Patent Description:
Sensors are widely used in modern Systems to measure or detect physical parameters, such as position, motion, force, acceleration, temperature, pressure, etc. While a variety of different sensor types exist for measuring these and other parameters, they all suffer from various limitations. For example, inexpensive low field sensors, such as those used in an electronic compass and other similar magnetic sensing applications generally consist of anisotropic magnetoresistance (AMR) based devices. In order to arrive at the required sensitivity and reasonable resistances that mesh well with CMOS, the sensing units of such sensors are generally in the order of square millimeters in size. For mobile applications, such AMR sensor configurations are costly, in terms of expense, circuit area, and power consumption.

Other types of sensors, such as magnetic tunnel junction (MTJ) sensors and giant magnetoresistance (GMR) sensors, have been used to provide smaller profile sensors, but such sensors have their own concerns, such as inadequate sensitivity and being effected by temperature changes. To address these concerns, MTJ sensors and GMR sensors have been employed in a Wheatstone bridge structure to increase sensitivity and to eliminate temperature dependent resistance changes. Indeed, two-axis magnetic field sensors have been developed for electronic compass applications to detect the earth's field direction by using a Wheatstone bridge structure for each sense axis. However, such field sensors typically include two opposite pinning directions for each sense axis, resulting in four different pinning directions which must be individually set for each circuit utilizing a magnet array with complex and unwieldy magnetization techniques, or employ a thick NiFe shield/flux concentrating layer to direct the local direction of a lower Intermediate field requiring additional process complexity. <CIT> describes a process for generating multiple pinning directions in bulk wafer, and a Wheatstone bridge structure with a single pinning direction for each sense axis. The different pinning directions are typically set during an anneal process, but may have variations across a wafer and from device to device which result in bridge output with an undesirable offset. An average offset can be corrected by requiring a compensation angle in the orientation of the patterned devices of, for example, <NUM> degrees to achieve the zero offset. However, the offset Variation remains a problem and may increase with the compensation angle; therefore, minimizing this compensation angle for zero offset is important for minimizing the standard deviation of offset.

US patent application <CIT> discloses an underlying layer made of NiFeN disposed over the principal surface of a substrate. A pinning layer made of antiferromagnetic material containing Ir and Mn is disposed on the underlying layer. A reference layer made of ferromagnetic material whose magnetization direction is fixed through exchange-coupling with the pinning layer directly or via another ferromagnetic material layer, is disposed over the pinning layer. A nonmagnetic layer made of nonmagnetic material is disposed over the reference layer. A free layer made of ferromagnetic material whose magnetization direction changes in dependence upon an external magnetic field is disposed over the nonmagnetic layer. International application <CIT> discloses an MgO-based magnetic tunnel junction (MTJ) device which includes in essence a ferromagnetic reference layer, a MgO tunnel barrier and a ferromagnetic free layer. The microstructure of MgO tunnel barrier, which is prepared by the metallic Mg deposition followed by the oxidation process or reactive sputtering, is amorphous or microcrystalline with poor (<NUM>) out-of-plane texture. In the present invention at least only the ferromagnetic reference layer or both of the ferromagnetic reference and free layer is proposed to be bi-layer structure having a crystalline preferred grain growth promotion (PGGP) seed layer adjacent to the tunnel barrier. This crystalline PGGP seed layer induces the crystallization and the preferred grain growth of the MgO tunnel barrier upon post-deposition annealing. US patent application <CIT> discloses a tunnel magnetoresistive sensor that includes a pinned magnetic layer, an insulating barrier layer formed of Mg-O, and a free magnetic layer. A barrier-layer-side magnetic sublayer constituting at least part of the pinned magnetic layer and being in contact with the insulating barrier layer includes a first magnetic region formed of CoFeB or FeB and a second magnetic region formed of CoFe or Fe. The second magnetic region is disposed between the first magnetic region and the insulating barrier layer. US patent application <CIT> discloses a TMR element including a lower magnetic layer, an upper magnetic layer, and a tunnel barrier layer of crystalline insulation material sandwiched between the lower magnetic layer and the upper magnetic layer. The lower magnetic layer includes a first magnetic layer and a second magnetic layer sandwiched between the first magnetic layer and the tunnel barrier layer. The second magnetic layer is formed from a magnetic material containing at least one of Fe, Co and Ni.

Accordingly, a need exists for an improved sensor design and fabrication process for forming reference layers with substantially orthogonal magnetization directions having zero offset with a small compensation angle. There is also a need for a dual-axis sensor that can be efficiently and inexpensively constructed as an integrated Circuit structure for use in mobile applications. There is also a need for an improved magnetic field sensor and fabrication to overcome the problems in the art, such as outlined above. Furthermore, other desirable features and characteristics of the exemplary embodiments will become apparent from the subsequent detailed description and the appended Claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.

A sensor and fabrication process are provided for forming reference layers with substantially orthogonal magnetization directions having zero average offset, a small sensor to-sensor variation in offset, and small offset variation cross wafer with a small compensation angle in the orientation of the devices.

The invention is set forth in the independent claims. Preferred embodiments of the invention are further specified in the dependent claims.

The present Invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and.

Any implementation described herein as exemplary is not necessarily to be construed as preferred or advantageous over other implementations.

During the course of this description, like numbers are used to identify like elements according to the different figures that illustrate the various exemplary embodiments.

A method and structure are described for providing multi-axis pinning on a bulk wafer which may be used to form an integrated circuit sensor with different reference layers having substantially orthogonal pinning directions set with a single pinning material deposition and bulk wafer setting procedure, and fabricated with a pinned layer having two or more layers of specific materials resulting in a reduced compensation angle for a zero offset, and a reduced offset deviation. The reference layers are separated from a sensing element (or free layer) by a tunneling barrier, the reference layers comprising a synthetic antiferromagnet (SAF) structure, where the pinned layer next to the pinning layer is separated from the fixed layer next to the tunneling barrier by a coupling layer. In one exemplary embodiment, the pinned layer includes a layer of Cobalt and Iron (CoFe) material adjacent the coupling layer and a layer of Cobalt, Iron, and Boron (CoFeB) material adjacent the pinning layer. In another exemplary embodiment, the pinned layer includes two layers of CoFe material separated by a layer of CoFeB material. Both embodiments provide the pinned layer with improved soft magnetic characteristics, for examples small cocerivty Hc, better squareness of hysteresis loop, less openness at hard axis hysteresis loop, and low intrinsic anisotropy field. Since the procedure of setting multiple pinning directions was an anneal process during which there is no magnetic field applied, the more soft magnetic characteristic the pinned layer has, the less deviation the pinning direction has. The pinning direction deviation is one of the sources responsible for the offset deviation.

As a preliminary fabrication step, a stack of one or more layers of ferromagnetic and antiferromagnetic materials are etched into shaped reference layers having a twodimensional shape with a high aspect ratio, where the shape provides a distinction for the desired magnetization direction for each reference layer. Depending on the materials and techniques used, the final magnetization direction may be oriented along the short axis or the long axis of the shaped layer. For example, if the pinned SAF layer is formed with a slight imbalance and patterned into micron-scale dimensions, the magnetization will direct along the short axis. As will be appreciated by those skilled in the art, the SAF embodiment provides a number of benefits related to the use of pinned-SAF reference layers in magnetoelectronic devices. In other embodiments, by controlling the thicknesses and shape of the pinned SAF layers, the final magnetization may be directed along the long axis. Using shape anisotropy, different magnetization directions are induced in the reference layers with different shape orientation. In selected embodiments, the patterned reference layers are set first in the presence of an orienting field that is aligned between the desired magnetization directions for the reference layers. This orienting field is removed before the samples are heated. The heating temperature has to be high enough to generating a high enough pinning field from the pinning layer next to the pinned layer. The heating reduces the material component of the anisotropy and allow the shape anisotropy to dominate the magnetization directions. In this manner, once the orienting field is removed, the shape anisotropy directs the magnetization in the desired direction. Upon removing the orienting field, the magnetizations of the reference layers relax to follow the shape of the reference layers so as to induce a magnetization that is aligned along the desired axis of the shaped reference layer and later pinned when the pinning layer provides enough pinning field during the anneal process. For example, if two reference layers (pinned SAF) are shaped to have longer dimensions which are perpendicular to one another, then the induced magnetizations for the two reference layers will be close to being perpendicular to one another.

However, there are additional sources beyond the shape anisotropy that influence the magnetization of the reference layers and thus determine the final pinning direction. While the pinning steps take place at high temperature, thereby reducing the intrinsic anisotropy of the deposited ferromagnetic layers, a finite anisotropy is still present with a defined direction which competes with the shape anisotropy of the patterned reference layer. Also, due to field cycling over time, the support structure and/or pole pieces for the magnet oven utilized in the pinning anneal may become magnetized, applying a small residual field even in the absence of magnetizing current. This residual field will interact with the reference layer magnetization, and may be either zeroed out or overcome to create a small compensating field with a field that is applied in a direction to oppose this remnant field. Additionally, similar to the distribution of pinning strengths present in any real pinned ferromagnetic layer, there exists a local distribution in the temperature required to pin the ferromagnetic layer. This allows high temperature steps, that occur prior to the pinning anneal, to create a low level of local pinning sites that may influence the direction of the magnetization during the pinning anneal. Therefore competition between the intrinsic anisotropy, shape anisotropy, low level of early pinning, and a small remnant field present during the pinning anneal prevent a true orthogonal setting of the induced magnetizations. For accurate elimination of soft Fe effects in the final device, a true orthogonal setting is desired as any simple calibration for soft Fe effects will lose accuracy if non-orthogonalities are also present. This true orthogonal setting may be accomplished by one or more of: <NUM>) Tailoring the intrinsic anisotropy of the reference layer material by applying a field direction during the deposition to induce an anisotropy direction that is different from that used during the setting procedure. A similar method to tailor the intrinsic anisotropy is to apply an alternating field during the deposition of the magnetic layers in order to remove the possibility of introducing a low level of magnetic pinning in the pinned layer during the high energy deposition of the ferromagnetic layers, which will counteract the desired pinning direction during the magnetic field anneal. A third tailoring possibility is to produce a rotating field during the deposition of the magnetic material to remove any preferred anisotropy direction. Additionally, the magnetic materials with low intrinsic anisotropy field can be used as pinned layer for improving true orthogonal setting. <NUM>) Applying a small field during the pinning portion of the magnetic anneal to either eliminate the residual field in the magnet or to provide a slight negative net field for proper compensation of the intrinsic anisotropy. <NUM>) Forming the reference layers with a non-orthogonal axis wherein the final resultant setting direction is truly orthogonal. When the reference layers with a non-orthogonal alignment are used, the offset of the response curve of signal vs. magnetic field direction can be reduced, however, the offset standard deviation cross the wafer is high. Therefore, there is a need to improve offset standard deviation cross wafer.

If the reference layers have not been annealed above the pinning temperature (which is the lower of either the antiferromagnetic blocking temperature or the antiferromagnetic crystalline phase formation temperature) of the antiferromagnet, then a single anneal process is applied to set the induced magnetizations for the two reference layers. With this approach, a single pinning material deposition step and a single anneal step are performed to set all induced magnetizations for the reference layers without requiring additional magnetic layers, thereby providing a bulk wafer setting procedure that simplifies and reduces the manufacturing cost and complexity, as compared to previously known methods utilizing either two different antiferromagnetic pinning layers or a magnetic array to set the pinning directions. The simplified process for forming reference layers with different pinned directions allows independent magnetic sensor elements to be formed at a minimal spacing within the sensor die, and as a result, different sensor configurations within a single die of minimal area can have different sensitive axes.

An illustrative embodiment of the present invention will now be described in detail with reference to the accompanying figures. While various details are set forth in the following description, it will be appreciated that the present invention may be practiced without these specific details, and that numerous implementation-specific decisions may be made to the invention described herein to achieve the device designer's specific goals, such as compliance with process technology or design-related constraints, which will vary from one implementation to another. While such a development effort might be complex and time-consuming, it would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. In addition, selected aspects are depicted with reference to simplified cross sectional drawings without including every device feature or geometry in order to avoid limiting or obscuring the present invention. It is also noted that, throughout this detailed description, conventional techniques and features related to magnetic sensor design and operation, Magnetoresistive Random Access Memory (MRAM) design, MRAM operation, semiconductor device fabrication, and other aspects of the integrated circuit devices may not be described in detail herein. While certain materials will be formed and removed to fabricate the integrated circuit sensors as part of an existing MRAM fabrication process, the specific procedures for forming or removing such materials are not detailed below since such details are well known and not considered necessary to teach one skilled in the art of how to make or use the present invention. Furthermore, the circuit/component layouts and configurations shown in the various figures contained herein are intended to represent exemplary embodiments of the invention. It should be noted that many alternative or additional circuit/component layouts may be present in a practical embodiment.

<FIG> shows a magnetic field sensor <NUM> formed with first and second differential sensors <NUM>, <NUM> for detecting the component directions of an applied field along a first axis <NUM> (e.g., the y-axis direction) and a second axis <NUM> (e.g., the x-axis direction), respectively. As depicted, each sensor <NUM>, <NUM> is formed with unshielded sense elements that are connected in a bridge configuration. Thus, the first sensor <NUM> is formed from the connection of a plurality of sense elements <NUM>-<NUM> in a bridge configuration over a corresponding plurality of pinned layers <NUM>-<NUM>, where each of the pinned layers <NUM>-<NUM> is magnetized in the y-axis direction. In similar fashion, the second sensor <NUM> is formed from the connection of a plurality of sense elements <NUM>-<NUM> in a bridge configuration over a corresponding plurality of pinned layers <NUM>-<NUM> that are each magnetized in the x-axis direction that is perpendicular to the magnetization direction of the pinned layers <NUM>-<NUM>. In the depicted bridge configuration <NUM>, the sense elements <NUM>, <NUM> are formed to have a first easy axis magnetization direction and the sense elements <NUM>, <NUM> are formed to have a second easy axis magnetization direction, where the first and second easy axis magnetization directions are orthogonal with respect to one another and are oriented to differ equally from the magnetization direction of the pinned layers <NUM>-<NUM>. As for the second bridge configuration <NUM>, the sense elements <NUM>, <NUM> have a first easy axis magnetization direction that is orthogonal to the second easy axis magnetization direction for the sense elements <NUM>, <NUM> so that the first and second easy axis magnetization directions are oriented to differ equally from the magnetization direction of the pinned layers <NUM>-<NUM>. In the depicted sensors <NUM>, <NUM>, there is no shielding required for the sense elements, nor are any special reference elements required. In an exemplary embodiment, this is achieved by referencing each active sense element (e.g., <NUM>, <NUM>) with another active sense element (e.g., <NUM>, <NUM>) using shape anisotropy techniques to establish the easy magnetic axes of the referenced sense elements to be deflected from each other by <NUM> degrees. The configuration shown in <FIG> is not required to harvest the benefits of the dual axis pinning technique, and is only given as an example. For example, thin shields may also be used to suppress the sensor response of two of four identical sensor elements to achieve a differential response.

By positioning the first and second sensors <NUM>, <NUM> to be orthogonally aligned, each with the sense element orientations deflected equally from the sensor's pinning direction and orthogonal to one another in each sensor, the sensors can detect the component directions of an applied field along the first and second axes.

As seen from the foregoing, a magnetic field sensor may be formed from differential sensors <NUM>, <NUM> which use unshielded sense elements <NUM>-<NUM>, <NUM>-<NUM> connected in a bridge configuration over respective pinned layers <NUM>-<NUM>, <NUM>-<NUM> to detect the presence and direction of an applied magnetic field. With this configuration, the magnetic field sensor provides good sensitivity, and also provides the temperature compensating properties of a bridge configuration.

To provide additional insight into the structure and formation of the magnetic field sensor of the exemplary embodiments, <FIG> provides a simplified schematic perspective view of an exemplary field sensor <NUM> formed by connecting four MTJ sense elements <NUM>, <NUM>, <NUM>, <NUM> in a Wheatstone bridge circuit. The bridge circuit may be manufactured as part of an existing MRAM or thin-film sensor manufacturing process with only minor adjustments to control the magnetic orientation of the various sensor layers. In particular, the depicted MTJ sensors <NUM>, <NUM>, <NUM>, <NUM> are formed with pinned layers <NUM>, <NUM>, <NUM>, <NUM> that are each magnetically aligned in a single pinned direction, and with sense layers <NUM>, <NUM>, <NUM>, <NUM> that are aligned to have different magnetization directions from the magnetization direction of the pinned layers <NUM>, <NUM>, <NUM>, <NUM>. As formed, each MTJ sensor (e.g., <NUM>) includes a pinned electrode <NUM> formed within reference layers shown in <FIG> and subsequently discussed in more detail, an insulating tunneling dielectric layer <NUM>, and a sense electrode <NUM> formed with one or more upper ferromagnetic layers. The pinned and sense electrodes are desirably magnetic materials whose magnetization direction can be aligned. Suitable electrode materials and arrangements of the materials into structures commonly used for electrodes of magnetoresistive random access memory (MRAM) devices and other MTJ sensor devices are well known in the art. For example, the pinned layers <NUM>, <NUM>, <NUM>, <NUM> are formed with a layer of CoFe. In the following embodiments, these pinned layer may be formed with a layer of CoFe and a layer of CoFeB (<FIG>), or two layers of CoFe separated by a layer of CoFeB (<FIG>) to a combined thickness in the range <NUM> to <NUM>Å, and in selected embodiments in the range <NUM> to <NUM>Å.

In an exemplary implementation (<FIG>), each of the pinned layers (e.g., <NUM>, <NUM>, <NUM>, and <NUM>) is formed with two ferromagnetic layers: an amorphous ferromagnetic layer such as CoFeB, and a crystalline ferromagnetic layer such CoFe, over an anti-ferromagnetic pinning layer. In another exemplary implementation (<FIG>), each pinned layer (e.g., <NUM>, <NUM>, <NUM>, <NUM>) includes a ferromagnetic stack component (e.g., a stack of CoFe, CoFeB and CoFe) which is <NUM> to <NUM>Å thick. An underlying anti-ferromagnetic pinning layer is approximately <NUM>Å thick. The lower anti-ferromagnetic pinning materials may be PtMn, though other materials, such as PtPdMn, IrMn and RhMn, may be used. As formed, the pinned layers function as a fixed or pinned magnetic layer when the direction of its magnetization is pinned in one direction that does not change during normal operating conditions. As disclosed herein, the heating qualities of the materials used to pin the pinned layers <NUM>, <NUM>, <NUM>, <NUM> can change the fabrication sequence used to form these layers.

More specifically and by way of example, referring to <FIG>, the structure <NUM> includes a pinned layer <NUM> formed between a pinning layer <NUM> comprising PtMn and a coupling layer <NUM> comprising Ruthenium (Ru). A fixed layer <NUM> is formed over the coupling layer <NUM> and a tunnel barrier <NUM> comprising AlOx is formed between the fixed layer <NUM> and a free layer <NUM> comprising NiFe. A cap layer <NUM> may be formed over the free layer <NUM>. Optionally, a diffusion barrier (not shown) comprising AlOx may be formed over the free layer <NUM> prior to formation of the cap layer <NUM>. The pinned layer <NUM> includes an amorphous ferromagnetic layer <NUM>, preferably of CoFeB where B is <NUM> to <NUM>% by atomic weight, formed between a crystalline ferromagnetic layer <NUM> of CoFe (where Fe comprises <NUM>-<NUM> by atomic weight) and the pinning layer <NUM>. The amorphous ferromagnetic layer <NUM> of CoFeB has a thickness from <NUM> to <NUM>Å. Typically, CoFeB alloys have softer magnetic properties compared with CoFe alloys, so CoFeB/CoFe pinned layers provide a better soft magnetic properties. The soft magnetic CoFeB/CoFe pinned layer leads to less non-orthogonal aligned references layers for true orthogonal pinning directions and zero offset, and more significantly reduced offset deviation cross wafer, which strongly impact the manufacturability of these magnetic sensors.

By way of another example and referring to <FIG>, the structure <NUM> includes the pinning layer <NUM>, coupling layer <NUM>, fixed layer <NUM>, tunnel barrier <NUM>, free layer <NUM>, and cap layer <NUM> as in the structure <NUM> of <FIG>. However, the pinned layer <NUM> formed between the pinning layer <NUM> and the coupling layer <NUM> in <FIG> includes an amorphous ferromagnetic layer <NUM>, preferably of CoFeB where B is <NUM> to <NUM>% by atomic weight, formed between a first crystalline ferromagnetic layer <NUM> of, for example, CoFe and a second crystalline ferromagnetic layer <NUM> of, for example, CoFe, where the Fe of both layers <NUM>, <NUM> comprises10. <NUM>-<NUM> by atomic weight. The amorphous ferromagnetic layer <NUM> has a thickness in the range of <NUM> to <NUM>Å, while the crystalline ferromagnetic layer <NUM> has a thickness in the range of <NUM> to <NUM>Å. Similar to the pinned layer shown in <FIG>, the magnetic layer formed with a CoFeB layer inserted in a CoFe layer provides softer magnetic pinned layer. Furthermore, this magnetically pinned structure also has strong exchange coupling with pinning layer such as PtMn, hence high pinning field. This soft magnetic pinned structure leads to <NUM>% reduction of offset deviation cross wafer.

<FIG> shows pre-annealed M-H loops of CoFeB/CoFe pinned layer structures with different CoFeB thickness. The sample <NUM> has a CoFe thickness of <NUM>Å while the samples <NUM> and <NUM> have the CoFeB/CoFe thickness (in Å) of <NUM>/<NUM> and <NUM>/<NUM>, respectively. The coercive field of the CoFeB/CoFe layer is significantly reduced with increasing CoFeB thickness up to <NUM>Å, indicating improved soft magnetic properties. Furthermore, the width of switching is also reduced with a thick CoFeB layer, indicating less dispersion of magnetization directions. The squareness of hysteresis loops are also improved with a CoFeB layer. The pinned structure shown in <FIG> has the similar soft magnetic properties as shown in <FIG>, i.e., reduced coercive field and switching width. Additionally, the hard-axis M-H loops shows small coercivity and small anisotropy field as increasing CoFeB layer thickness for pinned layer structure as shown in <FIG>.

The pinned reference layer (e.g., <NUM>, <NUM>, <NUM>, <NUM>) may be formed having a magnetization direction (indicated by the arrow) that aligns along the long-axis of the patterned reference layer(s). However, in other embodiments, the pinned reference layer may be implemented with a synthetic anti-ferromagnetic (SAF) layer which is used to align the magnetization of the pinned reference layer along the short axis of the patterned reference layer(s). As will be appreciated, the SAF layer may be implemented in combination with an underlying anti-ferromagnetic pinning layer, though with SAF structures with appropriate geometry and materials that provide sufficiently strong magnetization, the underlying anti-ferromagnetic pinning layer may not be required, thereby providing a simpler fabrication process with cost savings. For example, <FIG> depicts a pinned reference layer <NUM> formed from an imbalanced synthetic anti-ferromagnet (SAF) having two differing ferromagnet layers <NUM>, <NUM> separated by a Ruthenium spacer layer <NUM>, where the ferromagnetic layers above <NUM> and below <NUM> the ruthenium layer <NUM> have different magnetic moments. Either or both of the ferromagnetic layers may be formed with CoFe (Cobalt Iron) or any desired ferromagnetic alloy. For example, CoFe may be used for the lower layer and CoFeB may be used for the upper layer in an exemplary embodiment. At certain periodic thicknesses of the Ruthenium spacer layer <NUM>, the two ferromagnet layers <NUM>, <NUM> will be exchange coupled so that the anti-parallel state is the low energy state. As a result, the net magnetic moment is minimized and the immunity to external field response is strengthened. An exemplary implementation and micromagnetic simulation of an imbalanced SAF stack <NUM> is shown in <FIG>, where the imbalanced SAF <NUM> includes a fixed layer <NUM> formed with CoFeB to a thickness of approximately <NUM> Angstroms, a spacer layer <NUM> formed with Ruthenium to a thickness of approximately <NUM> Angstroms, and a pinned layer <NUM> formed with CoFeB to a thickness of approximately <NUM> Angstroms. With this exemplary SAF structure, a net moment is created which will respond to the externally applied magnetic field H <NUM> as shown in <FIG>. For a reference layer formed with a SAF that has micron scale dimensions (e.g., greater than approximately <NUM> along the short axis), the magnetization tends to align anti-parallel along the short axis instead of along the long axis, hence the short axis sets the pinning direction. This results from the fact that the lowest energy state is for the two layers to close their magnetic flux along the short axis of the patterned shape. In remanence (e.g., after the setting field is removed), the magnetic moment of the largest moment layer (e.g., the lower pinned layer <NUM> in this example) aligns so that it is along the short axis of the SAF in the direction that has a positive projection onto the setting field angle (to the right in this example). Conversely, the magnetic moment of the smaller moment layer (e.g., the upper fixed or reference layer <NUM> in this example) aligns in the opposite direction from the pinned layer <NUM>, as shown in <FIG>.

Referring again to <FIG>, the upper or sense layers <NUM>, <NUM>, <NUM>, <NUM>, may be formed with one or more layers of ferromagnetic materials to a thickness in the range <NUM> to <NUM>Å, and in selected embodiments in the range <NUM> to <NUM>Å. The upper ferromagnetic materials may be magnetically soft materials, such as NiFe, CoFeB, NiFeX, CoFeX (X is non magnetic element) and the like. In each MTJ sensor, the upper ferromagnetic layers <NUM>, <NUM>, <NUM>, <NUM> function as a sense layer or free magnetic layer because the direction of their magnetization can be deflected by the presence of an external applied field, such as the Earth's magnetic field. As finally formed, the upper or sense layers <NUM>, <NUM>, <NUM>, <NUM> may be formed with a single ferromagnetic layer having a magnetization direction (indicated with the arrows) that aligns along the long-axis of the patterned shapes <NUM>, <NUM>, <NUM>, and <NUM>.

The pinned and sense electrodes may be formed to have different magnetic properties. For example, the pinned electrodes <NUM>, <NUM>, <NUM>, <NUM> may be formed with an anti-ferromagnetic film exchange layer coupled to a ferromagnetic film to form layers with a high coercive force and offset hysteresis curves so that their magnetization direction will be pinned in one direction, and hence substantially unaffected by an externally applied magnetic field. The ferromagnetic film can be a single magnetic layer or SAF layers where two ferromagnetic layers anti-ferromagnetically coupled through a coupling layer. In contrast, the sense electrodes <NUM>, <NUM>, <NUM>, <NUM> may be formed with a magnetically soft material to provide different magnetization directions having a comparatively low anisotropy and coercive force so that the magnetization direction of the sense electrode may be altered easily by an externally applied magnetic field. In selected embodiments, the strength of the pinning field is about two orders of magnitude larger than the anisotropy field of the sense electrodes, although different ratios may be used by adjusting the respective magnetic properties of the electrodes using well known techniques to vary their composition.

As shown in <FIG>, the pinned layers <NUM>, <NUM>, <NUM>, <NUM> in the MTJ sensors are formed to have a first exemplary magnetization direction in the plane of the pinned layers <NUM>, <NUM>, <NUM>, <NUM> (identified by the vector arrows pointing toward the top of the drawing of <FIG>). As described herein, the magnetization direction for the pinned layers <NUM>, <NUM>, <NUM>, <NUM> may be obtained using shape anisotropy of the pinned electrodes, in which case the shapes of the pinned layers <NUM>, <NUM>, <NUM>, <NUM> may each be longer in the direction of the "up" vector arrow. In particular, the magnetization direction for the pinned layers <NUM>, <NUM>, <NUM>, <NUM> may be obtained by first heating the shaped pinned layers <NUM>, <NUM>, <NUM>, <NUM> in the presence of a orienting magnetic field which is oriented non-orthogonally to the axis of longest orientation for the shaped pinned layers <NUM>, <NUM>, <NUM>, <NUM> such that the applied orienting field includes a field component in the direction of the desired pinning direction for the pinned layers <NUM>, <NUM>, <NUM>, <NUM>. The magnetization directions of the reference layers are aligned, at least temporarily, in a predetermined direction. However, by appropriately heating the pinned layers during this treatment and removing the orienting field without reducing the heat, the magnetization of the reference layers relaxes along the desired axis of orientation for the shaped pinned layers <NUM>, <NUM>, <NUM>, <NUM>. Once the magnetization relaxes, the reference layers can be annealed and/or cooled so that the magnetization direction of the pinned layers is set in the desired direction for the shaped pinned layers <NUM>, <NUM>, <NUM>, <NUM>. A true orthogonal setting may be accomplished by forming the reference layers with a non-orthogonal axis wherein the final induced magnetizations are truly orthogonal. With this approach, the formation of the magnetization direction for the pinned layers <NUM>, <NUM>, <NUM>, <NUM> can readily be reconciled with the fabrication steps used to form other pinning electrodes having distinct magnetization direction(s).

<FIG> is a flow chart that illustrates an exemplary method <NUM> of fabricating MTJ field sensors which have orthogonal reference layers using a bulk wafer setting procedure to form shaped reference electrodes including two or more pinned layers for providing a reduced compensation angle for a zero offset. It should be appreciated that the method <NUM> may include any number of additional or alternative tasks, the tasks shown in <FIG> need not be performed in the illustrated order, and method <NUM> may be incorporated into a more comprehensive procedure or process having additional functionality not described in detail herein. Moreover, one or more of the tasks shown in <FIG> could be omitted from an embodiment of the method <NUM> as long as the intended overall functionality remains intact. At step <NUM> a substrate structure is provided using known semiconductor processing techniques. As will be appreciated, the substrate structure includes a substrate that is covered by a base insulator layer, where one or more active circuit elements, such as digital or analog integrated circuits, may be formed on or in the substrate. The substrate and active circuit(s) may be formed as part of a front-end semiconductor fabrication process.

Next at step <NUM>, the MTJ layers are formed by depositing the sensor layers <NUM>, <NUM>, <NUM>, <NUM> over the base insulator layer using a multi-step fabrication process. At this point, the sensor layers may be formed as part of a back-end magnetics fabrication process which occurs after the front-end semiconductor fabrication process. At step <NUM>, the first of the sensor layers is formed by depositing the reference layers over the insulating layer, wherein the reference layers include a pinned layer formed between an antiferromagnetic pinning layer and a ferromagnetic fixed layer. The pinned layer includes a crystalline ferromagnetic layer over an amorphous ferromagnetic layer, or an amorphous ferromagnetic layer formed between two crystalline ferromagnetic layers. Though not shown, the pinning layer may be deposited in electrical contact with an underlying conductive contact layer. The materials chosen to form the pinning layer and the pinned layer should be such that the resultant ferromagnetic layer will have a relatively high pinning strength, and should be sufficiently refractory so as to withstand annealing temperatures (e.g., <NUM> to <NUM> degrees Celsius) if used to pin the magnetization direction of the pinned layer in a predetermined orientation. Additional sensor layers, such as the tunneling dielectric layer(s) <NUM>, <NUM>, <NUM>, <NUM> and the sense electrode layer(s) <NUM>, <NUM>, <NUM>, <NUM>, may also be deposited at this time using well known techniques. The intrinsic anisotropy (atomic-level pair ordering) of the reference layer <NUM>, <NUM>, <NUM>, <NUM>, in accordance with the first embodiment, may be tailored during deposition in order to obtain a true orthogonal directional difference between the pinned magnetizations. For the low level of compensation required, the direction of the pair-ordering anisotropy is set by an applied magnetic field during deposition. Methods of inducing a stronger anisotropy than by shape and intrinsic include growing a magnetic material with a preferred crystalline orientation and inducing by certain anisotropic film growth methods (for example, from shape asymmetry of the growing clusters or crystallites).

At step <NUM>, the deposited reference layer(s) are also patterned and selectively etched into elongated shapes having long axes drawn with different orientations. As a result of the etching, each shaped reference layer has a preferred shape anisotropy direction in the direction of the desired pinned magnetization direction. However, due to intrinsic anisotropy, SAF coupling strength and remenent field during anneal, the actual pinning direction may be different from that desired. For example, <FIG> shows a first reference layer <NUM> having a desired pinning (direction <NUM>) orthogonal to the long axis <NUM>. However, an actual pinning (direction <NUM>) is different from the desired pinning (direction <NUM>). A second reference layer <NUM> has a desired pinning (direction <NUM>) orthogonal to the long axis <NUM>. However, an actual pinning (direction <NUM>) is different from the desired pinning (direction <NUM>). In order to obtain a true orthogonal angle of the actual pinned magnetizations, a determination may be made from empirical pre-obtained results of the pinned magnetizations in relation to the long axes allowing for a positioning of the patterned elongated shapes in a direction to one another to provide the true orthogonal angle. The patterning (step <NUM>) of the reference layers <NUM>, <NUM> shown in <FIG> may be adjusted by modifying the angle between the long axes <NUM>, <NUM> that results in the actual pinning (directions <NUM>, <NUM>) being orthogonal as desired (directions <NUM>, <NUM>).

At step <NUM>, the etched reference layers, for materials such as PtMn which undergo a phase transition as the temperature crosses its transformation temperature, are heated or annealed below the pinning transition temperature in the presence of a orienting field that is applied with a direction that is between the different desired pinned magnetization directions for the different reference layers. For example, if two orthogonally oriented pinned layers are being formed, the applied orienting field may be oriented half-way between the desired orientations of the orthogonal pinned layers. Stated more generally, the applied orienting field should be oriented so that it includes a field component in the direction of each of the desired pinning directions for the reference layers. The properties of the materials used to form the reference layer will control how heat is to be applied. The heat step should be controlled so that the magnetizations of the shaped reference layers are free to follow the external magnetic field.

At step <NUM>, the orienting magnetic field is removed, and at step <NUM>, an anneal temperature at or above the pinning transition temperature is maintained for a predetermined duration, e.g., two hours. In the absence of an applied field, the high aspect ratio patterns provide a shape anisotropy that forces the applied magnetization in the shaped reference layers to relax along the respective anisotropy axes of the shaped reference layers. During anneal, the pinning layer such as PtMn, PtPdMn after phase transformation provides exchange coupling with the pinned layer. For other pinning layers such as IrMn, RhMn (which do not require phase transformation for exchange coupling), the anneal temperature has to be higher than the block temperature of the pinning layer in order to have the shaped reference layer pinned at desired directions. The direction of this applied magnetization may be fine tuned (so as to provide truly orthogonal magnetization directions) by applying a compensating field. <FIG> illustrates first and second reference layers <NUM>, <NUM>, each having a relaxation preference (directions) <NUM>, <NUM> respectively. The application of a compensating field <NUM> overcomes the influence provided by the combination of intrinsic anisotropy and magnet residual field resulting in the actual pinning <NUM>, <NUM> directed in the desired directions and orthogonal to one another.

At step <NUM>, the wafer is then cooled in zero, or compensating field so that the shape-induced magnetizations in the reference layers are pinned, thereby providing multiple orientations of reference layers. By cooling the reference layers below the blocking temperature after the shape-induced magnetizations are obtained in the zero or small compensating field, the magnetizations of the reference layers become pinned, and will remain rigidly pinned in their respective directions, at least for typical applied field strengths.

Using the techniques disclosed herein, first and second differential sensors (e.g., sensors <NUM>, <NUM> shown in <FIG>) may be fabricated together on a monolithic integrated circuit by forming the first reference layers <NUM>-<NUM> with a first pinning direction and simultaneously forming the second reference layers <NUM>-<NUM> with a second pinning direction that is orthogonal to the first pinning direction. These techniques may be further illustrated with a description of an exemplary process flow, beginning with <FIG> which depicts a partial cross-sectional view of a multi-sensor structure in which a stack of MTJ sensor layers <NUM>, <NUM>, <NUM> have been formed over a substrate <NUM> and base insulator layer <NUM>. When the sensors are to be integrated with semiconductor circuitry, conductive vias through the insulator layer <NUM>, made by methods known by those skilled in the art, will connect conductive portions of the sensors to the underlying circuitry in the substrate <NUM>. Depending on the type of transistor device being fabricated, the substrate <NUM> may be implemented as a bulk silicon substrate, single crystalline silicon (doped or undoped), or any semiconductor material including, for example, Si, SiC, SiGe, SiGeC, Ge, GaAs, InAs, InP as well as other Group III-IV compound semiconductors or any combination thereof, and may optionally be formed as the bulk handling wafer. In addition, the substrate <NUM> may be implemented as the top semiconductor layer of a semiconductor-on-insulator (SOI) structure. Though not shown, one or more circuit elements may be formed on or in the substrate <NUM>. In addition, a base insulator layer <NUM> is formed by depositing or growing a dielectric (e.g., silicon dioxide, oxynitride, metal-oxide, nitride, etc.) over the semiconductor substrate <NUM> using chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD), physical vapor deposition (PVD), atomic layer deposition (ALD), ion beam deposition (IBD), thermal oxidation, or combinations of the above.

Over the base insulator layer <NUM>, the stack of sensor layers is sequentially formed by depositing a first conductive layer (not shown) to serve after etching as a conductive line, lower ferromagnetic layers <NUM> (to serve after etching as the lower reference ferromagnetic layer), one or more dielectric layers <NUM> (to serve after etching as the tunnel barrier layer), one or more upper ferromagnetic layers <NUM> (to serve after etching as the upper sense ferromagnetic layer), and a second conductive layer (not shown) to serve after etching as the conductive line. Each of the layers may be blanket deposited using known techniques, such as CVD, PECVD, PVD, ALD, IBD, or combinations thereof to a predetermined thickness. In this way, the stack of sensor layers covers the entire wafer so that the stack is formed in the "Sensor <NUM>" area where a first type of sensor (e.g., x-axis sensors) will be formed, and is also formed in the "Sensor <NUM>" area where a second type of sensor (e.g., y-axis sensors) will be formed. In addition, the sensor stack may be formed in "Other" areas where a sensor having any desired orientation will be formed.

<FIG> illustrates processing of the sensor device structures subsequent to <FIG> after the stack of MTJ sensor layers <NUM>, <NUM>, <NUM> have been selectively etched, thereby defining predetermined shapes for the remnant sensor layers over the substrate <NUM> in each of the sensor areas. Any desired pattern and etching processes may be used to form the patterned sensor layers, including depositing a hardmask layer, such as silicon nitride (not shown), over the upper ferromagnetic layer(s) <NUM>, depositing and patterning a photoresist layer (not shown) to pattern the hardmask layer, and then selectively etching (e.g., with a reactive ion etching process) the exposed sensor layers using a photoresist layer (not shown) to form the openings <NUM>, <NUM>. To more clearly see how the selective etch process creates different predetermined shapes from the stack of MTJ sensor layers, reference is made to <FIG>, which provides a simplified top view of the sensor device structures depicted in <FIG>. As depicted in <FIG>, the openings <NUM>, <NUM> in the sensor layer stack <NUM>, <NUM>, <NUM> define the sensor layers <NUM>, <NUM>, <NUM> in the first sensor area to have a first shape that is oriented to have an easy axis in the desired pinning direction for the finally formed reference layer <NUM>. In similar fashion, the openings <NUM>, <NUM> define the shape of the sensor layers <NUM>, <NUM>, <NUM> in the second sensor area so that they have an easy axis in the desired pinning direction for the finally formed reference layer <NUM>. While the openings <NUM>, <NUM> can be used to define orthogonally oriented shapes <NUM>, <NUM>, any desired orientation can be achieved by properly patterning and controlling the etch process. For example, sensor layers <NUM>, <NUM>, <NUM> in the "other" sensor area may be defined to have another shape that is oriented to have an easy axis in the desired pinning direction for the finally formed reference layer <NUM>. In addition to being formed as long and narrow shapes, additional shaping may be provided so that each of the pinned reference layers performs more like a single magnetic domain. In <FIG>, the additional shaping is shown in the reference layers <NUM>, <NUM>, <NUM>, which are shaped to have pointed ends that taper. The magnetization directions (shown as a question mark) for various shaped reference layers <NUM>, <NUM> and <NUM> are unknown prior to the presentence of an orienting field.

Once the shaped reference layers <NUM>, <NUM>, <NUM> are formed, the desired pinning direction for the reference layers may be induced by first heating or annealing the wafer in the presence of a orienting field that is oriented between the orientations of the reference layers <NUM>, <NUM>, <NUM>, and then removing the field while maintaining a high anneal temperature. The result of heating and orienting the reference layers is shown in <FIG>, which illustrates processing of the sensor device structures subsequent to <FIG> when the etched reference layers <NUM>, <NUM>, <NUM> are heated in the presence of orienting field. As shown in <FIG>, the orienting field is aligned in a direction <NUM> that is between the desired magnetization directions for the finally-formed reference layers. However, at this stage in the process, the magnetizations of the reference layers <NUM>, <NUM>, <NUM> (as shown with the magnetization vectors <NUM>, <NUM>, <NUM>, respectively), follow the external magnetic field <NUM> when the field is high enough.

<FIG> illustrates the magnetization orientations of the reference layers in the sensor device structures subsequent to <FIG> after the orienting field <NUM> is removed and the etched stack of MTJ sensor layers are cooled. By cooling the wafer in a zero or small compensating field, the respective shapes of the reference layers <NUM>, <NUM>, <NUM> provide shape anisotropy that causes the magnetization of each reference layer to relax along a desired direction. Thus, the magnetization <NUM> of first reference layer <NUM> follows its shape so that it is aligned with the desired dimension of the shaped reference layer <NUM> (e.g., in the y-axis direction), thereby forming the desired pinning direction for the finally-formed reference layer <NUM>. In similar fashion, the desired pinning direction for the finally-formed reference layer <NUM> is induced when the magnetization <NUM> of second reference layer <NUM> follows its shape anisotropy (e.g., in the x-axis direction). Of course, any desired pinning direction can be induced by properly shaping the reference layer, as shown with the reference layer <NUM> where the magnetization <NUM> follows the shape anisotropy of reference layer <NUM> (e.g., at <NUM> degrees from the y-axis).

<FIG> illustrates processing of pinned and reference layers <NUM> that are formed by etching imbalanced SAF stacks <NUM>, <NUM>, <NUM>, annealing the layers <NUM> at low temperature in the presence of a orienting field that is oriented between the short-axis orientations of the reference layers, and then removing the orienting field <NUM> (as indicated with the dashed line field arrow), further annealing at the high temperature for PtMn providing pinning field, and the cooling the etched stack of MTJ sensors, thereby causing the magnetization of the etched reference layers <NUM>, <NUM>, <NUM> to be pinned along their respective short axes. As illustrated, the magnetization orientations of the etched reference layers <NUM>, <NUM>, <NUM> are pinned along the short axis of the etched reference layers. Thus, in the imbalanced SAF stack <NUM>, the reference layer magnetization <NUM> and pinned layer magnetization <NUM> are substantially anti-parallel to each other and orthogonal to the long axis of the etched reference layer <NUM>. Similarly, the reference layer magnetization <NUM> and pinned layer magnetization <NUM> in the imbalanced SAF stack <NUM> are substantially anti-parallel to each other and orthogonal to the long axis of the etched reference layer <NUM>, and likewise for the etched reference layer <NUM>. With the imbalanced SAF stack embodiment depicted in <FIG>, the long axis of reference layer <NUM> is patterned orthogonal to the direction used for a single reference layer <NUM> shown in <FIG> in order to provide a final reference direction that is midway between the orthogonal directions of reference layers <NUM> and <NUM>.

To further illustrate the resulting formation of multiple orientations in different, finally-formed reference layers, reference is now made to <FIG> which provides a cross-sectional view of the etched stack of MTJ sensor layers depicted in <FIG>. As depicted in <FIG>, the etched sensor layer stack <NUM>, <NUM>, <NUM> in the first sensor area has a reference layer that is pinned in a first pinning direction (e.g., "into" the plane of the drawing in <FIG>), the etched sensor layer stack <NUM>, <NUM>, <NUM> in the second sensor area has a reference layer that is pinned in a second pinning direction (e.g., to the "right" in the plane of the drawing in <FIG>), and the etched sensor layer stack <NUM>, <NUM>, <NUM> in the other sensor area has a reference layer that is pinned in yet another pinning direction (e.g., at <NUM> degrees from the plane of the drawing in <FIG>).

At this point in the fabrication process, each of the upper ferromagnetic or layer(s) <NUM>, <NUM>, <NUM> (and the tunnel barrier layer(s)) will have been selectively etched into the same shape as the underlying reference layer. However, where the final shape of the sense layers will be smaller than the underlying pinned layers, a second etch sequence may be used to define the final shapes of the different sense layers from the remnant portions of the upper ferromagnetic layer(s) <NUM>, <NUM>, <NUM>. The second etch sequence defines high aspect ratio shapes for the sense layers by using a patterned mask and etch process (e.g., reactive ion etching) to remove all unmasked layers down to and including the unmasked upper ferromagnetic layer(s), but leaving intact the underlying shaped pinning layers. The defined high aspect ratio shapes for the sense layers are oriented so that each sense layer has a shape anisotropy axis. In other words, the long axis for each sense layer is drawn to create the desired easy axis magnetization direction.

Briefly, a current becomes spin-polarized after the electrons pass through the first magnetic layer in a magnet/non-magnet/magnet trilayer structure, where the first magnetic layer is substantially fixed in its magnetic orientation by any one of a number of methods known in the art. When the spin-polarized electrons tunnel through the insulator layer , the probability of the electron tunneling depends on the relative orientation of the free layer and the fixed layer. When a magnetic field is applied to field sensitivity axis for the sensor shown in <FIG>, the free layer rotates toward along the magnetic field direction, the relative orientation of fixed and free layers is changed, and therefore the MTJ resistance is changed. When these sensors are biased, an output signal can be detected.

During fabrication of the MTJ array architecture <NUM>, each succeeding layer is deposited or otherwise formed in sequence and each MTJ device <NUM> may be defined by selective deposition, photolithography processing, etching, etc. using any of the techniques known in the semiconductor industry. Typically the layers of the MTJ are formed by thin-film deposition techniques such as physical vapor deposition, including magnetron sputtering and ion beam deposition, or thermal evaporation. During deposition of at least a portion of the MTJ, a magnetic field is provided to set a preferred anisotropy easy-axis into the material (induced intrinsic anisotropy). In addition, the MTJ stack is typically annealed at elevated temperature while exposed to a magnetic field directed along the preferred anisotropy easy-axis to further set the intrinsic anisotropy direction and to set the pinning direction when an antiferromagnetic pinning layer is used. The provided magnetic field creates a preferred anisotropy easy-axis for a magnetic moment vector in the ferromagnetic materials. In addition to intrinsic anisotropy, memory elements patterned into a shape having aspect ratio greater than one will have a shape anisotropy, and the combination of this shape and the intrinsic anisotropy define an easy axis that is preferably parallel to a long axis of the memory element.

The exemplary embodiments described herein may be fabricated using known lithographic processes as follows. The fabrication of integrated circuits, microelectronic devices, micro electro mechanical devices, microfluidic devices, and photonic devices involves the creation of several layers of materials that interact in some fashion. One or more of these layers may be patterned so various regions of the layer have different electrical or other characteristics, which may be interconnected within the layer or to other layers to create electrical components and circuits. These regions may be created by selectively introducing or removing various materials. The patterns that define such regions are often created by lithographic processes. For example, a layer of photo resist material is applied onto a layer overlying a wafer substrate. A photo mask (containing clear and opaque areas) is used to selectively expose this photo resist material by a form of radiation, such as ultraviolet light, electrons, or x-rays. Either the photo resist material exposed to the radiation, or that not exposed to the radiation, is removed by the application of a developer. An etch may then be applied to the layer not protected by the remaining resist, and when the resist is removed, the layer overlying the substrate is patterned. Alternatively, an additive process could also be used, e.g., building a structure using the photo resist as a template.

Although the described exemplary embodiments disclosed herein are directed to various sensor structures and methods for making same, the present invention is not necessarily limited to the exemplary embodiments which illustrate inventive aspects of the present invention that are applicable to a wide variety of semiconductor processes and/or devices. Thus, the particular embodiments disclosed above are illustrative only and should not be taken as limitations upon the present invention, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein.

However, the benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature or element of any or all the claims.

Claim 1:
A magnetoresistive thin-film magnetic field sensor (<NUM>, <NUM>), comprising:
an antiferromagnetic pinning layer (<NUM>);
a pinned layer (<NUM>) comprising:
an amorphous ferromagnetic layer (<NUM>) comprising CoFeB disposed over the pinning layer (<NUM>);
a first crystalline ferromagnetic layer (<NUM>) comprising CoFe disposed over the amorphous ferromagnetic layer (<NUM>); and
a second crystalline ferromagnetic layer (<NUM>) comprising CoFe disposed between the amorphous ferromagnetic layer (<NUM>) and the pinning layer (<NUM>);
a nonmagnetic coupling layer (<NUM>) disposed over the pinned layer (<NUM>);
a ferromagnetic fixed layer (<NUM>) disposed over the nonmagnetic coupling layer (<NUM>); and
a dielectric tunnel barrier layer (<NUM>) disposed over the ferromagnetic fixed layer (<NUM>);
a ferromagnetic sense layer (<NUM>) disposed above the dielectric tunnel barrier layer (<NUM>), wherein the ferromagnetic sense layer (<NUM>) comprises a magnetically soft material.