Abstract:
A method of forming a semiconductor layer of a semiconductor device including interposing a reticle between an energy source and a semiconductor wafer, the reticle including at least two duplicate mask patterns each having a different bias, and passing energy through an opening in a shutter and through one of the at least two duplicate mask patterns using the energy source to form an image on the semiconductor wafer. The one of the at least two duplicate mask patterns is chosen based on a required bias. The at least two duplicate mask patterns are disposed in a side by side relationship to one another and extend parallel or transverse to the shutter opening.

Description:
RELATED APPLICATION 
     This application is a continuation-in-part of U.S. patent application Ser. No. 10/866,976, filed Jun. 14, 2004. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to photomasks used in photolithography processes, such as semiconductor wafer fabrication processes. In particular, the present invention relates to photomasks with multiple versions of the same mask pattern having different biases to compensate for process errors that occur during exposure in a photolithography process. 
     BACKGROUND OF THE INVENTION 
     There are a wide variety of photomasks known in the art, as well as diverse uses to which they can put, as described in, e.g., U.S. Pat. Nos. 6,472,107 and 6,567,588. Among the many types of photomasks used in the semiconductor industry, binary and phaseshift photomasks are quite common. A typical binary photomask is comprised of a substantially transparent substrate  2  and opaque layer  4 , in which a pattern is formed, as shown in a cross sectional illustration of an unprocessed binary photomask in  FIG. 1A . Further, the opaque layer  4  may also have an anti reflective (“AR”) coating  6 . The pattern of the opaque material in the opaque layer  4  and AR material in the AR coating  6  on the substantially transparent substrate  2  may be a scaled negative of the image desired to be formed on the semiconductor wafer. For a typical chrome on glass (“CoG”) or binary photomask, the substantially transparent substrate  2  is comprised of quartz. The opaque material  4  is comprised of chromium (“Cr”) and the AR material is comprised of chromium oxide (“CrO”). 
     A binary photomask used in the production of semiconductor devices is formed from a “blank” photomask. As shown in  FIG. 1A , a prior art blank photomask  1  is commonly comprised of at least four layers. The first layer  2  is a substantially transparent substrate, such as quartz, commonly referred to as the substrate. The next layer above the substantially transparent layer  2  is an opaque layer  4 , which is comprised of Cr in the case of a typical CoG photomasks. Thereafter, although not always necessary, there may be an AR layer  6  integral to the opaque layer, which in the case of CoG photomasks is comprised of CrO. A layer of photosensitive resist material  8  resides as the top layer. In the case of CoG photomasks, the photosensitive resist material  8  is typically a hydrocarbon polymer, the various compositions and thicknesses of which are well known in the art. Other layers may also be present for alternative reasons, as is described, for example, in U.S. Pat. No. 6,472,107. Similarly, other materials may be used as is well known in the art. 
     The desired pattern of opaque material to be created on the photomask may be defined by an electronic data file loaded into an exposure system which typically scans an electron beam (E beam) or laser beam in a raster fashion across the blank photomask. One such example of a raster scan exposure system is described in U.S. Pat. No. 3,900,737. Other imaging systems can be used that do not use raster scanning, such as shaped vector tools. As the E beam or laser beam is scanned across the blank photomask, the exposure system directs the E beam or laser beam at addressable locations on the photomask as defined by the electronic data file. In the case of a positive photoresist, the areas that are exposed to the E beam or laser beam become soluble, while the unexposed portions remain insoluble. In the case of a negative photoresist, the unexposed areas become soluble, while the exposed portions remain insoluble. As shown in  FIG. 1B , after the exposure system has scanned the desired image onto the photosensitive resist material, the soluble photosensitive resist is removed by means well known in the art, and the insoluble photosensitive resist material  8   a  remains adhered to the next layer (e.g., the AR layer  6 ). 
     After undergoing the foregoing photolithographic process, as illustrated in  FIG. 1C , the exposed layer of AR material  6  and the underlying layer of opaque material  4  are no longer covered by the photosensitive resist material  8   a  and are removed by a well known etch process. Only the portions of the layer of AR material  6   a  and the layer of opaque material  4   a  residing beneath the remaining photosensitive resist material  8   a  remain affixed to the substantially transparent substrate  2 . This initial or base etching may be accomplished by either a wet etching or dry etch process, both of which are well known in the art. 
     Another type of photomask used for transferring images to a semiconductor wafer is commonly referred to as a phaseshift photomask. Phaseshift photomasks are generally preferred over binary photomasks when the design to be transferred to the semiconductor wafer includes smaller, tightly packed feature sizes which are below the resolution capabilities of optical equipment being used. Phaseshift photomasks are engineered to be 180 degrees out of phase with light transmitted through etched areas on the photomask so that the light transmitted through the openings in the photomask is equal in amplitude. 
     One type of phaseshift photomask is commonly referred to as an embedded attenuated phaseshift mask (EAPSM). Other types of phaseshift masks are also known, and the teachings of the present invention may be equally applied thereto. As shown in  FIG. 2A , a typical blank EAPSM  31  may be comprised of four layers. The first layer is a typically a substantially transparent material  33  (such as quartz, for example) and is commonly referred to as a substrate. The next layer is typically an embedded phaseshifting material (“PSM layer”)  35 , such as molybdenum silicide (MoSi), tantalum silicon nitride (TaSiN), titanium silicon nitride (TiSiN), zirconium silicon oxide (ZrSiO), or other known phase materials. The next layer is typically an opaque material  37 , such as chromium, which may optionally include an anti reflective coating such as chromium oxynitride (CrON). The top layer is a photosensitive resist material  39 , as is well known in the art. 
     The method for processing a conventional EAPSM is now described. As with binary photomasks, the desired pattern of the opaque material to be created on the EAPSM is typically scanned by an electron beam (E beam) or laser beam in a raster or vector fashion across a blank EAPSM  31 . As the E beam or laser beam is scanned across the blank EAPSM  31 , the exposure system directs the E beam or laser beam at addressable locations on the EAPSM. In the case of a positive photoresist material, the areas that are exposed to the E beam or laser beam become soluble, while the unexposed portions remain insoluble. In the case of a negative photoresist, the unexposed areas become soluble, while the exposed portions remain insoluble. 
     As is done with binary photomasks and as shown in  FIG. 2B , after the exposure system has scanned the desired image onto the photosensitive resist material  39 , the soluble photosensitive resist material is removed by means well known in the art, and the insoluble photosensitive resist material  39   a  remains adhered to the opaque material  37 . Thus, the pattern to be formed on the EAPSM is formed by the remaining photosensitive resist material  39   a.    
     The pattern is then transferred from the remaining photosensitive resist material  39   a  to the opaque layer  37  and PSM layer  35  via well known etching techniques, such as plasma assisted etch described above, by etching away the portions of the opaque layer and PSM layer not covered by the remaining photoresist. After etching is completed, the remaining photoresist material is stripped or removed as shown in  FIG. 2C . Other processing steps, such as partial or complete etching of the opaque layer  37   a , may be further performed to complete the fabrication of the phaseshift photomask. 
     Photomasks are used in the semiconductor industry to transfer micro scale images defining a semiconductor circuit onto a silicon or gallium arsenide substrate or wafer and the like. To create an image on a semiconductor wafer, the photomask is interposed between the semiconductor wafer, which includes a layer of photosensitive material, and a stepper, which houses an energy source, such as a lamp or a laser. The energy generated by the stepper passes through the transparent portions of the substantially transparent substrate not covered by the opaque material (and, if utilized, the anti reflective and/or phaseshift material) and causes a reaction in the photosensitive material on the semiconductor wafer. Energy from the stepper is prevented from passing through the opaque portions of the photomask. As with the manufacture of photomasks, when the photosensitive material is exposed to light it will react. Thereafter, the soluble photosensitive material is removed using processes well known in the prior art. The semiconductor wafer is then etched in a manner similar to that described above. After further processing, a semiconductor product is formed. 
     As semiconductor chip features become exponentially smaller and the number of transistors per device become exponentially larger, large burdens have been placed on lithography processes. Resolution of anything smaller than a wavelength of exposure radiation is generally quite difficult, and pattern fidelity can deteriorate dramatically in sub-wavelength lithography. The resulting semiconductor features may deviate significantly in size and shape from the ideal pattern drawn from the circuit designer. This will decrease process yield and increase cost of the overall photomask manufacturing process. 
     The semiconductor industry is driven by a desire to lessen processing time and increasing process yield while maintaining or even reducing current costs. In particular, regarding lithograpy techniques using photomasks, the semiconductor industry has attempted to reduce process errors to increase yield by compensating for these process errors in the photomasks themselves. For example, when an image is transferred to a wafer by a 4 stepper tool using a photomask with a critical dimension (CD) of 100 nm, the resulting device layer on the wafer may have a line width of 28 nm. Accordingly, the semiconductor manufacturer will often request that the CD of the photomask be adjusted (or “biased”) so that, when the photomask pattern is developed on the semiconductor wafer, the resulting product will have the desired line width of 25 nm instead of 28 nm. 
     As a further example, U.S. Patent Application Publication No. 2003/0134205 (“the &#39;205 application”) discloses a process for manufacturing a photomask in which, for each pitch within a semiconductor design, a bias needed at the pitch that maximizes a common process window for all the pitches is calculated based on the given critical dimension (CD) of the mask design. The &#39;205 application combines this biasing with optical proximity correction to appropriately modify the original layout of the photomask. However, techniques such as that disclosed in the &#39;205 application are costly and increase turn-around time due to the required inspection steps and correction analysis. 
     Other techniques have been adopted to decrease cost of the photomask manufacturing process, which do not relate directly to reducing process errors or increasing yield. Such techniques often involve using multiple mask patterns on a common reticle or plate. For example, U.S. Pat. No. 6,421,111 discloses a multiple image reticle including a two dimensional array of spaced images, which obviates the need for rotation of the reticle to expose various levels of circuitry on a semiconductor wafer. 
     Similarly, U.S. Patent Application Publication No. 2004/0072083 discloses a photomask including a plurality of mask patterns, each used in an associated photolithography step and corresponding to an associated semiconductor layer, where the mask patterns are arranged so that the photomask is always used oriented in substantially the same direction. 
     Finally, U.S. Pat. No. 5,604,059 discloses a mask structure including a plurality of duplicating first device patterns and a plurality of duplicating second device patterns. The first device patterns are used to expose a first part of a semiconductor pattern and the second device patterns are used to expose a second part of the semiconductor pattern over the exposed first part. 
     In none of the prior art references is there disclosed the use of single photomask reticle having multiple versions of the same mask pattern, where different biasing is used. 
     It is an object of the present invention to provide a reticle that increases process yield and decreases turn-around time by compensating for process errors. 
     It is a further object of the present invention to provide an improved reticle which has multiple versions of the same mask pattern with different biasing. 
     Other objects and advantages of the present invention will become apparent from the following description. 
     SUMMARY OF THE INVENTION 
     It has now been found that the above and related objects of the present invention are obtained in the form of several related aspects, including providing a single reticle having multiple versions of the same mask pattern with different biasing. 
     A method of forming a semiconductor layer of a semiconductor device according to an exemplary embodiment of the invention includes interposing a reticle between an energy source and a semiconductor wafer, the reticle including at least two duplicate mask patterns each having a different bias, and passing energy through an opening in a shutter and through one of the at least two duplicate mask patterns using the energy source to form an image on the semiconductor wafer. The one of the at least two duplicate mask patterns is chosen based on a required bias. The at least two duplicate mask patterns are disposed in a side by side relationship to one another and extend parallel to the shutter opening. 
     A method of forming a semiconductor layer of a semiconductor device according to another exemplary embodiment of the invention includes interposing a reticle between an energy source and a semiconductor wafer, the reticle including at least two duplicate mask patterns each having a different bias, and passing energy through an opening in a shutter and through one of the at least two duplicate mask patterns using the energy source to form an image on the semiconductor wafer. The one of the at least two duplicate mask patterns is chosen based on a required bias. The at least two duplicate mask patterns are disposed in a side by side relationship to one another and extend transverse to the shutter opening. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and related objects, features and advantages of the present invention will be more fully understood by reference to the following, detailed description of the preferred, albeit illustrative, embodiment of the present invention when taken in conjunction with the accompanying figures, wherein: 
         FIGS. 1A-1C  are vertical sectional views showing various steps of a method of forming a conventional photomask; 
         FIGS. 2A-2C  are vertical sectional views showing various steps of a method of forming another conventional photomask; 
         FIG. 3  is a plan view of a conventional mask pattern formed on a reticle plate; 
         FIG. 4  is a plan view of multiple duplicate mask patterns having different biases formed on a reticle according to an exemplary embodiment of the present invention; 
         FIG. 5  is a plan view of multiple duplicate mask patterns having different biases formed on a reticle according to another exemplary embodiment of the present invention; 
         FIGS. 6A-6D  are vertical sectional views showing various steps of a method of manufacturing a reticle having multiple duplicate mask patterns with different biases according to an exemplary embodiment of the invention; 
         FIG. 7A  is a plan view of a shuttle plane over a reticle having multiple duplicate mask patterns with different biases according to an exemplary embodiment of the invention; and 
         FIG. 7B  is a plan view of a shuttle plane over a reticle having multiple duplicate mask patterns with different biases where the reticle is rotated 90° with respect to the shuttle plane according to an exemplary embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention generally relates to the use of multiple copies of the same mask pattern on a reticle using different biases, e.g., +5 nm, 0 nm, and −5 nm. The invention is equally applicable to different ranges and number of copies, as long as two or more copies have different biases on the same photomask. This example would allow a photomask customer/semiconductor manufacturer to choose three (or more) alternative masks patterns depending on the biasing that is appropriate for the particular stepper equipment for which the reticle is to be used. In essence, multiple field biases offer the end user a measure of “tuning” for their imaging systems. 
     In various exemplary embodiments of the invention, multiple mask patterns are formed on a reticle plate. Each mask pattern is duplicated any number of times on the plate, with each duplicate mask pattern having a different bias. For example, a first mask pattern on the plate can have a bias of 0 nm, a duplicate second mask pattern can be formed along side the first mask pattern with a bias of +5 nm, and a duplicate third mask pattern can be formed along side the first mask pattern with a bias of −5 nm. The number and location of the various patterns relative to each other and the relative biases are provided merely as an illustrative example and are not intended to limit the scope of the present invention. Thus, each mask pattern can have any suitable bias that satisfies process parameters required by the customer. By way of illustration, suitable biases may be (−10 nm, 0 nm, +10 nm), (−15 nm, 0 nm, +15 nm) and (−20 nm, 0 nm, +20 nm). The various exemplary embodiments of the invention are not restricted to 2-3 duplicates of a single pattern, and any number of duplicate patterns having different biases that can fit on a single reticle can be implemented. Further, the bias does not have to be restricted to a plus bias and a negative bias disposed around a central zero bias, but can be one sided (e.g., 0 nm, +5 nm, +10 nm), or asymmetric (e.g., −5 nm, 0 nm, +5 nm, +10 nm) and can be any incremental value (e.g., +1 nm, +2 nm or −7.5 nm, +2.5 nm, +12.5 nm). 
     Further, in various exemplary embodiments, the duplicate reticle mask patterns can be laid out on the reticle plate in any suitable manner. For example, a first group of duplicate mask patterns can be formed on the plate along with a second group of duplicate mask patterns, so that each layer of the semiconductor device associated with a duplicate mask pattern can be formed using multiple biases. Further, the duplicate mask patterns having the different biases can be formed on the same reticle with mask patterns that are not formed in duplicate. In this case, preferably only important and time sensitive masks would be formed multiple times with different biases, and other masks that are not as crucial would not be formed in duplicate. Of course, the application of the present invention is not intended to be limited to only important and time sensitive masks. Further, a first group of duplicate mask patterns can be formed on the plate along with a second group of duplicate mask patterns, with the first group being oriented in a first direction and the second group being oriented in a second direction. Similarly, any number of groups of one or more mask patterns that can fit on the reticle can be used, with any appropriate combination or orientation as long as at least one group has at least two members with different biases. 
       FIGS. 3 and 4  are provided to explain the general concept of various exemplary embodiments of the present invention.  FIG. 3  illustrates a conventional mask pattern  110  formed on a reticle  100 . The mask pattern  110  is biased 0 nm, i.e., not biased, so that the exposure process to form a semiconductor layer using the reticle  1  is susceptible to process errors. For example, the mean to nominal specification of the semiconductor layer formed using the reticle  100  may be +10 nm due to process errors. Any “non-repairable” defect in the writing area of the reticle will result in a repeat of the value-added steps of the reticle manufacturing process, making them non-valued added cost adders. 
       FIG. 4  illustrates an example of multiple mask patterns  210 ,  220  and  230  formed on a reticle  200  according to an exemplary embodiment of the present invention. In the present embodiment, the reticle  200  includes three duplicate mask patterns having different biases. However, any number of duplicate mask patterns can be formed depending on the overall area of the reticle and the capabilities of the stepper used to expose the semiconductor device layers. The multiple mask patterns include a first mask pattern  210 , a second mask pattern  220  and a third mask pattern  230 . The first mask pattern  210  is formed in the middle region of the reticle  200 , the second mask pattern  220  is formed adjacent to the first mask pattern  210  at one side region of the reticle  200 , and the third mask pattern  230  is formed adjacent to the first mask pattern  210  at another side region of the reticle  200 . The first, second and third mask patterns  210 ,  220  and  230  are duplicate mask patterns that can be used to form the same semiconductor layer, each having a different bias. The appropriate biasing of each duplicate mask pattern  210 ,  220  and  230  is based on customer latitude. For example, if the mean to nominal specification is +10 nm, the appropriate biasing may be 0 nm for the first photomask pattern  210 , +10 nm for the second photomask pattern  220  and −10 nm for the third photomask pattern  230 . Thus, the customer can choose from the three mask patterns  210 ,  220  and  230  the appropriately biased mask pattern that compensates for the process errors that occur during the semiconductor device fabrication steps. The choice of which photomask pattern has which bias is not a critical aspect of the present invention, such that any suitable order may be chosen. Other amounts of biasing may also be selected within the scope of the present invention. 
     The reticle  200  illustrated in  FIG. 4  affords the customer greater flexibility regarding optimization of the photolithography process results. As an example, the customer may choose one of the mask patterns that compensates for manufacturing tendencies, but for some reason results in a single defect killer. The customer then has two other mask patterns to choose from that avoids the killer defect, and which at the same time provides appropriate biasing within manufacturing tolerances. The mask pattern that avoids killer defects can be used in conjunction with appropriate biasing of the stepper equipment to optimize the photolithography process. 
       FIG. 5  illustrates multiple mask patterns formed on a reticle according to another exemplary embodiment of the present invention. In the present embodiment, the reticle  300  includes two sets of three duplicate mask patterns, each duplicate pattern in each set having a different bias. As shown in  FIG. 5 , the reticle  300  includes a first set  310  of mask patterns and a second set  320  of mask patterns. The first set  310  of mask patterns includes a first mask pattern  330 , a second mask pattern  340  and a third mask pattern  350 . The second set  320  of mask patterns includes a fourth mask pattern  360 , a fifth mask pattern  370  and a sixth mask pattern  380 . The first mask pattern  330  is formed in the middle region of the reticle  300 , the second mask pattern  340  is formed adjacent to the first mask pattern  330  at one side region of the reticle  300 , and the third mask pattern  350  is formed adjacent to the first mask pattern  330  at another side region of the reticle  300 . Similarly, the fourth mask pattern  370  is formed in the middle region of the reticle  300 , the fifth mask pattern  380  is formed adjacent to the fourth mask pattern  370  at one side region of the reticle  300 , and the sixth mask pattern  380  is formed adjacent to the fourth mask pattern  360  at another side region of the reticle  300 . The first, second and third mask patterns  330 ,  340  and  350  in the first set  310  of mask patterns are duplicate mask patterns that can be used to form the same semiconductor layer, where each of the first, second and third mask patterns  330 ,  340  and  350  has a different bias. Similarly, the fourth, fifth and sixth mask patterns  360 ,  370  and  380  in the second set  320  of mask patterns are duplicate mask patterns that can be used to form another semiconductor layer, where each of the fourth, fifth and sixth mask patterns  360 ,  370  and  380  has a different bias. Of course, the present invention is not limited to two sets of duplicate mask patterns having different biases. Any number of sets can be disposed on the reticle as long as each set contains at least two duplicate mask patterns having different biases. Preferably, each set would contain at least three duplicate mask patterns, where one pattern has no bias, one pattern has negative bias (e.g., −5 nm, −10 nm, −15 nm, −20 nm, etc.) and one pattern has positive bias (e.g., +5 nm, +10 nm, +15 nm, +20 nm, etc.). 
       FIGS. 6A-6D  show various steps of a method of manufacturing a reticle having multiple duplicate mask patterns with different biases according to an exemplary embodiment of the invention. As shown in  FIG. 6A , blank reticle  400  is comprised of a transparent material layer  410 , an opaque layer  420  and a photosensitive resist material layer  430 . The transparent material layer  410  is preferably made of quartz. The opaque layer  420  is formed on the transparent material layer  410  and is formed of a layer of Cr opaque material  422  and an integral layer of CrO AR material  424  formed on top of the layer of Cr opaque material  422 . The layer of photosensitive resist material  430  resides on top of the opaque material layer  420 . The photosensitive resist material  430  is typically a hydrocarbon polymer, the various compositions and thicknesses of which are well known in the art. The desired pattern of opaque material  420  to be created on the reticle  400  may be defined by an electronic data file loaded into an exposure system which typically scans an electron beam (E-beam) or laser beam in a raster fashion across the blank reticle. In exemplary embodiments of the invention, the data for each of the layers or mask patterns on the reticle can be duplicated and biased to produce multiple duplicate mask patterns having different biases in the final reticle  400 . As the E-beam or laser beam is scanned across the blank reticle, the exposure system directs the E-beam or laser beam at addressable locations on the reticle as defined by the electronic data file. The areas of the photosensitive resist material that are exposed to the E-beam or laser beam become soluble while the unexposed portions remain insoluble. As shown in  FIG. 6B , after the exposure system has scanned the desired image onto the photosensitive resist material, the soluble photosensitive resist is removed by means well known in the art, and the unexposed, insoluble photosensitive resist material  430  remains adhered to the CrO AR material  424 . Accordingly, the remaining photosensitive resist material  430  forms patterns including patterns  432  that duplicate with different biasing. 
     As illustrated in  FIG. 6C , the exposed opaque material layer  420  no longer covered by the photosensitive resist material  430  in the reticle  400  is removed by a well known etching process, and only the portions of the opaque material layer  420  residing beneath the remaining photosensitive resist material  430  remain affixed to quartz substrate  410 . Accordingly, the duplicating multiple patterns  432  with different biases in the photosensitive resist material  430  are transferred to the opaque material layer  420  to form corresponding multiple duplicating mask patterns  422  in the opaque material layer  420 . This initial or base etching may be accomplished by either a wet-etching or dry-etching process both of which are well known in the art. 
     As shown in  FIG. 6D , after the etching process is completed the photosensitive resist material  430  in the reticle  400  is stripped away by a process well known in the art. 
     In the various exemplary embodiments of the present invention in which a reticle having multiple duplicate masks with different biases is used to process a semiconductor layer, various issues may need to be addressed depending on the particular stepper being used to expose the semiconductor layer through the reticle. For example, one issue is that radial error may dominate uniformity/registration gains related to reduced field size. Another issue is that spherical aberration of the scanner lens may be too large when overlaying smaller fields to larger fields. Still another issue is that the scanner shuttle plane may be too small to adequately cover all the multiple duplicate mask patterns. This issue is illustrated in  FIGS. 7A and 7B .  FIG. 7A  shows a shutter  500  of a scanner concentric to the center line of the reticle plane CL of a reticle  600  having three duplicate mask patterns with different bias. Due to the shutter limitations, the image fields may need to be rotated 90° as shown in  FIG. 7B , which would allow for use of any one of the three fields of the reticle  600 . 
     Now that the preferred embodiments of the present invention have been shown and described in detail, various modifications and improvements thereon will become readily apparent to those skilled in the art. For example, the present invention is not limited to CoG photomasks, but also may be applied to other types of binary photomasks. Similarly, the present invention is not limited to EAPSM, but may also apply to other types of phaseshift photomasks, including by way of example, but not limited to, AAPSM (alternating aperture phase shift mask). Furthermore, application of the present invention is not limited to reticles having multiple versions of only one mask pattern with different biases. It could also apply to reticles having multiple versions of multiple mask patterns where each version of these mask patterns have different biases. Further, the inventive concept of multiple duplicate mask patterns having different biases to improve yield is not solely applicable to scanner technology. The concept is applicable to almost any microlithography approach. 
     The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all changes that come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.