Abstract:
A method for creating and transporting low-energy ions for use in plasma processing of a semiconductor wafer is disclosed. In an exemplary embodiment of the invention, the method includes generating plasma from a gas species to produce a plasma exhaust. The plasma exhaust is then introduced into a processing chamber containing the wafer. The ion content of the plasma exhaust is enhanced by activating a supplemental ion source as the plasma is introduced into the processing chamber, thereby creating a primary plasma discharge therein. Then, the primary plasma discharge is directed into a baffle plate assembly, thereby creating a secondary plasma discharge exiting the baffle plate assembly. The strength of an electric field exerted on ions contained in the secondary plasma discharge is reduced. In so doing, the reduced strength of the electric field causes the ions to bombard the wafer at an energy insufficient to cause damage to semiconductor devices formed on the wafer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional application of application Ser. No. 09/905,043, filed Jul. 31, 2001, now U.S. Pat. No. 6,761,796, which is a continuation in part of application Ser. No. 09/828,055, filed on Apr. 6, 2001, now abandoned, the disclosures of which are incorporated by reference herein in their entirety. 
    
    
     BACKGROUND 
     The present invention relates generally to the plasma processing of semiconductor wafers and, more specifically, to a low-energy ion generation and transport mechanism for use in plasma ashing systems. 
     In the manufacture of integrated circuits, photolithography techniques are used to form integrated circuit patterns on a substrate. Typically, a semiconductor substrate is coated with a photoresist material, portions of which are exposed to ultraviolet (UV) radiation through a mask to image a desired circuit pattern on the photoresist. The portions of the photoresist left unexposed to the UV radiation are removed by a processing solution, leaving only the exposed portions on the substrate. In certain instances, these remaining exposed portions are baked using UV light during a photostabilization process to enable the photoresist to withstand subsequent processing. 
     After such processing, in which the integrated circuit components are formed, it is generally necessary to remove the remaining photoresist from the wafer. In addition, residue that may have been introduced on the substrate surface through processes such as etching must be removed. Typically, the photoresist is “ashed” or “burned” in the presence of atomic oxygen and other gases, and the ashed or burned photoresist, along with the residue, is “stripped” or “cleaned” from the surface of the substrate. 
     One manner of removing photoresist and residues is by directing a radio frequency (RF) energized or microwave-energized plasma at the substrate surface. In the case of a microwave-energized plasma, the plasma is formed by a gas mixture that is transported through a plasma tube that passes through a resonant microwave cavity. Microwave energy within the cavity is introduced into the plasma tube to excite the gas mixture therein and form a plasma. The exited plasma exhaust containing reactive species passes from the tube into a process chamber, in which resides a photoresist-coated semiconductor substrate to be ashed. This type of asher is known as a “downstream asher”, where the resist coated substrate is physically removed from the plasma generator, which is known as an “upstream” plasma source. 
     In semiconductor applications where a relatively high dose of ion implantation has been imparted to a resist-coated wafer (e.g., ≧1×10 15  cm −2 ), the top layer of the photoresist turns into a highly carbonized crust which becomes impervious to the diffusion of trapped solvents from the remaining resist below. As a result, this crust must be carefully removed by the asher (generally at low wafer temperatures) in order to prevent the solvents from explosively exiting the crust. Otherwise, such a condition leads to the creation of “poppers” on the photoresist. The residue often left on the wafer surface by poppers is difficult to remove, and may be a potential source of contaminating particles on the wafer and within the tool chamber. A low temperature process relying solely upon the atomic species to chemically remove the crust, however, is inherently inefficient and compromises the asher&#39;s throughput (as measured by number of wafers processed per unit time). 
     A known method for enhancing the ash rate of the carbonized crust at low temperatures employs the use of ion bombardment. A conventional ion source in an asher uses a platen (or electrostatic chuck), which is typically biased at radio frequency (RF) by an RF source. Once activated, the RF source creates a capacitive discharge above the wafer. This secondary discharge then creates ion-electron pairs immediately above the wafer, from which ions are then accelerated by a capacitive “sheath” created above the wafer surface. Because the capacitive sheath may have an electric field potential as high as 40–50 eV or higher, the ions may strike the wafer at these high energies. However, such high-energy ion bombardment can cause extensive damage to the devices formed on the wafers. Also, the extensive heating of the wafer due to the heavy ion bombardment may lead to inconsistent wafer temperatures between wafer to wafer operations. 
     BRIEF SUMMARY 
     The previously discussed drawbacks and deficiencies of the prior art are overcome or alleviated by a method for creating and transporting low-energy ions for use in plasma processing of a semiconductor wafer. In an exemplary embodiment of the invention, the method includes generating plasma from a gas species to produce a plasma exhaust. The plasma exhaust is then introduced into a processing chamber containing the wafer. The ion content of the plasma exhaust is enhanced by activating a supplemental ion source as the plasma is introduced into the processing chamber, thereby creating a primary plasma discharge therein. Then, the primary plasma discharge is directed into a baffle plate assembly, where a secondary plasma discharge is created as the plasma exits the baffle plate assembly. The strength of the sheath potential exerted on ions contained in the secondary plasma discharge is reduced, the sheath potential resulting from the primary plasma discharge. The resulting reduced strength of the electric field accelerates the ions through a lower potential, thereby causing ion bombardment on the wafer at an energy insufficient to cause damage to semiconductor devices formed on the wafer. 
     In a preferred embodiment, the reduction of acceleration of ions through the sheath potential in the secondary plasma discharge is achieved by locating the supplemental ion source so as to have the baffle plate assembly disposed between the primary plasma discharge and the wafer. In addition, the baffle plate assembly is configured so as to cause the secondary plasma discharge to be shaped in substantially a micro-jet formation. The baffle plate assembly includes an upper baffle plate and a lower baffle plate, with said lower baffle plate further having a plurality of chamfered holes located therethrough, to provide uniform ion impingement on the surface of the wafer, thereby preventing charging effects from damaging the wafer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring to the exemplary drawings wherein like elements are numbered alike in the several Figures: 
         FIG. 1  is a schematical cross-sectional view of a plasma source and supplemental ion energizer which may be used with a process chamber of a photoresist asher; 
         FIG. 2  is a cross-sectional view of the supplemental ion energizer of shown in  FIG. 1 , taken along the line  2 — 2 ; 
         FIG. 3  is a cross-sectional view of an alternative embodiment of the supplemental ion energizer in  FIG. 2 ; 
         FIG. 4  illustrates a conventional plasma ashing system having a supplemental ion source, and a high-energy capacitive sheath created directly above a semiconductor wafer when an RF biased electrostatic chuck is activated to generate the ions; 
         FIG. 5  is a cross-sectional schematic of the process chamber of  FIG. 1 , illustrating the isolation of the high-energy capacitive sheath from the semiconductor wafer, in accordance with an embodiment of the invention; 
         FIG. 6  is a top view of a baffle plate assembly in accordance with an embodiment of the invention; 
         FIG. 7  is a side cross-sectional view of the baffle plate assembly shown in  FIG. 6 , taken along the line  7 — 7 ; 
         FIG. 8  is a top cross-sectional view of the lower baffle plate of the baffle plate assembly, taken along the line  8 — 8 ; 
         FIG. 9  is a cross-sectional, detailed view of the chamfered holes of the lower baffle plate; and 
         FIG. 10  is a schematic illustrating the micro-jet, low-energy ion formation through the chamfered holes of the lower baffle plate. 
     
    
    
     DETAILED DESCRIPTION 
     A novel low-energy ion generation and transport mechanism is disclosed herein; this mechanism enhances the chemical decomposition and subsequent volatilization of a carbonized top layer of an ion-implanted photoresist in a uniform manner, without also exposing the wafer to the potentially harmful effects of high-energy ion bombardment due to high sheath voltages. Such a “soft” ion assisted technique takes advantage of the synergy between the ions generated by a supplemental ion source in the ash tool and the chemical reactants already present, thereby producing a faster reaction than can be achieved by either, or the simple sum of these components. 
     It is believed that the ions contribute both “physical” kinetic energy, as well as “chemical” internal energy released upon reaction, thereby effectively lowering the activation energy for surface reactions. By shielding the wafer from the high sheath potentials characteristic of a conventional capacitive discharge, the ions are subjected to a substantially weaker electric field when passing through the sheath potential formed at the wafer. Thus, the ions strike the wafer at energies that are insufficient to do significant damage to the wafer devices, but still enable the ion-assisted chemical process of removing a carbonized crust formed on implanted photoresist. This scheme is also particularly suited for systems that do not need a wafer chuck for operation. 
     While ion implanted resist is mentioned here as an example of an application for soft ion bombardment, many other applications of such ion bombardment may be foreseen. This includes, but is not limited to removal of post etch residue, anisotropic resist removal, selective removal of photoresist in the presence of low-k material, anisotropic etch, etc. 
     Referring initially to  FIG. 1 , there is shown schematic of a photoresist asher  10 , in accordance with an embodiment of the invention. Asher  10  includes a gas box  12 , a microwave power generator assembly  14 , a process chamber  16  in which is heated a semiconductor substrate or workpiece such as a wafer  18 , and a radiant heater assembly  20  (for heating the wafer  18 ) situated at the bottom of the process chamber  16 . A temperature probe  24 , such as a thermocouple, is used to monitor the temperature of the wafer  18 . A vacuum pump  26  is used to evacuate the process chamber  16  for processes requiring vacuum conditions. A monochromator  28  is used to monitor the optical emission characteristics of gases within the chamber to aid in process endpoint determination. 
     In operation, a desired mixture of gases is introduced into a plasma tube  32  from gas box  12  through an inlet conduit  34 . The plasma tube  32  is made of a material such as quartz or sapphire. The gases forming the desired mixture are stored in separate supplies (not shown) and mixed in the gas box  12  by means of valves  36  and piping  38 . One example of a desired gas mixture is nitrogen-based forming gas (primarily nitrogen with a small percentage of hydrogen) with or without oxygen. Optionally, a fluorine containing gas such as carbon tetrafluoride may be added to the gas mixture to improve ashing rates for certain processes. In such a case, sapphire is a preferred material for plasma tube  32 . Sapphire resists the etching of the inner surface of plasma tube  32 , caused by the presence of reactive fluorine atoms and ions in the plasma. Additional details regarding the use of sapphire in fluorine assisted stripping may be found in U.S. Pat. No. 6,082,374 to Huffman, et al., the contents of which are incorporated herein by reference. 
     The desired gas mixture is energized by the microwave power generator assembly  14  to form a reactive plasma that will ash photoresist on the wafer  18  in the process chamber  16  when heated by the radiant heater assembly  20 . A magnetron  40  generates microwave energy (at about 2.45 GHz) which is coupled to a waveguide  42 . The microwave energy is then fed from the waveguide through apertures (not shown) in a microwave enclosure  44 , which surrounds the plasma tube  32 . As an alternative to microwave energy, the desired gas mixture may be energized by a radio frequency (RF) power source (not shown) in place of magnetron  40 , as is known in the art. 
     In the embodiment shown in  FIG. 1 , the plasma tube  32  is made of alumina (Al2O3) or single crystal sapphire to accommodate fluorine plasma chemistries. An outer quartz cooling tube  46  surrounds the sapphire plasma tube  32 , and is slightly separated therefrom. Pressurized air is fed into the gap between the tubes  32  and  46  to effectively cool the plasma tube  32  during operation. The microwave enclosure  44  is segmented into sections shown by phantom lines  45 . Segmentation of the enclosure  44  allows uniform microwave power distribution across the length of the tube  32 , and protects it from overheating by preventing an unacceptably large thermal gradient from developing along its axial length when suitable input power is provided. Unlike quartz, sapphire is inclined to crack when heated unevenly. Thus, each segment of the enclosure  44  is separately fed with microwave energy that passes through the quartz tube  46  and the sapphire plasma tube  32  passing therethrough. Additional details regarding this enclosure maybe found in U.S. Pat. No. 5,961,851 to Kamarehi, et al., the contents of which are incorporated herein by reference. Alternatively, a microwave applicator with a single resonant cavity specially designed to resonate in the TM012 mode may be used. Additional details regarding such an applicator may be found in U.S. Pat. No. 6,057,645 to Srivastava, et al., the contents of which are incorporated herein by reference. 
     Regardless of the material used for plasma tube  32  (quartz or sapphire) or the type of microwave applicator, the gas mixture within the plasma tube  32  is energized to create a plasma. Microwave traps  48  and  50  are provided at the ends of the microwave enclosure  44  to prevent microwave leakage. 
     As stated previously, a conventional downstream asher deliberately generates a plasma upstream from the substrate, such that the “afterglow” plasma impinging on the substrate is rich in reactive atomic species but has a poor concentration of ions. A supplemental ion source  74 , therefore, generates another plasma closer to the substrate having a substantial ionized content impinging on the surface of the wafer. The supplemental ion source  74  may be independently operable from the plasma generated by microwave power generator assembly  14  to accommodate different plasma characteristic requirements (e.g., low ion content or high ion content) within separate steps of a single process. It will be further appreciated that the supplemental ion source  74  may be incorporated into any type of upstream plasma based asher. 
     In the embodiment shown in  FIG. 1 , the supplemental ion source  74  is provided in the form of a coil antenna assembly  76  located between a dielectric window  52  and the plasma tube  32 . Alternatively, a planar antenna can be used. The dielectric window  52  in asher  10  is made of quartz or a similar dielectric material (like the ceramic alumina Al2O3) so that the signal emitted by coil antenna assembly  76  may pass therethrough and into the process chamber  16 . As best seen in  FIG. 2 , the coil antenna assembly  76  has a metallic (e.g., copper) coil antenna  78  embedded within a base  80 . The base  80  is preferably made of a fluoropolymer resin such as polytetrafluoroethylene (PFTE), more commonly referred to as Teflon®. Teflon® is a registered trademark of E.I. du Pont de Nemours and Company. 
     The coil antenna  78  should preferably reside outside of the process chamber  16  so that there is no contact with the energetic plasma, and no resulting particulate or sputtered metal contamination problem. The coil antenna  78 , which is generally planar in shape, is also sufficiently large so as to cover the entirety (or substantially the entirety) of wafer  18 . 
     The antenna  78  is energized by an RF signal output from RF signal generator  82 . In a preferred embodiment, the RF signal operates at 13.56 megahertz (MHz). However, the operating frequency may be any radio frequency that is permitted within the ISM bands. Disposed between the RF signal generator  87  and the antenna  78  is a matching network  84  that minimizes reflected power from the antenna  78  back into the RF generator  82 . The connections between the RF generator  82  and the matching network  84 , as well as between the matching network  84  and antenna  78 , may be made with coaxial cables or waveguides. Alternatively, other mechanisms of energizing the coil may be used, such as a self-contained, frequency-tuned RF generator and amplifier. 
       FIG. 3  illustrates an alternative embodiment of the supplemental ion source  74 , which is provided in the form of a plate antenna assembly  90 . The plate antenna assembly  90  has a metallic (e.g., copper or aluminum) plate antenna  92  that is generally circular in shape, and is provided with a central aperture  93  for alignment with plasma tube  32 . The plate antenna  92 , which is also generally planar in shape, is again sufficiently large to cover the entirety (or substantially the entirety) of wafer  18 . The resulting plasma discharge supplemented by antenna assembly  90  is primarily capacitive in nature, whereas the plasma discharge supplemented by antenna assembly  76  is partially inductive. 
     It will be appreciated that although  FIG. 1  shows the use of an RF source for the supplemental energizer, the use of other energizing (e.g., microwave) sources is contemplated for the supplemental ion energizer  74 . 
     Referring again to  FIG. 1 , after passing through supplemental ion source  74 , the energized plasma enters the process chamber  16  through an opening  51  in the dielectric window  52 . An apertured, dual-layered baffle plate assembly  54 , comprised of upper baffle plate  54   a  and lower baffle plate  54   b , evenly distributes the reactive plasma across the surface of the wafer  18  being processed. The radiant heater assembly  20  comprises a plurality of tungsten halogen lamps  58  residing in a reflector  64  that reflects and redirects the heat generated by the lamps toward the backside of the wafer  18  positioned within the process chamber  16  on quartz pins  68 . The thermocouple  24 , being in close contact with the wafer  18 , provides a feedback loop to lamps  58 , such that active temperature control of the wafer  18  may be maintained. One or more temperature sensors  72 , such as thermocouples, are mounted on the exterior of dielectric window  52  to provide an indication of chamber temperature. 
     In a conventional supplemental energizer of a typical plasma asher  101 , the semiconductor wafer  102  is typically secured upon a chuck  104 , as shown in  FIG. 4 . When ion bombardment is needed, the chuck  104  is typically biased at a certain RF potential (in order to provide supplemental ion generation), and a capacitive plasma is created directly above the wafer  102 . The wafer  102  and chuck  104  are then enveloped in an energetic capacitive sheath  106 . This energetic sheath  106  elevates the plasma to a positive potential relative to the wafer, and hence the ions (that are mostly positively charged having lost one or more electrons) are then attracted to the surface of wafer  102 . The potential of the sheath  106  is the mechanism by which the ions are accelerated at high energy and thus bombard the wafer  102 . 
     In contrast, the present invention embodiments allow the wafer to be protected from the formation of a high-energy sheath. In addition, a dense plasma is formed near the wafer. Referring now to  FIG. 5 , there is shown a cross-sectional schematic of the process chamber  16 , illustrating the capacitive sheath created during the plasma process. The wafer  18 , being mounted on quartz pins  68  (and thermocouple  24 ), is not biased at RF potentials. Once the microwave-energized plasma (or input gas, if the microwave plasma is not being operated) enters the process chamber  16  through an opening  51 , the RF antenna  74  ignites an energetic capacitive (or a combination of inductive and capacitive) discharge with its associated energetic sheath  106 . However, unlike a conventional process chamber, the sheath  106  (created due to the primary RF discharge) is located on the opposite side of baffle plate assembly  54  as the wafer  18 . Thus, wafer  18  is not subjected to high-energy ions accelerated by energetic sheath  106 . Instead, the ions diffuse to the wafer  18  and impact the wafer  18  with a maximum energy associated with a “floating potential” sheath  108  surrounding wafer  18 . It should be noted that neither the sheaths formed between the baffle plates in baffle plate assembly  54  (and the holes therein), nor the plasma are shown in  FIG. 5 . 
     Referring generally now to  FIGS. 6 through 10 , there is shown the detailed baffle plate assembly  54  in accordance with another aspect of the present invention. Again, baffle plate assembly  54  includes the upper baffle plate  54   a , made from a dielectric material such as quartz or ceramic. The upper baffle plate  54   a  may also be coated with sapphire so as to make it resistive to fluorine related etching. As shown in the top view of  FIG. 6 , upper baffle plate  54   a  has an impingement disk  110  disposed at the center thereof. The impingement disk  110  causes the primary plasma discharge to impinge thereupon and stream through openings  112  in the upper baffle plate  54   a  and into an interior plenum  113  in a uniform manner, as seen in  FIG. 7 . Baffle plate assembly  54  further includes a lower baffle plate  54   b , preferably made from a material such as anodized aluminum, and is grounded to act as the opposing capacitive electrode to antenna  78  or  92 . Lower baffle plate  54   b  also has a plurality of holes  114  therein, through which a secondary plasma discharge exits. The holes  114  in lower baffle plate  54   b  are preferably equivalent in number with the openings  112  in upper baffle plate  54   a , and are spatially aligned with respect to one another. 
     As shown more particularly in  FIG. 8 , a series of channels or grooves  116  run in a generally V-shaped configuration through the lower baffle plate  54   b  and in between the holes  114 . The channels  116  provide a cooling mechanism for baffle plate assembly  54  by the circulation of water or other coolant material therethrough. It will be noted that the channels  116  and holes  114  are configured so as to prevent any channel  116  from being punctured and causing leaks. 
     Referring now to  FIG. 9 , there is illustrated a detailed cross-sectional drawing of the lower baffle plate holes  114 . Each hole  114  has a first diameter d 1  on an inner surface  118  (or plenum side) of lower baffle plate  54   b  and a second diameter d 2  on an outer surface  120  (or wafer side) of lower baffle plate  54   b . The first diameter d 1  is larger than the second diameter d 2 . As can be seen, the plenum side of the holes  114  are generally frustoconical in shape, being tapered inward by a 90 degree chamfer until the interior diameter thereof is equal to d 2 . This occurs roughly halfway through the thickness of the lower baffle plate  54   b , and thus holes  114  may be characterized as having both a frustoconical section  117  and a cylindrical section  119 . The cylindrical section  119  of hole  114  has a height represented by d 3  in  FIG. 9 . 
     Finally,  FIG. 10  illustrates the micro-jet, low-energy ion generation region through the chamfered holes  114  of the lower baffle plate  54   b . As a result of the primary plasma discharge in the plenum above the baffles, a local sheath  122  is created in the holes  114 , thereby resulting in an electric field. Electrons and ions from the primary discharge enter the holes  114  and create a current path (indicated by dashed arrows  123 ) through each hole and to the outer surface  120  of the lower baffle plate  54   b . As the current lines (arrows  123 ) converge approaching the holes  114 , the current density increases causing the formation of a denser plasma in the hole, thereby forming the plasma jet which has a narrow plasma sheath. The increased plasma density of the microjet may also increase the neutral temperature which reduces the density of neutrals in the holes. The combination of these effects may increase the electron temperature and change the chemistry of the discharge in the micro-jet. In addition, the ions are also accelerated by the sheath and strike the inner surface  124  of the holes  114 , thereby ejecting secondary electrons. The narrower sheaths associated with high plasma density permit the acceleration of the electrons across the sheath  122  with few collisions resulting in the creation of very energetic electrons in the micro-jet. The secondary electrons gain enough energy so as to collide with neutral gas molecules, thereby ionizing them and creating a micro-jet shaped discharge  126  through the holes. 
     The formation of micro-jets in the grounded baffle plate  54   b  has the unique property of increasing the plasma density near the larger area electrode. In conventional capacitively coupled discharges, the plasma density is highest at the smaller electrodes because of the higher electric fields and greater RF current density at the smaller electrode, this smaller electrode being where the wafer resides. The use of the perforated lower baffle plate  54   b  as an electrode with holes therein that facilitate micro-jet formation increases the plasma density and power dissipation at the larger area electrode. The greater density decreases the plasma sheath thickness, thereby increasing the capacitance across the sheath and increasing the effective area ratio between the lower baffle plate  54   b  and the antenna  78  or  92 . The increased area ratio decreases the ion bombardment energy of the baffle plate which minimizes the sputtering of the baffle plate and corresponding contamination of the wafer with sputtering products. At the same time, the increased plasma density in the region of the lower baffle plate  54   b  near the wafer and greater electron temperature increases both the ion bombardment flux of the wafer and can induce unique plasma chemistries. Thereby, surface reactions are possible on the wafer that could not have otherwise occurred without the micro-jets. This ion bombardment occurs in a uniform manner such that charging effects on the wafer are mitigated. 
     The micro-jet discharge  126  is the primary source of ions that ultimately impinge on the wafer. Because the wafer is biased at its floating potential (and not that of the RF capacitive-electrode sheath potential of the secondary discharge as in conventional ashers), the ions strike the wafer at energies insufficient to cause damage. In addition, it has also been empirically determined that a low aspect ratio (diameter d 2  divided by height d 3 ) of the holes  114  enhances the micro-jet discharge. However, if the height of the holes  114  (i.e., thickness of the lower baffle plate  54   b ) is made too small, the capacity for water cooling of the lower baffle plate is eliminated. On the other hand, if the diameter of a hole is made too large so as to lower the aspect ratio, the effectiveness of the baffle plate in uniformly dispersing the plasma discharge is diminished. Accordingly, the holes  114  of the present invention embodiment(s) are configured so as to provide a lower aspect ratio for effective generation and transport of an ion rich plasma, through the holes to the wafer, while still allowing effective baffle plate cooling. 
     The sizing and aspect ratio of the holes  114  needed to generate reliable micro-jets therein is a function of the process conditions, including parameters such as plasma power, pressure, gas composition, etc. In this process, the “lighting” of micro-jets in the holes  114  is required to achieve uniform processing. The process described achieves the uniform and reliable lighting of the micro-jets to produce such uniform processing. This differs from other applications, such as etch tunnels, in which a perforated plate with smaller holes that do not reliably form micro-jets is used to create a “field-free region” within the tunnel where wafers can be processed with minimal ion bombardment. Similarly, the present invention embodiments differ from other prior art wherein a perforated plate with larger holes is used to make a simple capacitive electrode in which the plasma passes through the holes in the plate without the formation of micro-jets. 
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.