Abstract:
The subject matter discloses a system for regenerating and treating slurry in a semiconductors manufacturing environment, the system comprising a removing module for removing the slurry from the manufacturing environment a receiving module for receiving the slurry and storing it. The system also comprises an addition unit for adding chemical materials to the slurry received by the receiving module, thus creating a qualified regenerated slurry and a pipe for maneuvering the slurry containing the added chemical materials to the manufacturing to environment for another use.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Technical Field 
         [0002]    The present disclosure relates to manufacturing wafer in general, and to use of a slurry material in particular. 
         [0003]    2. Discussion of the Related Art 
         [0004]    In the fabrication of semiconductor products, such as CPUs and various memory devices, the Chemical Mechanical Process (CMP) technique is a global planarization process, which can provide flat and polished surface for a semiconductor wafer. This main semiconductor technology provides planarization of semiconductor wafers. 
         [0005]    In a CMP process, the surface of the wafer that is to be polished, also referred to as the process surface of the wafer, is placed upside down on a polishing pad on a CMP machine. The polishing pad can be rotated to rub against the process surface of the wafer. During the CMP process, a chemical agent, which is customarily referred to as slurry, is applied on the wafer to assist the polishing pad in polishing the process surface of the wafer. Other CMP processes use an abrasive containing pad while chemicals and water are applied to the pad to assist the polish. 
         [0006]    Slurry constitutes of an abrasive material such as fumed or colloidal silica, dispersed alumina, or dispersed Ceria suspended in water. Additional chemicals are added to the mixture—either acidic or alkaline solution, depending on the substrate needed to be polished. Typical acid would be Hydrogen peroxide and typical base would be KOH but not limited to the above. Additional chemicals added to the slurry may be proprietary of the slurry manufacturer to enhance certain feature of the polish such as polish rate selectivity. 
         [0007]    Slurry is continuously poured during the polish step of the substrate while the substrate passes on mainly fresh slurry continuously. Spent slurry remaining on the pad after the substrate pass is typically discarded off the polish pad sideways and drained out. After the completion of each CMP process, the slurry remnants left on the polishing pad should be cleaned away prior to conducting a subsequent CMP process on another wafer. 
         [0008]    A system and process for regeneration and reuse of slurry is hence a long felt need. 
       SUMMARY OF THE PRESENT INVENTION 
       [0009]    It is an object of the subject matter to disclose a system for regeneration of slurry used in a semiconductors manufacturing environment. The system comprises a detecting element for detecting when the slurry can be taken from the manufacturing environment and a removing module for removing the slurry from the manufacturing environment. The system also comprises an activating unit for determining when to remove the slurry material from the manufacturing environment. 
         [0010]    In some embodiments, the removing module operates continuously and the system further comprises a maneuvering mechanism for maneuvering the removing module according to commands. In some embodiments, the removing module is a suction mechanism for sucking the slurry from the manufacturing environment into a container. 
         [0011]    In some embodiments, the system further comprises a pump for separating the air from the collected slurry. In some embodiments, the system further comprises a conveying pipe for conveying the collected slurry to the manufacturing environment for reuse. 
         [0012]    It is another object of the subject matter to disclose a method for regeneration of slurry used in a semiconductors manufacturing environment. The method comprises receiving an indication concerning the time in which the slurry can be taken from the manufacturing environment and determining when to remove the slurry material from the manufacturing environment. The method also comprises activating a collection mechanism for collecting the slurry from the manufacturing environment and collecting the slurry from the manufacturing environment. 
         [0013]    In some embodiments, the method further comprises a step of detecting the manufacturing environment before sending an indication concerning the time in which the slurry can be taken from the manufacturing environment. In some embodiments, the method further comprises a step of maneuvering the collection mechanism to an area in the manufacturing environment before collecting slurry to facilitate collection of the slurry. The method may also comprise a step of maneuvering the collection mechanism when collecting the slurry. 
         [0014]    It is another object of the subject matter to disclose a system for treating slurry in a semiconductors manufacturing environment. The system comprises a receiving module for receiving the slurry from the manufacturing environment after the slurry was used for manufacture and an addition unit for adding chemical materials to the slurry received by the receiving module, thus creating qualified regenerated slurry. The system also comprises a pipe for maneuvering the slurry containing the added chemical materials to the manufacturing environment for another use. 
         [0015]    In some embodiments, the addition unit continuously adds the chemical materials while receiving frequent indications from a detecting module that detects parameters of materials within the slurry. In some embodiments, the detecting module provides a binary indication, either more of a specific material is required to be added to the slurry or not. In some embodiments, the slurry is circulated in a pipe connected to a container, and wherein the slurry is detected in more than one location in the pipe. 
         [0016]    It is another object of the subject matter to disclose a method of treating slurry collected from a semiconductor manufacturing environment. The method comprises receiving the slurry collected from the manufacturing environment and adding chemical materials to the collected slurry, such that the collected slurry added with the chemical materials is qualified for reuse in the manufacturing environment. The method also comprises conveying the slurry added with chemical material to an element in the manufacturing environment. 
         [0017]    In some embodiments, addition of the chemical materials is performed continuously after frequently detecting parameters of materials in the slurry. In some embodiments, only a portion of the difference between the desired amount of a material and a current amount of a material is added to the slurry. In some embodiments, the element in the manufacturing environment is a slurry tank from which slurry is provided to the semiconductor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    Exemplary non-limited embodiments of the disclosed subject matter will be described, with reference to the following description of the embodiments, in conjunction with the figures. The figures are generally not shown to scale and any sizes are only meant to be exemplary and not necessarily limiting. Corresponding or like elements are designated by the same numerals or letters. 
           [0019]      FIG. 1  schematically illustrates a manufacturing environment of semiconductors, in accordance with an exemplary embodiment of the disclosed subject matter; 
           [0020]      FIG. 2  shows a collection mechanism, according to some exemplary embodiments of the disclosed subject matter; 
           [0021]      FIG. 3  shows an apparatus for treating collected slurry, according to some exemplary embodiments of the disclosed subject matter; 
           [0022]      FIG. 4  shows a flow of collecting the slurry from a manufacturing environment of semiconductors, according to some exemplary embodiments of the disclosed subject matter; and, 
           [0023]      FIG. 5  shows a flow of treating the collected slurry, according to some exemplary embodiments of the disclosed subject matter. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0024]    The technical problem dealt with in the disclosed subject matter is to reduce costs of wafer manufacture. Another technical problem is to reduce the amount of slurry material used in wafer manufacture. Another technical problem is to regenerate slurry material and to minimize changes on the material before reuse. 
         [0025]    One exemplary technical solution suggested in the subject matter is a system comprising a suction mechanism for sucking slurry material or other byproducts of the wafer manufacturing process. Such suction mechanism may be connected to an activating module for determining when to suck the byproduct from the pad, the wafer or generally from the manufacturing environment. Such activating module may be a maneuvering unit that maneuvers the suction mechanism when suction is enabled, for example between subsequent CMP processes. The activating module may receive data from detectors within the manufacturing environment to indicate when the suction mechanism may perform suction. The activating module may alternatively receive data from the manufacturing unit. The suction mechanism may perform sucking only in predefined periods, for example when suction is enabled, or perform suction continuously and a maneuvering unit may maneuver the suction mechanism towards the pad, wafer or surface from which the material or byproducts are sucked from, when suction is enabled. 
         [0026]      FIG. 1  schematically illustrates a manufacturing environment of semiconductors, in accordance with an exemplary embodiment of the disclosed subject matter. The manufacturing environment  100  comprises a pad  110  on which a wafer  115  is mounted when polished. The pad  110  may be positioned on a rotating plate  120  such that the wafer  115  is capable of rotating when polished to provide uniform polishing of the wafer  115 . 
         [0027]    The slurry material is provided on the pad surface using a slurry pipe  130  having a slurry-dispensing tip  135 . The amount of slurry material and the flow of said slurry material may be determined a processing module (not shown) that may be located in the body  140  of the manufacturing machine. Wafer is sliding on the slurry wetted surface of the pad and is chemically mechanically polished at that process. After polishing the wafer, the slurry material and the polish byproducts are spread on the manufacturing environment  100 , for example on the pad  110  and drained to a drainage tube  150  or vessel (not shown). 
         [0028]    A collection apparatus  170  is provided by the disclosed subject matter for collecting the slurry material and some other byproducts of the manufacturing environment  100 . The collection apparatus  170  may collect slurry material from the pad  110 , a plate  140  on which the pad  110  is mounted, the drainage tube  150  or other elements in the manufacturing environment  100  desired by a person skilled in the art. The collection apparatus  170  may comprise a suction module for sucking the slurry material from the manufacturing environment  100  into a slurry container (not shown). Such suction module may operate continuously or intermittently, according to the semiconductor, the environment or the production is specifications. The collection apparatus  170  may be a spoon-like mechanical tool for mechanically collecting the slurry material from the pad without using the power required for sucking fluid into a container. In some exemplary embodiments of the system, slurry is pumped into the pipe  170 . The pipe may be flexible at some sections to allow bending. The tip of the pipe is made in a way to create a vacuum chamber, ahead of a gliding surface. The vacuum chamber enables airflow from the front end of the tip into the vacuum chamber and later into the pipe. The slurry droplets will be raised by the gliding surface. The tip is held in place loosely by a mechanical arm. The mechanical arm can be moved towards the surface of the pad and back. The motion has to allow total separation of the head from the pad surface so when needed, pumping of slurry will be stopped. 
         [0029]    The collection apparatus  170  may be connected to a container within the manufacturing environment  100 , from which the slurry is provided to the slurry pipe  130 . As a result, the collected slurry is regenerated in the same machine. Using the slurry in the machine from which it is collected provides for efficient and selectivity in the slurry material, for example, as opposed to embodiments in which the slurry material is collected from various and different machines or manufacturing environments into a central container. The mix of various slurry materials is then supplied to the various machines from the central container, while losing the specific qualities and characteristics of each slurry material when mixed with other materials in the central container. 
         [0030]      FIG. 2  shows a collection mechanism, according to some exemplary embodiments of the disclosed subject matter. In some exemplary embodiments, the collection mechanism  200  is a suction mechanism. The suction mechanism  220  is provided to suck the slurry material from the manufacturing environment  260 , for example from a main region  270  containing the pad (such as  110  of  FIG. 1 ) and the wafer (such as  115  of  FIG. 1 ). The suction mechanism  220  of an exemplary embodiment of the disclosed subject matter sucks air into a container  250  (collection and separation tank) continuously. In some cases, the slurry can be sucked intermittently, for example between subsequent CMP processes, or polishing events. 
         [0031]    In some exemplary embodiments of the disclosed subject matter, a maneuvering unit  230  connected to the suction mechanism  220  may be provided, for maneuvering the suction mechanism  220  only when slurry is to be sucked. Maneuvering the suction mechanism  220  is preferred when the time for activating said suction mechanism is longer than a predefined threshold, for example 0.2 seconds, in which the slurry can be collected but the suction is disabled. As a result, suction is continuous and operates in an area where the slurry is not collected from the manufacturing environment. When slurry can be collected, the maneuvering unit  230  maneuvers the suction mechanism  220  towards the main region  270  or to any region in the manufacturing environment  260  desired by a person skilled in the art. 
         [0032]    The suction mechanism  220  is connected to a collection and separation container  250  that receives the collected slurry. The collection and separation container  250  is connected to an air pump  255  that exhausts air from collection and separation container  250  to create vacuum in the collection and separation container  250 . The vacuum is the source of collecting energy for the suction mechanism  220 . The air pump  255  may operate continuously or intermittently, based on the conditions determined at a control unit  240 . The collection and separation container  250  may function as a liquid separator separating the slurry from the air coming from the suction mechanism  220 . The process of separation is a standard procedure known to a person skilled in the art. The collection and separation container  250  may be of a conic shape in its lower portion, to facilitate to absorption of heavy particles to the floor of the container  250 . The collection and separation container may be made of metal or plastic materials, such as polypropylene or polyethylene. 
         [0033]    In some embodiments of the disclosed subject matter, the collection and separation container  250  may be connected to the manufacturing environment  260 , such that the slurry can be reused in the same manufacturing machine or in another machine. The slurry is conveyed to the manufacturing environment  260  using a conveying pipe  280  and a liquid pump  290  that extracts the slurry out of the vacuum environment maintained in the collection and separation container  250 . The conveying pipe  280  connects to the bottom of the collection and separation container  250  and an element in the manufacturing environment  100 , for example the slurry pipe  238 . 
         [0034]    The liquid pump  290  may operate continuously to re-circulate slurry for treatment or according to signals arriving from level sensors attached to the collection and separation container  250 . In some exemplary embodiments of the disclosed subject matter, two level sensors are connected to the collection and separation container  250 . One sensor provides indication to a controller to open a valve or switch on the liquid pump  290  and allow slurry to exit to the next stage. The other sensor provides indication to close the above valve, thus preventing any air from entering the rest of the system. Other connections between the collection and separation container  250  and the slurry pipe  238  may include chemical, mechanical or electrical processes to improve the collected slurry material. As a result, the regenerated output from the slurry pipe  238  will be of a sufficient quality, or have chemical characteristics equivalent to the characteristics of the slurry material in the previous CMP process and will be explained hereafter. 
         [0035]    In some exemplary embodiments of the disclosed subject matter, the system comprises a re-circulation pipe  285  connects the conveying pipe  280  and the collection and separation container  250 . The re-circulation pipe  285  enables correction of the slurry while some of it is supplied for reuse by the conveying pipe. Some of the slurry is conveyed back to the collection and separation container  250 , where it is corrected by adding chemical materials, as disclosed below. 
         [0036]    The maneuvering unit  230  is connected to a power device  240  and may move the suction mechanism  220  laterally, radially, circularly or a combination thereof. In some exemplary embodiments, the length of the maneuvering unit  230  may be of a range between 20 to 120 centimeters. The power device  240  may provide energy to both the maneuvering unit  230  and the suction mechanism  220 . Such power may be electrical, mechanical, hydraulic and the like. The movement of the suction mechanism  220  can be made for example by pneumatic pistons or electrical motors that will transfer the suction mechanism  220  up and away from the manufacturing environment  260 . The maneuvering unit  230  may be of an articulated shape containing two or more connected arms, such that at least a portion of the arms move when moving the suction mechanism  220 . Alternatively, the maneuvering unit  230  may be a telescopic tube in which a plurality of interconnected concentric cylinders are located one inside the other, such that extending one or more cylinders from the cylinder in which they are contained provides extension of the maneuvering unit. 
         [0037]    The maneuvering unit  230  may be operated according to a command. A processing unit (not shown) may provide such command outside the manufacturing system. For example, the processing unit within the maneuvering unit  230  receives data, according to which, suction is required for consequent 3 seconds once every 32 seconds, the time required to finish the polish process or another manufacturing process in which slurry cannot be collected. 
         [0038]    In some cases, the data from the manufacturing unit may activate the collection unit, for example the suction mechanism  220 . In such case, the suction mechanism  220  operates intermittently and requires a command or data to activate. In another exemplary embodiment, the power device  240  generates or provides power to the air pump  255 , according to the specification of the collection apparatus. When the power device  240  stops providing energy to the suction mechanism, suction is not performed. As a result, when the suction to mechanism is provided over the main region  270 , the power device activates the suction mechanism  220 . Such activation may be a result of a signal or data received from the manufacturing unit. 
         [0039]    In accordance with some exemplary embodiments of the disclosed subject matter, the collection apparatus  200  is not connected to the manufacturing unit and one or more sensors  215  provide the activation of the collection mechanism, such as the suction mechanism  220 . Such one or more sensors  215  may be light sensors, humidity, heat, flow, may detect chemical reactions or any other optical or physical attribute desired by a person skilled in the art. For example, the sensors may detect that the slurry pipe  238  stopped providing slurry for 1.2 seconds. As a result, a signal or data are transmitted from the one or more sensors  215  to the maneuvering unit  230 , the power device  240  or to the collection unit, according to the specification of the collection apparatus  200 , as disclosed above. 
         [0040]    In some exemplary embodiments of the disclosed subject matter, activation of the collection mechanism, such as the suction mechanism  220 , is achieved using predefined programs or patterns that activate the activation mechanism of the collection apparatus  200 . For example, the manufacturing environment  100  operates for consecutive 10 hours. At that time, collection of slurry is required for the 10 th -12 minutes of every hour. No activation is required by the manufacturing unit or by sensors, since the collection mechanism is programmed to collect the slurry material in the predefined periods. As such, in the predefined periods either the collection mechanism begin working, or the maneuvering unit  230  maneuvers the collection mechanism towards the main region  270 . 
         [0041]      FIG. 3  shows an apparatus for treating collected slurry, according to some exemplary embodiments of the disclosed subject matter. The treating apparatus  300  is connected to the collection and separation container  250  for collecting the slurry material collected by the collection apparatus  200 . In the embodiment in which slurry is sucked, the slurry is provided to the collection and separation container  250  from the manufacturing environment via a pipe tip of the collection mechanism as explained above. The slurry is conveyed from the collection and separation container  250  using the liquid pump  290 . 
         [0042]    The treating apparatus  300  may comprise a treatment tank  320 . In some cases, the functions performed in the treatment tank  320  may be performed in the collection and separation container  250 . In such case, the treating apparatus  300  requires only one tank combining the functions of both the treatment tank  320  and the collection and separation container  250 . The treatment tank  320  may be connected to pump  330  for re-circulating the slurry material in the treatment tank  320 . In some exemplary embodiments of the disclosed subject matter, level sensors are connected to the tanks, such the treatment tank  320 . One sensor provides indication to a controller (not shown) to open a valve in one of the tanks and allow slurry to exit from the tanks in order to prevent from filling the system in slurry. The other sensor may provide indication to close slurry supply valve to the slurry pipe ( 238  of  FIG. 2 ), thus preventing any air from entering the system. The sensors may indicate to open and close the valves as a function of parameters values. Such parameters may be the volume of material in the treatment tank, humidity, transparency, color, density or other parameters desired by a person skilled in the art. 
         [0043]    In some exemplary embodiments of the disclosed subject matter, the treating apparatus  300  further comprises a chemical addition unit  350 . Such chemical addition unit  350  adds chemical materials to the slurry contained in a container of the treating apparatus, for example the treatment tank  320 . The chemical addition unit  350  may contain one or more tanks in which chemical materials or compounds are contained. The chemical addition unit  350  may contain a computerized module for receiving a command from an external unit, such as a manufacturing unit or a detecting module  340 , and provide the chemical materials according to the command. The chemical materials may be liquid, paste, cream, gas, or the like. The detecting module  340  comprises one or more sensors that continuously detect quality or level of one or more parameters. The determination of the materials to be added to the collected slurry may be done in any module in or communicating with the treating apparatus  300 , for example in the chemical addition unit  350  or the detecting module  340 . The parameters detected may be humidity, temperature, density, color, transparency, other optical property, chemical reactions with a specific material or object, electrical conductivity, viscosity and the like. 
         [0044]    In the exemplary embodiments of the disclosed subject matter, the treating apparatus  300  further comprises a conveyor (such as  280  as  FIG. 2 ) to convey the corrected slurry material to the manufacturing unit. The conveyor may comprise a pump or a valve connected to a pipe connected to the manufacturing unit. Alternatively, the correction is performed in the manufacturing unit and the conveying is done from the collection apparatus to the manufacturing unit as the slurry is substantial to the material collected from the manufacturing environment. 
         [0045]    In some exemplary embodiments of the disclosed subject matter, the correction is performed by a measure significantly smaller than the difference between the measured materials and the desired measures. For example, in case the sensors measure concentration of 1.2 percent of a material, and the desired concentration is 2 percents, the sensors or another computerized entity in the treating apparatus  300  may determine the difference in terms of volume or mass. For example, the mass required is 200 grains. In such case, the chemical addition unit  350  may add about 10 grams in each second, and simultaneously receive indications from the detecting module  340  as to the concentration of the specific material in the slurry. As such, the amount of material provided to by the chemical addition unit  350  is gradual and correction of the material is continuous and gradual. As a result, the chemical addition unit  350  may provide less material than in a case of providing the entire difference between the desired amount and current amount. Further, frequent detections by the detecting module  340  results in more accurate corrections, and provide detections in different portions of the slurry located in the pipes and in the tanks. 
         [0046]      FIG. 4  shows a flow of collecting slurry from a manufacturing environment of semiconductors, according to some exemplary embodiments of the disclosed subject matter. Step  405  provides for receiving indication concerning the time to collect slurry material. A power device that activates the collection apparatus, by the collection apparatus, may perform receiving the indication or by the maneuvering unit, that maneuvers the collection mechanism. The indication may be provided from a sensing device in the manufacturing environment, from the manufacturing unit and the like. The indication provides for a time in which slurry can be collected from the manufacturing environment. 
         [0047]    Step  410  provides for determining when to activate the collection mechanism. Such determination may be, for example, that slurry is to be collected for the next 34 seconds. Such determination may be a function of indication received from a sensor, from a module in the manufacturing environment, from the amount of slurry already collected and the like. 
         [0048]    Step  415  provides for activating the collection apparatus. For example, when the power device is enabled to supply power to the maneuvering unit. Alternatively, the collection mechanism operates intermittently and requires activation to start collecting slurry. In the case of intermittent operation, the collection mechanism may not require maneuver. 
         [0049]    In step  425 , the maneuvering unit maneuvers the collection apparatus that collects slurry from the manufacturing environment. The maneuvering stage may be radial, lateral and the like. Maneuvering may continue during collection of the slurry in case the collection mechanism, for example mechanical tool or suction mechanism, cannot collect a sufficient amount of slurry material when located in a specific location. 
         [0050]    In step  430 , the collection apparatus collects the slurry material from the manufacturing environment. The slurry material can be sucked, collected by a mechanical tool such as a spoon, or collected in other techniques desired by a person skilled in the art. The collection may be statically, for example positioning the suction unit in a specific location between subsequent CMP processes, or dynamic, in which the collection mechanism, for example spoon-like mechanical tool, is maneuvered while collecting the slurry material. 
         [0051]    In step  435 , the liquid slurry is separated from the air that has moved it into the collection and separation container. Air is drown from the top of the container while the heavy slurry remains at the bottom of the container. In step  440 , the slurry material is stored in a container connected to the collection mechanism. Such container may comprise a unidirectional valve for enabling only entrance of is the slurry material for a predefined period of time. In step  450 , the collected slurry material is conveyed to the manufacturing unit for reuse or for further treatment in the treatment unit. The convey of step  450  may be performed when detecting a predefined amount of slurry material in the container, or in case another parameter of the slurry material, such as color or density, has a sufficient value. 
         [0052]      FIG. 5  shows a flow of treating collected slurry, according to some exemplary embodiments of the disclosed subject matter. In step  505 , a container receives the slurry collected from the manufacturing environment. The container may be connected to the collection apparatus. For example, in case a suction mechanism is used, the pump of the suction mechanism is connected to a pipe that ends in a container which stores the slurry. The slurry in the container  505  is re-circulated constantly while passing through the next step. Slurry is pump from the bottom of the container  505  and returned to the top of it. By this, the slurry is constantly mixed with itself and with any added chemicals. 
         [0053]    In step  515 , the quality of the collected slurry is detected. For example, one or more sensors that are connected to the exit side of the container  505  detect the quality of predefined parameters. Such parameters may be humidity, density, amount of a specific material in the collected slurry, PH level, temperature or any other measurable attribute desired by a person skilled in the art. Next, in step  520 , according to a set of rules, a computerized unit within the system for treating the collected slurry, or communicating with the system, determines if a chemical material needed to be added to the slurry. 
         [0054]    In step  525 , a chemical addition unit adds chemical materials to the slurry material. Such chemical materials may be acidic material such as Hydrogen Peroxide or base type material such as Potassium hydroxide. The amount of each chemical material added is predetermined small steps to eventual reach the predetermined level. The size of the steps will take into account the volume of treated slurry and the time needed for response and can be calculated by those skilled in the art. The slurry material with the addition of the chemical material is not required to be identical to the slurry material provided in the previous CMP process. The quality required by the corrected slurry, with the addition of chemical material, is required to have substantially the same effect on the wafer and on other elements in the manufacturing environment, as the previously provided slurry. In step  530 , the slurry with the chemical addition is conveyed to an element in the manufacturing environment, for example a container connected to a slurry pipe such as pipe  238 . In step  540 , the quality of the corrected slurry material is detected. Such detection may be performed on the slurry before conveyed to the container. Alternatively, detection is performed after reuse of the corrected slurry, on the outcome of the CMP process, not on the slurry material. Such outcome may comprise the wafer, the pad, byproducts of manufacturing the semiconductors and the like. 
         [0055]    While the disclosure has been described with reference to exemplary embodiments, 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 without departing from the essential scope thereof. Therefore, it is intended that the disclosed subject matter not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but only by the claims that follow.