Patent Publication Number: US-7901845-B2

Title: Method for optical proximity correction of a reticle to be manufactured using character projection lithography

Description:
RELATED APPLICATIONS 
     This application: 1) is related to Fujimura, U.S. patent application Ser. No. 12/202,364, entitled “Method And System For Manufacturing A Reticle Using Character Projection Lithography” filed on Sep. 1, 2008; and 2) is related to Fujimura, U.S. patent application Ser. No. 12/202,366, entitled “Method And System For Design Of A Reticle To Be Manufactured Using Character Projection Lithography” filed on Sep. 1, 2008. 
     BACKGROUND OF THE DISCLOSURE 
     The present disclosure is related to lithography, and more particularly to the design and manufacture of a surface which may be a reticle, a wafer, or any other surface, using character or cell projection lithography. 
     In the production or manufacturing of semiconductor devices, such as integrated circuits, optical lithography may be used to fabricate the semiconductor devices. Optical lithography is a printing process in which a lithographic mask or reticle is used to transfer patterns to a substrate such as a semiconductor or silicon wafer to create the integrated circuit. Other substrates could include flat panel displays or even photomasks. Also, extreme ultraviolet (EUV) or X-ray lithography are considered types of optical lithography. The reticle or multiple reticles may contain a circuit pattern corresponding to an individual layer of the integrated circuit and this pattern can be imaged onto a certain area on the substrate that has been coated with a layer of radiation-sensitive material known as photoresist or resist. Once the patterned layer is transferred the layer may undergo various other processes such as etching, ion-implantation (doping), metallization, oxidation, and polishing. These processes are employed to finish an individual layer in the substrate. If several layers are required, then the whole process or variations thereof will be repeated for each new layer. Eventually, a combination of multiples of devices or integrated circuits will be present on the substrate. These integrated circuits may then be separated from one another by dicing or sawing and then may be mounted into individual packages. In the more general case, the patterns on the substrate may be used to define artifacts such as display pixels or magnetic recording heads. 
     In the production or manufacturing of semiconductor devices, such as integrated circuits, maskless direct write may also be used to fabricate the semiconductor devices. Maskless direct write is a printing process in which patterns are transferred to a substrate such as a semiconductor or silicon wafer to create the integrated circuit. Other substrates could include flat panel displays, imprint masks for nano-imprinting, or even photomask. Desired patterns of a layer are written directly on the surface, which in this case is also the substrate. Once the patterned layer is transferred the layer may undergo various other processes such as etching, ion-implantation (doping), metallization, oxidation, and polishing. These processes are employed to finish an individual layer in the substrate. If several layers are required, then the whole process or variations thereof will be repeated for each new layer. Some of the layers may be written using optical lithography while others may be written using maskless direct write to fabricate the same substrate. Eventually, a combination of multiples of devices or integrated circuits will be present on the substrate. These integrated circuits are then separated from one another by dicing or sawing and then mounted into individual packages. In the more general case, the patterns on the surface may be used to define artifacts such as display pixels or magnetic recording heads. 
     As indicated, the lithographic mask or reticle comprises geometric patterns corresponding to the circuit components to be integrated onto a substrate. The patterns used to manufacture the reticle may be generated utilizing CAD (computer-aided design) software or programs. In designing the patterns the CAD program may follow a set of predetermined design rules in order to create the reticle. These rules are set by processing, design, and end-use limitations. An example of an end-use limitation is defining the geometry of a transistor in a way in which it cannot sufficiently operate at the required supply voltage. In particular, design rules can define the space tolerance between circuit devices or interconnect lines. The design rules are, for example, used to ensure that the circuit devices or lines do not interact with one another in an undesirable manner. For example, the design rules are used so that lines do not get too close to each other in a way that may cause a short circuit. The design rule limitations reflect, among other things, the smallest dimensions that can be reliably fabricated. When referring to these small dimensions, one usually introduces the concept of a critical dimension. These are, for instance, defined as the smallest width of a line or the smallest space between two lines, those dimensions requiring exquisite control. 
     One goal in integrated circuit fabrication by optical lithography is to reproduce the original circuit design on the substrate by use of the reticle. Integrated circuit fabricators are always attempting to use the semiconductor wafer real estate as efficiently as possible. Engineers keep shrinking the size of the circuits to allow the integrated circuits to contain more circuit elements and to use less power. As the size of an integrated circuit critical dimension is reduced and its circuit density increases, the critical dimensions of its corresponding mask pattern approaches the resolution limit of the optical exposure tool used in optical lithography. As the critical dimensions of the circuit layout become smaller and approach the resolution value of the exposure tool, the accurate transcription between the mask pattern and the actual circuit pattern developed on the resist layer becomes difficult. To further the use of optical lithography to transfer patterns having features that are smaller than the light wavelength used in the optical lithography process, a process known as optical proximity correction (OPC) has been developed. OPC alters the original layout on the mask to compensate for distortions caused by effects such as optical diffraction and the optical interaction of features with proximate features. OPC includes all resolution enhancement technologies performed with a reticle. 
     OPC adds sub-resolution lithographic features to mask patterns to reduce differences between the original mask pattern, that is, the design, and the final transferred circuit pattern on the substrate. The sub-lithographic features interact with the original mask pattern and with each other and compensate for proximity effects to improve the final transferred circuit pattern. One feature that is used to improve the transfer of the pattern is a sub-resolution assist feature (SRAF). Another feature that is added to improve pattern transference is referred to as “serifs”. Serifs are small features that can be positioned on a corner of a pattern to sharpen the corner in the final transferred image. As the limits of optical lithography are being extended far into the sub-wavelength regime, the OPC features must be made more and more complex in order to compensate for even more subtle interactions and effects. However, as imaging systems are pushed closer to their limits, the ability to produce reticles with sufficiently fine OPC features becomes critical. Although adding serifs or other OPC features to a mask pattern is advantageous, it also substantially increases the total features count in the mask pattern. For example, adding a serif to each of the corners of a square adds eight more rectangles to a mask or reticle pattern. Adding OPC features is a very laborious task and requires costly computation time that results in more expensive reticles. Not only are OPC patterns complex, but since optical proximity effects are long range compared to minimum line and space dimensions, the correct OPC patterns in a given location depend significantly on what other geometry is in the neighborhood. Thus, for instance, a line end will have different size serifs depending on what is near it on the reticle. This is even through the objective might be to produce exactly the same shape on the wafer. These slight but critical variations are important and have prevented others from being able to form reticle patterns. It is conventional to discuss the OPC-decorated patterns to be written on a reticle in terms of main features, that is features that reflect the design before OPC decoration, and OPC features, where OPC features might include serifs, jogs, and SRAF. To quantify what is meant by slight variations, a typical slight variation in OPC decoration from neighborhood to neighborhood might be 5% to 80% of a main feature size. Note that for clarity, variations in the design of the OPC are what is being referenced. Manufacturing variations, such as line-edge roughness and corner rounding, will also be present in the actual surface patterns. When these OPC variations produce substantially the same patterns on the wafer what is meant is that the geometry on the wafer is targeted to be the same within a specified error, which depends on the details of the function that that geometry is designed to perform, e.g., a transistor or a wire. Nevertheless, typical specifications are in the 2%-50% of a main feature range. There are numerous manufacturing factors that also cause variations, but the OPC component of that overall error is often in the range just listed. 
     There are a number of technologies used for forming patterns on a reticle, including using optical or particle beam systems. The most commonly used system is the variable shape beam (VSB) type, where a precise electron beam is shaped and steered onto a resist-coated surface of the reticle. These shapes are simple shapes, usually limited to rectangles of certain minimum and maximum sizes and triangles with their three internal angles being 45 degrees, 45 degrees, and 90 degrees of certain minimum and maximum sizes. At pre-determined locations, doses of electrons are shot into the resist with these simple shapes. The total writing time for this type of system increases with the number of shots. A second type of system is a character projection system. In this case there is a stencil in the system that has in it a variety of shapes which may be rectilinear, arbitrary-angled linear, circular, annular, part circular, part annular, or arbitrary curvilinear shapes, and may be a connected set of complex shapes or a group of disjointed sets of a connected set of complex shapes. An electron beam can be shot through the stencil to efficiently produce more complex patterns (i.e., characters) on the reticle. In theory, such a system could be faster than an VSB system because it can shoot more complex shapes with each time-consuming shot. Thus, an E shot with an VSB system takes four shots, but could be done with one shot with a character projection system. Note that shaped beam systems can be thought of as a special (simple) case of character projection, where the characters are just simple characters, usually rectangles or 45-45-90 triangles. It is also possible to partially expose a character. This can be done by, for instance, blocking part of the particle beam. For example, the E described above can be partially exposed as an F or an I, where different parts of the beam are cut off by an aperture. For a very complex reticle, one must fracture the pattern into nearly billions and sometimes approaching trillions of elemental shapes. There are, for instance, simple rectangular shapes for a VSB system or a limited number of characters in a character projection system. The more total instances of elemental shapes (characters) in the pattern, the longer and more expensive the write time. However, for writing surfaces such as an OPC-decorated reticle where there are numerous fine variations among even the smaller patterns, such projection systems are today impractical. The number of characters that can be made available among which the selection of characters by the projection machine takes minimal time is limited, today only allowing about 10-1000 characters. When faced with the plethora of slightly varying OPC patterns that are required to be placed on a reticle, no system or method has been available which can accomplish this task. 
     Thus, it would be advantageous to reduce the time and expense it takes to prepare and manufacture a reticle that is used for a substrate. More generally, it would be advantageous to reduce the time and expense it takes to prepare and manufacture any surface. It would also be desirable to have a stencil mask that contains some of the complex characters needed to produce or generate a surface having various patterns that are required to be transferred to a surface. For example, it is possible that a surface can have thousands of patterns that have only slight differences among each other. In order to prepare a surface it is desirable to have a stencil mask that can generate many of these patterns having slight differences. As discussed more fully herein, this can be accomplished by using a stencil mask that contains a set of characters that can be combined, modified, or adjusted to generate the patterns with many of the slight variations. Thus, there exists a need for a method and a system for manufacturing a surface that eliminates the foregoing problems associated with preparing a surface. 
     SUMMARY OF THE DISCLOSURE 
     In one form of the present disclosure, a method for optical proximity correction of a design of a pattern on a surface is disclosed with the method comprising the steps of inputting desired patterns for the substrate and inputting a set of characters some of which are complex characters that may be used for forming the patterns on the surface. 
     In another form of the present disclosure, a method for optical proximity correction of a design of patterns on a surface is disclosed with the method comprising the step of inputting possible glyphs, the glyphs being based on predetermined characters, and the glyphs being determined using a calculation of varying a character dose or varying a character position or applying partial exposure of a character. 
     In still another form of the present invention, a system for optical proximity correction of a design of a pattern on a surface is disclosed in which the system comprises desired patterns for the substrate and a set of characters some of which are complex characters for forming some of the patterns on the surface. 
     These and other advantages of the present disclosure will become apparent after considering the following detailed specification in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cell projection system used to manufacture a surface; 
         FIG. 2A  illustrates a design of a pattern to be placed on a substrate; 
         FIG. 2B  illustrates a pattern formed in a reticle from the design shown in  FIG. 2A ; 
         FIG. 2C  illustrates a pattern formed in the photoresist of a substrate using the reticle of  FIG. 2B , illustrating that without optical proximity correction, the image is not nearly similar to the design shown in  FIG. 2A ; 
         FIG. 3A  illustrates an optical proximity corrected version of the pattern shown in  FIG. 2A ; 
         FIG. 3B  illustrates an optical proximity corrected version of the pattern shown in  FIG. 3A  after it is formed in the reticle; 
         FIG. 3C  illustrates a pattern formed in the photoresist of a silicon wafer using the reticle of  FIG. 3B ; 
         FIG. 4A  illustrates an ideal pattern to be placed on a substrate; 
         FIG. 4B  illustrates two basic stencil shapes; 
         FIG. 4C  illustrates the two basic stencil shapes shown in  FIG. 4B  in an overlapping manner; 
         FIG. 4D  illustrates a pattern formed on a reticle by use of the overlapping stencil shapes shown in  FIG. 4C ; 
         FIG. 4E  illustrates a pattern formed on a substrate by use of the pattern shown in  FIG. 4D ; 
         FIG. 5A  illustrates two basic stencil shapes in an overlapping manner where one of the stencil shapes consists of two disjointed squares; 
         FIG. 5B  illustrates a pattern formed on a reticle by use of the overlapping stencil shapes shown in  FIG. 5A ; 
         FIG. 5C  illustrates a pattern formed on a substrate by use of the pattern shown in  FIG. 5B ; 
         FIG. 6A  illustrates a stencil shape for forming a pattern on a reticle; 
         FIG. 6B  illustrates a pattern formed on a reticle by use of the stencil shape shown in  FIG. 6A ; 
         FIG. 6C  illustrates a pattern formed on a substrate by use of the pattern shown in  FIG. 6B ; 
         FIG. 7A  illustrates four stencil shapes used to form a pattern on a surface; 
         FIG. 7B  illustrates a pattern formed on a surface by use of the stencil shapes shown in  FIG. 7A ; 
         FIG. 8A  illustrates a set of characters formed on a stencil mask; 
         FIG. 8B  illustrates a pattern formed on a surface by use of the set of characters shown in  FIG. 8A ; 
         FIG. 8C  illustrates a set of adjustment characters; 
         FIG. 8D  illustrates, using solid and dotted line shapes, the varying degrees of doses by which each character and adjustment characters are exposed in the resist of a surface by use of the set of characters shown in  FIG. 8A  and the adjustment characters shown in  FIG. 8C ; 
         FIG. 8E  illustrates a pattern formed in a surface by use of the set of characters shown in  FIG. 8A  and the adjustment characters shown in  FIG. 8C ; 
         FIG. 9  illustrates a conceptual flow diagram of how to prepare a surface for use in fabricating a substrate such as an integrated circuit on a silicon wafer; 
         FIG. 10  illustrates another conceptual flow diagram of how to prepare a surface for use in fabricating a substrate such as an integrated circuit on a silicon wafer; 
         FIG. 11  illustrates a set of characters; 
         FIG. 12  illustrates a set of characters and adjustment characters with shape variation; 
         FIG. 13  illustrates a set of characters and adjustment characters with positional variation; 
         FIG. 14  illustrates a set of patterns created by shape variation of adjustment characters; 
         FIG. 15  illustrates a set of patterns created by various dosage amounts of adjustment characters; 
         FIG. 16  illustrates a set of patterns created by various dosage amounts of a single character; 
         FIG. 17  illustrates a set of patterns created by positional variation of adjustment characters; 
         FIG. 18  illustrates a conceptual flow diagram of how to prepare a surface for use in fabricating a substrate such as an integrated circuit on a silicon wafer; 
         FIG. 19  illustrates examples of glyphs; and 
         FIG. 20  illustrates examples of parameterized glyphs. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Referring now to the drawings, wherein like numbers refer to like items, number  10  identifies an embodiment of a lithography system, such as a particle beam writer system, in this case an electron beam writer system, that employs character projection to manufacture a surface  12  according to the present disclosure. The electron beam writer system  10  has an electron beam source  14  that projects an electron beam  16  toward an aperture plate  18 . The plate  18  has an aperture  20  formed therein which allows the electron beam  16  to pass. Once the electron beam  16  passes through the aperture  20  it is directed or deflected by a system of lenses (not shown) as electron beam  22  toward another rectangular aperture plate or stencil mask  24 . The stencil mask  24  has formed therein a number of apertures  26  that define various types of characters  28 . Each character  28  formed in the stencil mask  24  may be used to form a pattern in the surface  12 . An electron beam  30  emerges from one of the apertures  26  and is directed onto the surface  12  as a pattern  32 . The surface  12  is coated with resist (not shown) which reacts with the electron beam  30 . The pattern  32  is drawn by using one shot of the electron beam system  10 . This reduces the overall writing time to complete the pattern  32  as compared to using a variable shape beam (VSB) projection system or method. The surface  12  may be a reticle. The surface  12  may then be used in another device or machine, such as a scanner, to transfer the pattern  32  onto a silicon wafer to produce an integrated circuit or a chip. More generally, the reticle  12  is used in another device or machine to transfer the pattern  32  on to a substrate. 
     As indicated above, since semiconductor and other nano-technology manufacturers are reaching the limits of optical lithography it is difficult to transfer an ideal pattern onto a substrate. For example,  FIG. 2A  illustrates an ideal pattern  40 , which represents a circuit, to be formed in the resist of a substrate. When a reticle is produced that attempts to have the pattern  40  formed thereon, the reticle is not a perfect representation of the pattern  40 . A pattern  42  that may be formed in a reticle that attempts to represent the pattern  40  is shown in  FIG. 2B . The pattern  42  has more rounded and shortened features as compared to the pattern  40 . When the pattern  42  is employed in the optical lithography process a pattern  44  is formed in the photoresist on the substrate as depicted in  FIG. 2C . The pattern  44  is not very close to the ideal pattern  40 , demonstrating why optical proximity correction is required. 
     In an effort to compensate for the difference in the patterns  40  and  44  optical proximity correction is used. Optical proximity correction alters the reticle to compensate for distortions created by optical diffraction, optical interactions with neighboring shapes, and resist process effects.  FIGS. 3A-3C  show how optical proximity correction can be employed to enhance the optical lithography process to develop a better version of the pattern  44 . In particular,  FIG. 3A  illustrates a pattern  50  that is an altered version of the pattern  40 . The pattern  50  has a serif element  52  added to various corners of the pattern  50  to provide extra area in an attempt to reduce optical and processing effects that reduce the sharpness of the corner. When a reticle of the pattern  50  is produced it may appear in the reticle as a pattern  54  as shown in  FIG. 3B . When the optical proximity corrected pattern  54  is used in an optical lithography device an output pattern  56 , as depicted in  FIG. 3C , is produced. The pattern  56  more resembles the ideal pattern  40  than the pattern  44  and this is due to optical proximity correction. Although using optical proximity correction is helpful, it may require that every pattern be altered or decorated which increases the time and cost to produce a reticle or photomask. Also, the various patterns formed on the reticle may properly have slight differences between them when OPC is applied and this adds to the time and expense in preparing a reticle. Further, the large number of slight differences or variations in the patterns may make producing a reticle unmanageable using character projection systems because the number of required characters would be too large. 
     Referring now to  FIG. 4A , an ideal pattern  60 , such as a contact, that is to be placed on a substrate is shown. The ideal pattern  60  is in the shape of a square. In an attempt to provide a reticle that will transfer the pattern  60  onto the substrate as closely as possible the following steps are used.  FIG. 4B  shows two basic stencil shapes or characters  62  and  64  that can be used to write the ideal pattern  60  onto a reticle. The stencil shape  62  is a square shape  66  having a serif  68  positioned at each corner  70 ,  72 ,  74 , and  76 . The stencil shape  64  is an adjustment character that may be repositioned on the shape  62  to change or alter the shape of the serif  68  at one or more of the corners  70 ,  72 ,  74 , and  76 . For example, in  FIG. 4C  the stencil shape  64  is shown overlapping the corner  74  of the stencil shape  62 . When the stencil shapes  62  and  64  are used in a cell projection device, such as the electron beam writer system  10  shown in  FIG. 1 , to write a pattern onto a reticle, a pattern  78  as shown in  FIG. 4D  will appear. The pattern  78  has a corner  80  that is more elongated or pronounced that any of the other corners. This is due to the use of the stencil shape  64  to alter the corner  74 . The pattern  78 , which is on a photomask or a reticle, may be used in a conventional lithographic device to transfer the pattern  78  onto a substrate. For example, if pattern  78  on the reticle was the appropriate shape given the neighboring shapes that affect optical proximity correction for producing as close to the pattern  60  as possible on the substrate, a pattern  82  as depicted in  FIG. 4E , would be the result of the pattern  78  being transferred onto a substrate. The pattern  82  is similar to or an approximation of the ideal pattern  60 . 
     Various other patterns may be formed with the use of the stencil shapes  62  and  64 . For example, two instances of the shape  64  can be combined together as one character  90  used to overlap the corners  70  and  74  to form a pattern  92 , which is shown in  FIG. 5A . Stencil shapes  90  and  92  are overlapping shots that may produce pattern  94  in  FIG. 5B  on the reticle. When the pattern  94  on the reticle is the appropriate shape given the neighboring shapes that affect optical proximity correction for producing as close to the pattern  60  as possible on the substrate, a pattern  96  as is shown in  FIG. 5C  appears on the substrate when the pattern  94  on a reticle is used to project the substrate. The pattern  96  is substantially the same as the ideal pattern  60 . It is also possible and contemplated to change or vary the dose that is used in the electron beam writer system  10 , in order to further modify or adjust the various patterns that are formed on a reticle. As can be appreciated, with the use of a few stencil shapes a large number or diversity of shapes can be created on a surface such as a reticle. 
     With particular reference now to  FIG. 12 , a set of sixteen characters,  400 ,  402 ,  404 ,  406 ,  408 ,  410 ,  412 ,  414 ,  416 ,  418 ,  420 ,  422 ,  424 ,  426 ,  428 , and  430  are shown as the characters would appear on a surface after being projected by a character projection system. The “0 ear” pattern on the surface, as shown by the character  400 , was projected by a character whose design is shown in  FIG. 13  as “center CP”  450  to project a pattern on a design that is a square as shown in  FIG. 13  as “square”.  452 . The “2 ears” pattern, as shown by the character  414 , is projected by a character whose design is shown in  FIG. 13  as “ear at  23 ”  454  and is an example of an adjustment character. Similarly, the fifteen characters  402 ,  404 ,  406 ,  408 ,  410 ,  412 ,  414 ,  416 ,  418 ,  420 ,  422 ,  424 ,  426 ,  428 , and  430  projected in combination with character  400  may create fifteen patterns  472 ,  474 ,  476 ,  478 ,  480 ,  482 ,  484 ,  486 ,  488 ,  490 ,  492 ,  494 ,  496 ,  498 , and  500 , as depicted in  FIG. 14  on a surface. A pattern  470  ( FIG. 14 ) is created by projecting character  400  along with a certain dose. The fifteen patterns  472 ,  474 ,  476 ,  478 ,  480 ,  482 ,  484 ,  486 ,  488 ,  490 ,  492 ,  494 ,  496 ,  498 , and  500  of  FIG. 14  are glyphs formed by a combination of two character shots that are examples of a large variation of slightly different patterns that may be generated on the surface from a small set of characters. A potential reason for the need for the large variation is optical proximity correction for the eventual projection using optical lithography in the case where the surface is a reticle or a photomask. In order to project the square  452  as shown in  FIG. 13  on the substrate, because of the need for optical proximity correction, a large variation of slightly different patterns that are variations of the “O ear”  400  ( FIG. 12 ) need to be generated on the reticle. The present disclosure however is independent of the reason to need the large variation of slightly different patterns. 
     By varying the dose of the adjustment characters, the variety of patterns that can be shot on the surface through only these characters further increases.  FIG. 15  represents the cases  530 ,  532 ,  534 ,  536 , and  538  of the dose being varied by 0%, −30%, −60%, +50%, and +100% generating critical dimension variations from 10 nm to 19 nm. In addition, the dose of the center character represented by “0 ear”  400  ( FIG. 12 ) can also be varied to create further variation of slightly different patterns.  FIG. 16  represents the different shapes  550 ,  552 ,  554 ,  556 , and  558  that can be generated on the surface by varying the dose by −40%, −20%, 0%, +25%, and +50%. A shape  560  illustrates the overlapping of the shapes  550 ,  552 ,  554 ,  556 , and  558  to further demonstrate that slightly different patterns can be generated by varying the dose. Each of the patterns  550 ,  552 ,  554 ,  556 , and  558  may be glyphs, or patterns that are known to be available by combining a small number of character shots. A parameterized glyph may be used as a more compact representation with more generality to describe a number of glyphs in a single description. A pattern  560  demonstrates that a dose amount may be a parameter to represent multiple glyphs with one representation. A parameterized glyph that is a single description describing all of these possible glyphs  550 ,  552 ,  554 ,  556 , and  558  is a more compact and a more flexible representation. Slightly different patterns can also be generated by shooting the same basic patterns of the adjustment characters in different positions. Referring to  FIG. 17 , a pattern  580  and a pattern  582  are composed by placing the same 1 ear character, such as the character  404  shown in  FIG. 12 , at different locations to the 0 ear character, such as the character  400  in  FIG. 12 . By preparing a number of variations of the characters, in this case the variations of the center character and the variations of adjustment characters and varying the dose and relative positions, a very large number of slightly different patterns can be projected on the surface while using only two shots. With three or more shots, the number of available glyph patterns that can be projected on the surface increases geometrically. Other patterns, such as patterns  584 ,  586 ,  588 , and  590  are also shown in  FIG. 17 . For example, the pattern  584  is formed by combining character  400  ( FIG. 12 ) with the character for standard distance for 2 ears using an adjustment character  412  shown in  FIG. 12 . The pattern  586  is formed by combining character  400  with the character for a long distance for 2 ears using the adjustment character  432  of  FIG. 12 . The pattern  588  is formed by combining character  400  of  FIG. 12  with the character for a standard distance for 3 ears using the adjustment character  424  of  FIG. 12 . The pattern  590  is formed by combining character  400  of  FIG. 12  with the character for a long distance for 3 ears using an adjustment character  434  shown in  FIG. 12 . 
     With reference now to  FIG. 6A , another stencil pattern  100  is shown that can be used in an attempt to form a pattern on a substrate, such as a silicon wafer, to resemble the ideal pattern  60  as shown in  FIG. 4A . The stencil pattern  100  includes a stencil shape  102  having a serif  104  at each corner  106 ,  108 ,  110 , and  112 . The stencil pattern  100  also has a sub-resolution assist feature (SRAF)  114  positioned at a diagonal at each of the corners  106 ,  108 ,  110 , and  112 . The stencil pattern  100  is used to form a pattern  116  on a reticle, as is shown in  FIG. 6B . With reference now to  FIG. 6C , the pattern  116  is then used to form a pattern  118  on a substrate. The pattern  118  is similar to the ideal pattern  60 . 
       FIG. 7A  illustrates four stencil characters  150 ,  152 ,  154 , and  156  that may be used on a stencil mask to be combined to form a sophisticated shape or pattern  158  on a reticle as shown in  FIG. 7B . In particular, the first character  150  is shot or projected onto the reticle, then the second character  152  is shot, then the third character  154 , and finally the fourth character  156 . The characters are curvilinear in shape and not rectilinear in shape. In this manner, a complex pattern, such as the pattern  158 , may be formed on a reticle. The shapes on the stencil mask may be termed “characters” and the pattern formed on the reticle may be termed a “glyph”. It is also possible to use dose control in addition to shape variation to generate more slight variations of patterns formed on a reticle by using the same stencil characters, such as the characters  150 ,  152 ,  154 , and  156 . A combination of multiple characters may be overlapped with each other with different dose variations to increase the variation of possible shapes or patterns that may be generated. Additionally, the position of a character may be changed to increase the variation of possible shapes or patterns that may be generated. Since the shapes of the characters  150 ,  152 ,  154 , and  156  are curvilinear this reduces the number of shots that must be used by a particle beam writer system to shoot or project the characters  150 ,  152 ,  154 , and  156  onto a reticle to write a glyph pattern, such as the pattern  158 . For example, the pattern  158  can be shot by using only the four characters  150 ,  152 ,  154 , and  156 . While if rectilinear shapes were used many more shots or VSB shots would have to be used. As can be seen, being able to use characters instead of VSB shots reduces the time in preparing a reticle. It is also possible to use rectilinear shapes with curvilinear shapes to form a pattern on a reticle. While this feature of character projection is available in character projection systems for projecting surfaces that require a very large variety of shapes, the number of characters that can be made available as single components are not large enough. The present method and system combines multiple characters with dose, position, or partial projection variations with potentially overlapping shots to increase the number of glyph patterns available dramatically. By having a large number of glyphs as available patterns instead of the very limited number of characters as available patterns, more complex patterns can be projected on the surface without significantly affecting the shot count or write times. Alternatively, using the large number of thus available glyphs allows surfaces with highly complex shapes to be shot with far less number of shots and write times. 
     Referring now to  FIG. 8A , an example of a set of characters  200  that may be placed on a stencil mask is shown. The set of characters  200  may be used to form a pattern  202  on a reticle, as is illustrated in  FIG. 8B . The pattern  202  may be formed from one or more of the characters in the set of characters  200 . However, in an effort to better form an ideal pattern to be transferred on a silicon wafer by use of a reticle, adjustment characters or shots  204 , as seen in  FIG. 8C , may be used to further enhance the pattern  202 .  FIG. 8D  depicts an example of the pattern  202  in combination with the adjustment characters  204  that may be formed in the resist of a reticle. The adjustment characters  204  are shown in dotted lines to indicate a smaller dose for these characters  204  as compared to the dose used to shoot the other characters  202 .  FIG. 8E  shows a pattern  206  that is formed in a reticle by use of the set of characters  200  and the adjustment characters  204  with varying doses. A limited number of characters, such as the set of characters  200 , may be used to form a plurality of different shaped patterns or a plurality of slightly different shaped patterns. 
       FIG. 9  is a conceptual flow diagram  250  of how to prepare a reticle for use in fabricating a surface such as an integrated circuit on a silicon wafer. In a first step  252 , a physical design, such as a physical design of an integrated circuit is designed. This can include determining the logic gates, transistors, metal layers, and other items that are required to be found in a physical design such as that in an integrated circuit. Next, in a step  254 , optical proximity correction is determined. In an embodiment of this disclosure this can include taking as input a library of pre-calculated glyphs or parameterized glyphs. This can also alternatively, or in addition, include taking as input a library of pre-designed characters including complex characters that are to be available on a stencil  260  in a step  262 . In an embodiment of this disclosure, an OPC step  254  may also include simultaneous optimization of shot count or write times, and may also include a fracturing operation, a shot placement operation, a dose assignment operation, or may also include a shot sequence optimization operation, or other mask data preparation operations. Once optical proximity correction is completed a mask design is developed in a step  256 . Then, in a step  258 , a mask data preparation operation which may include a fracturing operation, a shot placement operation, a dose assignment operation, or a shot sequence optimization may take place. Either of the steps of the OPC step  254  or of the MDP step  258 , or a separate program independent of these two steps  254  or  258  can include a program for determining a limited number of stencil characters that need to be present on a stencil or a large number of glyphs or parameterized glyphs that can be shot on the surface with a small number of shots by combining characters that need to be present on a stencil with varying dose, position, and degree of partial exposure to write all or a large part of the required patterns on a reticle. It is to be understood throughout this disclosure that the mask data preparation step  258  or mask data preparation does not include OPC. Combining OPC and any or all of the various operations of mask data preparation in one step is contemplated in this disclosure. Mask data preparation step  258  which may include a fracturing operation may also comprise a pattern matching operation to match glyphs to create a mask that matches closely to the mask design. Mask data preparation may also comprise inputting patterns to be formed on a surface with the patterns being slightly different, selecting a set of characters to be used to form the number of patterns, the set of characters fitting on a stencil mask, and the set of characters based on varying character dose or varying character position or applying partial exposure of a character within the set of characters to reduce the shot count or total write time. A set of slightly different patterns on the surface may be designed to produce substantially the same pattern on a substrate. Also, the set of characters may be selected from a predetermined set of characters. In one embodiment of this disclosure, a set of characters available on a stencil in a step  270  that may be selected quickly during the mask writing step  262  may be prepared for a specific mask design. In that embodiment, once the mask data preparation step  258  is completed, a stencil is prepared in a step  260 . In another embodiment of this disclosure, a stencil is prepared in the step  260  prior to or simultaneous with the MDP step  258  and may be independent of the particular mask design. In this embodiment, the characters available in the step  270  and the stencil layout are designed in step  272  to output generically for many potential mask designs  256  to incorporate slightly different patterns that are likely to be output by a particular OPC program  254  or a particular MDP program  258  or particular types of designs that characterizes the physical design  252  such as memories, flash memories, system on chip designs, or particular process technology being designed to in physical design  252 , or a particular cell library used in physical design  252 , or any other common characteristics that may form different sets of slightly different patterns in mask design  256 . The stencil can include a set of characters, such as a limited number of characters that was determined in the step  258 , including a set of adjustment characters. Once the stencil is completed the stencil is used to generate a surface in a mask writer machine, such as an electron beam writer system. This particular step is identified as a step  262 . The electron beam writer system projects a beam of electrons through the stencil onto a surface to form patterns in a surface, as shown in a step  264 . The completed surface may then be used in an optical lithography machine, which is shown in a step  266 . Finally, in a step  268 , a substrate such as a silicon wafer is produced. As has been previously described, in a step  270  characters may be provided to the OPC step  254  or the MDP step  258 . The step  270  also provides characters to a character and stencil design step  272  or a glyph generation step  274 . The character and stencil design step  272  provides input to the stencil step  260  and to the characters step  270 . The glyph generation step  274  provides information to a glyphs or parameterized glyphs step  276 . Also, as has been discussed, the glyphs or parameterized glyphs step  276  provides information to the OPC step  254  or the MDP step  258 . 
     With reference now to  FIG. 10 , another conceptual flow diagram  300  of how to prepare a surface for use in fabricating a substrate such as an integrated circuit on a silicon wafer is shown. In a first step  302 , a physical design, such as a physical design of an integrated circuit is designed. This may be the ideal pattern that the designer wants transferred onto a substrate. Next, in a step  304 , optical proximity correction of the ideal pattern generated in the step  302  is determined. This can include selecting glyphs that need to be prepared. Optical proximity correction may also comprise inputting possible glyphs, the glyphs being based on predetermined characters, and the glyphs being determined using a calculation of varying a character dose or varying a character position or applying partial exposure of a character. Further, optical proximity correction may comprise selecting a glyph from the possible glyphs, computing the patterns on the substrate based on the selected glyph, and selecting another glyph if an error from the computation exceeds a predetermined threshold. The predetermined characters may be from a list of geometric patterns. Once optical proximity correction is completed a mask design is developed in a step  304 . Then, in a step  306 , a mask design is prepared. Once the mask design is prepared further enhancement of the mask design takes place in a mask data preparation step  308 . The mask data preparation step  308  can include a program for determining a limited number of stencil characters that need to be present on a stencil to be able to write all of the required patterns on a reticle. Mask data preparation may also comprise pattern matching to match glyphs to create a mask that matches closely to the mask design. Iterations, potentially including only one iteration where a correct-by-construction “deterministic” calculation is performed, of pattern matching, dose assignment, and equivalence checking may also be performed. These steps will assist in preparing an enhanced equivalent mask design. Once the mask is enhanced an equivalent mask design is generated in a step  310 . There are two motivations for tests that can be used to determine whether the equivalent mask design is really equivalent to the mask design. One motivation is to pass mask inspection. Another motivation is to confirm that the chip or integrated circuit will function properly once it has been fabricated. The closeness to which a pattern matching operation declares a match may be determined by a set of equivalence criteria. An equivalence criteria may be driven at least partially by litho-equivalence. Litho-equivalence may be determined by a set of predetermined geometric rules, a set of mathematical equations that declare a match, a partial match, or a no match, or by running a lithography simulation of the pattern on the surface design and a lithography simulation of a glyph and by comparing the two results using a set of predetermined geometric rules, or by a set of mathematical equations that declare a match, a partial match, or no match. The MDP step  308  may use a pre-determined set of available characters, glyphs, or parameterized glyphs to optimize for shot count or write time while insuring that a resulting equivalent mask design  310  is acceptable to the equivalence criteria. In another embodiment, OPC and MDP may be combined in a correct by construction method, in which case there may not be the mask design  306  generated separately from the equivalent mask design  310 . The equivalent mask design may be used to prepare a stencil as is shown in a step  312 . Once the stencil is completed the stencil is used to prepare a reticle in a mask writer machine, such as an electron beam writer system. This step is identified as a step  314 . The electron beam writer system projects a beam of electrons through the stencil onto a surface to form patterns in a surface. The surface is completed in a step  316 . The completed surface may then be used in an optical lithography machine, which is shown in a step  318  to transfer the patterns found on the surface to a substrate such as a silicon wafer to manufacture an integrated circuit. Finally, in a step  320 , a substrate such as a semiconductor wafer is produced. As has been previously described, in a step  322  characters may be provided to the OPC step  304  or the MDP step  308 . The step  322  also provides characters to a glyph generation step  326 . The character and stencil design step  324  provides input to the stencil step  312  or to a character step  322 . The character step  322  may provide input to the character and stencil design step  324 . The glyph generation step  326  provides information to a glyphs or parameterized glyphs step  328 . Also, as has been discussed, the glyphs or parameterized glyphs step  328  provides information to either the OPC step  308  or the MDP step  308 . 
     Referring now to  FIG. 18 , another conceptual flow diagram  700  of how to prepare a surface which is directly written on a substrate such as a silicon wafer is shown. In a first step  702 , a physical design, such as a physical design of an integrated circuit is designed. This may be an ideal pattern that the designer wants transferred onto a substrate. Next, in a step  704 , proximity effect correction (PEC), and other data preparation (DP) steps are performed to prepare input data to a substrate writing device, where the result of the physical design contains a multiplicity of patterns that are slightly different. The step  704  may also comprise inputting possible glyphs or parameterized glyphs from step  724 , the glyphs being based on predetermined characters from step  718 , and the glyphs being determined using a calculation of varying a character dose or varying a character position or applying partial exposure of a character in glyph generation step  722 . The step  704  may also comprise pattern matching to match glyphs to create a wafer image that matches closely to the physical design created in the step  702 . Iterations, potentially including only one iteration where a correct-by-construction “deterministic” calculation is performed, of pattern matching, dose assignment, and equivalence checking may also be performed. A stencil is prepared in a step  708  and is then provided to a wafer writer in a step  710 . Once the stencil is completed the stencil is used to prepare a wafer in a wafer writer machine, such as an electron beam writer system. This step is identified as the step  710 . The electron beam writer system projects a beam of electrons through the stencil onto a surface to form patterns in a surface. The surface is completed in a step  712 . Further, in a step  718  characters may be provided to the PEC and Data Prep step  704 . The step  718  also provides characters to a glyph generation step  722 . The character and stencil design step  720  provides input to the stencil step  708  or to a character step  718 . The character step  718  may provide input to the character and stencil design step  720 . The glyph generation step  722  provides information to a glyphs or parameterized glyphs step  724 . The glyphs or parameterized glyphs step  724  provides information to the PEC and Data Prep step  704 . The step  710  may include repeated application as required for each layer of processing, potentially with some processed using the methods described in association with  FIGS. 9 and 10 , and others processed using the methods outlined above with respect to  FIG. 18 , or others produced using any other wafer writing method to produce integrated circuits on the silicon wafer. 
       FIG. 11  shows various other basic template shapes or characters  350 ,  352 ,  354 ,  356 ,  358 ,  360 , and  362  that can be used as a set of characters on a stencil to form various patterns on a reticle. The stencil characters can be modified slightly by three methods when using character projection. The first way is to modify the shape and the size of the character. For example, variable character projection may be used where a single character can be varied by partially exposing a portion of the character. The second way is to modify the dose amount slightly when shooting a given shape and size of a character. A “dose” of a particle projection shot is the shutter speed, the length of time for which a given shot is being projected on the surface of a reticle. “Dose correction” is a process step in which the dose amount for any given character projection shot is modified slightly, for example, for proximity effect correction (PEC). In this particular embodiment, in addition or in combination with other dose correction, the dose is altered purposefully to slightly modify the size and shape of the characters projected onto the surface of a reticle to form patterns or glyphs on the reticle. It is also possible to modify the patterns shot onto a reticle by using multiple overlapping shots of the characters  350 ,  352 ,  354 , and  356 , to produce a large variety of patterns or glyphs. The patterns or glyphs may be rectilinear, near-rectilinear, linear, or curvilinear in shape. Further, it is also contemplated to modify the dose in combination with the use of overlapping characters to generate even more varieties of patterns or glyphs. Also, a set of stencil characters may be used with VSB shots, which are an example of a simple character, to form even more patterns or glyphs on a surface. VSB shots and characters may be combined with assigned dose amounts to generate a large variety of patterns or glyphs. The third method to modify the stencil characters slightly is with positional variation. The characters  358 ,  360 , and  362  show three variations of positions of the same character. By varying dose amounts in addition to the geometric shapes of the characters and relative position of the characters with respect to each other, the number of mask image fragments that can be quickly shot from a given collection of character projection characters is multiplied. A large number of glyphs that require a small number of characters can be made available to project complex patterns with reduced shot count and write times. 
     By use of a set of characters, complex shapes including connected or unconnected groups of rectilinear shapes, shapes combining edges of arbitrary angles, and shapes that include arbitrary curvatures may be formed. Arbitrary curvatures may include circles, semi-circles, and quarter-circles. A set of character projection characters are designed and are included in the stencil installed in a particle beam projection system that writes a reticle. An optical proximity correction system may be used to select a combination of character projection characters including potentially VSB shots with potential varying dose amounts and degrees of partial projection to generate a large number of patterns. A set of characters can be predesigned either specifically for a particular design or more generally for a set of designs and potential future designs with certain commonalties such as a particular semiconductor fabrication technology node. The optical proximity correction system may fracture overlapping characters each with variable dose amounts. This allows for complex shapes to be created on the reticle. 
     It is also possible that the optical proximity correction system can start with a large library of pre-computed or pre-calculated glyphs. The optical proximity correction system can then attempt to use the available glyphs as much as possible in performing optical proximity correction transformation of the original physical design of the integrated circuit to the reticle design. Glyphs may be each marked with an associated shot count and write time optimization value or values and an optical proximity correction system, a mask data preparation system, or some independent program may optimize for shot count or write time by selecting the lower shot count or write time. This optimization may be performed in a greedy manner where each glyph is chosen to optimize what is the best glyph to choose for shot count or write time with a certain order in which to choose glyphs to match a pattern, or in an iterative optimization manner such as with simulated annealing where exchanges of glyph selection optimizes the overall shot count or write time. It is possible that some desired patterns to be formed on a reticle may still remain unmatched by any available glyphs and such patterns may need to be formed by use of VSB shots. 
     Referring now to  FIG. 19 , examples of glyphs  1000 ,  1002 ,  1004 , and  1006  that may be used by optical proximity correction, fracturing, proximity effect correction, or any other steps of mask data preparation are shown. These glyphs  1000 ,  1002 ,  1004 , and  1006  may or may not be generated by a combination of the same characters or they may also be glyphs generated from four different characters. Regardless of the method of creating the glyphs, the glyphs represent possible patterns that are known to be possible patterns on the surface that can be generated with a small number of shots or write times. Each glyph may have associated with it the specification for characters required to generate the glyph, the partial exposure instructions for each of the characters, projected required dose of each character, and relative positions of the characters. 
       FIG. 20  shows examples of parameterized glyphs  1010  and  1012 . The glyph  1010  demonstrates a general shape described with a specification of a dimension that can be varied, in this case the length X being varied from length unit values between 10 and 25. The glyph  1012  demonstrates the same general shape in a more restrictive way where the length X can only be one of the specific values, for example, 10, 15, 20, or 25. The parameterized glyph  1010  demonstrates that these descriptions allow for a large variety of possible glyphs that is not practical with the enumeration method of glyphs that are not parameterized. 
     An example of a parameterized glyph description for the glyph  1010  may be as follows:
         pglyph upsideDownLShape (x: nanometers where ((x=10) or ((x&gt;10) and (x&lt;25)) or (x=25)));   rect (0, 0, 5, 15);   rect (0, 15, x, 20);   end pglyph;       

     An example of a parameterized glyph description for the glyph  1012  may be as follows:
         pglyph upsideDownLShape2 (x: nanometers where ((x=10) or (x=15) or (x=20) or (x=25)));   rect (0, 0, 5, 15);   rect (0, 15, x, 20);   end pglyph;       

     These example descriptions are based on parameters that yield a logical test that determines which values of parameters meet a certain criteria such as “where ((x 10) or (x=15) or (x=20) or (x=25))” or “where ((x=10) or ((x&gt;10) and (x&lt;25)) or (x=25)).” There are many other ways to describe a parameterized glyph. Another example that demonstrates a constructive method is as follows:
         pglyph upsideDownLShape2 (x: nanometers);   glyphFor (x=10, x+x+5; x&gt;25)   {   rect (0, 0, 5, 15);   rect (0, 15, x, 20);   }   end pglyph;.       

     While the specification has been described in detail with respect to specific embodiments, it will be appreciated that those skilled in the art, upon attaining an understanding of the foregoing, may readily conceive of alterations to, variations of, and equivalents to these embodiments. These and other modifications and variations to the present system and method for manufacturing a reticle using character projection lithography may be practiced by those of ordinary skill in the art, without departing from the spirit and scope of the present subject matter, which is more particularly set forth in the appended claims. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to be limiting. Thus, it is intended that the present subject matter covers such modifications and variations as come within the scope of the appended claims and their equivalents.