PERPENDICULAR ANISOTROPY

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a nonprovisional of U.S. provisional patent

application serial number 61/680,579 filed on August 7, 2012, incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This invention was made with Government support under Grant No.

HR001 1 -10-C-0153 awarded by the Defense Advanced Research Projects Agency (DARPA). The Government has certain rights in this invention.

INCORPORATION-BY-REFERENCE OF MATERIAL

SUBMITTED ON A COMPACT DISC

Not Applicable

NOTICE OF MATERIAL SUBJECT TO COPYRIGHT PROTECTION

[0004] A portion of the material in this patent document is subject to

copyright protection under the copyright laws of the United States and of other countries. The owner of the copyright rights has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the United States Patent and Trademark Office publicly available file or records, but otherwise reserves all copyright rights whatsoever. The copyright owner does not hereby waive any of its rights to have this patent document maintained in secrecy, including without limitation its rights pursuant to 37 C.F.R. § 1 .14. BACKGROUND OF THE INVENTION

[0005] 1 . Field of the Invention

[0006] This invention pertains generally to magneto-resistance (MR)

sensors, and more particularly to magneto-resistance (MR) sensors which may incorporate spin valve or magnetic tunnel junction (MTJ) devices.

[0007] 2. Description of Related Art

[0008] Magneto-resistance (MR) sensors based on spin valve and magnetic tunnel junction (MTJ) devices have been widely used in applications such as magnetic field sensors, position and rotation sensors, as well as medical applications because of their large signal, low power consumption, and long endurance.

[0009] In magnetic sensors, the central issue is the linear and reversible response to the magnetic field. At present, one popular method is to align the magnetizations of a sensing layer and a reference layer to be initially perpendicular to each other in the plane of the film. This can be obtained by introducing an external or internal transverse bias field, or by utilizing shape anisotropy and step edge anisotropy. However, such designs complicate sensor design, and suffer from magnetization fluctuation and magnetic noise when reducing sensor size. This limits the scalability and integration for applications.

[0010] Another approach employs out-of-plane magnetized materials to achieve a linear and hysteresis-free response. For example, linear response and large linear field range have been obtained in sensors with a perpendicularly magnetized reference layer and in-plane sensing layer. A sensor with a high linearity up to applied field of 500 Oe has been demonstrated by using Pt/CoFe out-of-plane magnetized sensing layer. However, multilayer-based structures using transition metals such as Pt- and Pd- multilayer structures exhibit small MR due to their low spin polarization, further resulting in low sensitivity.

BRIEF SUMMARY OF THE INVENTION

[0011] An aspect of the present invention is a nanoscale linear magnetoresistance sensor with an in-plane magnetized reference layer and a sensing layer with interfacial perpendicular anisotropy. The sensor exhibits high TMR and a tunable free layer magnetization (e.g.

magnetization direction tunable to be in-plane, canted, or out-of-plane), which leads to high sensitivity and linearity. The sensor exhibits a large field sensitivity (demonstrated up to 0.02%/Oe) and a high linear field range (demonstrated up to 600Oe). It is appreciated that actual sensitivity and linear field range values may exceed those demonstrated.

[0012] The present invention incorporates CoFeB materials with

perpendicular magnetic anisotropy for MgO based devices such as spin valve and magnetic tunnel junction (MTJ) devices in magnetic field sensors including speed, position and rotation sensors (e.g. used for the automotive industry), hard disk drive read heads, medical applications, etc.

[0013] The sensors of the present invention exhibit high MR value (typically larger than -10%) and large interfacial perpendicular anisotropy (IPA) (typically larger than 0.2 mJ/m ² ). The utilization of CoFeB with large IPA for building sensors produces high sensitivity due to its high MR ratio when used in MgO based MTJ structures.

[0014] Another aspect is a nanoscale MTJ sensor with a CoFeB sensing layer that exhibits perpendicular anisotropy. A unique material structure and device design are used to realize a superior sensor with an out-of-plane magnetization free layer (or partially in-plane free layer with perpendicular anisotropy) and in-plane reference layer. Interfacial anisotropy is used in layered material structures based on dielectric oxides (e.g. MgO) and metallic ferromagnets (e.g. Fe-rich CoFeB such as Co ₄ oFe ₄ oB ₂ o or

Co2oFe6oB2o)- The use of the interfacial anisotropy allows for simultaneous realization of high perpendicular anisotropy (hence a very linear sensor response) and a high magnetoresistance ratio (hence a high sensitivity in the sensor), achieving overall superior performance to the state of the art.

[0015] Additionally, the interfacial anisotropy (and hence the sensor

response in terms of sensitivity and linearity) of the sensor of the present invention may be tuned by application of a DC bias voltage to the device, allowing for voltage tunability of the sensor response.

[0016] In another embodiment, a very high resistance tunnel structure (for example with a thick MgO > 1 nm) may be used to achieve a high- resistance sensor, which allows for ultralow power consumption due to small leakage currents.

[0017] In one aspect, the device of the present invention comprises a tunnel magnetoresistance (TMR) sensor with an in-plane magnetized reference layer and (partial or complete) out-of-plane magnetized free (sensing) layer, separated by a dielectric oxide (for example MgO).

[0018] In a preferred embodiment, the perpendicular anisotropy originates at the interface of the free layer with its neighboring layers, and is therefore sensitive to its thickness. The device exhibits high TMR (i.e. larger than -10%) to achieve high sensitivity. The device linearity is sensitive to the free layer thickness, and the sensor response (sensitivity and/or linearity) can be tuned by an applied voltage,

[0019] In one embodiment, the dielectric barrier may be made thick (e.g. > 1 nm) to allow only for a small leakage current, allowing for low-power operation and energy-efficient voltage-controlled tuning.

[0020] In another embodiment, the free (sensing) layer material may

comprise an Fe-rich metallic film such as Fe, or CoFe, or CoFeB.

[0021] In a further embodiment, the free layer may be additionally capped (or seeded) by an appropriate metal layer that may serve both as part of a measurement contact, as well as to promote the perpendicular anisotropy. Such metal layer may comprise Ta, Ru, Hf, Pt, Pd, Cu, or the like material.

[0022] In a further embodiment, the free layer may be additionally capped (or seeded) by an appropriate oxide layer that may serve both as part of a measurement contact, as well as to promote the perpendicular anisotropy or voltage control of the sensor response. Such oxide layer may comprise MgO, TaOx, HfOx, or the like material.

[0023] The TMR sensor of the present invention may be integrated as part of a circuit to sense its resistance as a function of applied magnetic field for various applications. It may also be included as part of a sensor array to achieve high sensitivity and directionality, or as part of one of a range of established resistance-based sensing circuits (such as a Wheatstone bridge).

[0024] Further aspects of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

[0025] The invention will be more fully understood by reference to the

following drawings which are for illustrative purposes only:

[0026] FIG. 1 is a schematic diagram of a layer structure and magnetic

configuration of a magnetoresistance sensor in accordance with the present invention.

[0027] FIG. 2 is a schematic diagram of the sensor of FIG. 1 with a capping layer.

[0028] FIG. 3A through FIG. 3E show plots of resistance as a function of the applied magnetic field for the sensor of FIG. 1 with different sensing layer thicknesses.

[0029] FIG. 4A shows a plot of the resistance as a function of applied

magnetic field curve for a sensor with a CoFeB sensing layer of t _C0 FeB =

1 .36 nm in the low magnetic field regime.

[0030] FIG. 4B shows the nonlinearity in the magnetoresistance for different

CoFeB sensing layer thicknesses. The light gray box in FIG. 4B illustrates the range where nonlinearity is less than 1 %.

[0031] FIG. 5A and FIG. 5B show plots of the field sensing range and

sensitivity, respectively, as a function of the CoFeB sensing layer thickness.

DETAILED DESCRIPTION OF THE INVENTION

[0032] FIG. 1 is a schematic diagram of a layer structure 10 and magnetic configuration of a magnetoresistance sensor in accordance with the present invention. The sensing structure primarily comprises a free (sensing) layer

12 separated from a reference layer 16 by a dielectric barrier 14. Dielectric barrier 14 preferably comprises a dielectric oxide (e.g. MgO).

[0033] In a preferred embodiment, the reference layer 16 is part of a

synthetic antiferromagnetic layer, composed of the reference layer 16, the pinned layer 20, and a spacer layer 18, where the spacer layer 18 preferably comprises a metal layer such as Ru.

[0034] In another embodiment, the reference layer 16 comprises a Co-rich layer (e.g. Co6oFe2oB2o or Co ₄ oFe ₄ oB ₂ o)- The magnetization of the Co-rich reference layer 16 is aligned parallel to the film plane (e.g. direction 32 in

FIG. 1 ).

[0035] In a further embodiment, the sensing (free) layer 12 comprises an Fe-rich layer such as Co2oFe6oB2o or Co ₄ oFe ₄ oB ₂ o- The sensing layer 12 comprises a thickness t _C0 FeB within the range of approximately 0.8 nm ~ 2.3 nm), to allow for tuning its perpendicular anisotropy by changing the thickness of the sensing layer. In some embodiments, depending on material selection or layer configuration, the thickness may range from 1 .30 nm to ~ 1 .85 nm. Below a certain transition thickness, the magnetization (i.e. magnetic moment) direction 30 of the sensing layer 12 is perpendicular to the film plane. In this scheme, there is no need for a sensor with a large aspect ratio or external bias field, which is necessary for in-plane sensors in order to achieve linear output.

[0036] The structure 10 shown in FIG. 1 is shown in use in a magnetic

tunnel junction configuration, with two ferromagnetic layers 16, 20 separated by a metallic spacer (e.g. coupling or spacer layer 18 comprising

Ru). Such a configuration comprises a ferromagnetic pinned layer 20 (e.g. comprising having magnetization direction 34 opposite to magnetization direction 32 of reference layer 16). An extra anti-ferromagnetic layer 22 (e.g. PtMn or IrMn) may be included to pin the orientation of the

ferromagnets. The magnetic field H to be sensed by the device is in plane for this device 10.

[0037] The dielectric barrier 14 may be made thick (e.g. > 1 nm) to allow only for a small leakage current, allowing for low-power operation and energy-efficient voltage-controlled tuning.

[0038] While the free (sensing) layer 12 comprises Co ₂ oFe ₆ oB2o or

Co ₄ oFe ₄ oB ₂ o in a preferred embodiment, it is contemplated that sensing layer 12 material may comprise other Fe-rich metallic films such as Fe, or

CoFe, or CoFeB.

[0039] It is also appreciated that other layer configurations from those

depicted in FIG. 1 and FIG. 2 are contemplated. For example, dielectric or MgO layer may be positioned above sensing layer 12. Furthermore, the device 10 may be fabricated in reverse order, with sensing layer 12 at the bottom and anti-ferromagnetic layer 22 at the top.

[0040] Layer configuration 10 provides better scalability, a simple structure and high sensitivity. Furthermore, the tunability of the perpendicular anisotropy of the CoFeB sensing layer 12 by adjusting its thickness allows one to easily design various magnetic sensors for specific applications.

[0041] Referring to the layer configuration 50 of FIG. 2, the free layer 12 may be additionally capped (or seeded) by an appropriate metal or dielectric layer 24 that may serve both as part of a measurement contact, as well as to promote the perpendicular anisotropy or voltage control of anisotropy. In one embodiment, layer 24 may comprise a metal such asTa,

Ru, Hf, Pt, Pd, or the like material. In an alternative embodiment, layer 24 may comprise an oxide layer, comprising e.g. MgO, TaOx, HfOx, or the like material.

[0042] Referring to the layer configurations 10 of FIG. 1 and 50 of FIG. 2, in both cases the perpendicular anisotropy of the CoFeB sensing layer 12 may be tuned by application of a voltage (i.e. electric field) across the dielectric barrier 14, allowing one to tune the magnetic sensor properties using an applied voltage.

[0043] Experimental Results

[0044] The magnetic multilayers were deposited on a Si/SiO2 substrate (not shown) in a physical vapor deposition (PVD) system. The completed stack was annealed for 2 hours at 300 °C in 1 T. Electron-beam lithography and ion milling were used to define and etch the MTJ layers resulting in circular nanopillars with nominal diameters of 60 nm, 80 nm, and 100 nm. The experimental results for the devices with the different diameters show similar behavior and all data shown below are for 80 nm circular

nanopillars. The magneto-transport measurements were performed by a G- S-G probe at room temperature.

[0045] FIG. 3A through FIG. 3E show plots of the resistance as a function of applied magnetic field by varying the sensing (free) layer 12 thickness (tcoFee)- For a device with t _C0 FeB = 1 -84 nm (shown in FIG. 3E), normal magnetoresistance (MR) behavior was observed with MR ratio of 82% [MR = (R _A p-Rp)/Rp, where R _A p and R _P are the resistances for an antiparallel and parallel magnetization configuration, respectively.] This suggests that the magnetization (i.e. magnetic moment) direction 30 of the CoFeB sensing layer 12 is in the plane of the films. The slight shift in the MR curve is presumably due to the dipolar coupling, as well as orange peel coupling, between the reference and the sensing layers (this can be eliminated by choosing a compensated synthetic antiferromagnet for layers 16, 18, and 20).

[0046] With decreasing sensing layer 12 thickness (tcoFee), the out-of-plane magnetization component of the sensing layer 12 continues to increase, and for t _C0 FeB = 1 -64 nm (shown in FIG. 3C), the linear response around zero field with hysteretic behavior is presented, indicating that the CoFeB sensing layer 12 exhibits a transition from in-plane magnetization to out-of- plane magnetization in zero-applied magnetic field.

[0047] Further decreasing the thickness of the CoFeB sensing layer 12 to tcoFeB = 1 -53 nm (shown in FIG. 3B), the sensor response becomes linear and hysteresis-free around zero-applied field, indicating a stable single domain perpendicular state.

[0048] At tcoFeB = 1 -36 nm (shown in FIG. 3A), the linearity extends to a

large applied field of more than 1000 Oe. The CoFeB sensing layer 12 thickness dependence of the MR loops correlates to perpendicular anisotropy (magnetic moment 30) at the sensing layer 12 (CoFeB)/dielectric oxide layer 14 (MgO) interface, which has been confirmed by vibrating sample magnetometry (VSM) measurement.

[0049] FIG. 4A shows the MR ratio as a function of the applied magnetic field for a sensor with sensing layer 12 thickness (t _C0 FeB) = 1 -36 nm in a range of fields of - 400 Oe < H < 400 Oe, which indicates that the magnetic field sensitivity is 0.017% MR/Oe. This value is significantly larger than that (0.0018% MR/Oe) previously reported for a sensor utilizing an out-of-plane magnetized sensing layer because of the significantly enhanced MR ratio in the case of the sensor 10 of the present invention.

[0050] The nonlinearity, defined as 100x[R(H)-R ₀ -r ₀ H]/R(H) , is plotted in

FIG. 4B, where R ₀ and r ₀ are coefficients obtained from linear regression of R(H) as a function of the applied magnetic field H. The nonlinearity depends strongly on the CoFeB sensing layer 12 thickness. To better understand the t _C0 FeB dependence of the field sensitivity and field sensing range, the data corresponding to the nonlinearity of smaller than 1 % were chosen for quantitative analysis. The linear field range increases while the field sensitivity decreases with decreasing t _C0 FeB, as shown in FIG. 5A.

[0051] For tcoFeB = 1 .64 nm, the field sensitivity is up to 0.036% MR/Oe, but the linear field range is limited to 150 Oe. This is, however, larger than the range reported for sensors (0.0047% MR/Oe) with super-paramagnetic

CoFeB layer with a similar field range. When the CoFeB sensing layer 12 thickness is less than 1 .53 nm, the field sensitivity only shows a small reduction (from 0.019 %MR/Oe at 1 .53 nm to 0.0165 %MR/Oe at 1 .36 nm).

[0052] The linear field range substantially increases from 450 to 650 Oe as the CoFeB sensing layer 12 decreases from 1 .53 to 1 .36 nm. This changed trend is similar to that reported in the art for spin valve sensors with out-of- plane Pt/CoFe sensing layer, but a higher field sensitivity can be obtained in the sensors according to this invention, due to the larger MR effect with MgO barrier 14 and high spin polarization CoFeB layer 12.

[0053] The following discussion details the underlying mechanisms of the free layer 12 thickness (t _C0 FeB) dependence of the field sensitivity and nonlinearity. For the sensor of the present invention, where the magnetization of the sensing free layer 12 is expected to coherently rotate toward the film plane, the resulting change in the sensor resistance is expressed as:

R(H) -R(0) where H _K = 2K /μοΜ ₈ , K = Κ, tcoFeB -Μ ₈ ² /μο, Mo is permeability in free space, Ms is the saturation magnetization, and K, is the interfacial anisotropy. Thus for small rotation angles, the field sensitivity can be expressed as: [0054] From Eqn. 2 the field sensitivity is dependent on the MR ratio and H _k . However, for the sensor configuration 10 of the present invention, the MR ratio does not change significantly, while H _k increases strongly as the CoFeB sensing layer thickness is reduced. Therefore, H _k is dominant in the tcoFeB dependence of the field sensitivity, as shown in the FIG. 4B. As for nonlinearity, it can be seen from Eqn. 1 that the nonlinearity increases as the external magnetic field and MR increase, while it decreases as H _k increases, i.e., the nonlinearity decreases with the decrease of t _C0 FeB as shown in FIG. 4B.

[0055] Optimizing the field sensitivity, field sensing range, linearity and

reversibility (i.e. critical parameters for magnetic field sensors), our results show that, the devices with t _C0 FeB≤ 1 -60 nm are promising for magnetic field sensors. In addition, the ability of tuning the sensor performance by changing the sensing layer thickness and applying voltages across the device presents a powerful method to design magnetic sensors for specific applications.

[0056] It should be appreciated that detailed values of MR, interfacial

anisotropy, and its thickness and voltage dependence vary greatly with the deposition and annealing conditions of the films. Hence, the specific numbers in this description are given for illustration and should not be understood to limit the scope of this invention.

[0057] In summary, nano-scale MTJ sensors were fabricated with an in- plane magnetized reference layer and a perpendicular anisotropy sensing layer and the effect of thickness of the sensing layer on the magnetic sensor performance was systematically investigated. A sensitivity value of around 0.02% MR/Oe in the nano-scale MTJ was obtained with a sensing layer thickness smaller than 1 .60 nm.

[0058] Furthermore, the field sensing range can be controlled merely by changing the thickness of the CoFeB sensing layer 12, or by application of a voltage across the device (more precisely across the dielectric barrier 14), which allows the MTJs to be designed for various magnetic field sensors. In addition, the nano-scale size and simple structure of the sensors of the present invention make them easy to integrate with complementary metal- oxide-semiconductor technology for nano-scale low-power-consumption sensors.

[0059] From the discussion above it will be appreciated that the invention can be embodied in various ways, including the following:

[0060] 1 . A magneto-resistance (MR) sensor, comprising: an in-plane

magnetized reference layer; and a free sensing layer comprising interfacial perpendicular anisotropy.

[0061] 2. A sensor as in any of the previous embodiments, wherein the

sensing layer is tunable to vary the direction of the sensing layer

magnetization.

[0062] 3. A sensor as in any of the previous embodiments, wherein the

magnetization direction is tunable to be in-plane, canted, or out-of-plane to modify sensor sensitivity and linearity.

[0063] 4. A sensor as in any of the previous embodiments, wherein the

interfacial anisotropy of the sensing layer may be tuned by varying the thickness of the sensing layer.

[0064] 5. A sensor as in any of the previous embodiments, wherein the thickness of the sensing layer ranges from approximately 1 .30 nm to 1 .85 nm.

[0065] 6. A sensor as in any of the previous embodiments, wherein the interfacial anisotropy of the sensing layer is configured to be tuned by application of a bias voltage to the sensor.

[0066] 7. A sensor as in any of the previous embodiments, wherein the reference layer comprises a synthetic anti-ferromagnetic layer.

[0067] 8. A sensor as in any of the previous embodiments, wherein the reference layer comprises a Co-rich layer.

[0068] 9. A sensor as in any of the previous embodiments, wherein the reference layer comprises CoFeB.

[0069] 10. A sensor as in any of the previous embodiments, wherein the sensing layer comprises an Fe-rich metallic film.

[0070] 1 1 . A sensor as in any of the previous embodiments, wherein the sensing layer comprises CoFeB.

[0071] 12. A sensor as in any of the previous embodiments, wherein the sensing layer and reference layer are separated by a dielectric barrier.

[0072] 13. A sensor as in any of the previous embodiments, wherein the dielectric barrier comprises MgO.

[0073] 14. A sensor as in any of the previous embodiments, wherein the dielectric barrier has a thickness greater than 1 nm to form a high resistance tunnel structure.

[0074] 15. A sensor as in any of the previous embodiments, wherein the free layer is capped by a metal layer to promote the perpendicular anisotropy or the voltage control of perpendicular anisotropy.

[0075] 16. A sensor as in any of the previous embodiments, wherein the sensor consists of a device comprising a spin valve or magnetic tunnel junction.

[0076] 17. A nanoscale linear magneto-resistance (MR) structure,

comprising: an in-plane magnetized reference layer; a free layer comprising interfacial perpendicular anisotropy; and a dielectric barrier separating the sensing layer and reference layer; wherein the free layer comprises a sensing layer for sensing resistance as a function of applied magnetic field; and wherein the sensing layer is tunable to vary the direction of the sensing layer magnetization.

[0077] 18. A structure as in any of the previous embodiments, wherein the magnetization direction is tunable to be in-plane, canted, or out-of-plane to modify the sensitivity and linearity of the structure.

[0078] 19. A structure as in any of the previous embodiments, wherein the interfacial anisotropy of the sensing layer may be tuned by varying the thickness of the sensing layer.

[0079] 20. A structure as in any of the previous embodiments, wherein the thickness of the sensing layer ranges from approximately 1 .30 nm to 1 .85 nm.

[0080] 21 . A structure as in any of the previous embodiments, wherein the interfacial anisotropy of the sensing layer is configured to be tuned by application of a DC bias voltage to the structure.

[0081] 22. A structure as in any of the previous embodiments, wherein the reference layer comprises a synthetic anti-ferromagnetic layer.

[0082] 23. A structure as in any of the previous embodiments, wherein the reference layer comprises Co-rich layer.

[0083] 24. A structure as in any of the previous embodiments, wherein the reference layer comprises Co6oFe2oB2o- [0084] 25. A structure as in any of the previous embodiments, wherein the sensing layer comprises an Fe-rich metallic film.

[0085] 26. A structure as in any of the previous embodiments, wherein the sensing layer comprises Co2oFe6oB2o-

[0086] 27. A structure as in any of the previous embodiments, wherein the dielectric barrier comprises MgO.

[0087] 28. A structure as in any of the previous embodiments, wherein the dielectric barrier has a thickness greater than 1 nm to form a high resistance tunnel structure.

[0088] 29. A structure as in any of the previous embodiments, wherein the free layer is capped by a metal layer to promote the perpendicular anisotropy or voltage control of perpendicular anisotropy.

[0089] 30. A structure as in any of the previous embodiments, wherein the structure consists of a sensor for use with a device comprising a spin valve or magnetic tunnel junction.

[0090] 31 . A nanoscale tunnel magneto-resistance (TMR) sensor,

comprising: an in-plane-magnetized, Co-rich reference layer; an Fe-rich free layer comprising interfacial perpendicular anisotropy; and a dielectric barrier separating the sensing layer and reference layer; wherein the free layer comprises a sensing layer for sensing resistance as a function of applied magnetic field; and wherein the sensing layer is tunable to vary the direction of the sensing layer magnetization to be in-plane, canted, or out- of-plane.

[0091] 32. A sensor as in any of the previous embodiments, wherein the interfacial anisotropy of the sensing layer may be tuned by varying the thickness of the sensing layer.

[0092] 33. A sensor as in any of the previous embodiments, wherein the thickness of the sensing layer ranges from approximately 1 .30 nm to 1 .85 nm.

[0093] 34. A sensor as in any of the previous embodiments, wherein the interfacial anisotropy of the sensing layer is configured to be tuned by application of a bias voltage to the structure.

[0094] 35. A sensor as in any of the previous embodiments, wherein the dielectric barrier comprises a dielectric oxide.

[0095] Although the description above contains many details, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. Therefore, it will be appreciated that the scope of the present invention fully encompasses other embodiments which may become obvious to those skilled in the art, and that the scope of the present invention is accordingly to be limited by nothing other than the appended claims, in which reference to an element in the singular is not intended to mean "one and only one" unless explicitly so stated, but rather "one or more." All structural, chemical, and functional equivalents to the elements of the above-described preferred embodiment that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Moreover, it is not necessary for a device or method to address each and every problem sought to be solved by the present invention, for it to be encompassed by the present claims. Furthermore, no element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element herein is to be construed under the provisions of 35 U.S.C. 1 12, sixth paragraph, unless the element is expressly recited using the phrase "means for."