Abstract:
Aspects of the present disclosure describe cylindrical molds that may be used to produce cylindrical masks for use in lithography. A structured porous layer may be deposited on an interior surface of a cylinder. A radiation-sensitive material may be deposited over the porous layer in order to fill pores formed in the layer. The radiation-sensitive material in the pores may be cured by exposing the cylinder with a light source. The uncured resist and porous layer may be removed, leaving behind posts on the cylinder&#39;s interior surface. It is emphasized that this abstract is provided to comply with the rules requiring an abstract that will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.

Description:
FIELD OF THE INVENTION 
     Aspects of the present disclosure relate to a master mold for use in the manufacture a lithography mask and methods for manufacturing the same. 
     BACKGROUND 
     This section describes background subject matter related to the disclosed embodiments of the present invention. There is no intention, either express or implied, that the background art discussed in this section legally constitutes prior art. 
     Nanostructuring is necessary for many present applications and industries and for new technologies and future advanced products. Improvements in efficiency can be achieved for current applications in areas such as solar cells and LEDs, and in next generation data storage devices, for example and not by way of limitation. 
     Nanostructured substrates may be fabricated using techniques such as e-beam direct writing, Deep UV lithography, nanosphere lithography, nanoimprint lithography, near-field phase shift lithography, and plasmonic lithography, for example. 
     Earlier authors have suggested a method of nanopatterning large areas of rigid and flexible substrate materials based on near-field optical lithography described in International Patent Application Publication No. WO2009094009 and U.S. Pat. No. 8,182,982, which are both incorporated herein in their entirety. According to such methods, a rotatable mask is used to image a radiation-sensitive material. Typically the rotatable mask comprises a cylinder or cone with a mask pattern formed on its surface. The mask rolls with respect to the radiation sensitive material (e.g., photoresist) as radiation passes through the mask pattern to the radiation sensitive material. For this reason, the technique is sometimes referred to as “rolling mask” lithography. This nanopatterning technique may make use of Near-Field photolithography, where the mask used to pattern the substrate is in contact with the substrate. Near-Field photolithography implementations of this method may make use of an elastomeric phase-shifting mask, or may employ surface plasmon technology, where the rotating mask surface includes metal nano holes or nanoparticles. In one implementation such a mask may be a near-field phase-shift mask. Near-field phase shift lithography involves exposure of a radiation-sensitive material layer to ultraviolet (UV) light that passes through an elastomeric phase mask while the mask is in conformal contact with a radiation-sensitive material. Bringing an elastomeric phase mask into contact with a thin layer of radiation-sensitive material causes the radiation-sensitive material to “wet” the surface of the contact surface of the mask. Passing UV light through the mask while it is in contact with the radiation-sensitive material exposes the radiation-sensitive material to the distribution of light intensity that develops at the surface of the mask. 
     In some implementations, a phase mask may be formed with a depth of relief that is designed to modulate the phase of the transmitted light by π radians. As a result of the phase modulation, a local null in the intensity appears at step edges in the relief pattern formed on the mask. When a positive radiation-sensitive material is used, exposure through such a mask, followed by development, yields a line of radiation-sensitive material with a width equal to the characteristic width of the null in intensity. For 365 nm (Near UV) light in combination with a conventional radiation-sensitive material, the width of the null in intensity is approximately 100 nm. A polydimethylsiloxane (PDMS) mask can be used to form a conformal, atomic scale contact with a layer of radiation-sensitive material. This contact is established spontaneously upon contact, without applied pressure. Generalized adhesion forces guide this process and provide a simple and convenient method of aligning the mask in angle and position in the direction normal to the radiation-sensitive material surface, to establish perfect contact. There is no physical gap with respect to the radiation-sensitive material. PDMS is transparent to UV light with wavelengths greater than 300 nm. Passing light from a mercury lamp (where the main spectral lines are at 355-365 nm) through the PDMS while it is in conformal contact with a layer of radiation-sensitive material exposes the radiation-sensitive material to the intensity distribution that forms at the mask. 
     Another implementation of the rotating mask may include surface plasmon technology in which a metal layer or film is laminated or deposited onto the outer surface of the rotatable mask. The metal layer or film has a specific series of through nanoholes. In another embodiment of surface plasmon technology, a layer of metal nanoparticles is deposited on the transparent rotatable mask&#39;s outer surface, to achieve the surface plasmons by enhanced nanopatterning. 
     The abovementioned applications may each utilize a rotatable mask. The rotatable masks may be manufactured with the aid of a master mold (fabricated using one of known nanolithography techniques, like e-beam, Deep UV, Interference and Nanoimprint lithographies). The rotatable masks may be made by molding a polymer material, curing the polymer to form a replica film, and finally laminating the replica film onto the surface of a cylinder. Unfortunately, this method unavoidably would create some “macro” stitching lines between pieces of polymer film (even if the master is very big and only one piece of polymer film is required to cover entire cylinder&#39;s surface one stitching line is still unavoidable). 
     Thus, there is a need in the art to produce a master mask that is capable of producing replicas that do not have stitching lines. 
     SUMMARY 
     Aspects of the present disclosure describe a mold and methods for manufacturing molds that may be useful in the fabrication of lithography masks, for example, near-field optical lithography masks for “Rolling mask” lithography, or masks for nanoimprint lithography. In rolling mask lithography, a cylindrical mask is coated with a polymer, which is patterned with desired features in order to obtain a mask for phase-shift lithography or plasmonic printing. The features that are patterned into the polymer may be patterned through the use of the molds described in the present application. The molds may include patterned features that protrude from an interior surface of an optically transparent cylinder. The protruding features may range in size from about 1 nanometer to about 100 microns, preferably from about 10 nanometers to about 1 micron, more preferably from about 50 nanometers to about 500 nanometers. The mask can be used to print features ranging in size from about 1 nanometer to about 1000 nanometers, preferably about 10 nanometers to about 500 nanometers, more preferably about 50 nanometers to about 200 nanometer 
     A first aspect of the present disclosure describes a mold that may be made with a porous mask. A layer of structured porous material may be deposited or grown on an interior surface of an optically transparent cylinder. One example of grown porous material is a porous alumina fabricated using anodization of aluminum layer (Anodized Aluminum Oxide—AAO). The interior of the cylinder may then be coated with a radiation-sensitive material. The radiation-sensitive material will fill in the pores that are formed in the structured porous material. The radiation-sensitive material may then be developed by exposing the exterior of the cylinder with a light source. Exposure from the exterior allows the radiation-sensitive material that has filled the pores to be cured without curing the remaining resist. The uncured resist and the porous mask material may be removed, thereby forming a mold that has posts extruding from its interior surface. 
     According to an additional aspect of the present disclosure, an epitaxial layer may be grown on the interior surface of the cylinder. Structured porous material may then be deposited or otherwise formed on the epitaxial layer. The epitaxial layer may then be grown using the pores in the porous layer as a guide. The epitaxial layer may be grown to a thickness greater than the structured porous layer, or the structured porous layer may be etched back to leave the epitaxial post behind. According to certain aspects of the present disclosure, the epitaxial material may be a semiconductor material. Each of the epitaxial posts may be configured to be a light emitting diode (LED). The LED posts may further be configured to be individually addressable such that radiation may be selectively produced by individual posts. 
     According to an additional aspect of the present disclosure, the mold may be formed with a self-assembled monolayer of nanospheres. The monolayer may be formed over a layer of radiation-sensitive material that has been formed on the interior surface of a cylinder. The radiation-sensitive material may then be exposed by a light source located in the interior of the cylinder. The self-assembled monolayer masks portions of the radiation-sensitive material during exposure. The exposed regions may then be removed by a developer. The radiation-sensitive material that was shielded by the self-assembled monolayer may then be cured and in order to form posts that are made from a glass-like substance. 
     According to an additional aspect of the present disclosure, a self-assembled monolayer of nanospheres formed may comprise quantum dots. The quantum dots may be formed over a layer of radiation-sensitive material that has been formed on the interior surface of a cylinder. The quantum dots may be used to expose the radiation-sensitive material directly below each dot. As such, there may be no need for an external light source. The developer may then remove the unexposed portions of the radiation-sensitive material. The exposed portions of the radiation-sensitive material may then be cured to form a glass-like substance. 
     According to an additional aspect of the present disclosure, a self-assembled monolayer of nanospheres may be formed on the exterior surface of the cylinder and a radiation-sensitive material may be formed on the interior surface of the cylinder. A light source positioned outside of the cylinder may be used to produce the radiation that exposes the radiation-sensitive material. The nanospheres may mask portions of the radiation-sensitive material from the radiation. The exposed portions may be removed with a developer, thereby leaving behind posts. The posts may be cured to produce a glass-like material. 
     According to an additional embodiment of the present invention the self-assembled monolayer may comprise quantum dots. The quantum dots may be formed on an exterior surface of a cylinder. The quantum dots may be used to expose portions of a radiation-sensitive material that has be formed on an interior surface of the cylinder. As such, there may be no need for an external light source. The developer may then remove the unexposed portions of the radiation-sensitive material. The exposed portions of the radiation-sensitive material may then be cured to form a glass-like substance. The radiation-sensitive material that has been formed on the interior surface of a cylinder. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating “Rolling mask” near-field nano lithography. 
         FIG. 2  is an overhead view of a cylinder master mold with protrusions extending out from the interior surface according to an aspect of the present disclosure. 
         FIGS. 3A-3G  are schematic diagrams that show the process of forming the master mold according to aspects of the present disclosure. 
         FIGS. 4A-4D  are schematic diagrams that show the process of forming the master mold according to additional aspects of the present disclosure that utilize an epitaxial seed layer. 
         FIGS. 5A,5B,5B ′, 5 C are schematic diagrams that show the process of forming the master mold according to additional aspects of the present disclosure that utilize self-assembled monomers formed on the interior of the master mold. 
         FIGS. 6A,6B,6B ′, 6 C are schematic diagrams that show the process of forming the master mold according to additional aspects of the present disclosure that utilize self-assembled monomers formed on the exterior surface of the master mold. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     As a preface to the detailed description, it should be noted that, as used in this specification and the appended claims, the singular forms “a”, “an”, and “the” include plural referents, unless the context clearly dictates otherwise. 
     In the following Detailed Description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology, such as “top,” “bottom,” “front,” “back,” “leading,” “trailing,” “above”, “below”, etc., is used with reference to the orientation of the figure(s) being described. Because components of embodiments of the present invention can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. 
     A “Rolling mask” near-field nanolithography system has been described in International Patent Application Publication Number WO2009094009, which has been incorporated herein by reference. One of the embodiments is shown in  FIG. 1 . The “rolling mask” consists of glass (e.g., fused silica) frame in the shape of hollow cylinder  11 , which contains a light source  12 . An elastomeric film  13  laminated on the outer surface of the cylinder  11  has a nanopattern  14  fabricated in accordance with the desired pattern. The rolling mask is brought into a contact with a substrate  15  coated with radiation-sensitive material  16 . 
     A nanopattern  14  can be designed to implement phase-shift exposure, and in such case is fabricated as an array of nanogrooves, posts or columns, or may contain features of arbitrary shape. Alternatively, nanopattern can be fabricated as an array or pattern of nanometallic islands for plasmonic printing. The nanopattern on the rolling mask can have features ranging in size from about 1 nanometer to about 100 microns, preferably from about 10 nanometers to about 1 micron, more preferably from about 50 nanometers to about 500 nanometers. The rolling mask can be used to print features ranging in size from about 1 nanometer to about 1000 nanometers, preferably about 10 nanometers to about 500 nanometers, more preferably about 50 nanometers to about 200 nanometers. 
     The nanopattern  14  on the cylinder  11  may be manufactured with the use of a master mold. Aspects of the present disclosure describe the master methods and methods for forming a mold that has features that will form a nanopattern  14  that has holes or depressions. In order to form holes or depressions in the rolling mask, the master mold may have protrusions, such as posts. 
       FIG. 2  is an overhead view of a master mold  200  according to an aspect of the present disclosure. The master mold  200  is a hollow cylinder  220  that has an exterior surface  221  and an interior surface  222 . The cylinder  220  may be made from a material that is transparent to radiation that is in the visible and/or ultraviolet wavelengths. By way of example, and not by way of limitation, the cylinder may be a glass such as fused silica. The master mold  200  has protrusions  233  that extend inwards from the interior surface  222  toward an interior of the cylinder  220 , e.g., extending from the interior surface  222  towards a central axis of the cylinder. 
       FIGS. 3A-3G  are cross sectional views of the master mold  200  as seen along the line  3 - 3  shown in  FIG. 2 . Each figure depicts a processing step used in the fabrication of the master mold  200  according to aspects of the present disclosure. 
       FIG. 3A  is a depiction of the master mold after a structured porous layer  330  on an interior surface of the cylinder  320 . By way of example, and not by way of limitation, the, cylinder  320  may be made of a transparent material, such as fused silica. It is noted that fused silica is commonly referred to as “quartz” by those in the semiconductor fabrication industry. Although quartz is common parlance, “fused silica” is a better term. Technically, quartz is crystalline and fused silica is amorphous. The structured porous layer  330  contains a high density of cylindrical pores  329  that are aligned perpendicular to the surface on which the structured porous layer is disposed. The size and density of the pores  329  may be in any range suitable for the desired features of the mask pattern, e.g., as discussed above with respect to  FIG. 2 . By way of Example and not by way of limitation, the nanostructured porous layer  330  may be a layer of anodic aluminum oxide (AAO) that has been formed on an interior surface  322  of the cylinder  320 . AAO is a self-organized nanostructured material containing a high density of cylindrical pores that are aligned perpendicular to the surface on which the AAO layer is disposed. The AAO may be formed by depositing a layer of aluminum on the interior surface  322  of a cylinder  320  made of fused silica and then anodizing the aluminum layer. Alternatively, the cylinder  320  may be made completely from aluminum, and then internal or external surfaces of such a cylinder could be anodized to form a porous surface. Anodizing the aluminum layer may be done by passing an electric current through an electrolyte (often an acid) with the aluminum layer acting as a positive electrode (anode). 
     In alternative implementations, the nanostructured porous layer may be fabricated using a self-assembled monolayer or by direct writing techniques, such as laser ablation or ion beam lithography. 
     As shown in  FIG. 3A , the pores  329  may not penetrate through the entire depth of the layer  330 . If the pores  329  do not extend through the structured porous layer  330  down to the interior surface  322  of the cylinder, the material of the structured porous layer may be etched back with an etch process. If the etch process is isotropic, the original size of the pores  329  must be made small enough to account for growth during the etching process. For example, if the final diameter of the pores is desired to be 300 nm, and the original diameter of the pores  329  is 50 nm, then the isotropic etch must remove 125 nm of porous material in order to enlarge the diameter of the pores  329  to 300 nm. Additionally, if the etch process is isotropic, only 125 nm of material may be removed from the bottom of the pore in order to extend the pore to the interior surface  322  of the cylinder. If more material needs to be removed in order to reach the interior surface  322 , then the diameter of the pores  329  may become larger than desired.  FIG. 3B  depicts the enlarged pores  329  that completely extend through the nanostructured porous layer  330 . 
     After the pores  329  have been etched to the proper dimensions and depths, a radiation-sensitive material  331  may be deposited over the nanostructured porous layer  330  and the exposed portions of the interior surface  322 , as shown in  FIG. 3C . By way of example, and not by way of limitation, the radiation-sensitive material  331  may be deposited by dipping, spraying, rolling, or any combination thereof. By way of example, and not by way of limitation, the radiation-sensitive material  331  may be a photoresist or a UV curable polymer. Examples of suitable photoresists include commercially available formulations such as TOK iP4300 or Shipley 1800 series from Dow Chemical Co. Examples of suitable UV-curable materials include UV polymerizable adhesives for polymers and glass. Additionally, the radiation-sensitive material  331  contains silicon and other constituents that enable the material to be annealed after it has cured in order to produce a glass-like material. Other constituents that may be used to help form the glass-like material include Oxygen and Silicon. The radiation-sensitive material  331  may be a solid film, or it may be a liquid layer as long as it does not flow excessively during exposure. 
     Next,  FIG. 3D  shows the cured material  332  in the pores  329 . The radiation-sensitive material  331  is cured by exposure to a radiation  323  from a radiation source (not shown). By way of example, and not by way of limitation, the radiation  323  may be produced by a radiation source that produces ultraviolet light or the radiation  323  may be produced by a radiation source that produces light in the visible spectrum. The radiation source may be located outside of the cylinder and may emit radiation  323  that passes through the wall of the cylinder  320 . The illumination through the cylinder  320  limits the exposure to the material  331  deposited in the AAO pores  329 . Additionally, the exposure cures the material  331  to a depth of roughly twice the exposure wavelength. By way of example, when an ultraviolet wavelength is used for curing, then the cured material  332  may have a thickness of approximately 600 nm. The curing sensitivity of the radiation-sensitive material  331  must be sufficiently high to allow the radiation-sensitive material inside the pores  329  to be cured before the material  331  above the pores  329  is cured. Also, the depth of the pores  329  may be greater than the projected thickness of the cured material  332  in order to prevent exposure of the radiation-sensitive material  331  directly above the pores  329 . 
       FIG. 3E  shows the master mold  300  after the excess radiation-sensitive material has been removed after the cured material  332  has been formed. The remaining unexposed radiation-sensitive material  321  may be removed with a developer or other solvent. Thereafter, as shown in  FIG. 3F , the cured material  332  is annealed in order to form a glass-like material  333 . Finally, once the annealing is completed, the AAO layer  330  may be selectively etched away with a wet etching process.  FIG. 3G  depicts the final structure of the master mold  300 . The glass-like material  333  protrudes from the interior surface  322  of the cylinder  320 . 
     According to an additional aspect of the present disclosure, the protrusions may be formed through an epitaxial growth process.  FIG. 4A  is an overhead view of a master mold  400 . The master mold  400  is a hollow cylinder  420  that has an exterior surface  421  and an interior surface  422 . The cylinder  420  may be made from a material that is transparent to radiation that is in the visible and/or ultraviolet wavelengths. By way of example, and not by way of limitation, the cylinder may be a glass such as fused silica. An epitaxial seed layer  424  may be formed on the interior surface  422 . By way of example, and not by way of limitation, the epitaxial seed layer  424  may be a semiconductor material such as silicon or gallium arsenide (GaAs). The master mold  400  has protrusions  433  that extend outwards from the epitaxial seed layer  424 . The protrusions may be the same material as the epitaxial seed layer  424 .  FIGS. 4B-4D  are cross-sectional views of the master mold  400  along the line  4 - 4 . 
       FIG. 4B  is a depiction of a structured porous layer  430  that is deposited over the epitaxial seed layer  424 . As shown in  FIG. 4B , the pores  429  might not penetrate through the entire depth of the structured porous layer  430 . 
     When the pores  429  do not extend through the structured porous layer  430  down to the epitaxial seed layer  424 , then the structured porous layer material may be etched back with an etch process. If the etch process is isotropic, the original size of the pores  429  must be made small enough to account for growth during the etching process. For example, if the final diameter of the pores is desired to be 300 nm, and the original diameter of the pores  329  is 50 nm, then the isotropic etch must remove 125 nm of aluminum in order to enlarge the diameter of the pores  429  to 300 nm. Additionally, if the etchant is an isotropic etchant, only 125 nm of material may be removed from the bottom of the pore in order to extend the pore to the epitaxial seed layer  424 . If more material needs to be removed in order to reach the epitaxial seed layer  424 , then the diameter of the pores  429  may become larger than desired.  FIG. 4C  depicts the enlarged pores  429  that completely extend through the structured porous layer  430 . 
     Once the pores  429  have been completed, the protrusions  433  may be formed with an epitaxial growth process, such as, but not limited to vapor-phase epitaxy (VPE). The growth of the protrusions  433  is guided by the pores  429  in the structured porous layer  430 . The protrusions  433  may be grown to a height that allows them to protrude beyond the structured porous layer  430 . However, the protrusions  433  may be shorter than the structured porous layer  430 , if the structured porous layer will be subsequently etched back in order to expose the protrusions  433 . 
     According to aspects of the present disclosure, protrusion  433  formed through epitaxial growth of a semiconductor material may further be configured to be LEDs. Each of the protrusions  433  may be individually addressable such that each may be controlled to emit light as desired. This is beneficial for use as a master mold, because the molding process no longer requires an external light source. The protrusions  433  may function as a physical mold, and may be used to cure the photomask that is being molded at the same time. Further, the ability to control individual protrusions allows for a single master mold to be utilized in order to form multiple different patterns by selecting which protrusions will also cure the material in the photomask. 
     According to yet another additional aspect of the present disclosure, a self-assembled monolayer may be used as a mask to pattern the protrusions  533  in a master mold  500 .  FIGS. 5A-5C  are cross-sectional views of a master mold  500  at different processing steps during the mold&#39;s fabrication.  FIG. 5A  depicts the formation of a self-assembled monolayer (SAM)  540  formed over a radiation-sensitive material  531  on the interior surface  522  of the cylinder  520 . By way of example, and not by way of limitation, the SAM  540  may be formed from metal nanospheres, or quantum dots. By way of example, and not by way of limitation, the radiation-sensitive material  531  may be photoresist or a UV curable polymer. Additionally, the radiation-sensitive material  531  contains silicon and other constituents that enable the material to be annealed in order to produce a glass-like material. 
     Next, at  FIG. 5B , the radiation-sensitive material  531  is exposed with radiation  523  from a radiation source (not shown). Plasmonic lithography may be utilized, e.g., if the SAM  540  comprises metal nanospheres. The metal nanospheres may be used as plasmonic mask antennae. The portions of the radiation-sensitive material  531  that are exposed to radiation may become soluble to a developer solvent used to develop the radiation-sensitive material. The portion of the radiation-sensitive material that is unexposed  532  may remain insoluble to the developer solvent. It is noted that alternative aspects of the present disclosure include use of a reverse tone process in which portions of the radiation-sensitive material  531  that are exposed to radiation become insoluble to a developer and portions of the radiation-sensitive material that are not so exposed remain soluble to the developer. Alternative aspects of the present disclosure where the SAM  540  comprises quantum dots may not need an additional light source to expose the radiation-sensitive material  531 . As shown in  FIG. 5B ′ the quantum dots in the SAM  540  may be activated in order to expose the radiation-sensitive material  531 . When the exposure is made by the quantum dots, the radiation-sensitive material may be cured by the exposure. The non-exposed portions of the radiation-sensitive material  531  may therefore be removed by the developer. Finally, in  FIG. 5C  the protrusions  533  are annealed in order to convert the cured radiation-sensitive material  532  into glass-like material. 
     Alternative aspects of the present disclosure include implementations in which the mask itself is made with light emitting diodes (LEDs). Such a mask may be implemented, e.g., using a polymer mask with an array of holes smaller than features that are desired to be printed, with a corresponding layer of LEDs above it. A specific subset of the LEDs may be turned on to define the pattern to be printed. 
     According to an additional aspect of the present disclosure, a SAM  640  may be formed on the exterior surface  621  of the cylinder  620  as show in  FIG. 6A . The SAM  640  may be substantially similar to the SAM  540 . The formation of a SAM  640  on the exterior surface allows for the light used for the exposure to originate from outside of the cylinder  620  as shown in  FIG. 6B . In  FIG. 6B , the radiation-sensitive material  631  may be exposed with radiation  623  that is emitted by a radiation source (not shown) that is located outside of the cylinder  620 . Alternatively, if the SAM  640  comprises quantum dots, then the radiation source that produces the radiation  623  may be omitted, and the quantum dots may be used to expose the radiation-sensitive material  631  instead, as shown in  FIG. 6B ′. Finally,  FIG. 6C  shows the removal of the non-exposed radiation-sensitive material, and the annealing of the protrusions  633  to form the glass-like material. 
     While the above is a complete description of the preferred embodiments of the present invention, it is possible to use various alternatives, modifications, and equivalents. Therefore, the scope of the present invention should be determined not with reference to the above description but should, instead, be determined with reference to the appended claims, along with their full scope of equivalents. Any feature, whether preferred or not, may be combined with any other feature, whether preferred or not. In the claims that follow, the indefinite article “A” or “An” refers to a quantity of one or more of the item following the article, except where expressly stated otherwise. The appended claims are not to be interpreted as including means-plus-function limitations, unless such a limitation is explicitly recited in a given claim using the phrase “means for”. Any element in a claim that does not explicitly state “means for” performing a specified function, is not to be interpreted as a “means” or “step” clause as specified in 35 USC §112, ¶6.