Patent Publication Number: US-7909428-B2

Title: Fluid ejection devices and methods of fabrication

Description:
BACKGROUND OF THE INVENTION 
     Inkjet printing technology is used in many commercial products such as computer printers, graphics plotters, copiers, and facsimile machines. One type of inkjet printing, known as “drop on demand,” employs one or more inkjet pens that eject drops of ink onto a print medium such as a sheet of paper. Printing fluids other than ink, such as preconditioners and fixers, can also be utilized. The pen or pens are typically mounted to a movable carriage that traverses back-and-forth across the print medium. As the pens are moved repeatedly across the print medium, they are activated under command of a controller to eject drops of printing fluid at appropriate times. With proper selection and timing of the drops, the desired pattern is obtained on the print medium. 
     An inkjet pen generally includes at least one fluid ejection device, commonly referred to as a printhead, which has a plurality of orifices or nozzles through which the drops of printing fluid are ejected. Adjacent to each nozzle is a firing chamber that contains the printing fluid to be ejected through the nozzle. Ejection of a fluid drop through a nozzle may be accomplished using any suitable ejection mechanism, such as thermal bubble or piezoelectric pressure wave to name a few. Printing fluid is delivered to the firing chambers from a fluid supply to refill the chamber after each ejection. 
     To increase print quality and functionality, it is desirable to be able to eject printing fluid of different drop weights from a single printhead. This can be accomplished by designing some of the nozzles in a printhead to eject lower weight drops and other nozzles to eject higher weight drops. However, the different configurations used for the low drop weight nozzles and the high drop weight nozzles make it difficult to optimize overall nozzle performance. For example, the ability to provide adequate refill speeds for the high drop weight nozzles can be compromised by the ability to generate sufficient drop velocity for the low drop weight nozzles, and vice versa. Accordingly, dual drop weight range on a single printhead die is limited by an inherent tradeoff between refill speed and drop velocity. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an inkjet pen. 
         FIG. 2  is a perspective view of an inkjet printhead. 
         FIG. 3  is a cross-sectional view of the printhead taken along line  3 - 3  of  FIG. 2 . 
         FIGS. 4-8  are cross-sectional views illustrating the steps of a first embodiment of fabricating a printhead. 
         FIGS. 9-11  are cross-sectional views illustrating the steps of a second embodiment of fabricating a printhead. 
         FIGS. 12 and 13  are cross-sectional views illustrating the steps of a third embodiment of fabricating a printhead. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Representative embodiments of the present invention include a fluid ejection device in the form of a printhead used in inkjet printing. However, it should be noted that the present invention is not limited to inkjet printheads and can be embodied in other fluid ejection devices used in a wide range of applications. 
     Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  shows an illustrative inkjet pen  10  having a printhead  12 . The pen  10  includes a body  14  that generally contains a printing fluid supply. As used herein, the term “printing fluid” refers to any fluid used in a printing process, including but not limited to inks, preconditioners, fixers, etc. The printing fluid supply can comprise a fluid reservoir wholly contained within the pen body  14  or, alternatively, can comprise a chamber inside the pen body  14  that is fluidly coupled to one or more off-axis fluid reservoirs (not shown). The printhead  12  is mounted on an outer surface of the pen body  14  in fluid communication with the printing fluid supply. The printhead  12  ejects drops of printing fluid through a plurality of nozzles  16  formed therein. Although only a relatively small number of nozzles  16  is shown in  FIG. 1 , the printhead  12  may have two or more columns with more than one hundred nozzles per column, as is common in the printhead art. Appropriate electrical connectors  18  (such as a tape automated bonding, “flex tape”) are provided for transmitting signals to and from the printhead  12 . 
     Referring to  FIGS. 2 and 3 , the printhead  12  includes a substrate  20 , a thin film stack  22  disposed on top of the substrate  20 , and a fluidic layer assembly  24  disposed on top of the thin film stack  22 . At least one ink feed hole  26  is formed in the substrate  20 , and the nozzles  16  are arranged around the ink feed hole  26 . The nozzles  16  are formed in the fluidic layer assembly  24  and comprise a group of low drop weight nozzles  16   a  and a group of high drop weight nozzles  16   b . In the illustrated embodiment, the low drop weight nozzles  16   a  are arranged in a first column on a first side of the ink feed hole  26  (the left side in  FIG. 3 ), and the high drop weight nozzles  16   b  are arranged in a second column on a second side of the ink feed hole  26  (the right side in  FIG. 3 ). 
     Associated with each nozzle  16   a ,  16   b  is a firing chamber  28 , a feed channel  30  establishing fluid communication between the ink feed hole  26  and the firing chamber  28 , and a fluid ejector  32  which functions to eject drops of printing fluid through the nozzle  16   a ,  16   b . In the illustrated embodiment, the fluid ejectors  32  are resistors or similar heating elements. It should be noted that while thermally active resistors are described here by way of example only, the present invention could include other types of fluid ejectors such as piezoelectric actuators. The nozzles  16   a ,  16   b , the firing chambers  28 , the feed channels  30  and the ink feed hole  26  are formed in the fluidic layer assembly  24 , which is fabricated as multiple layers (as described below). The resistors  32  are contained within the thin film stack  22  that is disposed on top of the substrate  20 . As is known in the art, the thin film stack  22  can generally include an oxide layer, an electrically conductive layer, a resistive layer, a passivation layer, and a cavitation layer or sub-combinations thereof. Although  FIGS. 2 and 3  depict one common printhead configuration, namely, two rows of nozzles about a common ink feed hole, other configurations may also be formed in the practice of the present invention. 
     The fluidic layer assembly  24  has a first side  34  that faces the substrate  20  and a second side  36  that faces away from the substrate  20 . In the illustrated embodiment, the second side  36  is non-planar or stepped. In this case, the fluidic layer assembly  24  includes a step or raised portion  38  formed on the second side  36 , such that the fluidic layer assembly  24  comprises the raised portion  38 , which is relatively thick, and a thinner base portion  40 . 
     The low drop weight nozzles  16   a  are formed in the base portion  40 , and the high drop weight nozzles  16   b  are formed in the raised portion  38 . The high drop weight nozzles  16   b  have larger cross-sectional areas than the low drop weight nozzles  16   a  to provide larger drop weights. Furthermore, because the raised portion  38  is thicker than the base portion  40 , the high drop weight nozzles  16   b  are longer or deeper than the low drop weight nozzles  16   a . As shown in  FIG. 3 , the nozzles  16   a ,  16   b  have a substantially vertical bore profile. That is, the walls of the nozzle bores are substantially perpendicular to the first and second sides  34  and  36 . The nozzles  16   a ,  16   b  can alternatively have a tapered bore profile. If the nozzles have tapered bore profile, this will preferably be in the form of a convergent taper in which the nozzle opening is larger on the first side  34  than the second side  36 . 
     To eject a droplet from one of the nozzles  16   a ,  16   b , printing fluid is introduced into the associated firing chamber  28  from the ink feed hole  26  (which is in fluid communication with the printing fluid supply (not shown)) via the associated channel  30 . The associated resistor  32  is activated with a pulse of electrical current. The resulting heat from the resistor  32  is sufficient to form a vapor bubble in the firing chamber  28 , thereby forcing a droplet through the nozzle  16   a ,  16   b . The firing chamber  28  is refilled after each droplet ejection with printing fluid from the ink feed hole  26  via the feed channel  30 . 
     By virtue of being longer and having a larger cross-sectional area, the high drop weight nozzles  16   b  are able to eject larger droplets without compromising refill speed or drop velocity. Similarly, the low drop weight nozzles  16   a  can eject smaller droplets without sacrificing refill speed or drop velocity because they are shorter and have a smaller cross-sectional area. Accordingly, the printhead  12  provides excellent dual drop weight range on a single printhead die. 
     Referring to  FIGS. 4-8 , one process for fabricating an inkjet printhead  12  is described. The process starts with a substrate  20 , which is typically a single crystalline or polycrystalline silicon wafer. Other possible substrate materials include gallium arsenide, glass, silica, ceramics, or a semiconducting material. The substrate  20  has a first planar surface  42  and a second planar surface  44 , opposite the first surface. The thin film stack  22  is formed or deposited on the first surface  42  of the substrate  20  in any suitable manner, many such techniques being well known in the art. As mentioned above, the thin film stack  22  contains the fluid ejectors  32  and typically includes some or all of an oxide layer, an electrically conductive layer, a resistive layer, a passivation layer, and a cavitation layer. 
     Next, the fluidic layer assembly  24 , which will ultimately define the nozzles  16   a ,  16   b , the firing chambers  28  and the feed channels  30 , is formed on top of the thin film stack  22 . In the embodiment of  FIGS. 4-8 , the fluidic layer assembly  24  is fabricated in three layers: a chamber layer, a first bore layer and a second bore layer. These three layers are formed of any suitable photoimagable materials. One such suitable material is a photopolymerizable epoxy resin known generally in the trade as SU8, which is available from several sources including MicroChem Corporation of Newton, Mass. SU8 is a negative photoresist material, meaning the material is normally soluble in developing solution but becomes insoluble in developing solutions after exposure to electromagnetic radiation, such as ultraviolet radiation. All three layers can be made from the same material, or one or more of the layers can be made of different photoimagable materials. By way of example, this embodiment is described with all three layers comprising a negative photoresist material. However, it should be noted that positive photoresists could alternatively be used. In this case, the mask patterns used in the photoimaging steps would be reversed. 
     Fabrication of the fluidic layer assembly  24  begins by applying a layer of a photoresist material to a desired depth over the thin film stack  22  to provide a chamber layer  46 , as shown in  FIG. 4 . The chamber layer  46  is then imaged by exposing selected portions to electromagnetic radiation through a first mask  48 , which masks the areas of the chamber layer  46  that are to be subsequently removed and does not mask the areas that are to remain. Because the chamber layer  46  is a negative photoresist material (by way of example), the portions subjected to radiation undergo polymeric cross-linking, which is depicted in the drawings with double hatching, and become insoluble. In the illustrated embodiment, the area of the chamber layer  46  that will be removed is an area in the center of the chamber layer  46  that corresponds to the firing chambers  28  and the feed channels  30 . 
     After the light exposure, the chamber layer  46  is developed to remove the unexposed chamber layer material and leave the exposed, cross-linked material. This creates a developed area or void  50 , as seen in  FIG. 5 . The void  50  resulting from the removed chamber layer material will eventually form the firing chambers  28  and the feed channels  30 . The chamber layer  46  can be developed using any suitable developing technique which includes, for example, using an appropriate agent or developing solution such as propylene glycol monomethyl ether acetate (PGMEA) or ethyl lactate. 
     Referring to  FIG. 6 , a sacrificial fill material  52  is applied so as to fill the void  50 . The fill material  52  is then planarized, such as through a resist etch back (REB) process or a chemical mechanical polishing (CMP) process. This planarization process removes any excess fill material to bring the fill material  52  in the void  50  flush with the upper surface of the chamber layer  46 . Next, another layer of a photoresist material is applied to a desired depth on the upper surface of the chamber layer  46  to provide a first bore layer  54 . The fill material  52  keeps first bore layer material out of the void  50 . The first bore layer  54  is possibly, although not necessarily, made of the same material as the chamber layer  46 . 
     The first bore layer  54  is then imaged by exposing selected portions to electromagnetic radiation through a second mask  56 , which masks the areas of the first bore layer  54  that are to be subsequently removed and does not mask the areas that are to remain. The areas of the first bore layer  54  that are to be removed are a series of relatively small regions of unexposed, soluble material that will become the nozzles  16   a ,  16   b . In the illustrated embodiment, this comprises a series of first regions  58   a  (only one shown in  FIG. 6 ) that will become the low drop weight nozzles  16   a  and a series of second regions  58   a  (only one shown in  FIG. 6 ) that will become a lower portion of the high drop weight nozzles  16   b . The first and second regions  58   a ,  58   b  are aligned with corresponding fluid ejectors  32 . The second mask  56  can be patterned such that the first regions  58   a  will be smaller in cross-sectional area than the second regions  58   b , so that the high drop weight nozzles  16   b  will have larger cross-sectional areas than the low drop weight nozzles  16   a . For example, the first regions  58   a  can be sized to be 13 microns in diameter, while the second regions  58   b  can be sized to be 20 microns in diameter. 
     The exposure is carried out at a predetermined focus offset (i.e., the difference between the nominal focal length of the photoimaging system and the relative positioning of the wafer) that provides a desired profile for the regions  58   a ,  58   b  and thus a desired bore profile for the nozzles  16   a ,  16   b . In the illustrated example, exposure is performed at a relatively high focus offset (e.g., about 7-15 microns) to provide a convergent profile. The first bore layer  54  is typically not developed at this point in the process. 
     Turning to  FIG. 7 , another layer of photoresist material is applied to a desired depth on top of the first bore layer  54  to provide a second bore layer  60 . The second bore layer  60  is possibly, although not necessarily, made of the same material as the chamber layer  46  and/or the first bore layer  54 . The second bore layer  60  is then imaged by exposing selected portions to electromagnetic radiation through a third mask  62 , which masks the areas of the second bore layer  60  that are to be removed and does not mask the areas that are to remain. The areas of the second bore layer  60  that are to be removed include a series of third regions of unexposed, soluble material  58   c , wherein each third region  58   c  is aligned with, and located above, a corresponding one of the second regions  58   b  in the first bore layer  54 . The third regions  58   c  are sized similarly to the second regions  58   b  and are formed with a similar convergent profile. 
     The second bore layer  60  includes a larger region  64  that surrounds the third regions  58   c  and is subjected to the electromagnetic radiation so as to undergo polymeric cross-linking and become insoluble in developing solutions. The region  64 , which is not subsequently removed, becomes the raised portion  38  of the fluidic layer assembly  24 . The region  64  typically extends the entire length of the second bore layer  60  and has a width that is substantially equal to the desired width of the raised portion, which could be 150 microns, for example, or could be as large as half the die or more. The portions of the second bore layer  60  lying outside of the region  64  are additional areas to be removed and are thus not exposed to electromagnetic radiation. 
     After the first and second bore layers  54  and  60  have been exposed, they are jointly developed (again using any suitable developing technique), to remove the unexposed, soluble bore layer material and leave the exposed, insoluble material, as shown in  FIG. 8 . This results in the fluidic layer assembly  24  collectively made up by the chamber layer  46 , the first bore layer  54 , and the second bore layer  60  wherein the remaining portion of the first bore layer  54  makes up the base portion  40  and the remaining portion of the second bore layer  60  defines the raised portion  38 . The raised portion  38  is thus formed on the second side  36 , with the low drop weight nozzles  16   a  being formed in the base portion  40  and the high drop weight nozzles  16   b  being formed in the raised portion  38 . In addition, the fill material  52  filling the void  50  in the chamber layer  46  is also removed, leaving a substantially closed space defining the firing chambers  28  and the feed channels  30  that are in fluid communication with the nozzles  16   a ,  16   b . The ink feed hole  26  is then formed in the substrate  20  using any suitable technique, including wet etching, dry etching, deep reactive ion etching (DRIE), laser machining, and the like. 
     Turning now to  FIGS. 9-11 , another process for fabricating an inkjet printhead  12  is described. The initial steps for preparing the substrate  20 , the thin film stack  22 , and the chamber layer  46  (including the void  50  and the fill material  52 ) are essentially the same as described above and, as such, are not repeated here. As in the first embodiment, the layers comprising the fluidic layer assembly  24  can be formed of any suitable photoimagable materials. By way of example, the layers in this embodiment will also be described as comprising a negative photoresist material, although positive photoresists could alternatively be used. 
     Once the chamber layer  46  has been applied and processed, a layer of photoresist material is applied to a desired depth on the upper surface of the chamber layer  46  to provide a first bore layer  54 , as shown in  FIG. 9 . The fill material  52  again keeps first bore layer material out of the void  50  in the chamber layer  46 . The first bore layer  54  is possibly, although not necessarily, made of the same material as the chamber layer  46 . 
     The first bore layer  54  is then imaged by exposing selected portions to electromagnetic radiation through a fourth mask  66 , which masks certain areas of the first bore layer  54  and does not mask the remaining areas. The areas that are not masked, and are thus exposed to radiation, undergo polymeric cross-linking and become insoluble in developing solutions. In this exposure, the entire left side (as seen in  FIG. 9 ) of the first bore layer  54  is exposed except for a first series of relatively small regions of soluble material  58   a  (only one shown in  FIG. 9 ) that will become the low drop weight nozzles  16   a . In the illustrated embodiment, the first regions  58   a  are aligned with corresponding fluid ejectors  32  and are formed using a suitable focus offset to provide convergent profiles. The right side of the first bore layer  54  is not exposed at this time. 
     Referring to  FIG. 10 , the first bore layer  54  is further imaged by exposing selected portions to electromagnetic radiation through a fifth mask  68 , which masks certain other areas of the first bore layer  54  and does not mask the remaining areas. In this exposure, the entire right side of the first bore layer  54  that was not previously exposed is exposed except for a second series of relatively small regions of soluble material  58   b  (only one shown in  FIG. 10 ) that will become the high drop weight nozzles  16   b . In the illustrated embodiment, the second regions  58   b  are aligned with corresponding fluid ejectors  32  and are formed with a low focus offset (e.g., about 4 microns or less) to create a divergent profile. This will prevent any mixing of the fill material  52  and the unexposed first bore layer material. 
     The fourth and fifth masks  66  and  68  can be patterned such that the first regions  58   a  will be smaller than the second regions  58   b , so that the high drop weight nozzles  16   b  will have larger cross-sectional areas than the low drop weight nozzles  16   a . For example, the first regions  58   a  can be sized to be 13 microns in diameter, while the second regions  58   b  can be sized to be 20 microns in diameter. The first bore layer  54  is typically not developed at this point in the process. 
     Referring to  FIG. 11 , another layer of photoresist material is applied to a desired depth on top of the first bore layer  54  to provide a second bore layer  60 . The second bore layer  60  is possibly, although not necessarily, made of the same material as the chamber layer  46  and/or the first bore layer  54 . The second bore layer  60  is then imaged by exposing selected portions to electromagnetic radiation through a sixth mask  70 , which masks the areas of the second bore layer  60  that are to be removed and does not mask the areas that are to remain. Selected portions of the first bore layer  54  are also cross-linked by this exposure, thus reducing the amount of soluble material in the second regions  58   b . The areas of the second bore layer  60  that are to be removed include a series of third regions of soluble material  58   c , wherein each third region  58   c  is aligned over a corresponding one of the second regions  58   b  in the first bore layer  54 . The third regions  58   c  are formed using a focus offset that provides a convergent profile. 
     The second bore layer  60  includes a larger region  64  that surrounds the third regions  58   c  and is subjected to the electromagnetic radiation so as to undergo polymeric cross-linking and become insoluble in developing solutions. The region  64 , which is not subsequently removed, becomes the raised portion  38  of the fluidic layer assembly  24 . The region  64  typically extends the entire length of the second bore layer  60  and has a width that is substantially equal to the desired width of the raised portion, which could be 150 microns for example. The portions of the second bore layer  60  lying outside of the region  64  are additional areas to be removed and are thus not exposed to electromagnetic radiation. 
     After the first and second bore layers  54  and  60  have been exposed, they are jointly developed (again using any suitable developing technique), to remove the unexposed, soluble bore layer material and leave the exposed, insoluble material. This results in the fluidic layer assembly  24  (collectively made up by the chamber layer  46 , the first bore layer  54 , and the second bore layer  60 ) having the raised portion  38  formed on the second side  36 , with the low drop weight nozzles  16   a  formed in the base portion  40  and the high drop weight nozzles  16   b  formed in the raised portion  38 . In addition, the fill material  52  filling the void  50  in the chamber layer  46  is also removed, leaving a substantially closed space defining the firing chambers  28  and the feed channels  30  that are in fluid communication with the nozzles  16   a ,  16   b . The ink feed hole  26  is then formed in the substrate  20  using any suitable technique, including wet etching, dry etching, deep reactive ion etching (DRIE), laser machining, and the like. 
     Turning now to  FIGS. 12 and 13 , yet another process for fabricating an inkjet printhead  12  is described. Again, the initial steps for preparing the substrate  20 , the thin film stack  22 , and the chamber layer  46  (including the void  50  and the fill material  52 ) are essentially the same as described above and, as such, are not repeated here. As in the first two described embodiments, the layers comprising the fluidic layer assembly  24  can be formed of any suitable photoimagable materials. By way of example, the layers in this embodiment will also be described as comprising a negative photoresist material, although positive photoresists could alternatively be used. 
     Once the chamber layer  46  has been applied and processed, a layer of photoresist material is applied to a desired depth on the upper surface of the chamber layer  46  to provide a first bore layer  54 , as shown in  FIG. 12 . The fill material  52  again keeps first bore layer material out of the void  50  in the chamber layer  46 . The first bore layer  54  is possibly, although not necessarily, made of the same material as the chamber layer  46 . 
     The first bore layer  54  is then imaged by exposing selected portions to electromagnetic radiation through a seventh mask  72 , which masks certain areas of the first bore layer  54  and does not mask the remaining areas. The areas that are not masked, and are thus exposed to radiation, undergo polymeric cross-linking and become insoluble in developing solutions. In this exposure, the entire left side of the first bore layer  54  (as seen in  FIG. 12 ) is exposed except for a first series of relatively small regions of soluble material  58   a  (only one shown in  FIG. 12 ) that will become the low drop weight nozzles  16   a . In the illustrated embodiment, the first regions  58   a  are aligned with corresponding fluid ejectors  32 . The right side of the first bore layer  54  is not exposed at this time. 
     Referring to  FIG. 13 , another layer of photoresist material is applied to a desired depth on top of the first bore layer  54  (before developing the first bore layer  54 ) to provide a second bore layer  60 . The second bore layer  60  is possibly, although not necessarily, made of the same material as the chamber layer  46  and/or the first bore layer  54 . The second bore layer  60  is then imaged by exposing selected portions to electromagnetic radiation through an eighth mask  74 , which masks the areas of the second bore layer  60  that are to be subsequently removed and does not mask the areas that are to remain. This exposure step also exposes certain areas in the portion on the right side of the first bore layer  54  that were not previously exposed. The areas of the first and second bore layers  54  and  60  that are to be removed include a second series of relatively small regions of soluble material  58   b  in the first bore layer  54  and a third series of relatively small regions of soluble material  58   c  in the second bore layer  60  (only one of each shown in  FIG. 13 ) that will become the high drop weight nozzles  16   b . Accordingly, between the two exposures, the entire first bore layer  54 , except for the first and second regions  58   a  and  58   b , is exposed to radiation. In the illustrated embodiment, the second and third regions  58   b  and  58   c  are aligned with each other and with corresponding fluid ejectors  32 . The seventh and eighth masks  72  and  74  can be patterned such that the first regions  58   a  will be smaller than the second and third regions  58   b  and  58   c , so that the high drop weight nozzles  16   b  will have larger cross-sectional areas than the low drop weight nozzles  16   a . For example, the first regions  58   a  can be sized to be 13 microns in diameter, while the second and third regions  58   b  and  58   c  can be sized to be 20 microns in diameter. 
     The second bore layer  60  includes a larger region  64  that surrounds the second regions  58   b  and is subjected to the electromagnetic radiation so as to undergo polymeric cross-linking and become insoluble in developing solutions. The region  64 , which is not subsequently removed, becomes the raised portion  38  of the fluidic layer assembly  24 . The region  64  typically extends the entire length of the second bore layer  60  and has a width that is substantially equal to the desired width of the raised portion, which could be 150 microns for example. The region  64  is preferably large enough to overlap (as shown in  FIG. 13 ) the portion of the first bore layer  54  that was exposed during the first exposure step. The remaining portions of the second bore layer  60  are additional areas to be removed and are thus not exposed to electromagnetic radiation. 
     After the first and second bore layers  54  and  60  have been exposed, they are jointly developed (again using any suitable developing technique), to remove the unexposed, soluble bore layer material and leave the exposed, insoluble material. This results in the fluidic layer assembly  24  (collectively made up by the chamber layer  46 , the first bore layer  54 , and the second bore layer  60 ) having the raised portion  38  formed on the second side  36 , with the low drop weight nozzles  16   a  formed in the base portion  40  and the high drop weight nozzles  16   b  formed in the raised portion  38 . In addition, the fill material  52  filling the void  50  in the chamber layer  46  is also removed, leaving a substantially closed space defining the firing chambers  28  and the feed channels  30  that are in fluid communication with the nozzles  16   a ,  16   b . The ink feed hole  26  is then formed in the substrate  20  using any suitable technique, including wet etching, dry etching, deep reactive ion etching (DRIE), laser machining, and the like. 
     While specific embodiments of the present invention have been described, it should be noted that various modifications thereto could be made without departing from the spirit and scope of the invention as defined in the appended claims.