Patent Publication Number: US-6984015-B2

Title: Ink jet printheads and method therefor

Description:
FIELD OF THE INVENTION 
   The invention is directed to printheads for ink jet printers and more specifically to improved printhead structures and methods for making the structures. 
   BACKGROUND 
   Ink jet printers continue to be improved as the technology for making the printheads continues to advance. New techniques are constantly being developed to provide low cost, highly reliable printers which approach the speed and quality of laser printers. An added benefit of ink jet printers is that color images can be produced at a fraction of the cost of laser printers with as good or better quality than laser printers. All of the foregoing benefits exhibited by ink jet printers have also increased the competitiveness of suppliers to provide comparable printers in a more cost efficient manner than their competitors. 
   As advances are made in print quality and speed, a need arises for an increased number of ejection devices on the surface of the semiconductor substrate. There is also a desire to provide substrates that can eject more than one color ink. Multi-color ink substrates require multiple feed slots for providing different color inks to the associated ejection devices. Having multiple slots in a semiconductor substrate often weakens the substrate. In order to increase the strength of the substrate, the slots are often spaced-apart an amount sufficient to provide more substrate structure between the slots. However, increasing the spacing between the slots requires additional substrate area which increases the cost of the printheads. Thus, there continues to be a need for improved manufacturing processes and techniques which provide improved printhead components that can be provided at a lower cost. 
   SUMMARY OF THE INVENTION 
   With regard to the above and other objects the invention provides a semiconductor substrate for an ink jet printhead. The substrate includes a silicon substrate having a thickness ranging from about 500 to about 900 microns and having a first surface and a second surface opposite the first surface. One or more ink feed slots are formed in the silicon substrate from the first surface to the second surface thereof. The ink feed slots have a first width dimension, opposing first ends, and a first length dimension between the opposing first ends adjacent the first surface of the substrate. Stress relieving openings are provided adjacent the opposing first ends of the ink feed slots. The stress relieving openings provide an overall feed slot length dimension, have a radius greater than the first width dimension of the ink feed slots and have a radius to first length dimension ratio ranging from about 1:60 to about 1:250. 
   In another aspect the invention provides a method for making an ink jet printhead. The method includes providing a semiconductor substrate having a thickness ranging from about 500 to about 900 microns and having a first surface, and a second surface opposite the first surface. One or more ink feed slots are micromachined in the semiconductor substrate for ink flow communication from the second surface to the first surface of the substrate. The slots have a first width dimension, opposing first ends, and a first length dimension between the opposing first ends adjacent the first surface of the substrate. Stress relieving openings are micromachined adjacent the opposing ends of the ink feed slots. The stress relieving openings provide an overall feed slot length dimension and have a radius greater than the first width dimension of the ink feed slots. A nozzle plate is attached to the semiconductor substrate to provide the ink jet printhead. 
   An advantage of at least one of the embodiments of the invention is that multiple, relatively long ink feed slots can be provided in a relatively narrow silicon substrate while providing a substrate that is less prone to cracking or breaking. Without desiring to be bound by theory, it is believed that the stress relieving openings at the ends of the ink feed slots provide relieve for stress concentrations induced in the substrate during adhesive curing and cooling steps of a printhead manufacturing process. The openings also are effective to reduce stress concentrations induced in the substrate due to an impact load caused by dropping or striking an ink cartridge assembly containing the substrate. 
   For the purposes of the invention, the term “micromachining” shall include a wide variety of slot formation processes including, but not limited to, sand blasting, chemical etching, dry etching, deep reactive ion etching, laser ablation, and the like. Accordingly, the invention is adaptable to most conventional micromachining processes for forming openings or slots in semiconductor substrates. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further advantages of the invention will become apparent by reference to the detailed description when considered in conjunction with the figures, which are not to scale, wherein like reference numbers indicate like elements through the several views, and wherein: 
       FIG. 1  is a plan view not to scale of a conventional semiconductor chip for a printhead containing multiple ink feed slots; 
       FIG. 2  is a perspective, cross-sectional view, not to scale, of a conventional semiconductor chip for a printhead containing multiple ink feed slots; 
       FIG. 3  is a plan view, not to scale, of a semiconductor chip according to an embodiment of the invention for a printhead containing multiple ink feed slots as viewed from a first surface of the chip; 
       FIG. 4  is a perspective, cross-sectional view, not to scale, of a portion of a semiconductor chip according to an embodiment of the invention containing multiple ink feed slots; 
       FIG. 5  is a plan view, not to scale, of a semiconductor chip according to an embodiment of the invention for a printhead containing multiple ink feed slots as viewed from a second surface of the chip; 
       FIG. 6  is a cross-sectional view, not to scale, through a semiconductor substrate and nozzle plate according to an embodiment of the invention; 
       FIG. 7  is a perspective view, not to scale, of an ink cartridge containing a semiconductor substrate according to an embodiment of the invention; and 
       FIG. 8  is a graph of impact energy versus percentage substrate cracks comparing conventional substrates with a substrate according to the invention for various impact energies. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Industry trends and competitive forces in the industry continue to drive manufacturers to reduce the cost of ink jet printheads and increase the print speed and quality. One method for decreasing the cost of the printheads is to provide narrower semiconductor substrates containing multiple ink feed slots formed therein. In order to increase print speed, the slots are usually made as long as possible so that more ink ejectors can be placed adjacent the slots for ejecting ink. 
   A conventional semiconductor substrate  10  for an ink jet printhead containing multiple ink feed slots formed in a relatively narrow semiconductor substrate is illustrated in  FIGS. 1 and 2 . The substrate in  FIG. 1  is viewed from device side  12  thereof and contains multiple ink feed slots  14 ,  16 , and  18  formed through the thickness (T) of the substrate  10 . Each of the ink feed slots  14 ,  16 , and  18  preferably provides a different color ink to an ink ejector on the device surface  12  of the semiconductor substrate  10 . The overall dimensions of the substrate are typically from about 3 to about 7 millimeters wide by from about 8 to about 15 millimeters long. However, chip lengths of up to about 30 millimeters are also contemplated by the invention. 
   As shown in  FIGS. 1 and 2 , the ink feed slots  14 ,  16 , and  18  are relatively narrow, i.e., about 80 microns wide. Accordingly, ends  20 ,  22 , and  24  of the ink feed slots  14 ,  16 , and  18  have a relatively small radius. In the case of 80 micron wide ink feed slots  14 ,  16 , and  18 , the maximum radius of the ends  20 ,  22 , and  24  is about 40 microns. 
   A disadvantage of the foregoing prior art design is that such substrates  10  are weaker and more likely to crack during the manufacturing process than wider substrates having shorter ink feed slots formed therein. Stress is introduced into the substrates  10  when the substrates  10  are attached to a plastic or metal ink cartridge body using a thermally curable adhesive. The substrates  10 , typically silicon substrates  10 , have a relatively low coefficient of thermal expansion compared to metals and plastics. 
   During the attachment process, the substrate  10  and cartridge body are heated to an elevated temperature sufficient to cure the adhesive. As the adhesive cures, the substrate  10  is locked in place on the cartridge body. As the substrate/cartridge body assembly cools to room temperature, the cartridge body often shrinks more than the substrate  10  creating a slight outward bowing of the substrate  10  away from the cartridge body. The stress due to such deformation of the substrate  10  tends to be greatest at the ends  20 ,  22 , and  24  of the ink via. As a ratio of the radius of the ends  20 ,  22 ,  24  to a length of the ink vias  14 ,  16 , and  18  decreases, the potential for cracking of the substrates  10  increases. 
   Additionally, striking or dropping the substrate  10  induces stresses in the substrate. Hence, the potential for cracking of the substrate  10  also increases when a cartridge assembly containing the substrate  10  is struck or dropped. 
   With reference now to  FIGS. 3–5 , a semiconductor substrate  26  according to an embodiment of the invention is provided. The substrate  26  includes one or more ink feed slots, preferably at least two and most preferably at least three ink feed slots, such as slots  28 ,  30 ,  32 , micromachined through a thickness (T 1 ) of the substrate  26  extending between a first surface  34  ( FIG. 3 ) and a second surface  36  ( FIG. 5 ) thereof The ink feed slots  28 ,  30 , and  32  provide ink flow communication between an ink reservoir and ink ejection devices on the first surface  34  of the substrate  26 . Each of the ink feed slots  28 ,  30 , and  32  may provide a different color ink to the ejection devices, or may all provide the same color ink to the ejection devices depending on the particular application. 
   The substrate  26  is preferably a silicon substrate having thickness (T 1 ) ranging from about 500 to about 900 microns. Various metal insulating and passivating layers are deposited on the first surface  34  of the substrate to provide ink ejection devices and logic devices for ejecting ink from a printhead using the substrate  26 . 
   Ink ejection devices  38  are typically disposed along a length (L 1 ) of the ink feed slots  28 ,  30 , and  32  on one or both sides thereof ( FIG. 6 ). Ink is fed from the ink feed slots  28 ,  30 , and  32  through ink channels  40  into ink chambers  42  in a thick film layer or nozzle plate  44  attached to the substrate  26 . Upon activation of the ink ejectors  38 , ink is caused to flow through nozzle holes  46  in the nozzle plate  44  toward a print media, such as a sheet of paper. 
   Referring again to  FIG. 3 , stress relieving openings  48  and  50  are provided on opposing ends  52  and  54  of each of the ink feed slots  28 ,  30 , and  32 . The stress relieving openings  48  and  50  preferably have a radius dimension (R) that is greater than a first width (W 1 ) dimension of the ink feed slots  28 ,  30 , and  32 . The first width dimension (W 1 ) of the ink feed slots  28 ,  30 , and  32  preferably ranges from about 80 to about 250 microns, while the stress relieving openings  48  and  50  preferably have a radius dimension ranging from about 150 to about 300 microns. 
   With respect to the openings  52  and  54 , a ratio of radius dimension (R) to the length (L 1 ) of the ink feed slots  28 ,  30 , and  32  preferably ranges from about 1:60 to about 1:250. While circular stress relieving openings  48  and  50  are shown, the invention is not limited to circular stress relieving openings  48  and  50 . Accordingly, the stress relieving openings  48  and  50  may be substantially, circular, oval, or arcuate in shape, for example. 
   The slots  28 ,  30 , and  32  and stress relieving openings  48  and  50  may be micromachined through the thickness (T 1 ) of the substrate  26  with relatively constant first width (W 1 ) and radius (R) dimensions using, for example, deep reactive ion etching (DRIE). However, in order to maximize the area of the surface  36  of the substrate  26  available for electrical tracing and logic devices, the slots  28 ,  30 , and  32  are preferably formed in a two step micromachining process. In the two step micromachining process, a relatively narrow slot  28  having the first width dimension (W 1 ) ( FIG. 4 ) is formed from the device surface side  34  of the substrate  26 . The relatively narrow slot  28  is preferably micromachined to a depth (D 1 ) through the thickness (T 1 ) of the substrate  26  that ranges from about 5 to about 50% of the thickness (T 1 ) of the substrate  26 . 
   In another micromachining process, a relatively wider slot  56  having a second width dimension (W 2 ) is formed from the second surface  36  of the substrate  26 . The relatively wider slot  56  is preferably formed to a depth (D 2 ) through the thickness (T 1 ) of the substrate  26  ranging from about 50 to about 95% of the thickness (T 1 ) of the substrate  26 . As shown in  FIG. 3 , the relatively wider slot  56  has a second length (L 2 ) that is preferably greater than length (L 1 ) of the slots  28 ,  30 , and  30  between opposing ends  52  and  54  thereof. The relatively wider slot  56  is provided to reduce ink flow restriction through the thickness (T 1 ) of the substrate  26 . 
   The length (L 2 ) of the relatively wider slot  56  is preferably less than an overall length (L 3 ) provided by the ink feed slot  28  and openings  48  and  50 . Accordingly, as shown in  FIG. 5 , there is provided additional supporting substrate area  58  between an end  60  of the relatively wider slot  56  and an edge  62  of the substrate  26 . The additional supporting substrate area  58  is believed to resist deformation of the substrate  26  during manufacturing processes. Because the relatively wider slot  56  has a length dimension (L 2 ) preferably less than the overall length dimension (L 3 ) of the slots  28 ,  30 , and  32 , a shelf area  64  is provided in the slots  28 ,  30 , and  32  as shown in  FIGS. 3 and 4 . 
   The slots  28 ,  30 , and  32  and relatively wider slots  56  are preferably formed in the substrate  26  subsequent to depositing passivating, insulating, conductive, resistive, and protective layers on the substrate  26  to form the ink ejection devices such as heater resistors  38  on the surface  34  of the substrate. Next, the nozzle plate  44  is adhesively attached to the substrate  26  to provide a nozzle plate/substrate assembly  44 / 26  as shown generally in  FIG. 6 . A flexible circuit  66  ( FIG. 7 ) is attached to the nozzle plate/substrate assembly  44 / 26  to provide a printhead  68 . The printhead  68  is preferably adhesively attached to a cartridge body  70  to provide an ink jet cartridge  72  for use in an ink jet printer. 
   The nozzle plate  44  contains a plurality of the nozzle holes  46  each of which are in fluid flow communication with the ink chambers  42 . The nozzle plate  44  is made of a material selected from metal such as nickel or a polymeric material such as a polyimide available from Ube Industries, Ltd of Tokyo, Japan under the trade name UPILEX. A preferred material for the nozzle plate  44  is a polymeric material and the nozzle holes  46  are made such as by laser ablating the polymeric material. 
   The nozzle plate  44  and ink chambers  42  are preferably aligned optically so that each nozzle hole  46  in the nozzle plate  44  aligns with one of the ink ejection devices  38  in the ink chamber  42 . Misalignment between the nozzle holes  46  and the ink ejection device  38  may cause problems such as misdirection of ink droplets from the printhead  68 , inadequate droplet volume or insufficient droplet velocity. Accordingly, nozzle plate/substrate assembly  44 / 26  alignment is critical to the proper functioning of an ink jet printhead  68 . 
   A particularly preferred method for forming ink feed slots  28 ,  30 , and  32  in a silicon semiconductor substrate  26  is a dry etch technique, preferably deep reactive ion etching (DRIE) or inductively coupled plasma (ICP) etching. This technique employs an etching plasma comprising an etching gas derived from fluorine compounds such as sulfur hexafluoride (SF 6 ), tetrafluoromethane (CF 4 ) and trifluoroamine (NF 3 ). A particularly preferred etching gas is SF 6 . A passivating gas is also used during the etching process. The passivating gas is derived from a gas selected from the group consisting of trifluoromethane (CHF 3 ), tetrafluoroethane (C 2 F 4 ), hexafluoroethane (C 2 F 6 ), difluoroethane (C 2 H 2 F 2 ), octofluorobutane (C 4 F 8 ) and mixtures thereof. A particularly preferred passivating gas is C 4 F 8 . 
   In order to conduct dry etching of the ink feed slots  28 ,  30 , and  32  in the semiconductor substrate  26 , the substrate  26  is preferably coated on the first surface  34  thereof with an etch stop material selected from SiO 2 , a photoresist material, metal and metal oxides, i.e., tantalum, tantalum oxide and the like. Likewise, the substrate  36  is preferably coated on the second surface  36  with a protective layer or etch stop material selected from SiO 2 , a photoresist material, tantalum, tantalum oxide and the like. The SiO 2  etch stop layer and/or protective layer may be applied to the first and second surfaces  34  and  36  of the silicon substrate  26  by a thermal growth method, sputtering or spinning. A photoresist material may be applied to the substrate  26  by spinning the photoresist material onto the first or second surfaces  34  and  36  of the substrate  26 . The locations of the feed slots  28 ,  30 , and  32  in the substrate  26  may be patterned in the substrate  26  from either side of the substrate  26 , the opposite side being preferably provided with an etch stop material. 
   In a particularly preferred process, as described above, the ink feed slots  28 ,  30 , and  32  are patterned and etched in the substrate  26  using a two step etching process. In the first step, the ink feed slots  28 ,  30 , and  32  are etched from the first surface  34  of the substrate  26  to a depth, preferably less than about 50 microns. The first surface  34  is then coated with a photoresist layer or SiO 2  layer and the substrate  26  is dry etched from the second surface side  36  to complete the feed slots  28 ,  30 , and  32  through the chip. As a result of the two-step process, the feed slot locations and sizes are more precise. 
   In order to conduct a deep reactive ion etching process, the patterned substrate  26  is placed in an etch chamber having a source of plasma gas and back side cooling such as with helium and water. It is preferred to maintain the substrate  26  below about 400° C., most preferably in a range of from about 50° to about 80° C. during the etching process. In the process, a deep reactive ion etch (DRIE) or inductively coupled plasma (ICP) etch of the silicon is conducted using an etching plasma derived from SF 6  and a passivating plasma derived from C 4 F 8  wherein the substrate  26  is etched from the second surface  36  side toward the first surface  34  side. 
   During the etching process, the plasma is cycled between the passivating plasma step and the etching plasma step until the ink feed slots  28 ,  30 , and  32  reach the etch stop layer on the first surface  34 . Cycling times for each step preferably ranges from about 5 to about 20 seconds for each step. Gas pressure in the etching chamber preferably ranges from about 15 to about 50 millitorrs at a temperature ranging from about −20° to about 35° C. The DRIE or ICP platen power preferably ranges from about 10 to about 25 watts and the coil power preferably ranges from about 800 watts to about 3.5 kilowatts at frequencies ranging from about 10 to about 15 MHz. Etch rates may range from about 2 to about 10 microns per minute or more and produce slots  28 ,  30 , and  32  having side wall profile angles ranging from about 88° to about 92°. Etching apparatus is available from Surface Technology Systems, Ltd. of Gwent, Wales. Procedures and equipment for etching silicon are described in European Application No. 838,839A2 to Bhardwaj, et al., U.S. Pat. No. 6,051,503 to Bhardwaj, et al., PCT application WO 00/26956 to Bhardwaj, et al. 
   When the etch stop layer SiO 2  on the first surface  34  is reached, etching of the ink feed slots  28 ,  30 , and  32  terminates. The flow path through the ink feed slots  28 ,  30 , and  32  may be completed by blasting through the etch stop layer on the surface  34  in the location of the ink feed slots  28 ,  30 , and  32  using a high pressure water wash in a wafer washer. The finished substrate  26  preferably contains ink feed slots  28 ,  30 , and  32  which are located in the substrate  26  so that feed slots  28 ,  30 , and  32  are a distance ranging from about 40 to about 60 microns from their respective ink ejection devices  38 . 
   In another embodiment, the relatively wide slot  56  may be formed in the second surface  36  of the substrate  26  by chemically etching the silicon substrate  26  prior to or subsequent to forming ink feed slots  28 ,  30 , and  32  in the substrate  26 . Chemical etching of wide slots  56  may be conducted using KOH, hydrazine, ethylenediamine-pyrocatechol-H 2 O (EDP) or tetramethylammonium hydroxide (TMAH) and conventional chemical etching techniques. Prior to or subsequent to forming the wide slots  56 , the ink feed slots  28 ,  30 , and  32  are preferably formed in the silicon substrate  26  from the first surface  34  side as described above. When the wide slots  56  are made by chemical etching techniques, a silicon nitride (SiN) protective layer is preferably used to pattern the trench location on the second surface  36  side of the substrate  26 . Upon completion of the slots  56  formation, a protective layer of SiO or other protective material for dry etching silicon is applied to the second surface  36  side of the substrate  26  to protect the silicon material during the dry etch process. The wide slots  56  are preferably etched in the substrate  26  to a depth of about 50 to about 300 microns or more. In the alternative, the wide slots  56  may be formed in the substrate  26  by grit blasting to the desired depth under carefully controlled conditions. 
   As compared to wet chemical etching, the dry etching techniques described above may be conducted independent of the crystal orientation of the silicon substrate  26  and thus ink feed slots  28 ,  30 , and  32  may be placed more accurately in the substrates  26 . While wet chemical etching is suitable for a substrate thickness of less than about 200 microns, the etching accuracy is greatly diminished for a substrate thickness greater than about 200 microns. The gases used for DRIE techniques according to the invention are substantially inert whereas highly caustic chemicals are used for wet chemical etching techniques. The shape of the ink feed slots  28 ,  30 , and  32  made by DRIE is essentially unlimited whereas the shape of slots made by wet chemical etching is dependent on crystal lattice orientation. For example in a (100) silicon chip, KOH will typically only etch squares and rectangles without using advance compensation techniques. The crystal lattice does not have to be aligned for DRIE techniques. 
   The foregoing method for forming ink feed slots in a semiconductor substrate is described for example, in U.S. Pat. No. 6,402,301 to Powers et al., the disclosure of which is incorporated by reference thereto. Other methods for reactive ion etching are described in U.S. Pat. No. 6,051,503 to Haynes et al., incorporated herein by reference as if fully set forth. Useful etching procedures and apparatus are also described in EP 838,839 to Bhardwaj et al., WO 00/26956 to Bhardwaj et al. and WO 99/01887 to Guibarra et al. Etching equipment is available from Surface Technology Systems Limited of Gwent, Wales. 
   Semiconductor substrates having ink feed slots, such as substrate  26 , made according to the invention have exhibited greater resistance to stress cracks than substrates made according to a conventional process. With reference to  FIG. 8 , substrates having slots made according to the invention (curve A) having stress relieving features can absorb about 25% more impact energy than substrates made according to the conventional process (Curve B) that do not have stress relieving features. 
   In order to compare the improved characteristics of substrates made according to the invention with conventional substrates, substrates without and without the stress relieving openings were attached to center portions of test plaques using die bond adhesives. The substrates were silicon substrates having a length of about 13.7 millimeters, a width of about 4.6 millimeters, and at least one ink feed slot with a width of about 0.11 millimeters. The test plaques to which the chips were attached were about 20 millimeters square and made of modified polyphenylene oxide polymer such as available from GE Plastics of Pittsfield, Mass. under the trade name NORYL. 
   In a test to determine how stresses in the substrates are affected by a load on the substrates, a five pound load was placed on the test plaque to which the substrate was attached in a hybrid three point bend test along the center-line of the test plaque with an axis parallel to the ink feed slot in the substrate and parallel to the lateral supports for the test plaque. The lateral supports for the test plaque were spaced about 8.7 millimeters from the center-line of the test plaque. The axis of the load only extended part way along an axis parallel with the lateral support axes. A comparison of substrates without and without stress relieving openings and substrates having openings of different size are contained in the following table. 
   
     
       
         
             
             
             
           
             
               TABLE 1 
             
             
                 
             
             
               Sample 
               Radius of Stress 
               Maximum principal stress at 
             
             
               No. 
               Relieving Opening (microns) 
               end of ink feed slot (MPa) 
             
             
                 
             
           
          
             
                 
             
          
         
         
             
             
             
          
             
               1 
               None 
               898 
             
             
               2 
               70 
               450 
             
             
               3 
               112 
               343 
             
             
               4 
               150 
               292 
             
             
                 
             
          
         
       
     
   
   As shown by the foregoing results, substrates (Samples 2, 3 and 4) having stress relieving openings at the ends of the ink feed slots exhibited a significant decrease in stress under load compared to substrates (Sample 1) without stress relieving openings. The stress was decreased about 50% between Sample 1 and Sample 2, and about 67% between Sample 1 and Sample 4. The size of the stress relieving openings also had a beneficial effect on stress decrease. For example, substrates having slots with openings of 150 micron radius (Sample 4) at the ends of the slots exhibited a decrease in stress of about 35% compared to substrates having 70 micron radius openings at the ends of the slots. 
   Tests were conducted on substrates to determine stress in the substrates under thermal load. As above, the substrates were attached to test plaques using die bond adhesives. The ambient temperature was 110° C. and the test plaque assemblies were cooled down to 20° C. and the stresses were determined. The results are contained in the following table. 
   
     
       
         
             
             
             
           
             
               TABLE 2 
             
             
                 
             
             
               Sample 
               Radius of Stress 
               Maximum principal stress at 
             
             
               No. 
               Relieving Opening (microns) 
               end of ink feed slot (MPa) 
             
             
                 
             
           
          
             
                 
             
          
         
         
             
             
             
          
             
               1 
               None 
               6.34 
             
             
               2 
               95 
               5.06 
             
             
                 
             
          
         
       
     
   
   As shown by the foregoing samples, a substrate (Sample 2) with a slot having stress relieving openings of 95 micron radius at the ends of the ink feed slots exhibited a decrease in thermally induced stress of about 20% over a substrate (Sample 1) that did not contain stress relieving openings at the ends of the ink feed slots. 
   It will be recognized by those skilled in the art, that the invention described above may be applicable to a wide variety of micro-fluid ejection devices other than ink jet printing devices. Such microfluid ejection devices may include liquid coolers for electronic components, micro-oilers, pharmaceutical delivery devices, and the like. 
   Having described various aspects and embodiments of the invention and several advantages thereof, it will be recognized by those of ordinary skills that the invention is susceptible to various modifications, substitutions and revisions within the spirit and scope of the appended claims.