Patent Publication Number: US-2019198751-A1

Title: Method of forming tunnel magnetoresistance (tmr) elements and tmr sensor element

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates generally to magnetic field sensors. More specifically, the present invention relates to tunnel magnetoresistance (TMR) sensor elements and robust TMR sensor element fabrication methodology. 
     BACKGROUND OF THE INVENTION 
     Magnetic field sensor systems are utilized in a variety of commercial, industrial, and automotive applications to measure magnetic fields for purposes of speed and direction sensing, angular sensing, proximity sensing, and the like. Magnetic field sensors may be based on semiconductor materials (e.g., Hall sensors, magnetoresistors, and so forth) and ferromagnetic materials (e.g., ferromagnetic magnetoresistors and flux guides). Other magnetic field sensors may utilize optical, resonant, and superconducting properties. 
     Tunnel magnetoresistance (TMR) sensor elements exploit a magnetoresistive effect that occurs in a magnetic tunnel junction (MTJ) structure. An MTJ structure includes a metal-insulator-metal layer sandwich in which the metal layers are ferromagnetic and the insulator layer is very thin. Electrically, this forms a tunnel diode in which electrons can tunnel from one ferromagnet into the other. Such a tunnel diode exhibits transport characteristics that depend, not only on the voltage bias, but also on the magnetic states of the top and bottom ferromagnetic layers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying figures in which like reference numerals refer to identical or functionally similar elements throughout the separate views, the figures are not necessarily drawn to scale, and which together with the detailed description below are incorporated in and form part of the specification, serve to further illustrate various embodiments and to explain various principles and advantages all in accordance with the present invention. 
         FIG. 1  shows a simplified side view of an example of a tunnel magnetoresistance (TMR) sensor element; 
         FIG. 2  shows a simplified side view of another example of a TMR sensor element; 
         FIG. 3  shows a side sectional view of a structure at an initial stage of processing in accordance with prior art methodology; 
         FIG. 4  shows a side sectional view of the structure of  FIG. 3  at a subsequent stage of processing in accordance with the prior art methodology; 
         FIG. 5  shows a side sectional view of the structure of  FIG. 4  at a subsequent stage of processing in accordance with the prior art methodology; 
         FIG. 6  shows a side sectional view of the structure of  FIG. 5  at the subsequent stage of processing shown in  FIG. 5  in accordance with the prior art methodology; 
         FIG. 7  shows a side sectional view of the structure of  FIG. 6  at a subsequent stage of processing in accordance with the prior art methodology; 
         FIG. 8  shows a side sectional view of a structure at an intermediate stage of processing in accordance with an embodiment; 
         FIG. 9  shows a side sectional view of the structure of  FIG. 8  at a subsequent stage of processing; 
         FIG. 10  shows a side sectional view of the structure of  FIG. 9  at a subsequent stage of processing; 
         FIG. 11  shows a side sectional view of the structure of  FIG. 10  at a subsequent stage of processing; and 
         FIG. 12  shows a flow chart of TMR element fabrication in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In overview, the present disclosure concerns tunnel magnetoresistance (TMR) sensor element fabrication methodology and TMR sensor elements fabricated utilizing the methodology. More particularly, the method of fabrication entails a nitride or oxide spacer technique to protect the magnetic tunnel junction (MTJ) structure during etching processes. The fabrication methodology may achieve a robust and high yield process that enables production of very different magnetic stacks without the addition of further process steps, for improved magnetic sensor performance, reliability, cost savings, and so forth. 
     The instant disclosure is provided to further explain in an enabling fashion the best modes, at the time of the application, of making and using various embodiments in accordance with the present invention. The disclosure is further offered to enhance an understanding and appreciation for the inventive principles and advantages thereof, rather than to limit in any manner the invention. The invention is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims as issued. 
     It should be understood that the use of relational terms, if any, such as first and second, top and bottom, and the like are used solely to distinguish one from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Furthermore, some of the figures may be illustrated using various shading and/or hatching to distinguish the different elements produced within the various structural layers. These different elements within the structural layers may be produced utilizing current and upcoming microfabrication techniques of depositing, patterning, etching, and so forth. Accordingly, although different shading and/or hatching is utilized in the illustrations, the different elements within the structural layers may be formed out of the same material. 
     Referring to  FIG. 1 ,  FIG. 1  shows a simplified side view of an example of a tunnel magnetoresistance (TMR) sensor element  20 . More particularly, TMR sensor element  20  is an MTJ structure that includes magnetic layers  22 ,  24  separated by an insulator layer, referred to herein as a tunnel junction  26 . An electrode  28  embedded within, for example, a dielectric material  30 , may be in electrical communication with magnetic layer  22 . Electrode  28  is referred to herein as a top electrode  28 . Another electrode, referred to herein as a bottom electrode  32 , may be in electrical communication with magnetic layer  24 . 
     In this example, magnetic layer  22  may be a free layer and magnetic layer  24  may be a reference layer with a reference magnetization. In some configurations, the reference magnetization of the reference magnetic layer, e.g., magnetic layer  24 , may be generated by a first antiferromagnetic layer (e.g., iridium-manganese or platinum-manganese thin film) referred to herein as a pinning layer  34 . Pinning layer  34  may be coupled to an artificial antiferromagnet that can include two layers  36 ,  38  (e.g., cobalt-iron thin films) separated by a ruthenium (Ru) film  40 . The first layer  36  is referred to herein as a pinned layer  36  and the second layer  38  is referred to herein as a fixed layer  38 . Fixed layer  38  may be fixed to have a reference magnetization  42 , M FIX , that is established by the properties of pinning layer  34  and pinned layer  36 . Free layer  22  is “free” to respond to, i.e., sense, the applied magnetic field (e.g., an external magnetic field) to provide a sense magnetization  44 , M FREE . 
     In general, TMR sensor element  20  has a variable resistance in the presence of an external magnetic field. At a fixed voltage, this resistance depends upon the resistance between electrodes  28 ,  32 . Since electrodes  28 ,  32  are electrically coupled with sense and reference layers  22 ,  24  respectively, the resistance between electrodes  28 ,  32  depends upon the alignment of the magnetic moments of the sense and reference layers  22 ,  24 . For example, when the magnetic moments of sense and reference layers  22 ,  24  are parallel (i.e., the vectors lie along parallel lines and point in the same direction), the resistance of the junction may be at its lowest. However, the resistance of the junction may be at its highest when the magnetic moments are anti-parallel (i.e., the vectors lie along parallel lines but point in the opposite direction). And in between, the resistance of the junction varies as the cosine of the angle between magnetic moments. One or more MTJ structures, such as magnetoresistance sensor element  20 , may be utilized for sensing an external magnetic field. 
       FIG. 2  shows a simplified side view of another example of a TMR sensor element  46 . In this example, the arrangement of magnetic layers  22 ,  24  is reversed. Accordingly, TMR sensor element  46  includes magnetic layers  22 ,  24  separated by junction  26 . However, top electrode  28  is in electrical communication with magnetic layer  24  and bottom electrode  32  is in electrical communication with magnetic layer  22 . 
     Again, magnetic layer  22  is the free layer that is “free” to respond to, i.e., sense, the applied magnetic field (e.g., an external magnetic field) to provide sense magnetization  44 , M FREE . Likewise, magnetic layer  24  is the reference layer having pinning layer  34 , pinned layer  36 , and fixed layer  38 , in which fixed layer  38  is fixed to have a reference magnetization  42 , M FIX , that is established by the properties of pinning layer  34  and pinned layer  36 . It should be understood that more variations are possible. These variations may include, but are not limited to, having pinning layers on top and bottom with different pinning strength, not having separate pinning layers for perpendicular magnetizations, or alternative arrangements of layers as suited for a particular sensor response. 
     The following  FIGS. 3-7  demonstrate prior art methodology for fabrication of TMR sensor elements, such as TMR sensor element  20  ( FIG. 1 ) and TMR sensor element  46  ( FIG. 2 ). As will be discussed below, prior art fabrication methodology can lead to poor product yield and/or poor magnetic performance. 
       FIG. 3  shows a side sectional view of a structure at an initial stage  48  of processing in accordance with prior art methodology. The structure shown in  FIG. 3  includes a substrate  50 , a bottom electrode layer  52  formed on substrate  50 , and a TMR stack  54  formed on bottom electrode layer  52 . In this example, TMR stack  54  includes a first magnetic layer  56  (which may be a free layer or a reference layer), a tunnel barrier layer  58 , and a second magnetic layer  60  (which may be the other of the free layer or the reference layer). A mask, which may be a hard mask  62 , has been deposited and patterned. 
       FIG. 4  shows a side sectional view of the structure of  FIG. 3  at a subsequent stage  64  of processing in accordance with the prior art methodology. In general, TMR stack  54  is suitably etched to form individual TMR sensor elements (e.g., TMR sensor elements  20  or  46 ). Following formation of hard mask  62 , prior art methodology typically entails ion beam etching to suitably etch first magnetic layer  56  and tunnel barrier layer  58 . Unfortunately, in the case of a small incident angle of an ion beam  66  (i.e., a beam angle that is close to or approximately 90° relative to the surface of TMR stack  54 , re-deposition of metallic particles  68  can occur in response to ion beam etching. These metallic particles  68  may be redeposited at sidewalls  70  of first magnetic layer  56  and tunnel barrier layer  58 . Since tunnel barrier layer  58  is an electrically isolating layer, the re-deposition of metallic particles  68  at sidewalls  70  can result in current flow (shorting) from first magnetic layer  56  to second magnetic layer  60  via the redeposited metallic particles  68 . Thus, in operation, current flow could bypass tunnel barrier layer  58  resulting in reduced TMR effect. 
       FIG. 5  shows a side sectional view of the structure of  FIG. 4  at a subsequent stage  72  of processing in accordance with the prior art methodology. At subsequent stage  72 , another ion beam etch process may be performed at a relatively high incident angle of ion beam  66  (e.g., approximately 30-60°) relative to the surface of TMR stack  54  in order to remove the redeposited metallic particles  68  ( FIG. 4 ). This enables minimization of the presence of redeposited metallic particles  68  at sidewalls  70  thereby reducing the incidence of shorting between first and second magnetic layers  56 ,  60 . 
     Referring now to  FIG. 6 ,  FIG. 6  shows a side sectional view of the structure of  FIG. 5  at the subsequent stage  72  of processing in accordance with the prior art methodology. Like  FIG. 5 ,  FIG. 6  shows the subsequent ion beam etch process performed at the relatively high incident beam angle of ion beam  66 . Per convention, multiple TMR sensor elements are likely to be fabricated on substrate  50 . Thus, the high incident angle of ion beam  66  can result in etch rate (throughput) reduction and non-uniform etching due to shadowing effects. That is, a non-homogeneous removal of second magnetic layer  60  may occur. 
       FIG. 7  shows a side sectional view of the structure of  FIG. 5  at a subsequent stage  74  of processing in accordance with the prior art methodology. The continued etching of second magnetic layer  60  at the relatively high incident beam angle of ion beam  66  increases the risk of re-deposition of metallic particles  68  (not shown) from second magnetic layer  60  and shorting between first and second magnetic layers  56 ,  60 . Additionally, bottom electrode layer  52  may be patterned non-uniformly by ion beam  66 . 
     To summarize,  FIG. 4  reveals that a small incident angle of ion beam  66  can result in the re-deposition of metallic particles  68  and shorting between the first and second magnetic layers  56 ,  60  resulting in an undesirably low yield of sensor elements. As shown in  FIG. 6 , in the case of high incident angle of ion beam  66  with etch stop following removal of tunnel barrier layer  58 , a higher TMR effect may be achieved at the expense of non-homogeneous remains of the bottom, second magnetic layer  60 . If second magnetic layer  60  is the reference layer  24 , including pinning layer  34  as shown in  FIG. 1 , this could result in a net non-zero magnetic moment of reference layer  24  and disturbance of the free layer  22 . Alternatively, if second magnetic layer  60  is the free layer  22 , as shown in  FIG. 2 , this results in a poorly controlled shape of free layer  22  leading to anisotropy variations which can deteriorate the accuracy of the resulting TMR sensor element. Still further and as shown in  FIG. 7 , complete etch of the bottom, second magnetic layer  60  and metal re-deposition at the tunnel junction can also result in an undesirably low yield. Moreover, a strongly non-uniform pattern of bottom electrode layer  52  may result in local thinning which increases the parasitic resistance which could result in a lower sensor output signal. Methodology, discussed below, alleviates the problems associated with prior art TMR sensor element fabrication techniques to increase yield, reduce manufacturing costs, and enhance magnetic sensor performance. 
     Referring now to  FIG. 8 ,  FIG. 8  shows a side sectional view of a structure at an intermediate stage  76  of processing in accordance with an embodiment. The structure shown in  FIG. 8  includes substrate  50 , bottom electrode layer  52  formed on substrate  50 , and TMR stack  54  formed on bottom electrode layer  52 . Thus, TMR stack  54  with bottom electrode layer  52  is formed on substrate  50 . Again, TMR stack  54  includes first magnetic layer  56  (which may be a free layer or a reference layer), tunnel barrier layer  58 , and second magnetic layer  60  (which may be the other of the free layer or the reference layer). A dielectric material has been deposited, patterned, and etched to form hard masks  62  (two shown). At intermediate stage  76 , a first etch process is performed with stopping at a top surface  78  of second magnetic layer  60 . 
     In some embodiments, substrate  50  may be an active silicon substrate (i.e., s semiconductor chip having implemented an integrated circuit). As such, the structures of the TMR sensor elements, including their contacting electrodes may be monolithically integrated on top of an active silicon substrate  50  in order to reduce packaging costs, to reduce the number of bond pads, and to enable easier and more accurate matching of the magnetic and electrical elements. An appropriate processing route for monolithic integration consists of performing the TMR processing operations described herein in a CMOS-backend process. 
     In an embodiment, the first etch process may be a first ion beam etching process. An incident angle  80  of ion beam  66  may be approximately 30-60° from perpendicular to a surface  82  of substrate  50  to minimize re-deposition of metallic particles  68  ( FIG. 4 ) at sidewalls  70 . Thus, the first ion beam etching process removes material portions of first magnetic layer  56  and tunnel barrier layer  58  of TMR stack  54 , largely leaving second magnetic layer  60  un-etched and therefore intact. 
       FIG. 9  shows a side sectional view of the structure of  FIG. 8  at a subsequent stage  84  of processing. At stage  84 , a protective layer  86  is deposited over TMR stack  54  and top surface  78  of second magnetic layer  60 . In this example, protective layer  86  is deposited at a thickness that is sufficient to fully encapsulate TMR stack  54  and hard masks  62 . At least one of silicon nitride, silane-based silicon oxide, and a tetraethylorthosilicate (TEOS) oxide material may be utilized to form protective layer  86 . 
       FIG. 10  shows a side sectional view of the structure of  FIG. 9  at a subsequent stage  88  of processing. At stage  88 , a second etch process is performed to partially remove protective layer  86 . The second etch process may be an anisotropic etch process. In response to the second etch process, a portion  92  of second magnetic layer  60  is exposed from protective layer  86  and spacers  90  are formed from a remaining portion of protective layer  86  that surrounds sidewalls  70  of first magnetic layer  56  and tunnel barrier layer  58 . More particularly, following the second etch process, due to the anisotropic etch process, protective layer  86  remains surrounding sidewalls  70  to produce spacers  90  that reside on top surface  78  of second magnetic layer  60  immediately adjacent to sidewalls  70  and extend vertically from top surface  78  to additionally surround hard masks  62 . 
     In an embodiment, the second etch process may be a blanket etch process utilizing, for example, a fluorine chemistry (e.g., CF 4 ) with etch stop on second magnetic layer  60  or end point detection (EPD) on second magnetic layer  60 . Alternatively, any suitable etch process may be performed to remove protective layer  86 , while producing spacers  90 . Still further, an etch process may be performed to remove material portions of protective layer  86  and second magnetic layer  60 , as will be discussed below in connection with  FIG. 11 . 
       FIG. 11  shows a side sectional view of the structure of  FIG. 10  at a subsequent stage  94  of processing. At stage  94 , portions  92  ( FIG. 10 ) of second magnetic layer  60  that were exposed from protective layer  86  are removed. The presence of spacers  90  enables the utilization of, for example, a third etch process. This third etch process may be a second ion beam etching process in which ion beam  66  is directed at an incident beam angle that is less than ten degrees from perpendicular to surface  82  of substrate  50  to completely remove portions  92  of second magnetic layer  60  that were exposed from protective layer  86  during the second etch process. Alternatively, the second etch process discussed in connection with  FIG. 10  may be performed to remove both protective layer  86  and portions  92  of second magnetic layer  60  that are not covered or otherwise protected by spacers  90 . The remaining second magnetic layer  60  residing under spacers  90  thus extends laterally relative to first magnetic layer  56  and tunnel barrier layer  58 . This extended portion of second magnetic layer  60  is referred to herein as a step region  96 , and step region  96  extends laterally relative to first magnet layer  56  and tunnel barrier layer  58  by a distance  98  defined by a lateral width  100  of spacer  90 . 
     During the etch process, spacers  90  protect first magnetic layer  56  and tunnel barrier layer  58  from re-deposition of metallic particles  68  ( FIG. 4 ) to avoid a low TMR effect that could potentially occur by shorting across the tunnel junction. Further, the second ion beam etching process at a small incident beam angle enables the uniform removal of portions  92  of second magnetic layer  60  since there are no shadowing effects. The prevention of shadowing effects minimizes thickness variations of bottom electrode layer  52  (discussed in connection with  FIGS. 6-7 ) and corresponding stress effects. Lateral width  100  of spacers  90  determines the enlargement of second magnetic layer  60  relative to first magnetic layer  56 . That is, distance  98  of step region  96  relative to first magnetic layer  56  and tunnel barrier layer  58  can be well-defined by lateral width  100  of protective layer  86  ( FIG. 9 ). For example, distance  98  for step region  96  of twenty nanometers with an accuracy of +/− five nanometers may be achieved for spacers  90  formed from a silicon nitride protective layer  86 . 
     Following removal of portions  92  of second magnetic layer  60 , TMR sensor elements  102  remain on surface  82  of substrate  50 . Each TMR sensor element  102  includes a bottom magnet  104  (which may be a free or reference layer) formed from second magnetic layer  60  and including step region  96 , a top magnet  106  (which may be the other of the free or reference layer) formed from first magnetic layer  56 , and a tunnel junction  108  interposed between top and bottom magnets  104 ,  106  formed from tunnel barrier layer  58 . Additionally, spacers  90  remain fully surrounding sidewalls  70  of top magnet  106  and tunnel junction  108 . Only two TMR sensor elements  102  are shown in  FIG. 11  for simplicity. However, it should be understood that a typical sensor layout consists of a multiplicity of TMR sensor elements  102  electrically connected on a substrate. 
     A fabrication technique that includes spacers  90  enables second magnetic layer  60  to be uniformly etched. This uniform etching may be crucial when, for example, bottom magnet  104  is the reference layer for TMR element  102 . As discussed in connection with  FIGS. 1 and 2 , the reference layer typically includes the pinned and fixed layers  36 ,  38 . The footprint of the pinned and fixed layers  36 ,  38  should be the same to avoid any remaining net magnetic moments which could lead to worse magnetic performance. In the case that bottom magnet  104  is the free layer for TMR element  102 , a fabrication technique that includes spacers  90  enables proper definition of the geometry of the free layer which can reduce the anisotropy variations that may result from using prior art processes. Further, due to the subsequent etching process after formation of spacers  90 , a wide variety of magnetic stacks may be etched uniformly thus enabling the fabrication of, for example, magnetic stacks with two antiferromagnets which may be beneficial for realizing speed sensors with proper cross- sensitivity using TMR technology. 
     Referring to  FIGS. 8-12 ,  FIG. 12  shows a flow chart of TMR element fabrication process  110  in accordance with an embodiment. TMR element fabrication process  110  summarizes the operations described in connection with  FIGS. 8-11 . Thus, at a block  112 , substrate  50  (e.g., wafer) having bottom electrode layer  52  and TMR stack  54  formed thereon (in which TMR stack  54  includes first magnetic layer  56 , tunnel barrier layer  58 , and second magnetic layer  60 ). Further, a suitable material (e.g., a dielectric) has been deposited and patterned to form hard masks  62 . At a block  114 , a first ion beam etching process is performed to remove material portions of the top first magnetic layer  56  and tunnel barrier layer  58  (see  FIG. 8 ) at incident angle  80  of ion beam  66  being in a range of 30-60 degrees. At a block  116 , protective layer  86  is deposited over TMR stack  54  (see  FIG. 9 ). 
     At a block  118 , a second etch process is performed to partially remove protective layer  86 . In particular, a blanket anisotropic etch process may be performed to form spacers  90  and remove portions of protective layer  86  so as to expose portions  92  of the bottom second magnetic layer  60  from protective layer  86  (see  FIG. 10 ). Again, the second etch process may utilize a fluorine-based chemistry. At a block  120 , the exposed portions  92  of the bottom second magnetic layer  60  are removed. Block  120  may be performed in concurrence with etching protective layer  86  at block  118 . Alternatively, block  120  may be performed by performing a second ion beam etching process to remove the exposed portions  92  of the bottom second magnetic layer  60 . This second ion beam etching process may entail directing ion beam  66  at an incident beam angle that is less than ten degrees from perpendicular to surface  82  of substrate  50  (see  FIG. 11 ) to form TMR sensor elements  102 . Ellipses follow block  120  to denote that additional operations may be performed thereafter. Additional operations may entail a separate mask layer and etching operation for patterning bottom electrode layer  52 , etching hard masks  62  to form vias and filling the vias with an electrically conductive material to form top electrodes, encapsulation after formation of TMR sensor elements  102 , wafer level testing, singulation, and the like. Thereafter, TMR element fabrication process  110  ends. 
     Embodiments described herein entail tunnel magnetoresistance (TMR) sensor element fabrication methodology and TMR sensor elements fabricated utilizing the methodology. An embodiment of a method comprises performing a first etch process on a substrate having a tunnel magnetoresistance (TMR) stack formed on the substrate, the first etch process removing material portions of a first magnetic layer and a tunnel barrier layer of the TMR stack and stopping at a top surface of a second magnetic layer of the TMR stack. The method further comprises depositing a protective layer over the TMR stack, performing a second etch process to remove the protective layer, wherein in response to the second etch process, a portion of the second magnetic layer is exposed from the protective layer and a spacer is formed from a remaining portion of the protective layer, the spacer surrounding sidewalls of the first magnetic layer and the tunnel junction, and removing the second magnetic layer exposed from the protective layer. 
     Another embodiment of a method comprises performing a first ion beam etching process on a substrate having a tunnel magnetoresistance (TMR) stack formed on the substrate, the first ion beam etching process removing material portions of a first magnetic layer and a tunnel barrier layer of the TMR stack and stopping at a top surface of a second magnetic layer of the TMR stack. The method further comprises depositing a protective layer over the TMR stack, performing a blanket etch process to remove the protective layer, wherein in response to the blanket etch process, a portion of the second magnetic layer is exposed from the protective layer and a spacer is formed from a remaining portion of the protective layer, the spacer surrounding sidewalls of the first magnetic layer and the tunnel junction, and performing a second ion beam etching process to remove the second magnetic layer exposed from the protective layer. 
     An embodiment of a tunnel magnetoresistance (TMR) sensor element comprises a bottom magnet formed on a surface of a substrate, a tunnel junction formed on said bottom magnet, a top magnet formed on said tunnel junction, and a spacer fully surrounding sidewalls of said top magnet and said tunnel junction. 
     Embodiments described herein include nitride or oxide spacer structure that protects the magnetic tunnel junction (MTJ) structure during etching processes. The spacer structure can alleviate problems associated with re-deposition of metallic particles and shorting between the magnetic layers. Additionally, the embodiments enable fast etching of the bottom magnetic layer with a small incident angle of an ion beam thereby increasing the throughput and reducing manufacturing costs. Further, robust TMR patterning is enabled with high yield (high TMR effect) and good magnetic sensor performance due to a well-defined shape of the bottom magnetic layer. As a consequence of the well-defined shape of the bottom magnetic layer, uniform programming of a pinning layer may be enabled on wafer level thereby reducing test costs. Further, the fabrication methodology may achieve a robust and high yield process that enables production of very different magnetic stacks without the addition of further process steps, for improved magnetic sensor performance, reliability, cost savings, and so forth. 
     This disclosure is intended to explain how to fashion and use various embodiments in accordance with the invention rather than to limit the true, intended, and fair scope and spirit thereof. The foregoing description is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications or variations are possible in light of the above teachings. The embodiment(s) was chosen and described to provide the best illustration of the principles of the invention and its practical application, and to enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims, as may be amended during the pendency of this application for patent, and all equivalents thereof, when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.