Abstract:
CPP-GMR sensors and methods for making them are disclosed. In an implementation, a CPP-GMR sensor comprises: a substrate; an antiferromagnetic (AFM) layer formed on the substrate; a magnetic pin layer formed on the AFM layer; a first wire electrically coupled to the pin layer; a non-magnetic spacer layer formed on the pin layer, the spacer layer insulated from the first wire by electrical insulation material; a sensing layer formed on the spacer layer; a protective layer formed on the sensing layer; and a second wire formed on the protective layer, the second wire electrically coupled to the first wire through the protective layer, the sensing layer, the spacer layer and the pin layer.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates generally to giant magnetoresistance (GMR) sensors. 
       BACKGROUND 
       [0002]    GMR is a quantum mechanical magnetoresistance effect observed in thin-film structures composed of adjacent ferromagnetic and non-magnetic conductive layers. Magnetization of the adjacent ferromagnetic layers causes a change in the electrical resistivity of the GMR structure. The magnetization direction can be controlled by applying an external magnetic field to the GMR structure. Electric current can flow through the magnetic super lattice of the GMR structure using two different geometries. In a current-in-plane (CIP) geometry, the current flows in the plane of the layers. In a current-perpendicular-to-plane (CPP) configuration, the current flows perpendicular to the plane of the layers. 
         [0003]    Modern mobile devices (e.g., smart phones) often include an electronic compass to determine a user&#39;s direction of travel. Some conventional electronic compasses use CIP-GMR sensors to sense external magnetic fields. Because CIP-GMR sensors have a maximum GMR effect of about 15%, the dynamic range, sensitivity and signal-to-noise ratio (SNR) of CIP-GMR sensors may be too low for some electronic compass applications. 
       SUMMARY 
       [0004]    CPP-GMR sensors and methods for making them are disclosed. 
         [0005]    In an implementation, a CPP-GMR sensor comprises: a substrate; an antiferromagnetic (AFM) layer formed on the substrate; a magnetic pin layer formed on the AFM layer; a first wire electrically coupled to the pin layer; a non-magnetic spacer layer formed on the pin layer, the spacer layer insulated from the first wire by electrical insulation material; a sensing layer formed on the spacer layer; a protective layer formed on the sensing layer; and a second wire formed on the protective layer, the second wire electrically coupled to the first wire through the protective layer, the sensing layer, the spacer layer and the pin layer. 
         [0006]    In another implementation, a sensor system comprises: a first current-CPP-GMR sensor comprising: a first substrate; a first AFM layer formed on the first substrate; a first magnetic pin layer formed on the first AFM layer; a first wire electrically coupled to the first pin layer; a first non-magnetic spacer layer formed on the first pin layer, the first spacer layer insulated from the first wire by insulation material; a first sensing layer formed on the first spacer layer; a first protective layer formed on the first sensing layer; and a second wire formed on the first protective layer, the second wire electrically coupled to the first wire through the first pin layer, the first space layer, the first sensing layer and the first protective layer. The sensor system further includes: a second CPP-GMR sensor comprising: a second substrate; a second AFM layer formed on the second substrate; a second magnetic pin layer formed on the second AFM layer; a third wire electrically coupled to the second pin layer and to the first wire of the first sensing structure; a second non-magnetic spacer layer formed on the second pin layer, the second spacer layer insulated from the third wire by insulation material; a second sensing layer formed on the second spacer layer; a second protective layer formed on the second sensing layer; and a fourth wire formed on the second protective layer, the fourth wire electrically coupled to the second wire of the first sensor structure and the third wire through the second pin layer, the second space layer, the second sensing layer and the second protective layer. 
         [0007]    In yet another implementation, a method of fabricating a CPP-GMR sensor comprises: forming an AFM layer on a substrate; forming a magnetic pin layer on the AFM layer; forming a non-magnetic spacer layer on the pin layer; forming a sensing layer on the spacer layer; forming a protective layer on the sensing layer; forming a sensor stack from the layers; forming first electrical insulation material on exposed regions of the substrate at least partially surrounding the sensor stack; forming a wire on the first electrical insulation material, the wire electrically coupled to the pin layer in the stack; and forming a second electrical insulation material over the wire, the second insulation material electrically insulating the wire from the spacer layer of the sensor stack. 
         [0008]    In yet another implementation, a method of fabricating a CPP-GMR sensor comprises: forming an AFM layer on a substrate; forming first electrical insulation material on exposed regions of the substrate at least partially surrounding the sensor stack; forming a magnetic pin layer on the AFM layer; forming a non-magnetic spacer layer on the pin layer; forming a sensing layer on the spacer layer; forming a protective layer on the sensing layer; forming a sensor stack from the layers; forming a wire on the pin layer, the wire electrically coupled to the pin layer in the stack; and forming a second electrical insulation material over the wire, the second insulation material electrically insulating the wire from the spacer layer of the sensor stack. 
         [0009]    Particular implementations disclosed herein provide one or more of the following advantages. The CPP-GMR sensors for electronic compass applications disclosed herein provide a GMR effect that is higher than CIP-GMR sensors enabling a full scale range to be increased in proportion to the GMR effect. Higher GMR effect can also be used to increase sensitivity to external magnetic fields and SNR. Additionally, the change in resistivity has almost linear dependence on the external magnetic field over a large field range. 
         [0010]    The details of the disclosed implementations are set forth in the accompanying drawings and the description below. Other features, objects and advantages are apparent from the description, drawings and claims. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0011]      FIG. 1  is a cross-section view of a prior art CIP-GMR sensor. 
           [0012]      FIG. 2  is a cross-section view of an example CPP-GMR sensor. 
           [0013]      FIGS. 3A-3D  is an example process for fabricating a CPP-GMR sensor. 
           [0014]      FIGS. 4A-4E  is an example alternative process for fabricating a CPP-GMR sensor. 
           [0015]      FIG. 5A-5B  illustrate the use of CPP-GMR sensor in an electronic compass. 
           [0016]      FIG. 6  illustrates an alternate pin layer structure. 
       
    
    
       [0017]    The same reference symbol used in various drawings indicates like elements. 
       DETAILED DESCRIPTION 
       [0018]      FIG. 1  is a cross-section view of a prior art CIP-GMR sensor  100 . CIP-GMR  100  sensor includes silicon substrate  101 , binder layer  102 , antiferromagnetic (AFM) layer  103 , pin layer  104 , non-magnetic spacer layer  105 , sensing layer  106  and protective layer  107 . In sensing layer  106 , magnetization direction M 2  can be reoriented by an external magnetic field. The initial magnetization direction M 2  is defined by the magnetization direction M 1  in pin layer  104 , which is made of a magnetic material. Protection layer  107  protects the sensor stack from the environment. Current path  108  is in the plane of the layers and extends through spacer layer  105 . 
         [0019]    The change in resistivity of CIP-GMR sensor  100  when exposed to an external magnetic field is proportional to cos θ, where θ is the in-plane angle between M 1  of pin layer  104  and M 2  of sensing layer  106 . This change in resistivity is due to spin-dependent scattering at the interface between spacer layer  105  and pin layer  104  and spacer layer  105  and sensing layer  106 . The maximum GMR effect (ΔR/R) for CIP-GMR sensor  100  is about 15%. 
         [0020]      FIG. 2  is a cross-section view of an example CPP-GMR sensor  200 . CPP-GMR sensor  200  includes silicon substrate  201 , binder layer  202 , AFM layer  203 , pin layer  204 , non-magnetic spacer layer  205 , sensing layer  206  and protective layer  207 . In sensing layer  206 , magnetization direction M 2  can be reoriented by an external magnetic field. The initial magnetization direction M 2  is defined by the magnetization direction M 1  in pin layer  204 , which is made of a magnetic material. Protection layer  207  protects the sensor stack from the environment. Current path  208  enters pin layer  204  and is perpendicular to the plane of the layers. 
         [0021]    The change in resistivity of CPP-GMR sensor  200  is due to spin dependent scattering inside pin layer  204  and sensing layer  206 . The maximum GMR effect (ΔR/R) for CPP-GMR sensor  200  is about 30%-50%. The higher maximum GMR effect can be used to improve one or more parameters of the sensor by controlling the shape anisotropy which can be modified by the printed shape of the GMR sensor. For example, the full scale range can be increased roughly in proportion to ΔR/R. Higher ΔR/R can also be used to increase sensitivity and SNR. The mechanism for sensing is the same as CIP-GMR sensor  100  except for the current path  208  which is perpendicular to the plane of the layers. The change in resistivity ΔR has almost linear dependence on the external magnetic field over a large field range. 
         [0022]    There are several technical challenges in fabricating CPP-GMR sensor  200 . First, current must be injected into pin layer  204  to obtain the higher ΔR/R. If the current is injected through AFM layer  203  or below, the ΔR/R advantage over CIP-GMR disappears. To address this first technical challenge, CPP-GMR sensor  200  is fabricated so that current is injected into pin layer  204 , as described in reference to  FIGS. 3A-3D and 4A-4E . 
         [0023]    An additional technical challenge is that total resistance of the CPP-GMR sensor is significantly reduced in comparison to the CIP-GMR sensor as the GMR stack thickness is very thin (˜20 nm), whereas the sensor lateral dimensions can be in the μm range. Lower total resistance means higher measurement current thus higher power consumption. Also, low resistance of the sensor element means the GMR effect ΔR/R will be reduced as it becomes ΔR sensor (R sensor +R rest of circuit ). To address this second technical challenge, multiple small CPP-GMR sensors can be connected in series, as described in reference to  FIGS. 3D and 4E . 
         [0024]      FIGS. 3A-3D  is an example process for fabricating CPP-GMR sensor  200 . Note that the figures are not drawn to scale and the wire build up steps are skipped for simplicity. 
         [0025]    Referring to  FIG. 3A , the process begins by depositing on a silicon substrate (not shown) AFM layer  301 , pin layer  302 , non-magnetic spacer layer  303 , sensing layer  304  and protective layer  305  (also referred to as “capping” layer). In some implementations, protective layer  305  can be made of tantalum or Ru, non-magnetic spacer layer  303  can be made of copper, sensing layer  304  can be made of NiFe or cobalt alloys, AFM layer  301  can be made of IrMn or PtMn and pin layer  302  can be made of CoFe. 
         [0026]    A first photoresist layer is deposited on protective layer  305 , which is then patterned using a mask to define open areas. After the photoresist material is developed using photo-lithography technology, the structure is milled down to AFM layer  301  thereby defining CPP-GMR sensor stack  300 . The milling is followed by a first deposition of electrical insulation material  307  (e.g., Alumina), resulting in the structure shown in  FIG. 3B . A mass spectrometer can be used during the mill process to precisely control the mill depth, which in turn, allows accurate alignment of AFM layer  301  and electrical insulation material  307 . 
         [0027]    Next, the first photoresist layer is lifted off and a second photoresist layer  306  is applied and patterned on the structure, which is followed by wire deposition. The resulting structure is shown in  FIG. 3C . In the wire deposition step it is important to get a good electrical connection between pin layer  302  and wire  308 . A side cleaning mill can be performed on the structure before wire deposition is performed to ensure a good electrical connection. In some implementations, shorting between spacer layer  303  and wire  308  can be avoided by ensuring that the thickness of wire  308  is less than the thickness of pin layer  302 . In some implementations, to reduce the resistance of the wire, the thickness of the wire can be made much thicker away from the interface between the pin layer  302  and wire  308 . 
         [0028]    After wire deposition, the second photoresist layer  306  is lifted off and a second deposition of electrical insulation material  307  (e.g., Alumina) is performed, resulting in CPP-GMR sensor  310  shown in  FIG. 3D . This final deposition leaves deposits of electrical insulation material  307  over wire  308  at the pin layer interface and protection layer  305 . Care should be taken to ensure there is no shorting between spacer layer  303  and wire  308 . A side cleaning mill before the second deposition of electrical insulation material  307  can help remove any residues. The resulting CPP-GMR sensor  310  has wire  308  interfaced with pin layer  302 , providing current path  309  through CPP-GMR sensor  310  (perpendicular to plane) and exiting on top of protective layer  305 . 
         [0029]    To increase the total resistance of CPP-GMR sensor  310 , multiple prints of CPP-GMR sensor  310  can be coupled together in series, where a previous CPP-GMR sensor  310  segment in the series is coupled to wire  308  at the pin layer interface and a next CPP-GMR sensor  310  segment in the series is coupled to wire  308  on top of protective layer  305 , as shown in  FIG. 3D . 
         [0030]      FIGS. 4A-4E  is an example alternative process for fabricating CPP-GMR sensor  200 . Note that the figures are not drawn to scale and the wire build up steps are skipped for simplicity. 
         [0031]    Referring to  FIG. 4A , the process begins by depositing AFM layer  401  on a silicon substrate (not shown). A first photoresist layer  402  is applied on AFM layer  401 , which is patterned with a mask to define open areas. After the photoresist material is developed using photo-lithography technology, the structure is milled down to the substrate. The milling is followed by a first deposition of an electrical insulation material  403  (e.g., Alumina), as shown in  FIG. 4B . The first photoresist layer  402  is lifted off, which is followed by depositing of pin layer  404 , spacer layer  405 , sensing layer  406  and protective layer  407 , resulting in the structure shown in  FIG. 4C . Note that a strong coupling between pin layer  404  and AFM layer  401  is desired. A gentle mill maybe needed before deposition of layers  404 - 407  or a thin layer of AFM deposition can be added before deposition of layers  404 - 407 . 
         [0032]    Next, a second photoresist layer  402  can be applied on the structure and patterned using a mask to define open areas. After the photoresist material is developed the structure can be milled down to pin layer  404 , as shown in  FIG. 4D . In some implementations, a slight over mill can be performed so that electrical insulation material  403  can extend partially into pin layer  404 . A mass spectrometer can be used to monitor which layer is being milled. 
         [0033]    Next, connecting wires  410  are deposited, followed by a second deposition of electrical insulation layer  403  (e.g., Alumina) to electrically insulate wire  410  at the pin layer interface and on top of protective layer  407 . The resulting CPP-GMR sensor  411  has wire  410  interfaced with pin layer  404 , providing current path  409  through CPP-GMR sensor  411  (perpendicular to plane) and exiting on top of protective layer  407 . 
         [0034]    To increase the total resistance of CPP-GMR sensor  411 , multiple prints of CPP-GMR sensor  411  can be coupled together in series, where a previous CPP-GMR sensor segment is coupled to pin layer  404  and a next CPP-GMR sensor  411  segment is coupled to wire  410  on top of protective layer  407 , as shown in  FIG. 4E . 
         [0035]      FIGS. 5A-5B  illustrate the use of CPP-GMR sensors in an electronic compass. Referring to  FIG. 5A , in some implementations, three CPP-GMR sensors  501 ,  502 ,  503  can be mounted on circuit board  500  so that they sense an external magnetic field along three mutually orthogonal axes X, Y, Z of a reference coordinate frame. Each CPP-GMR sensor  501 ,  502 ,  503  can be coupled to signal generating circuitry  504  for generating a signal in response to a change in resistivity of the CPP-GMR sensor. In the example shown, each CPP-GMR sensor is coupled to a Wheatstone bridge and generates current  505  that is proportional to the change in resistivity of the CPP-GMR sensor. In another implementation, each of the resisters in the Wheatstone bridge can be made of CPP-GMR sensors, and pairs of resistors (e.g., R 1 -R 3 , R 2 -R 4 ) can respond in opposite directions to generate a voltage signal that is proportional to the external magnetic field being measured. The signal can be routed to additional circuitry on, for example, an application specific integrated circuit (ASIC) where the signal can be further processed. The processed signal can be output to a host processor where it can be used in an electronic compass application to enable an apparatus (e.g., smart phones, wearable computers, vehicle navigation systems) to determine a direction of travel of the apparatus. 
         [0036]      FIG. 6  illustrates an alternate pin layer structure. In an implementation, the pin layer shown in  FIGS. 2, 3D and 4D  can be replaced with a multilayer structure  600 , which may be easier to process (because it is physically thicker) while maintaining the magnetization direction in-plane. For example, magnetic layers  601 ,  603 ,  605  of structure  600  can have a thickness of about 20-30 Å of Cobalt-Iron (CoFe) alloy and antiferromagnetic coupling layers  602 ,  604  can be about 8+/−1 Å of Ruthenium (Ru). Coupling layers  602 ,  604  ensure that the magnetic layers on either side are strongly magnetically coupled in opposite directions. For example, magnetic layers  601 ,  603  are magnetically coupled by coupling layer  602  in opposite directions and magnetic layers  603 ,  605  are magnetically coupled in opposite directions by coupling layer  604 . 
         [0037]    While this document contains many specific implementation details, these details should not be construed as limitations on the scope of what may be claimed but rather as descriptions of features that may be specific to particular implementations. Certain features that are described in this specification in the context of separate implementations can also be implemented in combination in a single implementation. Conversely, various features that are described in the context of a single implementation can also be implemented in multiple implementations separately or in any suitable sub combination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can, in some cases, be excised from the combination, and the claimed combination may be directed to a sub combination or variation of a sub combination.