Abstract:
A method for particle beam lithography, such as electron beam (EB) lithography, includes predefining a stencil design having a plurality of cell patterns with information from a cell library, fabricating the stencil design, synthesizing a functional description into a logic circuit design after predefining the stencil design so that one or more characteristics of the stencil design are considered during synthesizing of the functional description into the logic circuit design, optimizing the logic circuit design, generating a layout design from the optimized logic circuit design, and forming the logic circuit on a substrate according to the stencil design and the layout design.

Description:
BACKGROUND 
       [0001]    1. Field 
         [0002]    The field of the present invention relates to particle beam lithography and, in particular, to logic design for cell projection particle beam lithography. 
         [0003]    2. Description of Related Art 
         [0004]    In today&#39;s semiconductor manufacturing process, optical lithography using photomasks is commonly used. However, mask cost, which is rapidly growing with shrinking feature size is becoming a serious problem for semiconductor manufacturing. To solve this issue, various approaches that do not use a photomask, such as ML2 (Mask less lithography), have been proposed. One of the promising approaches among various ML2 is electron beam direct writing (EBDW). However, the problem with this approach has been its relatively low throughput, which hindered its use for volume production. Thus, EBDW has been applied only for research and for interconnect layers of production ASICs. 
         [0005]    To reduce writing time of EB, a cell projection (CP) technique has been proposed, which uses a stencil mask that contains cell patterns and enables a cell pattern drawn by one shot, which reduces overall writing time compared to conventional VSB (Variable Shape Beam) method. 
         [0006]      FIG. 1  shows a concept of cell projection EB writing. An electron beam source  100  projects an electron beam  102  that is formed into a rectangular shape  108  with a rectangular aperture  104  formed in a first aperture  106  and then is projected through a second aperture  110  formed in a stencil mask  112 . 
         [0007]    A conventional VSB machine usually uses relatively simple patterns as the apertures  110  of stencil mask  112  to form rectangular or triangular shapes with variable sizes and project them on a surface wafer or substrate  116 , and chip patterns  114  on the wafer  116  are formed by combination of those simple rectangular or triangular shapes. On the other hand, in the case of cell projection, the stencil mask  112  may include more complex patterns as apertures  110  of the stencil mask  112 . Each aperture pattern  110  can be any complex pattern of 10×10 um2 in size, as an example. Typical examples of such patterns contained on stencil masks are patterns of standard cell library entities such as logic gates or flip-flops. 
         [0008]      FIG. 2  shows an example of the cell pattern layout  200  on a stencil mask  202  having a plurality of cell areas  204 . An example of the number of cell area is  100 , depending on the size of cell area. By choosing one of such cells, a cell pattern can be drawn on a wafer by one shot of EB exposure, compared to ten or more shots in the case of VSB, thus enabling writing time reduction. 
         [0009]    However, the problem of CP is the limitation of number of cells that can be contained in one stencil mask. Since the cell library of ASICs usually has 300 to 500 cells and the stencil mask should contain all the necessary orientations of each cell, all the cell patterns needed cannot be accommodated on the stencil mask. The limited cell number on the stencil results in the case that only a part of cells used in the IC chip can be drawn with CP, which results in a limited throughput improvement. 
         [0010]    Referring to  FIG. 3 , a conventional LSI design flow  300  includes logic design and stencil design. In general, logic synthesis is used as a technique for optimally designing logic circuits, which usually starts with a functional description  310 , such as in RTL (Register Transfer Level) description and various constraints  312  for the logic circuit design. The functional description  310  is first transformed into a logic circuit by logic transformation  322  and then converted by technology mapping  324  to the final logic circuit  330 , which consists of cell entities of a given cell library  304 , which is pre-designed  302  for a given process technology, and comprises a set of standard cells that have implementation details such as layout pattern and performance data. 
         [0011]    In the course of this logic synthesis process  320 , a set of parameters and constraints are used for optimization. Commonly used parameters and constraints include delay (timing information), power consumption, and area (physical size). After layout design  340 , in the case of cell projection EB, a stencil mask  354  for CP is designed  350  and fabricated  352 . In designing the stencil mask, frequently used cell patterns are included in the stencil as much as possible to reduce the EB writing time. Then, the layout design  340  and stencil design  350  are used for EB data preparation  360 , and EB data  362  therefrom is used along with the stencil  354  to EB write and fabricate  370  the logic circuit  330  according to the layout design  340  on a wafer or substrate  380 . This flow of LSI design and stencil design has been commonly applied for EB writing. 
         [0012]    However, due to stencil capacity limitation, a substantial number of cells cannot be included in the stencil, which must be drawn by VSB. Thus, writing time reduction by CP is limited. This is because no consideration on writing time is given during logic design process in the conventional design flow  300 . 
         [0013]    There currently exists a need to improve logic design for cell projection (CP) electron beam (EB) lithography. 
       SUMMARY 
       [0014]    This invention is related to a method and system for logic design of integrated circuits to be manufactured by the cell projection (CP) electron beam (EB) lithography, which is a form of particle beam lithography. One feature of this logic design method is to take into account the stencil information, which affects EB writing time, to optimize the logic circuit. One application of this method is for the case that the stencil is pre-designed, and logic design is optimized or at least improved using the stencil information, such as which cells are included in the stencil. Another application is for the case that the stencil is not pre-designed and the logic circuit, and then stencil, are optimally designed so as to minimize or at least reduce the writing time, considering the restrictions related to the stencil and the shot count by CP and VSB. In either case, short EB writing time is realized while the circuit functionality is the same. 
         [0015]    This invention may be applied to any type of particle beam lithography technologies, including electron beam (EB) lithography, optical laser beam lithography and X-ray beam lithography, which transcribe patterns using at least two apertures (stencil masks) and source of particle beams, such as optical laser beam and X-ray beam, as well as electron beam lithography. 
         [0016]    In one embodiment, a method and system for particle beam lithography include predefining a cell library having a plurality of cell patterns associated with characteristic parameters of a cell, predefining a stencil mask for Cell Projection (CP) having the plurality of cell patterns with information from the cell library, adding stencil information which relates to particle beam lithography of each cell for the stencil mask to the cell library, fabricating the stencil mask, synthesizing and optimizing a logic circuit from a given functional description and constraints, considering the stencil information that relates to particle beam lithography, generating a layout design from the optimized logic circuit design, and forming the logic circuit on a substrate using the layout design and stencil mask by the Cell Projection (CP) particle beam lithography. 
         [0017]    In one embodiment, a method and system for particle beam lithography include predefining a cell library having a plurality of cell patterns associated with characteristic parameters of a cell, predefining a stencil mask for Cell Projection (CP) having the plurality of cell patterns with information from a cell library, adding stencil information which relates to particle beam lithography of each cell for the stencil mask to the cell library, fabricating the stencil mask, synthesizing and optimizing a logic circuit from a functional description and constraints, considering the stencil information that relates to particle beam lithography, as a first optimization process, execute a timing analysis of the logic circuit, improving the logic circuit design by iteratively replacing cells to solve, or reduce, the problems which were found by above analysis, as a second optimization process, generating a layout design from the optimized logic circuit design, and forming the logic circuit on a substrate using the layout design and stencil mask by Cell Projection (CP) particle beam lithography. 
         [0018]    In one embodiment, a method and system for particle beam lithography include predefining a cell library having a plurality of cell patterns associated with characteristic parameters of a cell, predefining a stencil mask for Cell Projection (CP) having the plurality of cell patterns with information from the cell library, adding stencil information which relates to particle beam lithography of each cell for the stencil mask to the cell library, fabricating the stencil mask, synthesizing a logic circuit from a functional description and constraints in accordance with a logical component of the cell library without considering the stencil information, optimally mapping the logical cells in the circuit to corresponding physical cell implementation, which includes stencil information, generating a layout design from the optimized logic circuit design, and forming the logic circuit on a substrate using the layout design and the stencil mask by the Cell Projection (CP) particle beam lithography. 
         [0019]    In one embodiment, a method and system for particle beam lithography include predefining a cell library having a plurality of cell patterns associated with characteristic parameters of a cell, adding stencil restriction information which is needed to design a stencil mask for particle beam lithography to the cell library, defining ‘Write Time Reduction Efficiency’ for each cell, which reflects the difference of write time for the case of written by Cell Projection (CP) and for the case of written by variable shaped beam (VSB) methods, and adding to the cell library, synthesizing and optimizing a logic circuit from a given functional description and constraints, considering Write Time Reduction Efficiency, designing a stencil mask by selecting cell patterns to be included in the stencil mask under the given stencil mask restriction information, considering overall write time reduction efficiency of the stencil mask, fabricating the stencil mask, synthesizing and optimizing the logic circuit again from a functional description and constraints, considering the stencil information, generating a layout design from the optimized logic circuit design, and forming the logic circuit on a substrate using the layout design and the stencil mask by the Cell Projection (CP) lithography. 
         [0020]    In one embodiment, a data file for a cell library includes a plurality of cell patterns associated with characteristic parameters of a cell and stencil information which relates to particle beam lithography of each cell for a stencil mask to the cell library. 
         [0021]    In one embodiment, a computer program product for synthesizing and optimizing a logic circuit from a given functional description and constraints, considering the stencil information that relates to particle beam lithography. 
         [0022]    This invention introduces a ‘Write Time Figure’ which reflects the particle beam writing time of each cell as a parameter or constraint for optimization of the logic synthesis tool. Thus, the optimum solution in terms of writing time can be obtained by the characteristics of logic synthesis algorithm. 
         [0023]    In one embodiment, a data file for a cell library includes a plurality of cell patterns associated with characteristic parameters of a cell and stencil restriction information which is needed to design a stencil mask for particle beam lithography to the cell library. 
         [0024]    In one embodiment, a software program for synthesizing and optimizing a logic circuit from a given functional description and constraints, considering stencil restriction information which is needed to design a stencil mask for particle beam lithography. 
         [0025]    This invention introduces a ‘Write Reduction Efficiency’ (WTRE) which reflects the difference of write time for the case of Cell Projection (CP) and Variable Shaped Beam (VSB) as a parameter or constraint for optimization of logic synthesis tool. Thus optimum solution in terms of writing time reduction using cell projection can be obtained. 
         [0026]    These and other objects and advantages of the present teachings will become more fully apparent from the following description taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0027]      FIG. 1  shows a conventional electron beam writing machine and stencil mask configuration for projecting an image on a substrate. 
           [0028]      FIG. 2  shows an example of cell pattern layout on a stencil mask. 
           [0029]      FIG. 3  shows a conventional LSI design flow including logic design and stencil design. 
           [0030]      FIG. 4  shows one embodiment of an LSI design flow by present invention. 
           [0031]      FIGS. 5A-5D  show various implementations of a logic function. 
           [0032]      FIG. 6  shows another embodiment of an LSI design flow by present invention. 
           [0033]      FIG. 7  shows still another embodiment of an LSI design flow by present invention 
           [0034]      FIG. 8  shows one embodiment of an application of present invention to a stencil design after logic design. 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    Various embodiments of the invention are described herein with reference to the drawings. It should be noted that the drawings are not drawn to scale and that elements of similar structures or functions are represented by like reference numerals throughout the drawings. 
         [0036]    The following discussion mainly describes on EB direct writing (EBDW) as an application of particle beam lithography and should not be limited to only EB direct writing (EBDW), but the this invention can be applied to mask writing using an electron beam (EB) writer with cell projection (CP) capability, in a similar manner, resulting in an improved throughput. 
         [0037]    Accordingly, it should be appreciated that this invention may also be applied to other drawing technologies with transcribed patterns using at least two apertures (stencil masks) and using other types of particle beams other than an electron beam (EB), such as an optical (light) laser beam, an X-ray beam or any other particle beams that run straight and stimulate a sensitive material (resist) layer to form patterns on a substrate. 
         [0038]    This invention provides a method for logic design of integrated circuits that enables to greatly reduce the particle beam, such as electron beam (EB), writing time using cell projection (CP). The logic design method of this invention takes into account new information, or parameters, that affect particle beam writing time, to the process of logic optimization. 
         [0039]    It should be appreciated that this invention is related to the method for optimal design in the sense that the writing time be at a minimum by considering, for example, the characteristics of particle beam writing and restrictions from the stencil. Also, this invention provides the method of optimally designing stencils which provide short design times for most of circuits. 
         [0040]    Some conventional ideas have been provided to reduce the number of cells needed to draw LSI chip patterns to reduce particle beam writing time. However, most of the conventional methods do not consider particle beam writing time in the stage of logic design, but only some modifications are done in later stages in order to reduce number of cell patterns which are drawn by CP. 
         [0041]    In contrast, the invention optimizes the logic circuit using the ‘Write Time Figure’, which reflects particle beam writing time of each cell as the optimization parameter or constraints of logic synthesis. So, this invention is superior to conventional methods in the level of writing time improvement. 
         [0042]    An innovative point of this invention is to enable optimum design of logic circuits by taking into account not only the restrictions of stencil mask but also the shot count number, or writing time, of cells which are to be written by CP or by VSB, thus resulting in the minimum particle beam writing time. Another innovative point is that the stencil mask is defined before the logic design so that it provides shortest particle beam writing time compared to the previous method, where the logic circuit is designed independent from stencil. This invention can also be applied to designing a universally optimum stencil. 
         [0043]    This invention includes a number of variations as follows, by the combination of whether the stencil is pre-designed or not, limitations from the stencil design, and the timing of logic optimization. 
         [0044]    Referring to  FIG. 4 , in a method  400  of LSI design of this invention, a cell library  402  is predefined and a stencil  454  is also predefined by stencil design  450  and fabricated  452  prior to LSI design, and a logic circuit design  440  is achieved considering the characteristics of the stencil  454  so that the writing time becomes minimum. 
         [0045]    In one embodiment of method  400 , a stencil  454  is designed  450  so that it includes frequently used cell patterns which can be drawn by CP. Other cells which are not included in the stencil have to be drawn by VSB. 
         [0046]    In process  400 , logic synthesis  420  is utilized as a technique for optimally designing logic circuits, which starts with a functional description  410 , such as in RTL (Register Transfer Level) description and various constraints  412  for the logic circuit design. The functional description  410  is transformed into a logic circuit  430  by a logic transformation  422  and then converted by technology mapping  424  to the final logic circuit  430  which consists of cell entities of a given cell library  404 , which is pre-designed for a given process technology, and comprises stencil and writing time information along with a set of cells that have implementation details such as layout pattern and performance data. The layout design  440  and stencil design  450  are used for EB data preparation  460 , and EB data  462  therefrom is used along with the stencil  454  to EB write and fabricate  470  the logic circuit  430  according to the layout design  440  on a wafer or substrate  480 . 
         [0047]    In the course of logic synthesis  420 , stencil and writing time information are used, along with other parameters and constraints, for logic optimization. The layout design  440  and cell library with stencil information  402  are used for EB data preparation  460 , and EB data  462  therefrom is used along with the stencil  454  to EB write and fabricate  470  the logic circuit  430  according to the layout design  440  on a wafer or substrate  480 . 
         [0048]    In process  400 , a cell library  404  is prepared to comprise a number, such as a ‘Write Time Figure’, that reflects the EB writing time for all of the layers of each cell. This ‘Write Time Figure’ is usually calculated by a linear equation of the shot counts needed to draw each layer of the cell, and the simplest case is equal to the total shot count of all layers. The shot count of a cell, which is not included in the stencil  454 , is the number of fractured shapes for VSB, and the shot count of a cell, which is included in the stencil  454 , is 1 or a number of CP shots if the cell pattern is drawn by more than one cell projection shot. 
         [0049]    However, in one aspect, the ‘Write Time Figure’ can be any function of the shot counts of each layer of the cells. Moreover, in another aspect, if only some layers are to be drawn by CP EBDW, only shot counts of those layers can be taken into account. 
         [0050]    In one aspect, using such cell library  404 , logic synthesis  420  is carried out under the condition that the summation of ‘Write Time Figures’ of all cells in the circuit comprise a minimum. In other words, the summation of ‘Write Time Figures’ is used as the cost function of the logic optimization. This cost function for logic synthesis  420  is not limited to the summation of ‘Write Time Figures’ but can be any function of ‘Write Time Figures’ of all cells in the circuit. 
         [0051]    As a special case of above method, logic synthesis  420  can be done using only the cells that are included in the stencil  454 . In this case, the circuit is composed with cells which can be drawn by CP. In one aspect, this case is equivalent to a special case that the cells, which are not included in the stencil, have the infinitive, or large enough value, of the ‘Write Time Figure’. 
         [0052]    In the method  400  above, other parameters for logic optimization, such as delay, power and area, or any subset thereof, can be used in combination with the ‘Write Time Figure’. Those parameters can be used not only as the parameter to optimize but also as the constraints for optimization. For example, a logic synthesis may be done to minimize the ‘Write Time Figure’ and area, in order of priority, and under given constraints of circuit timing. In one aspect, detailed types of optimization parameters and constraints, and the way of specifying them depend on the specification of the logic synthesis tools used. 
         [0053]    Referring to  FIGS. 5A-5D , some examples are shown for how differently a logic function can be implemented with varying shot counts and for how the invention can optimally select the cell patterns.  FIG. 5A  is an example of a standard cell (AND-NOR-221) used in the logic circuit. If this cell is included as a cell pattern of the stencil mask, the pattern can be drawn by one shot using CP, but if it is not included in the stencil mask and drawn by VSB, total shot count becomes 138, as shown in  FIG. 5B . However, if the stencil mask includes other logic gates, such as inverting gates, and the shot count information are used by the logic synthesis tool, a more optimized, i.e. with less shot count, circuit can be obtained, such as shown in  FIG. 5C . However, the needed chip area will be doubled. So, if other cell patterns, such as non-inverting gates, are added to the stencil, and also the chip area is taken into account as one of the optimization parameter in addition to the shot count, another further optimized circuit, with less area penalty and with reasonably reduced shot count as shown by  FIG. 5D , can be obtained. 
         [0054]    Referring to  FIG. 6 , another method  600  of LSI design and stencil design may be utilized as an alternate to method  400  of  FIG. 4 . 
         [0055]    In the method  600 , a logic optimization process  624  can be divided into two or more steps, and some part of the optimization process can be done in later design stages by an iterative improvement method  634 . For example, logic synthesis  620 , as the first optimization step, may be done using only the ‘Write Time Figure’ to optimize in technology mapping, and the timing problematic paths may be found using a circuit timing analysis tool  632 . Then, a second optimization process of iterative improvement  634  may be carried out to solve those timing problems by iteratively replacing cells on the relevant path with other functionally equivalent cells, which have less delay but may have bigger ‘Write Time Figures. This optimization step improves the circuit performance so that it meets the given timing constraints, with possible slight penalty of write time and chip area. 
         [0056]    In one aspect, such iterative improvement  634  can be achieved in any other later stages of the chip design, such as after floor planning, after routing, or after physical verification. 
         [0057]    Referring to  FIG. 7 , in another embodiment of a method  700  of LSI design and stencil design of this invention, logic synthesis  720  and optimization for EBDW  726  may be separated. In one aspect, the logic synthesis  720  may be achieved using the “Logical” standard cell library in conventional manner, where only conventional parameters such as area, timing and power to optimize the circuit. Then, each cell is mapped to a corresponding “Physical” cell, which has physical implementation, layout pattern and stencil CP pattern  736 . The Physical cell library has the relation between logical cell and corresponding Physical cells, which can be plural and each one has the ‘Write Time Figure’ and stencil related information, as described above in reference to process  400  ( FIG. 4 ). This mapping is optimally achieved by using those information and optimization criteria described above in reference to process  400  ( FIG. 4 ). An example of such plural implementation is a two input gate is implemented by different layout patterns or by different stencil patterns. Another example is the case that different part of the same stencil pattern is used by a Partial Projection technique. 
         [0058]    In one or more of the methods described above, it may be supposed that a logic circuit is not changed after the logic design until EB writing for manufacturing. However, it should be appreciated that in actual LSI development, some cells in the circuit, or circuit connectivity itself, may be changed manually by some reason after logic design, or even after layout design. Also, in other case the design is done manually, without using logic synthesis tools, In such cases, similar method and criteria can be applied to select the new set of cells for the change so as to minimize the EB writing time. 
         [0059]    In one embodiment, this invention can also be applied to the case that the stencil mask is not defined before the logic design and that provides the method to optimally design the logic circuit, and then, optimally design the stencil mask so that EB writing time using Cell Projection becomes minimum, as shown in  FIG. 8 . There are also several variations described herein below. 
         [0060]    In one embodiment, referring to  FIG. 8 , to obtain an optimum circuit  842  and an optimum stencil  854 , the ‘Write Time Figures’ for both the cases of CP writing and VSB writing for each cell in the library is required. The ‘Write Time Figures’ for the cases of VSB and CP may be defined as wv(i) and wc(i), respectively, for cell Ci. The ‘Write Time Reduction Efficiency’ for cell Ci may be defined as DW(i), which is the difference of write time when drawn by VSB and those when drawn by CP; DW(i)=wv(i)−wc(i). 
         [0061]    In one aspect, under the condition that such parameters associated with each cell are stored in the cell library  404 , and a maximum capacity of the stencil mask, such as a maximum number of cell patterns or a maximum total area of cell patterns are given, the method  800  to design an optimum circuit  842  and stencil mask  854  with stencil design, information and fabrication  850 ,  856 ,  852  is as follows. 
         [0062]    In the first step ( 1 A) for the temporary logic design, logic synthesis  820  is carried out so that the cells with larger DW(i) is used as much as possible, so as to be drawn by Cell Projection. In one aspect, a logic synthesis algorithm works to minimize parameters. To utilize this nature, the modified parameter may be used as a tool. For example; to use IW(i)=(C−DW(i)), where C is a constant that makes any IW(i) positive, or IW(i)=1/DW(i), the logic synthesis is carried out so that the total summation of IW(i) becomes minimum or so that the cells with small IW(i) are used as much as possible. If the logic synthesis tool has the capability to maximize the summation of parameters, DW(i) can be used as an optimization parameter or constraint. In one aspect, other parameters including area, timing and power may also be considered simultaneously with IW(i) or DW(i). 
         [0063]    In one aspect, after logic synthesis  820 , stencil design  850 , and stencil information  850  for optimization is generated, the cell library  404  may be updated  858  with optimal stencil design and stencil information. 
         [0064]    In the following step ( 2 A), stencil design  850 , which comprises deciding which cells are to be included in the stencil mask  854 , can be achieved by the following procedure, when the maximum number of cells on a stencil mask is given as CN. In one aspect, for each cell, calculate ‘Weighted Write Time Reduction Efficiency’ (WTRE) which is defined by f(i)DW(i), where f(i) is number of instances of cell Ci in this circuit, and sort all cells by ‘Weighted WTRE’ in descending order. Select first CN cells which have highest ‘Weighted WTRE’. Those CN cells are ones which are to be included into the stencil mask  854 . 
         [0065]    In the final step ( 3 A) for final logic design, logic synthesis  842  is carried out again by the method described in the previous processes,  400 ,  600  or  700  using the stencil mask information  856  defined by above step ( 2 A). 
         [0066]    In one aspect, it should be appreciated that the detailed algorithm to solve the problem is not limited to those described above. 
         [0067]    In another embodiment, step ( 1 A) above can be replaced by another ways of logic synthesis or logic design. The simplest way is to do the logic synthesis by just usual manner, which does not consider ‘Write Time Cost’, or even to use an existing, or previously designed, logic circuit without doing logic synthesis again. 
         [0068]    In the above embodiments, only one logic circuit is considered to select the subset of cells to be written with CP. However, in another embodiment, more than one circuit implementations which meet design constraints may be created by logic synthesis  820  and utilized to obtain even more optimized solution. One example of the method for this is as follows. 
         [0069]    In a step ( 1 B) generate more than one logic circuit implementations with same functionality by the logic synthesis tool as described in the step ( 1 A) above. 
         [0070]    In a step ( 2 B), calculate for each cell Ci of the superset a ‘Globally Weighted Write Time Reduction Efficiency’, which is defined as f(i)DW(i), where f(i) is number of instances of cell Ci in all of the logic circuits, and sort all cells by ‘Weighted WTRE’ in descending order. 
         [0071]    In a step ( 3 B), select the first CN cells which have highest ‘Globally Weighted Write Time Reduction Efficiency’. Those CN cells are ones which are to be included into the stencil mask  854 . 
         [0072]    In one aspect, it should be appreciated that the detailed algorithm to solve the problem is not limited to those described above. 
         [0073]    By applying the methods described above, a universally optimum (optimum for many circuits) stencil can be designed, if more than one sample circuit for consideration. Namely, after designing an optimum subset of cells, which constitute a stencil, for each of given sample circuit by the method described above, select top CN cells which appear most frequently in all of those subsets. In one aspect, it should be appreciated that the actual algorithm to select CN cells is not limited to those described above. 
         [0074]    In on or more of the methods described above, it may be supposed that the logic circuit and used cells are not changed after logic design until EB writing step for manufacturing. However, in the actual LSI development, some cells in the circuit or circuit connectivity itself may be changed after logic design or even after layout design. In such cases, the similar criteria, as described above, can be applied to select the new set of cells for the change so as to minimize the EB writing time. 
         [0075]    This invention includes cell library or technology files that include stencil information and other information which affects EB writing time. Also, this invention includes the software tools or systems that enables above described methods automatically or semi-automatically. 
         [0076]    Although particular embodiments of the invention have been shown and described, it will be understood that it is not intended to limit the invention to the preferred embodiments, and it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than restrictive sense. The invention is intended to cover alternatives, modifications, and equivalents, which may be included within the scope of the invention as defined by the claims.