Abstract:
An enhanced Spindt-tip field emitter tip and a method for producing the enhanced Spindt-tip field emitter. A thin-film resistive heating element is positioned below the field emitter tip to allow for resistive heating of the tip in order to sharpen the tip and to remove adsorbed contaminants from the surface of the tip. Metal layers of the enhanced field emission device are separated by relatively thick dielectric bilayers, with the metal layers having increased thickness in the proximity of a cylindrical well in which the field emitter tip is deposited. Dielectric material is pulled back from the cylindrical aperture into which the field emitter tip is deposited in order to decrease buildup of conductive contaminants and the possibility of short circuits between metallic layers.

Description:
This application is a Divisional of U.S. application Ser. No. 09/972,430 filed on Oct. 5, 2001, now issued as U.S. Pat. No. 6,628,052. 
    
    
     TECHNICAL FIELD 
     The present invention is related to micro electron field emitter devices and, in particular, to enhanced Spindt tip emitters that may include a sharpening feature, an increased depth of dielectric layers between metal layers without concomitant increase in tip to aperture distances, and pull-back of dielectric surfaces from the emitter tip. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to design and manufacture of field emitter tips. A brief discussion of field emission and the principles of design and operation of field emitter tips is therefore first provided in the following paragraphs, with reference to FIG.  1 . 
     When a wire, filament, or rod of a metallic or semiconductor material is heated, electrons of the material may gain sufficient thermal energy to escape from the material into a vacuum surrounding the material. The electrons acquire sufficient thermal energy to overcome a potential energy barrier that physically constrains the electrons to quantum states localized within the material. The potential energy barrier that constrains electrons to a material can be significantly reduced by applying an electric field to the material. When the applied electric field is relatively strong, electrons may escape from the material by quantum mechanical tunneling through a lowered potential energy barrier. The greater the magnitude of the electrical field applied to the wire, filament, or rod, the greater the current density of emitted electrons perpendicular to the wire, filament, or rod. The magnitude of the electrical field is inversely related to the radius of curvature of the wire, filament, or rod. 
     FIG. 1 illustrates principles of design and operation of a field emitter tip. The field emitter tip  102  rises to a very sharp point  104  from a silicon-substrate cathode  106 , or electron source. A localized electric field is applied in the vicinity of the tip by a first anode  108 , or electron sink, having a disk-shaped aperture  110  above and around the point  104  of the field emitter tip  102 . A second cathode layer  112  is located above the first anode  108 , also with a disk-shaped aperture  114  aligned directly above the disk-shaped aperture  110  of the first anode layer  108 . This second cathode layer  112  acts as a lens, applying a repulsive electronic field to focus the emitted electrons into a narrow beam. The emitted electrons are accelerated towards a target anode  118 , impacting in a small region  120  of the target anode defined by the direction and width of the emitted electron beam  116 . Although FIG. 1 illustrates a single field emitter tip, field emitter tips are commonly micro-manufactured by microchip fabrication techniques as regular arrays, or grids, of field emitter tips. 
     Spindt tips are electron field emitter microdevices, such as the field emitter tip shown in FIG. 1, in which the conical emitter tip is deposited by sputter deposition of a suitable metal or metal alloy onto a substrate. The deposition is carried out following layering and patterning of the dielectric and metallic layers that form the extraction cathode layer and lensing anode layer ( 108  and  112  in FIG.  1 ). 
     Spindt tips are well known in the art, and techniques for fabricating Spindt tips have been developed by designers and manufacturers of field emission devices. However, current Spindt tip designs and fabrication techniques suffer from numerous recognized deficiencies. Current techniques lead to application of Spindt emitter tips relatively closely surrounded by a cylindrical well through the dielectric and metal layers perpendicular to the substrate on which the emitter tip is deposited. Undesirable electrostatic charges may build up on the dielectric surfaces of the well during Spindt tip operation. It is well known that the very fine points of field emitter tips may be contaminated with absorbed contaminants and/or deformed during usage, greatly effecting the current density of emitted electrons. Once fabricated, Spindt tips are notoriously difficult, or impossible, to sharpen and clean in order to restore optimal performance. Current fabrication techniques limit the width of dielectric layers separating metallic layers to approximately the height of the final Spindt tip, so that the point of the Spindt tip is positioned within or near the aperture of the electron extraction cathode, but because of the relatively strong electric fields employed to operate field emission devices, the maximum allowed width of the dielectric may be insufficient to completely prevent dielectric breakdown and shorts between positively and negatively charged metallic layers within the Spindt tip emission device. For these reasons, designers and manufacturers of Spindt tip field emitter tips have recognized the need for a design and manufacturing technique that avoids these recognized deficiencies. 
     SUMMARY OF THE INVENTION 
     One embodiment of the present invention is an enhanced electron field emitter Spindt tip with a built-in cleaning and sharpening feature, increased thickness of dielectric layers that increases the breakdown voltage threshold of the device, a greater distance between the field emitter tip and surrounding dielectric surfaces, and a method that allows for increased fabrication precision and that allows for economical and efficient addition of additional metallic layers that allow the direction of the electron beam emitted from the field emitter tip to be controlled. Additional fabrication precision is made possible by using two-layer dielectric bilayers within the device: a SiO 2  sublayer and a Si 3 N 4  surface layer that serves as a lateral oxide etch stop during etching of internal chambers. In the enhanced Spindt-tip device, the Si 3 N 4  surface layer also coats the dielectric portions of the walls of the cylindrical well in which the Spindt tip is deposited, and is pulled back from close proximity to the Spindt tip between the metallic layers. Pulling back the Si 3 N 4  surface layer prevents build-up of electrostatic charge during operation of the Spindt tip and allows for increasing thickness of the dielectric bilayer without, at the same time, increasing the distance between the point of the Spindt tip and the electron extraction anode aperture. A thin-film resistive heating layer is added to the surface of the substrate, between the base of the Spindt tip and the substrate surface. By passing current through the thin-film resistive heating element layer, the Spindt tip can be heated to high temperatures in order to both sharpen the tip and to remove contaminants adsorbed to the tip. Tip sharpening reduces the radius of the tip and correspondingly increases the current density of emitted electrons during operation. The method that represents one embodiment of the present invention for fabricating enhanced Spindt tips employs metal chemical-mechanical-planarization (“CMP”) in place of oxide CMP used in currently available methods to allow planarization of the metal layers and more precise control of the positioning of the point of the Spindt tip relative to the field extraction anode. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates principles of design and operation of a silicon-based field emitter tip. 
     FIG. 2A shows an initial substrate upon which one or more Spindt tips are fabricated in a cross-sectional view, and FIG. 2B shows the initial substrate in a perspective view. 
     FIG. 3A shows a cross-sectional view of the first step in enhanced field emitter tip fabrication, and FIG. 3B illustrates the first step in a perspective view. 
     FIGS. 4A-B show a first-metal interconnect on the surface of the substrate following the photolithographic etch step. 
     FIGS. 5A-B show the nascent field emitter tip following application of the thin-film resistive heating layer. 
     FIGS. 6A-B show the nascent field emitter tip following etching of the thin-film resistive heating layer. 
     FIGS. 7A-B illustrate the SiO 2  dielectric layer deposited over the thin-film resistive heating layer and substrate in cross-section and perspective, respectively. 
     FIGS. 8A-B show the cylindrical slot produced by the etching step. 
     FIGS. 9A-B illustrate the nascent field emitter device following deposition of the Si 3 N 4  layer above the SiO 2  layer in cross-section and perspective, respectively. 
     FIG. 10A illustrates the nascent field emission device following deposition and etching of the Si 3 N 4  layer in cross-section. 
     FIG. 10B illustrates the nascent field emission device following deposition of the second metal layer. 
     FIG. 11A illustrates deposition of the second SiO 2  layer. 
     FIG. 11B illustrates the nascent field emission device following patterning and etching of the second SiO 2  layer. 
     FIG. 12A shows the nascent field emission device following deposition of the second Si 3 N 4  layer. 
     FIG. 12B shows the nascent field emission device following patterning and etching of the second Si 3 N 4  layer. 
     FIG. 13A shows the nascent field emission device following deposition of the third metallic layer. 
     FIG. 13B shows the nascent field emission device following patterning and etching of the third metallic layer, the second oxide layer, the second metallic layer, and the first oxide layer to produce a final central, cylindrical well. 
     FIG. 14A shows the nascent field-emitter tip following this lateral etch. 
     FIG. 14B shows the final Spindt-tip field emitter tip. 
     FIG. 15 illustrates application of a next SiO 2  layer above the third metallic layer via TEOS deposition. 
     FIG. 16 shows a completed five-metal-layer field emission device produced by the above-described procedures. 
     FIG. 17 illustrates a computer display device based on field emitter tip arrays. 
     FIG. 18 illustrates an ultra-high density electromechanical memory based on a phase-change storage medium. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Several embodiments of the present invention are described below with reference to FIGS. 2-16. In FIGS. 2-9 both a cross-sectional view and a perspective view are shown of a region of a layered substrate that includes a nascent Spindt tip during the fabrication process. In FIGS. 10-16, only cross-sectional views are shown. These figures are not meant to imply particular dimensions or shapes of Spindt tip devices fabricated according to the method of the present invention. Instead, these figures are meant to illustrate the fabrication steps. The size and dimensions of particular Spindt-tip devices are controlled in the design of photolithographic patterning masks by controlling various parameters, including time, solution composition, ion fluxes, and other such parameters, during fabrication steps. Although the figures illustrate fabrication of a single Spindt-tip, the techniques are generally employed to simultaneously fabricate large numbers of Spindt-tips in arrays of field emitter tips. 
     FIG. 2A shows an initial substrate upon which one or more Spindt tips are fabricated in a cross-sectional view, and FIG. 2B shows the initial substrate in a perspective view. The initial substrate  202  may be an SiO 2  layer of a silicon wafer that may already include fabricated microelectronic devices and circuits. 
     FIG. 3A shows a cross-sectional view of the first step in enhanced field emitter tip fabrication, and FIG. 3B illustrates the first step in a perspective view. In the first step illustrated in FIGS. 3A-B, a first low-resistivity metallic layer  302  is deposited onto the initial substrate by any of a number of well-known metal deposition methodologies, including vacuum evaporation, physical vapor deposition (“PVD”), chemical vapor deposition (“CVD”), or low pressure chemical vapor deposition (“LPCVD”). In one embodiment, a Ti/TiN layer is deposited by an LPCVD technique to a thickness of approximately 0.15μ. 
     Next, a photoresist layer is applied to the first metal layer and patterned via well-known photolithography techniques. The first metal layer is then etched to produce eventual interconnects to each field emitter tip, and, when a tip heating feature is included as part of the field emitter tip design, a gap in the first-metal interconnect where the tip will be formed. FIGS. 4A-B show a first-metal interconnect on the surface of the substrate following the photolithographic etch step. The interconnect  402  remains after removal of most of the first metal layer ( 302  in FIG.  3 ). An interconnect gap  404  is shown, illustrating the fabrication technique used when a heating feature is included. 
     Next, in the case that a heating feature is included in the field emitter tip design, a thin-film resistive heating layer is applied to the surface of the interconnect and substrate. FIGS. 5A-B show the nascent field emitter tip following application of the thin-film resistive heating layer. The thin-film resistive heating layer  502  covers both the interconnect  402  and the exposed substrate  202  surface. After fabrication of the field emitter tip, current can be applied to the thin-film resistive heating layer in order to heat metallic field emitter tips fabricated on the surface of the resistive heating layer. The degree of heating necessary for tip sharpening and removal of contaminants varies with the material used for, and the size and shape of, the field emitter tip. In the case of a molybdenum field emitter tip, a temperature of approximately 400 C. may be necessary, while for a tungsten field emitter tip, a temperature of approximately 1400 C. may be necessary. Resistive heating of the field emitter tip can be applied during manufacture as well as periodically during use of the field emission device containing the resistive heating element. A sophisticated field emission device may include diagnostic logic to detect deterioration of electron current densities emitted by field emitter tips within the device, and to automatically apply resistive heating to tips operating at decreased performance levels. 
     In a next step, in the case that a heating feature is included in the field emitter tip design, the thin-film resistive heating layer is etched, via a photolithographic process, to expose the surface of the substrate not covered by the interconnect and outside the interconnect gap. FIGS. 6A-B show the nascent field emitter tip following etching of the thin-film resistive heating layer. Following the photolithographic process, the thin-film resistive heating layer  502  remains above the interconnect  402  and interconnect gap  404 . 
     Next, a SiO 2  dielectric layer is deposited on the nascent field emitter tip using tetraethyl orthosilicate (“TEOS”), Si(OC 2 H 5 ) 4 , in a plasma-enhanced chemical vapor deposition (“PECVD”) technique. FIGS. 7A-B illustrate the SiO 2  dielectric layer deposited over the thin-film resistive heating layer and substrate in cross-section and perspective, respectively. The deposited SiO 2  dielectric layer  702 , in one embodiment, is approximately 0.4μ in depth. 
     In the next step, a photoresist layer is applied to the SiO 2  dielectric layer and is patterned by photolithographic techniques to produce a ring-shaped area of exposed SiO 2 . This exposed ring is then etched via an anisotropic plasma etching method, or any of various other well-known anisotropic SiO 2  etch techniques to produce a cylindrical slot in the SiO 2  layer. FIGS. 8A-B show the cylindrical slot produced by the etching step. In one embodiment, the radial width of the cylindrical slot  802  produced by this anisotropic etch step is on the order of 0.3μ, and the cylindrical slot has a radius of approximately 1.5μ, so that the perpendicular axis of the Spindt field emitter tip to be fabricated on top of the initial substrate is 1.5μ from the walls of the cylindrical slot. 
     Next, a layer of Si 3 N 4  is deposited onto the SiO 2  dielectric layer in order to produce a first dielectric bilayer. FIGS. 9A-B illustrate the nascent field emitter device following deposition of the Si 3 N 4  layer above the SiO 2  layer in cross-section and perspective, respectively. The Si 3 N 4  layer  902  is, in one embodiment, deposited by an LPCVD technique in order to efficiently and completely fill the cylindrical slot produced in the previous anisotropic etching of the SiO 2  layer and because LPCVD technology produces an Si 3 N 4  layer with high breakdown voltage characteristics. In one embodiment, the Si 3 N 4  layer is deposited to a thickness of 0.15μ above the underlying SiO 2  layer, with the cylindrical slot  802  etched into the SiO 2  layer  702  completely filled with Si 3 N 4  as shown in FIGS. 9A-B. 
     Next, a photoresist layer is applied to the surface of the Si 3 N 4  layer and is patterned by well-known photolithographic techniques to enable etching of a cylindrical aperture centered above the perpendicular axis of the field emitter tip to be subsequently deposited. FIG. 10A illustrates the nascent field emission device following deposition and etching of the Si 3 N 4  layer in cross-section. In one embodiment, the cylindrical aperture  1002  etched into the Si 3 N 4  layer  902  has a radius of 1μ  1004 , significantly less than that of the cylindrical slot  802  etched into the underlying SiO 2  layer, now filled with Si 3 N 4 . 
     Next, a second metal layer is deposited on top of the Si 3 N 4  layer, filling the cylindrical aperture etched into the Si 3 N 4  layer in the previous step. FIG. 10B illustrates the nascent field emission device following deposition of the second metal layer. In one embodiment, the second metal layer  1006  is composed of Ti or TiN, deposited to a thickness of 0.4μ and is planarized via TiN chemical mechanical polishing (“CMP”) to a thickness of 0.3μ above the SiO 2  layer and 0.15μ above the Si 3 N 4  layer. The second metallic layer  1006  is considerably thicker in the region  1008  close to the axis  1010  of the field emitter tip than in the region  1012  above the first dielectric bilayer comprising the Si 3 N 4  layer  902  and the SiO 2  layer  702 . The Si 3 N 4  layer  902 , upon completion of the field emission device, will form vertical walls of a well following removal of a disk-like section of SiO 2    1014 . This vertical Si 3 N 4  surface is resistant to hydrofluoric acid etching of SiO 2  to open the internal chambers into which the field emitter tip is deposited, thus allowing for greater dimensional control over the sizes of the chambers etched between metallic layers. 
     Next, a second SiO 2  layer is deposited upon the second metallic layer via TEOS deposition, and this second SiO 2  layer is patterned and etched to create a second ring-like cylindrical slot identical, or similar to, the ring-like cylindrical slot  802  in the first SiO 2  layer  702 . The techniques to deposit and pattern the second SiO 2  layer  1102  are similar to those used to deposit and pattern the first SiO 2  layer, and will not be repeated in the interest of brevity. FIG. 1A illustrates deposition of the second SiO 2  layer. FIG. 11B illustrates the nascent field emission device following patterning and etching of the second SiO 2  layer. In FIG. 11B, the second ring-like cylindrical slot  1104  is aligned with the first ring-like cylindrical hole  802  in the first SiO 2  layer. 
     Next, a second Si 3 N 4  layer that comprises the top layer of a second dielectric bilayer is deposited on top of the second SiO 2  layer, and then is patterned and etched in the same fashion that the first Si 3 N 4  layer is deposited, patterned, and etched. FIG. 12A shows the nascent field emission device following deposition of the second Si 3 N 4  layer. FIG. 12B shows the nascent field emission device following patterning and etching of the second Si 3 N 4  layer. The second Si 3 N 4  layer  1202  is etched to produce a second cylindrical aperture  1204  aligned with the cylindrical aperture  1002  of the first Si 3 N 4  layer  902 . 
     Next, a third metallic layer is deposited on top of the second Si 3 N 4  layer and a portion of the underlying second SiO 2  layer, and is then patterned and etched to produce an aperture that will serve as the aperture of the lens cathode in the completed field emission device, shown as aperture  114  in FIG.  1 . FIG. 13A shows the nascent field emission device following deposition of the third metallic layer. The third metallic layer  1302 , like the second metallic layer  1006 , is thicker in the region close to the axis ( 1010  in FIG. 10B) of the field emitter tip than in the region above the second dielectric bilayer comprising the Si 3 N 4  layer  1202  and the SiO 2  layer  1102 . The third metallic layer is then patterned with photoresist, and an anisotropic etch is performed which etches sequentially the third metallic layer, the second oxide layer, the second metallic layer, and the first oxide layer. By etching the metallic layers in one etch step, one photomasking step is eliminated, and the metal patterns become self-aligned, thereby improving the relative alignment between the layers compared to what could be achieved with separate photomasking and etching steps. FIG. 13B shows the nascent field emission device following patterning and etching of the third metallic layer, the second oxide layer, the second metallic layer, and the first oxide layer to produce a final central, cylindrical well. The central, cylindrical well  1304  extends through to the thin-film resistive heating layer  502 . 
     In two final steps, a buffered oxide etch (“BOE”) employing a buffered hydrofluoric acid solution is used to laterally etch the SiO 2  layers back from the walls of the cylindrical well  1304 , created in the previous step, to the vertical Si 3 N 4  rings formed in the ring-like slots etched into the SiO 2  layers. FIG. 14A shows the nascent field emitter tip following this lateral etch. The lateral etch step removes the dielectric material from proximity to the field emitter tip, decreasing the chance of electrical shorts due to contamination of dielectric surfaces during operation of the field emission device and eliminating charge buildup on dielectric surfaces in the vicinity of the electron column. Note that, following the lateral etch, the walls of the central, cylindrical well  1304  comprise alternating rings of Si 3 N 4    1402 - 1405  and metal  1406 - 1409 . Then, in the final step for a three-metal-layer field device, a Spindt field emitter tip is deposited through the central aperture via sputter deposition to form the completed field emitter tip. FIG. 14B shows the final Spindt-tip field emitter tip. In one embodiment, the Spindt tip  1410  is composed of a molybdenum and nickel alloy, although molybdenum and tungsten can be used in two alternate embodiments. The conical shape of the field tip is produced by carefully controlling sputter deposition conditions. The Spindt tip  1410  is centrally positioned within the central, cylindrical well  1304  on top of the thin-film resistive heating layer  502 . 
     Additional dielectric and metallic layers can be added by repeating the SiO 2 , Si 3 N 4 , and metallic layer deposition and etching steps outlined above, following completion of the three-metal-layer device illustrated in FIG.  14 B. FIG. 15 illustrates application of a next SiO 2  layer above the third metallic layer via TEOS deposition. Note that TEOS deposition fills the aperture etched into the third metal layer  1502  and results in SiO 2  deposition along the edges  1504  of the aperture etched into the second metal layer as well as on the surface of the field emitter tip  1506 . Additional Si 3 N 4 , metallic, and SiO 2  layers can be added by the steps outlined above to produce a four-metal-layer field emission device or a five-metal-layer field emission device. FIG. 16 shows a completed five-metal-layer field emission device produced by the above-described procedures. Note that the SiO 2  deposits within the apertures and on the field emitter tip shown in FIG. 15 are removed during a final BOE wet etch. The five-metal-layer field emission device, the top two metal layers  1602 - 1604  may be used as orthogonal beam directing elements to steer the electron beam emitted by the field emitter tip to different positions on the target cathode ( 118  in FIG.  1 ). The fourth and fifth metal layers may be patterned with orthogonally arranged slots for electron deflection in two axes. 
     Silicon-based field emitter tips can be micro-manufactured by microchip fabrication techniques as regular arrays, or grids, of field emitter tips. Uses for arrays of field emitter tips include computer display devices. FIG. 17 illustrates a computer display device based on field emitter tip arrays. Arrays of silicon-based field emitter tips  1702  are embedded into emitters  1704  arrayed on the surface of a cathode base plate  1706  and are controlled, by selective application of voltage, to emit electrons which are accelerated towards a face plate anode  1708  coated with chemical phosphors. When the emitted electrons impact onto the phosphor, light is produced. In such applications, the individual silicon-based field emitter tips have tip radii on the order of hundreds of Angstroms and emit currents of approximately 10 nanoamperes per tip under applied electrical field strengths of around 50 Volts. 
     Silicon-based field emitter tips are also employed in various types of ultra-high density electronic data storage devices. FIG. 18 illustrates an ultra-high density electromechanical memory based on a phase-change storage medium. The ultra-high density electromechanical memory comprises an air-tight enclosure  1802  in which a silicon-based field emitter tip array  1804  is mounted, with the field emitter tips vertically oriented in FIG. 18, perpendicular to lower surface (obscured in FIG. 18) of the silicon-based field emitter tip array  1804 . A phase-change storage medium  1806  is positioned below the field emitter tip array, movably mounted to a micromover  1808  which is electronically controlled by externally generated signals to precisely position the phase-change storage medium  1806  with respect to the field emitter tip array  1804 . Small, regularly spaced regions of the surface of the phase-change storage medium  1806  represent binary bits of memory, with each of two different solid states, or phases, of the phase-change storage medium  1806  representing each of two different binary values. A relatively intense electron beam emitted from a field emitter tip can be used to briefly heat the area of the surface of the phase-change storage medium  1806  corresponding to a bit to melt the phase-change storage medium underlying the surface. The melted phase-change storage medium may be allowed to cool relatively slowly, by relatively gradually decreasing the intensity of the electron beam to form a crystalline phase, or may be quickly cooled, quenching the melted phase-change storage medium to produce an amorphous phase. The phase of a region of the surface of the phase-change storage medium can be electronically sensed by directing a relatively low intensity electron beam from the field emitter tip onto the region and measuring secondary electron emission or electron backscattering from the region, the degree of secondary electron emission or electron backscattering dependent on the phase of the phase-change storage medium within the region. A partial vacuum is maintained within the airtight enclosure  1802  so that gas molecules do not interfere with emitted electron beams. 
     Although the present invention has been described in terms of a particular embodiment, it is not intended that the invention be limited to this embodiment. Modifications within the spirit of the invention will be apparent to those skilled in the art. For example, as discussed above, Spindt-tip field emission devices can be produced with varying shapes, sizes, and geometries depending on the photolithography pattern masks employed in the various steps outlined above, ion-beam fluxes, and chemical solution and plasma compositions to which the various metallic, and dielectric layers are exposed during fabrication of a field emission device, as well as the times of exposure. A variety of different techniques can be employed for the anisotropic and isotropic etching steps as well as for layer deposition. A Spindt-tip field emitter device having arbitrary numbers of metallic layers interleaved with dielectric mono or bilayers can be produced by straightforward extensions of the above-described steps. 
     The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the invention. The foregoing descriptions of specific embodiments of the present invention are presented for purpose of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously many modifications and variations are possible in view of the above teachings. The embodiments are shown and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents: