Abstract:
A fluid purifier includes an enclosure provided with an inlet and an outlet, and a plurality of different carbon based purifying media disposed within the enclosure which are capable of removing different contaminants. In a non-limiting embodiment, the purifier includes three carbon-based materials ordered according to a specific sequence. In an aspect of the non-limiting embodiment, the material closer to the purifier inlet may include acid-impregnated carbons, the centrally-located material may include base-impregnated carbons, and the material closer to the outlet may include activated carbons. In a further non-limiting example, a method for purifying gas includes passing a gas through a carbon based purifying media including a first carbon based purifying media which is capable of removing a first species of contaminants, a second carbon based purifying media which is capable of removing a second species of contaminants, and a third carbon based purifying media which is capable of removing a third species of contaminants.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application claims the benefit of U.S. Provisional Application No. 60/681,905, filed on May 16, 2005, and which is incorporated herein, in its entirety, by reference. 
     
    
     BACKGROUND  
       [0002]     The invention relates to fluid purifiers, and more particularly to gas purifiers. By way of non-limiting example, the invention relates to in-line gas purifiers used in the semiconductor industry for such purposes as removing contaminants from purging gasses.  
         [0003]     There are a number of reasons for providing purging gasses for the semiconductor and other industries. For example, in the semiconductor industry, the high-precision optics of photolithography machines (e.g. stepper machines) and the equally high-precision optics of wafer inspection machines use purging gasses to ensure, among other things, that the optics are immersed in an ultra-clean operating environment.  
         [0004]     One form of purging gas is referred to as Clean Dry Air (“CDA”). CDA is sometimes synthesized by mixing highly purified oxygen and nitrogen (“synthetic air”), typically in the same proportion that they are found in natural air. Other forms of purging gas include purified nitrogen. However, even CDA, synthetic air, and purified nitrogen have been known to carry contaminants which can build up or react with, for example, high precision optics.  
         [0005]     Lithographic processes are widely used for the production of integrated circuits for electronic devices. Lithographic processes are also useful in a variety of other applications well known to those of skill in the relevant arts. If the radiant energy source is visible or near visible light (e.g. ultraviolet or “UV” light), the process is often referred to as photolithography.  
         [0006]     Photolithography machines are typically found in clean rooms. As such, they are often exposed to other processing machines (e.g. etching machines, deposition machines, etc.) which can generate gaseous, liquid, and particulate contaminants in the clean room environment. Photolithography machines are made by such companies as Nikon, Canon, and others.  
         [0007]     The photolithographic process uses photo-sensitive chemicals (often referred to as “photoresist”) that, once exposed to a radiant energy source, change in chemical composition. Photoresist is typically coated onto a semiconductor wafer and cured before the semiconductor wafer is inserted into a photolithography machine. The photoresist may then be selectively exposed to the radiant energy source of the photolithography machine, e.g. through a mask, such that the exposed portions of the photoresist undergo a chemical transformation.  
         [0008]     Since the optics of the photolithography machine are in close proximity to the photoresist, there is a possibility of generating contaminants from the photoresist and elsewhere during the photolithography process. The optical components are delicate and may be damaged if exposed to impurities that are produced as by-products during the photolithography phase. In particular gaseous impurities can create deposits on the optics, causing aberrations in their transmission properties. Moreover, energy transfer from the radiant energy source to the deposits on the optics may eventually lead to irreversible damage of the optics. This is particularly problematic due to the high cost of high-precision optical assemblies, which can cost many hundreds of thousands of dollars.  
         [0009]     As mentioned previously, certain wafer inspection tools also use high-precision optics and employ the use of purging gas. For example, KLA Tencor makes wafer inspection equipment which use high-precision optics. Some wafer inspection equipment use UV or deep ultraviolet (DUV) light to enhance the sensitivity of the wafer inspection equipment. Other wafer inspection tools use other radiant energy sources. The optical components of the wafer inspection tools have contamination risks similar to optical components in lithographic processes.  
         [0010]     Three exemplary classes of compounds that are detrimental to the optical components are acids, bases, and hydrocarbons. Examples of impurities that fall into these exemplary classes are SO 2  and H 2 S (acids), NH 3  and ammines (bases), and toluene and decane (hydrocarbons). A large number of other substances, whether known or unknown, are categorizable into one or more of the classes by those of ordinary skill in the material and/or chemical sciences.  
         [0011]     It may be desirable to develop a technique for purifying a purging gas to prevent or reduce the likelihood that impurities reach optical components, either from the ambient environment or from the purging gas itself. Attempts to address this problem include compressed air purification and filtering.  
         [0012]     U.S. Pat. No. 5,607,647 describes an air filtering system for use in a clean environment made by a two-media sequence. The first media is carbon impregnated with sulfuric acid, which is effective to remove basic impurities. (As used herein, a “basic impurity” is an impurity that is a base, and an “acidic impurity” is an impurity that is an acid.) As a consequence of the basic impurities removal, a characteristic volatile compound is released and removed by a second media. Therefore the process described in this patent is a two-step removal process for a single class of contaminants, namely basic impurities. U.S. Pat. No. 5,626,820 describes a similar concept applied to clean room air purification.  
         [0013]     U.S. Pub. No. 20030159586 describes a two media purification method in which acidic contaminants are removed by a first media, while removal of other impurities is carried out by a second media. U.S. Pat. No. 6,645,898 describes a “synergistic” effect for compressed air purification obtained by employing a ternary composition made by an electropositive metal, a late transition metal, and a high silica zeolite.  
         [0014]     The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.  
       SUMMARY  
       [0015]     The following embodiments and aspects thereof are described and illustrated in conjunction with systems, tools, and methods that are meant to be exemplary and illustrative, not limiting in scope. In various embodiments, one or more of the above-described problems have been reduced or eliminated, while other embodiments are directed to other improvements.  
         [0016]     A technique for purifying compressed air of acidic, basic, and hydrocarbon contaminants may utilize carbon-based media. In an embodiment, a system constructed according to this technique may include a plurality of different carbon-based media. Each of the media may be effective to remove a specific class of contaminants that could be detrimental to, by way of example but not limitation, lithographic optics. In another embodiment, a method according to the technique includes purification of compressed air for other impurity-sensitive industrial processes.  
         [0017]     In a non-limiting example, an apparatus includes an inlet for receiving gas from a gas source; a first zone, in gaseous communication with the inlet, that includes acid-impregnated carbon, wherein in operation the gas from the gas source flows through the inlet to the first zone; a second zone, in continuous gaseous communication with the first zone, that includes base-impregnated carbon, wherein in operation the gas flows to the second zone; a third zone, in continuous gaseous communication with the second zone, that includes activated carbon, wherein in operation the gas flows to the third zone; and an outlet, in gaseous communication with the third zone, for expelling the gas.  
         [0018]     In another non-limiting example, an apparatus includes a gas inlet, wherein, in operation, gas flows through the gas inlet; a single-stage purifier structure operationally coupled to the gas inlet, the single-stage purifier structure including a plurality of zones, including a first zone for removing at least some of a first contaminant from the gas and a second zone for removing at least some hydrocarbon contaminants from the gas after removal of the first contaminant; a gas outlet operationally coupled to the single-stage purifier structure, wherein, in operation, gas flows through the gas outlet after the first contaminant and the hydrocarbon contaminants have been removed.  
         [0019]     In another non-limiting example, a single-stage gas purifier includes a first carbon medium effective to remove at least some of a first contaminant from a fluid; a second carbon medium effective to remove at least some of a second contaminant from a fluid; a third carbon medium effective to remove at least some of a third contaminant from a fluid.  
         [0020]     In another non-limiting example, a single-stage gas purifier structure includes an acid-impregnated carbon medium effective to remove at least some of a basic contaminant from a fluid; a medium of activated carbon effective to remove at least some of a hydrocarbon contaminant from the fluid.  
         [0021]     In another non-limiting example, a single-stage gas purifier includes a base-impregnated carbon medium effective to remove at least some of an acidic contaminant from a fluid; a medium of activated carbon effective to remove at least some of a hydrocarbon contaminant from the fluid.  
         [0022]     In another non-limiting example, a method for purifying a purging gas includes flowing gas into a single-stage purifier; removing basic impurities as the gas flows through a first zone; removing acidic impurities as the gas flows through a second zone; removing hydrocarbon impurities as the gas flows through a third zone; and flowing the gas out of the single stage purifier.  
         [0023]     In another non-limiting example, a fluid purifier includes an enclosure provided with an inlet and an outlet, and a plurality of different carbon based purifying media disposed within fluid enclosure which are capable of removing different contaminants. In one embodiment, the fluid includes a gas. In another embodiment, the fluid includes a liquid. In another embodiment, the fluid includes a gas and a liquid. In another embodiment, the fluid includes solid particulate matter.  
         [0024]     In a non-limiting example, a gas purifier includes an elongated enclosure provided with an inlet end and an outlet end, a first carbon based purifying material disposed proximate the inlet end, a second carbon based purifying material, and a third carbon based purifying material disposed proximate the outlet end, where the second carbon based purifying material is disposed between the first carbon based purifying material and the third carbon based purifying material. The first, second, and third materials may comprise, by way of example but not limitation, carbon materials capable of sorbing or trapping basic, acidic, and hydrocarbon contaminants.  
         [0025]     In a further non-limiting example, a method for purifying gas includes passing a gas through a carbon based purifying media including a first carbon based purifying media which is capable of removing a first species of contaminants, a second carbon based purifying media which is capable of removing a second species of contaminants, and a third carbon based purifying media which is capable of removing a third species of contaminants. By way of example but not limitation, the species are selected from the group consisting essentially of basic, acidic and hydrocarbon species.  
         [0026]     In addition to the exemplary aspects and embodiments described above, further aspects and embodiments, including combinations and subcombinations thereof, will become apparent by reference to the drawings and by study of the following description.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]     Embodiments of the invention are illustrated in the figures by way of non-limiting examples.  
         [0028]      FIG. 1  depicts an exemplary purifier;  
         [0029]      FIG. 2  depicts an exemplary flowchart of a method for purifying gas using a single-stage purifier;  
         [0030]      FIG. 3  depicts another exemplary purifier; and  
         [0031]      FIG. 4  depicts another exemplary purifier.  
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0032]      FIG. 1  depicts a purifier structure  100  according to an exemplary embodiment. It may be noted that the depiction of the purifier structure  100  in the example of  FIG. 1  is for illustrative purposes only. The same holds true for other examples presented herein. The depicted dimensions of various components are not intended to be to scale.  
         [0033]     In the example of  FIG. 1 , the purifier structure  100  has an enclosure  102 , a first partition  104  between a first zone  112  and a second zone  114 , and a second partition  106  between the second zone  114  and a third zone  116 . The purifier structure  100  includes an inlet  108  into the first zone  112  and an outlet  110  out of the third zone  116 . The first, second, and third zones respectively include first media  122 , second media  124 , and third media  126 .  
         [0034]     In certain embodiments, the media are all different. In other embodiments, the media are all the same. In still other embodiments, two of the media are the same. Other embodiments have other combinations.  
         [0035]     The enclosure  102  may include metal, or be generally metallic. Alternatively, the enclosure  102  may be made of any suitable non-reactive material. One of skill in the art of materials science would recognize that the enclosure  102  could be made of many different kinds of metallic or non-metallic materials. By way of non-limiting example, the enclosure can be made from stainless steel or Teflon. In other embodiments, separate cases housing separate materials can be abutted, connected together by pipes, a combination of the two, or otherwise.  
         [0036]     In the example of  FIG. 1 , the inlet  108  is represented by an arrow pointing toward the first zone  112 . The inlet  108  may be of any appropriate area, from pin-hole sized (or multiple pin-hole sized) to a large opening. As will be appreciated by those skilled in the art, a tube or flange (not shown) may be provided to help form an aperture into the container. Alternatively, a simple aperture into the first zone  112  may serve as the inlet  108 . Alternatively, the inlet  108  may be covered with a material that is permeable to certain gasses, or with a molecular sieve. In general, the inlet  108  should be designed such that the first media  122  (or any other media) is unlikely to leak or fall out through the inlet  108 , but that at least some gasses are able to pass through the inlet  108 . In other embodiments this is not an issue.  
         [0037]     In an embodiment that uses a compressed air gas supply, the inlet  108  may, for example, include pipes, valves, etc. for connecting the inlet to a compressed air gas supply. Such connections are well-known so they are not described in detail herein.  
         [0038]     In the example of  FIG. 1 , the purifier structure  100  includes three different types of media, the first media  122 , the second media  124 , and the third media  126 . The first media  122  is closer to the inlet  108  than the other media  124 ,  126 , while the third media  126  is closer to the outlet  110  than the other media  122 ,  124 . While they are shown to be separated by partitions  104  and  106  in this example, in other embodiments, partitions may or may not be used. In some embodiments, the media (e.g. two or more carbon based materials) may be incidentally, slightly, substantially, or completely intermixed.  
         [0039]     In an embodiment, the first media  122  includes acid-impregnated carbon-comprising material that is effective to remove basic impurities. A suitable media of this type is, by way of example but not limitation, Chemsorb-1425 sold by the C*CHEM Company of Lafayette, Colo., USA. Chemsorb-1425 includes about 59% activated carbon and less than about 30% phosphoric acid. This is but one non-limiting embodiment; Other proportions and materials are possible. By way of example but not limitation, the first media  122  may include carbon treated with phosphoric acid, citric acid, sulfuric acid, or just about any kind of acidic substance.  
         [0040]     In another embodiment, the second media  124  includes base-impregnated carbon-comprising material that is effective to remove acidic impurities. A suitable media of this type is, by way of example but not limitation, Chemsorb-1202 sold by the C*CHEM Company. Chemsorb-1202 includes less than about 3% potassium iodide, less than about 5% potassium hydroxide, and about 82% activated carbon. This is but one non-limiting embodiment; other proportions and materials are possible.  
         [0041]     In another embodiment, the third media  126  includes activated carbons that are effective to remove hydrocarbons. A suitable media of this type is, by way of example but not limitation, Chemsorb-1000 by C*CHEM Company. Chemsorb 1000 includes less than about 90% activated carbon. The third media  126  may have a higher or lower proportion of activated carbon to other components. The third media  126  may be slightly acidic, slightly basic, or neutral. Alternatively, the third media  126  is simply more acidic than the second media  124  and less acidic than the first media  122 .  
         [0042]     In an embodiment, the first media  122  is more acidic than the third media  126 , which is more acidic than the second media  124 . In an alternative embodiment, the first media  122  could be most basic and the second media  124  most acidic. It has been discovered that the third media  126 , on the other hand, yields the best technical results when placed last in order, closest to the outlet  110 . Despite this discovery, it is understood that the third media  126  may be at any position relative to the first media  122  and the second media  124  and fall within the scope of alternative embodiments.  
         [0043]     In various embodiments, the media  122 ,  124 ,  126  include carbon. Advantageously, it has been discovered that carbon media do not significantly alter the moisture content of the gas that is purified in the purifier structure  100 . Moreover, high moisture loads do not substantially impair the purifier capacity when other impurities are removed. Thus, the use of carbon media can result in improvements over other media, depending upon the parameters of a given application. In other applications, other materials may be desirable.  
         [0044]     For example, carbon media do not typically require heating during operation. As another example, carbon media do not substantially interact with oxygen and moisture. It may be noted that materials other than carbon that have one or more of the advantages of carbon could replace the carbon media in other embodiments.  
         [0045]     In the example of  FIG. 1 , the first, second, and third media  122 ,  124 ,  126  are respectively located within the first, second, and third zones  112 ,  114 ,  116 . In the example of  FIG. 1 , the zones  112 ,  114 ,  116  are physically separated by the partitions  104 ,  106 . The partitions  104 ,  106  may include a particle filter, a metallic mesh, or some other physical structure effective to retain the first, second, and third media  122 ,  124 ,  126  within their respective zones without blocking the passage of gas through the zones. That is, the partitions serve as gas permeable dividers. In certain applications, one or more of the partitions  104  and  106  are optional.  
         [0046]     Although in the example of  FIG. 1 , the compartments are physically separated by the partitions  104 ,  106 , in other embodiments, the compartments may or may not be physically separated by the partitions  104 ,  106 . By way of example but not limitation, the media  122 ,  124 ,  126  may be layered without physically partitioning the zones  112 ,  114 ,  116 . As another example, the partitions  104 ,  106  may be gaps (see, e.g.,  FIG. 3 , described later). As another example, the partitions  104 ,  106  may include both physical partitioning and gaps. As another example, the media  122 ,  124 ,  126  may be monolithic or sintered structures, obviating the need for a physical partition. As another example, one or more of the media  122 ,  124 ,  126  may be a monolithic structure that serves as a partition (see, e.g.,  FIG. 4 , described later). In alternative embodiments, so degree of intermixing may occur.  
         [0047]     In an embodiment, the volume ratio of the media  122 ,  124 ,  126  to one another may be adjusted according to application requirements using the following formula: 
 
 A+B+C= 1, where 
 
         [0048]     A represents the volume ratio of acid-impregnated carbon (“media A”);  
         [0049]     B represents the volume ratio of base-impregnated carbon (“media B”);  
         [0050]     C represents the volume ratio of activated carbon (“media C”);  
         [0051]     0.1≦A≦0.8;  
         [0052]     0.1≦B≦0.8; and  
         [0053]     0.1≦C≦0.8.  
         [0054]     In a specific embodiment, the volume ratio of media A is about 0.2, media B is about 0.1 and media C is about 0.7. In an even more specific embodiment, the volume ratio of media A is about 0.19, media B is about 0.10 and media C is about 0.71. These are set forth by way of non-limiting example of particular preferred embodiments, and other ratios are applicable for these and other purposes.  
         [0055]     In operation, the purifier structure  100  is coupled to a gas source (not shown), which provides a gas. The gas passes through the inlet  108 , the first zone  112 , the second zone  114 , the third zone  116 , and the outlet  110 . In an embodiment, the first media  122  removes basic impurities from the gas while the gas is passing through the first zone  112 ; the second media  124  removes acidic impurities from the gas while the gas is passing through the second zone  114 ; and the third media  126  removes hydrocarbon impurities from the gas while the gas is passing through the third zone  116 .  
         [0056]     In an embodiment, the gas is pressurized CDA (“compressed air”), created either from purified air or as synthetic air. In an alternative embodiment, the gas is nitrogen. Nitrogen is sometimes used as an alternative to compressed air for specific applications. Other gasses may be chosen depending upon the parameters of a given application, as would be understood by one of skill in the relevant art.  
         [0057]     In an embodiment, the pressure drop from the inlet  108  to the outlet  110  is relatively large. Filter panels typically have less of a pressure drop. In applications wherein a pressure drop is desired, the purifier structure  100  may be desirable option. The reason for the relatively large pressure drop is, at least in part, the purifier structure  100  is a single-stage gas purifier. The gas is not captured as in multi-stage gas purifiers. Single-stage gas purifiers may be more durable than multi-stage gas purifiers because there are fewer or no moving parts. Advantageously, the exemplary purifier structure  100  is effective to remove 3 different classes of impurities (acid, base, and hydrocarbon) in a single stage.  
         [0058]      FIG. 2  depicts a flowchart  200  of an exemplary method of one embodiment for removal of basic, acidic, and hydrocarbon contaminants from gas using a single-stage gas purifier. The single-stage gas purifier may include an inlet through which unpurified gas flows, zones within the single-stage gas purifier where contaminants are removed from the gas, and an outlet through which purified gas (e.g., gas that has had at least some contaminants removed) flows.  
         [0059]     The single stage gas purifier may or may not be physically divided into three zones. The three zones are logically identifiable (even if not physically divided) by the proportion of a given media in the zone. For example, the first zone includes more of a first media than a second or third media; the second zone includes more of the second media than the first or third media; and the third zone includes more of the third media than the first or second media. In other embodiments, two or more of the media may be mixed or intermingled.  
         [0060]     In an embodiment, basic contaminants are removed from the gas using acid-impregnated carbon-comprising material. In another embodiment, acidic contaminants are removed from the gas using base-impregnated carbon-comprising material. In another embodiment, hydrocarbons are removed from the gas using activated carbons.  
         [0061]     In an embodiment, the flowchart  200  starts at block  202  where gas flows into a single-stage gas purifier. The gas may flow into the single-stage gas purifier via an inlet. The gas may be in the form of, by way of example but not limitation, compressed air. In another embodiment, the gas may be nitrogen (with or without impurities). In one aspect, the gas may be a purging gas for use with, by way of example but not limitation, lithography or wafer inspection tools.  
         [0062]     In an embodiment, the flowchart  200  continues at block  204  where basic impurities are removed from the gas as the gas passes through a first zone. The basic impurities are removed using carbon material impregnated with acid. The carbon does not react significantly with moisture or oxygen that may be in the air.  
         [0063]     In an embodiment, the flowchart  200  continues at block  206  where acidic impurities are removed from the gas as the gas passes through a second zone. The acidic impurities are removed using carbon material impregnated with base. In an alternative embodiment, blocks  204  and  206  may be swapped such that acidic impurities are removed before basic impurities.  
         [0064]     In an embodiment, the flowchart  200  continues at block  208  where hydrocarbon impurities are removed from the gas as the gas passes through a third zone. The hydrocarbons are removed using activated carbon material. In an alternative embodiment, block  208  may be swapped such that hydrocarbons are removed before acidic or basic impurities. However, it has been found that removing hydrocarbons after blocks  204  and  206  is superior in certain applications.  
         [0065]     In an embodiment, the flowchart  200  ends when, at block  210 , the gas flows out of the single-stage gas purifier. The gas may flow out of the single-stage gas purifier via an outlet. It may be noted that the flow of gas through the single-stage gas purifier may be continuous over time. Thus, the activity described at each of the block  202  to  210  could occur simultaneously for different portions of the gas flow.  
         [0066]      FIG. 3  depicts another exemplary purifier structure  300  according to an embodiment. In the example of  FIG. 3 , the purifier structure  300  includes an enclosure  302  (here formed as a serpentine pipe), an inlet  308 , and an outlet  310 . In the example of  FIG. 3 , a “partition”  304  comprises a gap that separates the zones  312 ,  316  from one another. The zones  312 ,  316  respectively include media  322 ,  326 . The media  322  may be, by way of example but not limitation, acid-impregnated carbon effective to remove basic impurities from a fluid that passes through the zone  312  or base-impregnated carbon effective to remove acidic impurities from a fluid that passes through the zone  312 . The media  326  may be, by way of example but not limitation, activated carbon effective to remove hydrocarbon contamination from the fluid. In other embodiments, these media can be reversed or partially or wholly intermingled, by way of non-limiting example.  
         [0067]     In an alternative embodiment, the purifier structure  300  may include three or more zones with media that is effective to remove three or more different contaminants.  
         [0068]     It may be noted that the example of  FIG. 3  seems to depict relatively large air gaps both before and after the media. This is for illustrative purposes only. Indeed, it may be desirable to reduce the pressure drop that occurs when using a single-stage purifier by putting the purification media relatively close together.  
         [0069]      FIG. 4  depicts an alternative purifier structure  400  according to an embodiment. In the example of  FIG. 4 , the purifier structure  400  includes an enclosure  402 , an inlet  408 , and an outlet  410 . In the example of  FIG. 4 , a relatively thick porous partition  404  (compared to, for example, a screen or mesh partition) separates the zones  412 ,  416  from one another. The zones  412 ,  416  respectively include media  422 ,  426 . The media  422  may be, by way of example but not limitation, acid-impregnated carbon effective to remove basic impurities from a fluid that passes through the zone  412 . The media  426  may be, by way of example but not limitation, activated carbon effective to remove hydrocarbon contamination from the fluid. Other embodiments use alternatives to activated carbon.  
         [0070]     Advantageously, the porous partition  404  may serve as both a medium for removing impurities and as a partition between the media  422  and the media  426 . The porous partition  404  may be by way of example but not limitation a monolithic carbon structure impregnated with base, and effective to remove acidic impurities from a fluid that passes through the porous partition  404 . The monolithic carbon structure can be, by way of non-limiting examples, a honeycombed structure, sintered carbon pellets, or some other form that enables fluid to pass through the structure while removing acidic impurities from the fluid.  
         [0071]     In an alternative embodiment, the media  422  may be a base-impregnated carbon effective to remove acidic impurities from the fluid and the porous partition  404  may be an acid-impregnated carbon structure effective to remove basic impurities from the fluid.  
         [0072]     As used herein, the term “embodiment” means an embodiment that serves to illustrate by way of example but not limitation. As used herein, the term “alternative” is used to describe an embodiment that is not equivalent to another embodiment.  
         [0073]     While a number of exemplary aspects and embodiments have been discussed above, those of skill in the art will recognize certain modifications, permutations, additions, and sub-combinations thereof. It is therefore intended that any claims hereafter introduced based upon these descriptions and drawings are interpreted to include all such modifications, permutations, additions, and sub-combinations as are within their true spirit and scope.