms have magnetization tilted in opposite directions  7  by the same angle α. The magnetization direction of the magnetic free layer direction  7  depends on the competition between shape anisotropy and the magnetic bias field  10 . When this half-bridge push-pull sensor is subjected to a magnetic field along the direction  9 , the magnetization  7  of the free layer of R 11  rotates into the direction parallel to the pinned layer magnetization direction  8 , and the resistance shows a corresponding decrease; simultaneously the free layer magnetization direction ( 7 ) of R 12  rotates away from magnetic pinned layer magnetization direction  8 , showing a corresponding increase in its resistance. When a constant voltage V BIAS  is applied to bias the bridge, the output voltage V OUT  of the bridge changes in response to the magnetic field. This is the expected response for a push-pull half bridge. 
     The biasing method for push-pull half bridge is as follows. First apply and remove a large magnetic field along direction  11  of push-pull half bridge as shown in  FIG. 7  to set the magnetization of the permanent magnets. The resulting magnetic field  10  at the gap  13  between permanent magnets  15  generated by virtual magnetic charges at the edges  14  of the gaps is perpendicular to gap edges. Arrow  10  in  FIG. 7  indicates the exact biasing direction. 
       FIG. 8  shows a design sketch for a push-pull full bridge. Magnetoresistive elements R 21 , R 22 , R 23  and R 24  comprising bridge arms are connected in full bridge structure. The magnitude of the angle is the same for all bridge arms. The magnetic pinning layers of the  4  bridge arms have the same magnetization direction  8 . The magnetic moments  7  of free layer of opposite bridge arm magnetoresistive elements (R 21  opposite to R 23 , R 22  opposite to R 24 ) are parallel. The magnetization direction of adjacent bridge arm free layers  7  (R 21  and R 22 ; R 22  and R 23 ; R 23  and R 24 ; R 24  and R 21 ) are different. If a positive magnetic field is applied to the push-pull full-bridge along its sensing direction  9 , then the magnetic moment  7  of free layer in magnetoresistive elements R 21  and R 23  will rotate into the pinned layer magnetization direction  8 , which causes the resistance of R 21  and R 23  to decrease; meanwhile, magnetic moment ( 7 ) of the free layers of magnetoresistive elements R 22  and R 24  will rotate into the direction opposite to pinned layer magnetization direction  8 , thereby causing the resistance of R 21  and R 23  to increase. When a constant voltage V BIAS  is applied to the bridge, the voltage between terminals V 1  and V 2  will change in opposition. This produces the push-pull effect. Ideally, if the resistances of R 21  and R 23  change as (R 1 +ΔR), while resistances of R 21  and R 23  change as (R 2 −ΔR), then the output of the sensor is: 
                       V   ⁢           ⁢   1     -     V   ⁢           ⁢   2       =           R   ⁢           ⁢   2     -     Δ   ⁢           ⁢   R     -     (       R   ⁢           ⁢   1     +     Δ   ⁢           ⁢   R       )           R   ⁢           ⁢   2     +     R   ⁢           ⁢   1         ⁢     V   bias               (   1   )               
Ideally, R 1 =R 2 &gt;ΔR, and the equation reduces to:
 
                       V   ⁢           ⁢   1     -     V   ⁢           ⁢   2       ≈           -   2     ⁢   Δ   ⁢           ⁢   R         R   ⁢           ⁢   2     +     R   ⁢           ⁢   1         ⁢     V   bias               (   2   )               
Thereby showing full-bridge push-pull response.
 
     The method of biasing the push-pull full bridge magnetic field sensor is explained as follows: First, apply and remove a magnetizing field along axis indicated by  11  to set the permanent magnets for push-pull full bridge as shown in  FIG. 8 . The resulting magnetic field  10  at the gap  10  between permanent magnet  15  and will be produced by virtual magnetic charge at the edges of the magnets, resulting in a bias field that is perpendicular to edge  14 . Arrow  10  in  FIG. 8  indicates the exact biasing direction. 
     As observed by comparing  FIGS. 7 and 8 , the direction of the pinned layer magnetization  8  of pinning layer in push-pull full bridge sensor is the same for all bridge arms. This shows it is possible to manufacture a push-pull full-bridge sensor on one single chip and one single process. There is no need for complicated multichip assembly process, local laser annealing, or double deposition. 
       FIG. 9  shows sketch of the layout for push-pull full-bridge magnetic field sensor. Several MTJ elements  1  are connected in series, as shown in the figure, to form the bridge arms. If the permanent magnet is properly magnetized, the free layer will be biased by magnetic field  10  produced by permanent magnet  15  on both sides of MTJ elements  1 . The sensor bridge has a sensitive direction  9  that is perpendicular to its pinning layer magnetization direction  8 . Pads  23  of the sensor can be connected to ASIC or pins of lead frame through bonding wires. 
       FIG. 10  shows an alternative design of push-pull bridge which sensitive direction is perpendicular to its easy axis, as well as its simulated output. The upper two figures are outputs of two adjacent magnetoresistive elements when the field from the permanent magnets is respectively 50 Oe and 100 Oe. The lower two figures are full bridge outputs with saturate field when using the corresponding 50 Oe and 100 Oe bias fields. 
       FIG. 11  shows a design of push-pull bridge which sensitive direction is parallel to its easy axis, as well as its simulated output. The upper two figures are outputs of two adjacent magnetoresistive elements when the bias field from the permanent magnet is respectively 50 Oe and 100 Oe. The lower two figures show the full-bridge outputs at the corresponding 50 Oe and 100 Oe bias fields. 
     Usually in the actual measurement, MTJ output curve is not ideal curve shown in  FIG. 2 , and there is a certain offset. In practice, the external field can be applied to saturate the sensor in opposite directions to provide a means for measuring the offset calibration.  FIG. 12  shows a design with the coil  22  disposed above the magnetoresistive element. When the coil  22  is energized the magnetic field produced is applied to the fee layer, similarly to the external field. This current controlled field is designed to be implemented after the chip package is prepared to calibrate and compensate offset of the output and thermal drift. It can be performed from time to time in accordance with the actual needs of the user. As shown, the calibration field generated line width of the wire 5 μm, and the calibration current reverse the wire width is 3 μm, the gap width between the wires 2.5 μm. 
       FIG. 13  shows the magnetic field produced by the preset and calibration coil, compared to the magnetization direction of the free layers. As shown, the sensor pad  23  may be connected to an ASIC circuit through lead or lead frame of the package pins. Pads  24  are used for coil inputs and outputs. 
     Although a few embodiments of the present invention have been described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the present invention, the scope of which is defined in the claims and their equivalents.