Abstract:
Hall Effect devices, memory devices, and Hall Effect device readout voltage increasing method. A hall effect device includes a conductive film layer capable an electrical current, a ferromagnetic layer having a configurable orientation and configured to cover a portion of the conductive film layer such that fringe magnetic fields can be generated by an edge portion of the ferromagnetic layer, a high permeability magnetic layer disposed below the conductive film layer. The fringe magnetic fields are drawn toward the high permeability magnetic layer such that the magnetic fields pass though the conductive film layer to enable closure of the magnetic fields.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention generally relates to memory devices. At least some embodiments of the invention relate to Hall effect devices, memory devices, and hall effect device readout voltage increasing methods.  
         BACKGROUND OF THE INVENTION  
         [0002]    Hall plates have long been used as magnetic field sensors to measure fields that are homogeneous over the area of the plate. There are also numerous devices that combine ferromagnetic films along with Hall plates. In a typical configuration, the Hall plate is embedded, by an appropriate doping technique, vertically (i.e. perpendicular to the substrate surface) in a semiconducting substrate. A ferromagnetic film is fabricated outside the region of the Hall plate, and is used to “focus” the flux of an external magnetic field onto the vertical Hall plate.  
           [0003]    The disadvantages of this device, which is used as a sensor with linear response, include limited sensitivity along with increased cost of the device. The sensitivity may be limited because the enhancement ratio of the magnetic field at the focus of the ferromagnetic layer relative to the applied field is relatively small. Moreover, the geometry of the device does not permit a memory effect while preventing the device from being implemented practically as a memory element.  
         SUMMARY OF THE INVENTION  
         [0004]    At least some embodiments of the invention relate to Hall effect devices, memory devices, and hall effect device readout voltage increasing methods.  
           [0005]    In one aspect, a hall effect device comprising a conductive film layer capable of carrying an electrical current, a ferromagnetic layer having a configurable magnetization orientation and configured to cover a portion of the conductive film layer such that fringe magnetic fields can be generated by an edge portion of the ferromagnetic layer, a high permeability magnetic layer disposed below the conductive film layer, and wherein the fringe magnetic fields are drawn towards the high permeability magnetic layer such that the magnetic fields pass through the conductive film layer to enable closure of the magnetic fields.  
           [0006]    In another aspect, a memory device comprising a first layer disposed on a second layer capable of carrying an electrical current, the first layer covering a portion of the second layer, a third layer disposed below the second layer, and wherein fringe magnetic fields generated at an edge of the first layer are drawn towards the third layer and pass through the second layer, thereby increasing a readout voltage of the memory device.  
           [0007]    In a further aspect, a hall effect device comprising a conductive film layer capable of carrying an electrical current, a ferromagnetic layer having a configurable magnetization orientation and configured to cover a portion of the conductive film layer such that fringe magnetic fields can be generated by an edge portion of the ferromagnetic layer, one or more ferromagnetic components formed in close proximity to an edge of the ferromagnetic layer, and wherein the conductive film layer is patterned using a mesa etch such that the one or more ferromagnetic components are located substantially beneath the level of the conductive film layer, and fringe magnetic fields generated from the edge of the ferromagnetic layer are drawn towards the one or more ferromagnetic components such that the magnetic fields pass through the conductive film layer to enable closure of the magnetic fields.  
           [0008]    In yet another aspect, a hall effect device readout voltage increasing method comprising forming a conductive film layer capable of carrying an electrical current, forming a ferromagnetic layer to cover the conductive film layer such that fringe magnetic fields can be generated by an edge portion of the ferromagnetic layer, and forming a high permeability magnetic layer below the conductive film layer such that the fringe magnetic fields are drawn towards the high permeability magnetic layer and pass through the conductive film layer thereby increasing the readout voltage.  
           [0009]    Other aspects of the invention are disclosed herein as is apparent from the following description and figures.  
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0010]    Preferred embodiments of the invention are described below with reference to the following accompanying drawings.  
         [0011]    [0011]FIG. 1 is a schematic representation of an exemplary Hybrid Hall Effect (HHE) device.  
         [0012]    [0012]FIG. 2 is a perspective view of a hybrid Hall Effect device according to one embodiment.  
         [0013]    [0013]FIG. 3 is a cross-sectional view of a portion of the HHE device according to another embodiment and showing an element with high magnetic permeability that can be used to facilitate magnetic flux return and also to increase the perpendicular component of magnetic field in an active region of the HHE device.  
         [0014]    [0014]FIG. 4 is a top view of the HHE device and showing a ferromagnetic component with a preferred shape anisotropy and two lateral elements of high magnetic permeability used to facilitate magnetic flux return in accordance with one embodiment.  
         [0015]    [0015]FIG. 5 is a cross-sectional view of a portion of HHE device according to another embodiment. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    This disclosure of the invention is submitted in furtherance of the constitutional purposes of the U.S. Patent Laws“to promote the progress of science and useful arts” (Article 1, Section 8).  
         [0017]    Referring to FIG. 1, there is shown an exemplary circuit schematic of the hybrid Hall Effect (HHE) device according to an exemplary embodiment of the present invention. The HHE device  10  includes a Hall plate  12  having terminals  14 ,  16 ,  18 , and  20 , the Hall plate  12  being cross-centered in a square. The Hall plate  12  is alternatively referred to as a carrier channel  12 . Terminals  14  and  16  are used for current bias I+ or I− or voltage bias, and the terminals  18 ,  20  are used as sense probes S 1  and S 2  for sensing a bipolar Hall voltage (or current). The device  10  includes a ferromagnetic film  22  which is electrically isolated from the Hall plate  12 . Magnetization  24  of the film  22  is typically in the plane of the film  22  and lies along an axis parallel with that of the bias current. Magnetic material may also be used for film  22  with magnetization perpendicular to the plane. In a preferred embodiment, the film  22  covers a portion of the area of the Hall plate  12  such that an edge  26  of the film  22  is over a central region of the Hall plate  12 . Other arrangements of arranging the film  22  over the Hall plate  12  are possible.  
         [0018]    In the case where magnetization is parallel to the plane of the film  22 , the magnetization may have two stable states along the axis parallel with the bias current, and each state corresponds to “up” or “down” fringe fields near the edge of the film  22 , a positive or negative Hall voltage (or current), and a binary bit of information “1” or “0”.  
         [0019]    [0019]FIG. 2 shows a perspective view of the HHE device shown in FIG. 1 according to one embodiment. The ferromagnetic film  22  is electrically isolated from the Hall plate  12  by an insulating layer  32 . In a preferred embodiment, the ferromagnetic film  22  covers a portion of the area of the Hall plate  12  such that an edge  26  of the film  22  is over a central region of the Hall plate  12 . In one embodiment, the insulating layer  32  may cover the portion of the Hall plate  12  that is directly beneath the film  22 . In another embodiment, the insulating layer  32  may cover all of the Hall plate  12  and may serve the additional function of passivating portions of the device  10  offering protection against degradation during after processing.  
         [0020]    The film  22  may be fabricated as one component of a bilayer or a multilayer, where a second layer may be a thin magnetic (ferromagnetic or antiferromagnetic) layer used to magnetically bias a first layer. The result of the magnetic bias can be a larger remanence or a hysteresis loop that is not symmetric with respect to zero applied field. Other layers for example, in the multilayer, may be buffer layers that may be used to improve the quality of growth of the ferromagnetic layer and/or bias layer, or a passivation layer for protecting the multilayer from environmental degradation. The film  22  may be fabricated from a nonmetallic compound in order to achieve a specific operational advantage. For example, device  10  made with a ferrite as the ferromagnetic film  22  was found to achieve faster switching times.  
         [0021]    In order to achieve larger readout voltages or currents, the Hall plate  10  may also be fabricated using materials with mobilities larger than those of Si or GaAs. Further details are set forth in co-pending Navy Cases 83,835 and 83,836 having U.S. application Ser. Nos. 10/176,002 filed on Jun. 21, 2002, and 10/126,664, filed on Apr. 22, 2002, respectively, the entire contents of which are incorporated herein by reference.  
         [0022]    Local and fringe magnetic fields from the edge  26  of the ferromagnetic film  22  are perpendicular to the plane of the Hall plate  12  may point “up” or “down” depending on the orientation of the magnetization of the film  22 , and have an average readout, value of B av  in the active region of the device  10 . In an exemplary embodiment, for constant bias current, the sensed Hall voltage or current has opposite polarity when the fringe fields are“up” compared with when they are “down.” The magnetization state  24  may be written (set) to be positive or negative by using the magnetic field associated with a positive or negative current pulse transmitted down an integrated wire (not shown) that is contiguous with the film  22 , and described in U.S. Pat. No. 5,652,445 to Johnson, the entire contents of which are incorporated herein by reference.  
         [0023]    The local magnetic fields at the edge  26  of the film  22  provide the mechanism that enables operation of the device  10 . The magnitude of these fields is proportional to the saturation magnetization M s  of the film  22 , and output of the device  10  is thus proportional to the value of M s .  
         [0024]    [0024]FIG. 3 is a cross-sectional view of a portion of the HHE device showing an element  48  with high magnetic permeability that can be used to facilitate magnetic flux return and also to increase the perpendicular component of magnetic field in an active region of the HHE device in accordance with another embodiment of the present invention. The HHE device  300  includes a material of high magnetic permeability deposited as a layer  48  on a substrate  50 . The substrate may be made of Si or GaAs. The high magnetic permeability layer  48  may be deposited under or in a buffer layer  46 . In one exemplary embodiment, layer  48  is a Permalloy, such as Ni 0.8 Fe 0.2 .  
         [0025]    A carrier channel  12  is grown on top of the buffer layer  46  (e.g., insulating layer) and functions as a hall plate. In one embodiment, the buffer layer  46 , the hall plate  12 , and insulating layer  44  are grown at the same time and in the sequence as illustrated in FIG. 3. Layers  46 ,  12 , and  44  are together referred to as a heterostructure  49  that includes a two-dimensional electron gas (2DEG). Another insulating layer  32  is optionally grown on insulating layer  44 . A ferromagnetic film  22  is grown either directly on the insulating layer  44  or on the optionally grown insulating layer  32 .  
         [0026]    In certain instances, when the heterostructure  49  is grown, the insulating layer  44  may become incompatible with the growth of the ferromagnetic film  22  on it, and in such instances, the optional insulating layer  32  may be grown on top of the insulating layer  44 .  
         [0027]    The local, magnetic fringe field at an edge  26  of the ferromagnetic film  22  generates a voltage (or current) signal that enables an operation of the HHE device  300 . As the size of the ferromagnetic film  22  shrinks, the region of high field magnitude is restricted to a volume very close to the edge  26  of the film  22 . As a result, the field B av  present at the plane of the carriers can be diminished. The inventors have determined that stray magnetic field lines from film  22  may be preferentially directed down towards the layer of carriers (e.g., carrier channel  12 ) by using the high permeability magnetic element layer  48  that facilitates closure of the magnetic flux. A plurality of magnetic field lines is referred to herein as magnetic flux.  
         [0028]    Referring again to FIG. 3, a single magnetic field line  52  is shown as originated at the edge  26  of the film  22  and is drawn downwards to the high permeability material  48 . The field line  52  closes to an opposite end of film  22 . Although a single field line is illustrated, it will be appreciated that a plurality of magnetic field lines (e.g., magnetic flux lines) may originate at the edge  26 . By forcing the magnetic flux lines downward toward the high magnetic permeability layer  48 , such magnetic flux lines are made to pass through the carrier channel  12 , thereby increasing the magnitude of the perpendicular component of flux lines at the plane of the carrier channel  12 . Since the perpendicular component provides the Lorentz force which in turn provides the readout voltage (B av ), the readout voltage (B av ) is thereby increased.  
         [0029]    It will be appreciated that for an application involving an array of individual devices  300 , the high magnetic permeability layer  48  would be individually patterned and aligned beneath each film  22 . The magnetic state of the film  22  would be set by fringe fields from write pulses applied to an integrated write wire (not shown), and coupling to the high magnetic permeability layer  48  would be so weak that the write process would not be affected. In one embodiment as shown in FIG. 3, it may be possible to fabricate the high magnetic permeability layer  48  as a continuous layer. The material of the high magnetic permeability layer  48  would break into domains, with a domain associated with each film  22  in an array of such elements associated with respective individual devices  300 . The walls at the edges of the domains may generate fringe fields that could degrade device performance. In some devices, it may not be possible to find an appropriate material that can be grown under or as a part of the buffer layer  46 . However, it would still be possible to provide a magnetic element for flux closure. The Hall Effect device  300  described above may be used as a memory device.  
         [0030]    Referring to FIG. 4, there is shown a top view of the HHE device and illustrating a ferromagnetic component with preferred shape anisotropy and two lateral elements of high magnetic permeability used to facilitate magnetic flux return in accordance with one embodiment. FIG. 4 includes a Hall plate  12 , with Hall cross  402 , and a ferromagnetic film  22  grown on the hall plate  12 . One or more ferromagnetic components  62 ,  64  may be fabricated in close proximity to the film  22  for facilitating magnetic flux return.  
         [0031]    [0031]FIG. 5 is a cross-sectional view of a portion of HHE device according to another embodiment. In an exemplary case, the Hall cross  402  (FIG. 4) may be patterned using a mesa etch that etches through the insulating layers  32 ,  44 , the carrier channel  12 , which functions as a Hall plate, and the buffer layer  46 , to a surface of the substrate  50 . The mesa etch might etch only a small distance past the interface between the carrier channel  12  and the buffer layer  46 . The ferromagnetic element  62  facilitating flux closure may then be microfabricated such that it is substantially beneath the level of the carrier channel  12 . Although FIG. 5 illustrates only one ferromagnetic component  62 , it will be appreciated that a second ferromagnetic component  64  may be used when further focusing of magnetic flux lines (i.e., a plurality of magnetic field lines  52 ) is desired. Appropriate materials for the ferromagnetic element(s)  62  or  64  include Permalloy (e.g., Ni 0.8 Fe 0.2 ). The field lines  52  from the edge  26  of the film  22  are drawn downwards towards the ferromagnetic element  62 , thereby increasing the magnitude of the perpendicular component of field at the plane of the carrier channel  12  (B av ) and thus increasing a readout voltage of the Hall Effect device  500 . It may be beneficial to planarize the Hall Effect device  500  prior to fabrication of write wires (not shown).  
         [0032]    Although FIG. 5 shows ferromagnetic component  62  as formed on the surface of the substrate  50 , the ferromagnetic component  62  may be fabricated in any portion of the buffer layer  46  without making contact with the carrier channel  12 . Likewise, the ferromagnetic film  22  may be formed on the insulating layer  44  and insulating layer  32  may be desirable in circumstances as described above.  
         [0033]    Shape anisotropy may be used to reduce the coercivity and therefore reduce the amplitude of current in the write pulse that sets the magnetization state of the ferromagnetic film  22 . A variety of magnetic anisotropies may be used to influence the magnetic characteristics of the ferromagnetic film  22 . One design criteria involves shape anisotropy. In one example, the inventors have determined that a long rectangle, with an aspect ratio of about 5 to 1, promotes formation of an easy magnetization axis along the long axis of the rectangle resulting in low coercivity and high remanence. In another example, the inventors have determined that an ellipse, with a similar aspect ratio of about 4 to 1 results in slightly lower coercivities than a rectangle.  
         [0034]    The inventors have fabricated prototype cells appropriate for very large scale integration (VLSI) with Permalloy and cobalt films with dimensions approximately equal to 1 micron by 5 microns. Prototype cells appropriate for ultra large scale integration (ULSI) were fabricated with Permalloy films with dimensions approximately equal to 0.5 microns by 2.5 microns. In one embodiment, the film  22  has been drawn with a shape approximating an ellipse.  
         [0035]    The inventors have determined that a ULSI prototype with a Permalloy film has achieved a coercivity of 25 Oe with a very high remanence.  
         [0036]    It will be appreciated that the HHE device of the present invention may also be used in any Hall Effect Device, such as for example, magnetic sensitive field effect transistor (MAGFET), the magnetotransistor, or any other Hall Effect sensing device.  
         [0037]    Various advantages of the HHE device of the present invention include suitability of the HHE device for use in high density memory and logic environments. An exemplary aspect of the present invention presents novel materials systems to be used in the fabrication of HHE devices with the effect of enhancing the operating speed and increasing the output signal level of the device. In another aspect, the present invention achieves substantial improvement over existing HHE devices because the remanence of the magnetic component layer is larger and therefore the bistable output voltage or current is larger. HHE device of the present invention also has advantages over existing HHE devices as the hysterisis loop of the ferromagnetic component is square, thus contributing to the efficiency of the write process. The coercivity of the ferromagnetic component is smaller, thereby lowering the power of the write process, the perpendicular component of the magnetic field is increased in the active region of the device, thereby increasing the output voltage or current. The switching times of the ferromagnetic component layer are smaller, and materials used to fabricate the HHE device are compatible with the fabrication requirements of support circuitry, such as for example, select, sense and amplification circuits.  
         [0038]    In compliance with the statute, the invention has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the invention is not limited to the specific features shown and described, since the means herein disclosed comprised preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpretec in accordance with the doctrine of equivalents.