Abstract:
A monolithic optical pellicle and method of making used to protect a photomask during photolithography processing. The monolithic optical pellicle is comprised of a pellicle plate having a recessed central portion integrally formed with a perimeter frame of the pellicle plate such that it is a one-piece optical pellicle. The monolithic optical pellicle comprises a material of sufficient rigidity to minimize distortions in and maximize durability of the pellicle when used in combination with the recessed portion having a thickness that prevents sagging thereof due to applied forces on the resultant monolithic optical pellicle. This recessed central portion is the optical pellicle portion of the present monolithic optical pellicle, while the integral perimeter frame is used to attach the monolithic optical pellicle at the desired stand-off distance to a photomask. The monolithic optical pellicle preferably comprises a material that is transparent to an exposure field at about 157 nm wavelengths.

Description:
BACKGROUND OF INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates generally to optical pellicles, and more particularly, to monolithic hard pellicles for use in photolithography and methods of forming the same.  
         [0003]     2. Description of Related Art  
         [0004]     Pellicles have played an important role in the fabrication of semiconductor circuits. A conventional pellicle is a thin, optically transparent membrane stretched over a rigid metal frame for attachment to a photomask via such frame. In so doing, the pellicle is attached to the photomask such that the membrane is positioned at a distance spaced away from the photomask. The pellicle is then used in photolithography to protect the patterned photomask surfaces from contamination by airborne particles. That is, smaller particles and dust falling onto the pellicle plate is out of focus during the photolithography process, and will not therefore be reproduced in the silicon wafer.  
         [0005]     To fabricate a pellicle, a very thin membrane, normally in the range of 0.5 to 3.0 microns and usually made of organic or fluorocarbon polymers, is first formed on a substrate. The membrane is then removed and mounted to the rigid frame. These pellicle frames take many shapes, depending upon the photolithography equipment with which the pellicle is ultimately to be used. The frame always encloses the entire printable area of the mask so that the pellicle it supports is able to keep particles off this critical area.  
         [0006]     An emerging photolithography technique is the use of radiation having a 157 nm (nanometer) wavelength to expose the desired circuit pattern from the photomask to the resist layer on the wafer substrate. Extension of optical lithography to 157 nm has received widespread support as it offers the prospect of improved resolution based on decades of optical lithography experience. However, the development of pellicles for use in 157 nm lithography has been challenging due to the high energy of the photons, such as those at energies of about 7.9 eV. This is because most common lithographic materials available for the fabrication of thin (0.5 μm to 2.0 μm) protective pellicles used at 365 nm, 248 nm, and 193 nm exposure wavelengths are either not transparent enough and strongly absorb at the 157 nm wavelength; or are not durable enough to withstand more than a few minutes of exposure without severe degradation at the 157 nm exposure wavelength.  
         [0007]     Since no polymers have been found with sufficient radiation durability to be used as pellicles to protect photomasks from contamination at the 157 nm exposure wavelength, thick or hard quartz (crystallized silica) plate pellicles continue to be used at the 157 nm wavelength. The hard pellicles for use at such wavelength are conventionally made by attaching, such as by bonding, sealing, encapsulating, and the like, modified fused silica plates to fused silica frames. The pellicle and frame unit is then mounted onto photomask substrates via the fused silica frames to protect the patterned photomask surfaces from contamination by airborne particles.  
         [0008]     This conventional process of forming pellicles by attaching a frame to a fused silica plate typically results in small distortions or stresses that can have various effects on the pellicle itself as well as the photomask being protected by such pellicle. For instance, when distortions or stresses occur as a result of attaching a frame to a mask substrate to form a hard pellicle, these distortions and/or stresses are transferred to the hard pellicle, which in turn, create optical distortions or aberrations that distort the projected light during wafer exposure. Further, such stresses may undesirably distort the reticle or even cause stress-induced birefringence or double refraction, i.e., splitting of a light wave into two unequally reflected or transmitted waves, which may result in problems with wafer printing.  
         [0009]     As such, conventional processes and the resultant thick and/or hard pellicles formed by such processes are not suitable for efficiently, effectively and reliably protecting photomasks from contamination at the 157 nm exposure wavelength. Consequently, there is a need for improved methods for forming pellicles for use at the 157 nm exposure wavelength.  
       SUMMARY OF INVENTION  
       [0010]     Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide improved monolithic pellicles for use in photolithography and methods of forming the same.  
         [0011]     It is another object of the present invention to provide methods of forming monolithic pellicles and the monolithic pellicles formed that are suitable for efficiently, effectively and reliably protecting photomasks from contamination during photolithography exposure and storage, and will therefore not image any contamination on the wafer undergoing processing.  
         [0012]     A further object of the invention is to provide methods of forming monolithic pellicles and the monolithic pellicles formed by such methods suitable for use at or below 157 nm exposure wavelength.  
         [0013]     It is yet another object of the present invention to provide methods of forming monolithic pellicles and the monolithic hard pellicles with a single thermal expansion such that the monolithic hard pellicles inherently have low stress levels.  
         [0014]     Another object of the present invention is to provide methods of forming monolithic pellicles and the monolithic hard pellicles that are optically compatible with a stepper exposure process.  
         [0015]     Still another object of the present invention is to provide methods of forming monolithic pellicles and the monolithic hard pellicles that are robust for mounting and remounting in mask manufacturing such that the pellicle itself is not damaged during removal and reuse processes, thereby avoiding the need for the conventional rework of plate/frame bonding of a pellicle.  
         [0016]     Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.  
         [0017]     The above and other objects, which will be apparent to those skilled in art, are achieved in the present invention, which is directed to an optical pellicle that includes a transparent plate having a first thickness, a recessed portion of the transparent plate having a second thickness less than the first thickness, and a perimeter of the transparent plate having a third thickness and entirely surrounding the recessed portion such that the transparent plate comprises a monolithic optical pellicle. The first thickness may be an original thickness of the transparent plate, whereby the third thickness of the perimeter is the same as the original thickness. Alternatively, the third thickness may be less than the first thickness.  
         [0018]     In this aspect, the transparent plate may include a material such as silica, modified silica, modified fused silica quartz, and the like. It is preferred that the monolithic optical pellicles comprise a single material having a single thermal expansion. It also may be a material that is transparent to an exposure field at about 157 nm wavelengths or even lower. For example, wherein the first thickness of the transparent plate ranges from about 3 mm to about 6 mm, the second thickness of the recessed portion may range from about 200 μm to about 900 μm.  
         [0019]     In accordance with the invention, it is preferred that the second thickness of the recessed portion is at least thick enough to prevent sagging thereof due to applied forces on the monolithic optical pellicle. It is also preferred that the monolithic optical pellicle comprise a material of sufficient rigidity for preventing any damage and distortion thereto. The recessed portion extends into the transparent plate from a single surface thereof and stops at a depth within the transparent plate. In so doing, the perimeter comprises a frame portion and the recessed portion comprises an optical pellicle portion of the monolithic optical pellicle, whereby the frame and optical pellicle portions are a uniform one-piece structure.  
         [0020]     The monolithic optical pellicle is also provided with first and second optically flat surfaces, as well as the perimeter of the pellicle being provided with a plurality of openings traversing through the perimeter of the transparent plate. These openings may have a variety of shapes and sizes, and are used for introducing a gas flow over the recessed portion of the transparent plate upon mounting the monolithic optical pellicle to a photomask.  
         [0021]     In a second aspect of the invention, the present invention is directed to a method of forming an optical pellicle. The method includes providing a pellicle plate of a transparent material having a first thickness. A portion of this transparent material is then removed to transform the pellicle plate into a monolithic optical pellicle. The resultant monolithic optical pellicle comprises a recessed portion of the pellicle plate having a second thickness less than the first thickness that is entirely surrounded and integrally formed with a perimeter frame of the pellicle plate having a third thickness.  
         [0022]     The transparent material comprises a material of sufficient rigidity for preventing any stresses and damage to occur in the monolithic optical pellicle. It may also comprise a single material having a single thermal expansion that is transparent to an exposure field at about 157 nm wavelengths or even lower.  
         [0023]     The first thickness may comprise an original thickness of the pellicle plate, whereby the third thickness of the perimeter frame also of comprises such original thickness. Alternatively, the perimeter frame of the pellicle plate may be adjusted for a standoff distance between the monolithic optical pellicle and a photomask to which the monolithic optical pellicle is to be mounted. This is accomplished by the perimeter frame initially having the first thickness, and removing a predetermined thickness from the first thickness of the perimeter frame to provide the perimeter frame with the third thickness, which is less than the first thickness. The second thickness of the recessed portion is preferably at least thick enough to prevent sagging of the recessed portion due to applied forces on the monolithic optical pellicle.  
         [0024]     In this aspect of the invention, the pellicle plate is transformed into the monolithic optical pellicle by providing the pellicle plate into a processing chamber. The pellicle plate is masked on a first side to cover only a perimeter area of the plate such that a central portion of the pellicle plate on the first side remains exposed. This central portion is then processed by removing the transparent material of the pellicle plate from the exposed central portion on the first side. In so doing, the mask protects the perimeter area of the pellicle plate such that the transparent material at the perimeter area is maintained. This removal process stops at a predetermined distance within the pellicle plate to integrally form the recessed portion and the perimeter frame on the first side of the pellicle plate. The mask may then be removed to provide the present monolithic optical pellicle.  
         [0025]     The present monolithic optical pellicle is further planarized on first and second surfaces thereof to provide the pellicle with first and second optically flat surfaces. A plurality of openings may be proved that traverse through the perimeter frame for introducing a gas flow over the recessed portion of the pellicle plate.  
         [0026]     In another aspect, the invention is directed to a method of protecting a photomask during photolithography. The method includes providing a photomask, and attaching a monolithic optical pellicle thereto for protecting the photomask during subsequent photolithography processing. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0027]     The features of the invention believed to be novel and the elements characteristic of the invention are set forth with particularity in the appended claims. The figures are for illustration purposes only and are not drawn to scale. The invention itself, both its organization and method of operation, may best be understood by reference to the detailed description which follows taken in conjunction with the accompanying drawings in which:  
         [0028]      FIG. 1A  is an elevational view of a starting solid plate substrate within a processing chamber for forming the monolithic hard pellicle of the invention.  
         [0029]      FIG. 1B  is an elevational view of the starting solid plate substrate of  FIG. 1A  having a mask on a surface thereof for processing in accordance with the invention.  
         [0030]      FIG. 2A  is an elevational view of the resultant monolithic hard pellicle of the invention processed in accordance with  FIG. 1 .  
         [0031]      FIG. 2B  is an elevational view of a portion of the monolithic hard pellicle of  FIG. 2A  showing the pellicle as a single, unified solid structure.  
         [0032]      FIG. 2C  is a top plan view of the monolithic hard pellicle of  FIG. 2A .  
         [0033]      FIG. 2D  is a cross sectional view of the monolithic hard pellicle of  FIG. 2A . 
     
    
     DETAILED DESCRIPTION  
       [0034]     In describing the preferred embodiment of the present invention, reference will be made herein to  FIGS. 1A-2D  of the drawings in which like numerals refer to like features of the invention.  
         [0035]     The present invention is directed to rigid monolithic hard pellicles for use in photolithography and methods of forming the same. An essential feature of the invention is that the present monolithic hard pellicles eliminate the need to bond a modified fused silica plate to a silica frame, and thereby advantageously avoid any induced stresses and/or distortions associated with conventional pellicles having a frame attached to a transparent membrane as discussed above. The monolithic hard pellicles of the invention may be advantageously removed and remounted without distorting, damaging or changing its properties, particularly at its area located at its frame portion and pellicle portion as it is a one-piece uniform structure.  
         [0036]     Referring to the drawings,  FIG. 1  illustrates a single, hard starting pellicle plate  10  for processing and forming the monolithic pellicle of the invention. This single pellicle plate  10  has sufficient rigidity for processing in accordance with the invention, and may comprise any known suitable material that is transparent to any desired wavelength range used in optical lithography processing, preferably those at 157 nm or even lower. Preferably, the starting pellicle plate  10 , for 157 nm applications and below, is a single substrate of silica, more preferably, of modified fused silica such as a “dry” (OH concentrations below 50 ppm by weight, preferably less than 1 ppm by weight), fluorine-doped silica glass.  
         [0037]     Rigidity of the starting single, hard pellicle plate substrate  10 , preferably starting fused silica plate, is achieved by providing a plate with sufficient original thickness, such as one having an original thickness ranging from about 3 mm to about 6 mm. This sufficient rigidity of the starting pellicle plate assists in preventing any stresses and/or distortions to the resultant monolithic pellicle. While the present invention is directed in particular to fused silica plates having an original thickness in the range of about 3 mm to about 6 mm, it should be appreciated that starting substrates of other materials, sizes and thickness may benefit from the foregoing invention.  
         [0038]     In forming the present monolithic pellicle, the single, rigid starting pellicle plate  10  is provided on a rotating platen  105  within a processing chamber  100 . The process chamber  100  may comprise any known substrate etching or cutting chamber such as, but not limited to, a plasma assisted chemical etching chamber, a quartz (fused silica) etching chamber, a wet processing chamber, and ion assisted milling chamber and the like. Preferably, the process used to form the present monolithic pellicle  20  is one that is an isotropic process such that only the desired pellicle material at the center of pellicle plate  10  is removed while leaving the perimeter of pellicle plate  10  unprocessed such that a frame portion of the resultant monolithic pellicle is integrally formed.  
         [0039]     For example, referring to FIGS.  1 A-B, a starting single pellicle substrate plate  10  may be processed in a plasma-assisted chemical etching chamber  100 . The present monolithic pellicle  20  of the invention is formed from this single, one-piece pellicle substrate plate  10  such that the resultant monolithic pellicle  20  maintains a one-piece structure. The starting pellicle substrate plate  10  has a predetermined surface quality, x, y-planar surface area coordinates and initial thickness (i.e., in the z-plane).  
         [0040]     The pellicle substrate plate  10  is positioned on and affixed to a pedestal  102  within the processing chamber  100 , and then a mask  50  is formed over an exposed surface of the pellicle substrate plate  10 . As shown in  FIG. 1B , the mask  50  is formed such that it entirely covers a perimeter of the exposed surface of the starting pellicle plate for forming the frame portion of the resultant one-piece, monolithic pellicle of the invention. The mask  50  may be formed by known techniques and may comprise a variety of known masks including, but not limited to, a photoresist mask, a physical hard mask and the like.  
         [0041]     Once the mask  50  has been provided on the pellicle plate  10 , surface areas  12  of pellicle plate  10  exposed through mask  50  are then removed to a desired depth within the pellicle plate  10  to form monolithic pellicle  20 , as is shown in FIGS.  2 A-D. This is preferably accomplished by providing a plasma over the structure of  FIG. 1B  whereby the plasma is adapted to remove only those exposed areas  12  of pellicle plate  10  to the desired depth, while leaving both the mask  50  and the pellicle plate  10  material residing there under substantially intact. For example, a bulk portion of the pellicle plate  10  may be removed by milling using a plasma-assisted chemical etch within the process chamber.  
         [0042]     The plasma is preferably of a chemistry that is selective to the material of mask  50  such that mask  50  protects the underlying pellicle plate material to form the frame  22  of monolithic pellicle  20 . In so doing, milling occurs on a single surface of the pellicle plate, whereby mask  50  protects the underlying peripheral area of the pellicle plate, such that material is removed only at the center or central portion of the single starting pellicle plate  10  and the resultant monolithic pellicle  20  having frame  22  is integrally formed as a one-piece structure.  
         [0043]     Removal of the bulk pellicle material may be accomplished by a timed etch, or alternatively, using marker layers to determine an etch stop point within the pellicle plate, i.e., to indicate when a desired location (the marker layer) has been passed. In accordance with the invention, wherein the starting single pellicle substrate plate  10  comprises a modified silica substrate, a marker layer may comprise a nitride layer provided within the modified silica substrate at a predetermined location during the silica substrate growth process.  
         [0044]     Referring to FIGS.  2 A-D, the removal of bulk pellicle plate material stops at a depth within the plate such that the center portion of the pellicle plate is recessed to a desired depth and does not traverse entirely through the original thickness of the pellicle plate. The amount, or depth, of bulk pellicle plate material removed at the center of the pellicle plate will vary depending upon the ultimate desired pellicle thickness at the center of the resultant monolithic pellicle  20 . This desired center pellicle thickness may be achieved by measuring, or predetermining, the original thickness of the starting single pellicle substrate plate, and removing a desired amount at the center, i.e., the pellicle region, of the single starting pellicle substrate based on its original thickness.  
         [0045]     For example, wherein the starting pellicle plate has an original thickness of about 4 mm to about 5 mm, bulk pellicle plate material may be removed at the center of the membrane such that remaining pellicle plate material  23  at the bottom of recessed portion  24  has a thickness ranging from about 200 μm to about 900 μm, more preferably from about 300 μm to about 800 μm, depending upon its ultimate use or mounting position. However, it should be appreciated that the remaining pellicle plate material  23  may have a thickness less than 200 μm or greater than 900 μm depending upon the predetermined desired thickness of the bottom optical pellicle portion of the resultant pellicle  20 , as well as its ultimate use.  
         [0046]     For a better understanding, wherein the monolithic pellicle  20  of the invention is to be vertically mounted on a vertical surface of a photomask, for protection thereof, the remaining pellicle plate material  23  at the bottom of recessed portion  24  may have a thickness of about 300 μm. The pellicle should be as thin as possible for transparency, but thick enough to be mechanically stable and substantially unaffected by gravity and other forces encountered during lithographic processing. Alternatively, wherein the monolithic pellicle  20  of the invention is to be horizontally mounted on a horizontal surface of a photomask, for protection thereof, the remaining pellicle plate material  23  at the bottom of recessed portion  24  is preferred to have a thickness of about 800 μm to compensate for the additional gravitational pull on the monolithic pellicle in this horizontal orientation.  
         [0047]     In either horizontal or vertical mounting of the monolithic pellicle  20 , it should be appreciated that it is desired that the bottom of recessed portion of the monolithic pellicle  20  be of sufficient thickness to prevent any sag of the bottom area of the recessed portion that may result from gravitational forces. However, if these forces are reduced by vertical mounting of the monolithic pellicle, or by other methods such as reducing the pellicle area or adding support struts, then the bottom area thickness of the recessed pellicle portion  24  can be made thinner in comparison to a bottom area thickness of a recessed pellicle portion of a horizontally mounted monolithic pellicle.  
         [0048]     The height or thickness of frame  22  is preferably dictated or based upon the original thickness of the starting pellicle plate, as a result of frame  22  being protected by mask  50  and not etched during the above etch process. That is, frame  22  may have the original thickness of the starting pellicle plate  10 . Alternatively, if desired, once the mask  50  has been removed, such as by known methods, the frame  22  may be thinned to a predetermined or desired thickness. It may be desirable to thin frame  22  in order to tune or adjust for the standoff distance between the present monolithic pellicle and the photomask to which it is to be mounted to. Wherein the present monolithic pellicle  20  is for use at an exposure field of about 157 nm wavelengths, the frame  22  may have a width  23  ranging from about 3 mm to about 10 mm and height  25  ranging from about 3 mm to about 6 mm, while the recessed optical pellicle portion  24  has a thickness ranging from about 200 μm to about 900 μm. Alternatively, the width of frame  22  may be customized to less than 3 mm or greater than 10 mm depending upon the exposure field size, such as those less than or greater than 157 nm exposure wavelength.  
         [0049]     Once the recessing of the central portion of the monolithic pellicle is complete, processing continues in accordance with the invention by removing mask  50  and planarizing the exposed inner surface  25  of the remaining pellicle plate material  23  at the bottom of recessed portion  24  to provide an optically flat, smooth surface area. In addition, the outer, bottom surface  26  of the monolithic pellicle is also planarized to an optically flat, smooth bottom surface area. Surface area  25  and  26  may be processed to planarity using a mapping technique, such as interferometer metrology, to locate and identify those regions of surface  25  and  26  that require planarization and then planarizing only these identified regions using a tool that allows for such focused, localized planarization. For example, a localized atmospheric plasma etch may be used to remove these localized variations in surface topography on both top surface  25  and bottom surface  26  of the monolithic pellicle  20 .  
         [0050]     Subsequently, surfaces  25 ,  26  of pellicle  20  may undergo a final optical surface polish process using known techniques. Thus, in accordance with the above description of the invention, the present monolithic pellicle  20  is provided with optical flatness on both the exposed inner surface area  25  at the bottom of recessed portion  24 , as well as at the pellicle&#39;s outer, bottom surface area  26 .  
         [0051]     The resultant monolithic pellicle  20  may have any of a variety of shapes including, but not limited to, rectangular, square, circular and the like. It comprises a rigid, single structure pellicle comprising frame  22  and recessed portion  24 . The frame  22  entirely surrounds the recessed portion  24  and maintains the original thickness of the starting pellicle plate  10 , or a thickness substantially close to the original thickness. In accordance with the invention, the single, rigid monolithic pellicle  20  is integrally formed as a one-piece, continuous structure. As such, it advantageously avoids the two-step attaching process of conventional approaches, i.e., attaching the pellicle to the frame before attaching the frame to the reticle. In so doing, the simplification of the present invention eliminates the risk of generating distortions or birefringence in the photomask associated therewith.  
         [0052]     Referring to FIGS.  2 C-D, the present monolithic pellicle  20  may then be mounted onto a photomask  40 . Prior to mounting, the monolithic pellicle  20  is provided with openings  30  for the introduction of gases for cleaning or exposure and for air pressure equalization. These openings  30  preferably traverse through the frame  22  of the monolithic pellicle for introducing a gas flow over the recessed optical pellicle portion  24  surface area facing the photomask  40  to which it is mounted. The sizes and locations of such openings  30  are positioned and provided through the frame  22  in a manner that allows the monolithic pellicle to maintain its sufficient rigidity and physical properties. Preferably, at least two openings  30  are provided through the frame  22  preferably by known drilling techniques such as, for example, known silica/quartz drilling techniques. The openings  30  may comprise a variety of shapes including, but not limited to, circular, oval, rectangular, and the like, as well as combinations thereof, and have dimensions ranging from about 1.0 mm to about 2.0 mm. Additionally, wherein the frame  22  is provided with a plurality of these openings  30 , the openings may be provided throughout the frame at distances ranging from about every 5.0 mm to about every 8.0 mm. The openings  30  may be fitted with air filters using known techniques. These air filters advantageously prevent particles from passing through the frame onto the reticle surface.  
         [0053]     Once openings  30  are formed in the hard monolithic pellicle  20 , it may then be mounted onto and bonded to a photomask  40  whereby the frame  22  acts as the mounting frame while the central recessed optical pellicle portion  24  acts as the optical pellicle for use at 157 nm wavelengths and below. The mounting of the monolithic pellicle  20  may be accomplished by known pellicle mounting techniques, such as by attaching frame portion  22  to the photomask  40  using a glue, adhesive, bonding agent, and the like.  
         [0054]     Once mounted, the hard monolithic pellicle  20  is easily removed from the photomask  40  for rework such that only the attaching/bonding means holding the frame to the photomask, i.e., glue, adhesive, bonding agent, and the like, must be removed. That is, as the frame portion  22  and optical pellicle portion  24  of the monolithic pellicle  20  are a single one-piece structure, they advantageously do not detach from one another during rework processing, as compared to conventional frames bonded to an optical pellicle which often detach from one another during such rework processes. Another advantage of the present rigid monolithic pellicle  20  is that it is not damaged or distorted from or during rework processing as a result of its sufficient rigidity.  
         [0055]     Thus, in accordance with the invention, the present monolithic pellicle  20  is integrally formed as a single, one-piece optical pellicle portion and frame portion that is particularly useful at or below 157 nm exposure wavelengths. As it is structurally a single, one-piece monolithic pellicle formed from a single pellicle plate, it avoids the conventional need for bonding a frame to an optical pellicle, as well as avoids material mismatch between the frame portion and optical pellicle portion. The monolithic pellicle  20  comprises a single material such that it has a single thermal expansion. Wherein the monolithic pellicle  20  includes a marker layer, the concentration of marker layer material is insignificantly small relative to the host material, i.e., the transparent material of the pellicle, such that marker layer material has essentially no measurable affect on the thermal properties. The monolithic pellicle  20  has inherently low stress levels due to its single thermal expansion, and is sufficiently robust to withstand any stresses endured as a result of mounting and/or remounting in mask manufacturing. Additionally, the hard monolithic pellicle  20  of the invention achieves advantages over the prior art by eliminating the need to bond an optical plate to a frame, and thereby reducing any risk of generating distortions or birefringence in the photomask to which it is mounted. Due to the present monolithic pellicle one-piece structure, it beneficially has an extended useful life as compared to conventional pellicles having a frame attached to an optical pellicle, which easily detach from one another, damage and/or distort thereby ending the conventional pellicle&#39;s useful life.  
         [0056]     While the present invention has been particularly described, in conjunction with a specific preferred embodiment, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. It is therefore contemplated that the appended claims will embrace any such alternatives, modifications, and variations as falling within the true scope and spirit of the present invention.