Abstract:
A method for generating mask layout data for lithography simulation includes prescribing original data defining an original layout of a mask and determining a deviation between the original layout and a subsequent layout of a mask derived from said original layout. On the basis of this deviation, new data defining a new layout is calculated. This new layout is more similar to the subsequent layout that it is to the original layout.

Description:
RELATED APPLICATIONS  
         [0001]    This application claims the benefit of the priority date of German application DE 100 44 257.9, filed on Sep. 7, 2000, the contents of which are herein incorporated by reference.  
         FIELD OF INVENTION  
         [0002]    The invention relates to lithographic processing, and in particular, to simulation of lithographic processes.  
         BACKGROUND  
         [0003]    In the practice of lithography, original data defining an original layout are prescribed. New data are then automatically calculated proceeding from the original data. The calculation is effected in such a way that the new data define a new mask layout whose geometry is more similar to a mask produced or producible using the original data than it is to the original layout.  
           [0004]    In the case of a known method carried out with the aid of the “Selid” program from Sigma-C, the successive steps of the production process for producing a photomask are simulated. Such steps include: writing the mask to a photoresist by means of a laser or electron beam writer; developing the photoresist; etching the mask; and performing reaction diffusion processes. The simulation of the mask production process requires an additional program that differs from the program used later for simulating exposure and resist development processes in a wafer. This additional program requires additional input parameters, some of which have to be determined experimentally in a complicated manner. Simulating the steps of the production process therefore requires additional expenditure of time and computation complexity, and significant data processing capability.  
         SUMMARY  
         [0005]    It is an object of the invention to specify a simple method for generating mask layout data for lithography masks, in which method the new layout, with a reduced outlay, continues to be very similar to a mask that is produced using the original data. Moreover, the intention is to specify an associated apparatus and an associated program.  
           [0006]    The invention is based on the recognition that deviations between a mask defined by a layout and either a mask produced according to this layout or a mask modeled proceeding from this layout with simulation of the production process can be attributed to the production process. These deviations depend on the geometry of the mask to be produced and can largely already be predicted on the basis of the geometry of the original layout. This makes it possible to rapidly take account of the influences of the production process while avoiding the need to simulate the individual steps of the production process In the case of the method according to the invention, in addition to the method steps mentioned in the introduction, the new data which are intended to be used for the lithography simulation are calculated on the basis of rules which are based on deviations in the geometry of a layout from a mask that is produced according to this layout. As an alternative to the production, the mask that is used for comparison purposes can also be modeled proceeding from the layout with simulation of the steps of the production process. In both cases, in the method according to the invention, the individual method steps of the production process of the mask are not simulated, however, during the calculation of the new data. The inputting of a multiplicity of process parameters for the simulation of the production process and the computationally complicated simulation itself are thus obviated.  
           [0007]    The deviations in the geometry can be calculated by means of simple geometric relationships. Differential equations, such as e.g. diffusion equations, do not have to be solved. As a result, the new mask data can be calculated with a computation complexity that is reduced by orders of magnitude in comparison with the simulation of the production process.  
           [0008]    In a development of the method according to the invention, the rules are geometric calculation specifications for defining the boundaries of a structure at a position of the new layout depending on the length and/or the area of a reference structure located at the same position in the original layout. Alternatively or cumulatively, the distance between the reference structure and the adjacent structures of the original layout is also included in the calculation specification. The length and the area of a structure determine the extent of the deviations to be determined. The adjacent structures allow conclusions to be drawn regarding the locations at which deviations will occur. This is because different effects occur during the mask writing process of closely adjacent structures than during the writing of adjacent structures at a greater distance from the reference structure.  
           [0009]    In one refinement, in accordance with a rule, a shortening value is determined for an elongate reference structure, which is also referred to as a line structure or as a line for short, at a position of the original layout. Depending on the shortening value, the structure located at the same position in the new layout is shortened in the longitudinal direction in comparison with the reference structure. The line shortening can be attributed to the absence of adjacent structures. Thus, the nature of the line shortening and the extent of the line shortening can be determined on the basis of the geometry of the original layout. Instances of line shortening are illustrated in FIGS.  2  to  4 , which are explained in more detail below.  
           [0010]    In another refinement, in accordance with a further rule, a cornered reference structure is determined at a position of the original layout. A corner can be defined with the aid of the angle between two meeting lines or straight edges. In a customary design the structures often have corners whose edges are at an angle of 90° with respect to one another (however, any desired angles are also conceivable). For the purpose of rounding a corner, at least one radius or curvature value is determined or input by an operator. Depending on the radius or curvature value, the new data are calculated in such a way that the structure located at the same position in the new layout has a rounded edge profile instead of the corner. The radius value can be determined, for example, directly from the width of a structure. The circle equation, for example, can then be used for calculating the position of the structure in the new layout. Instances of corner rounding are explained below with reference to FIGS.  2  to  5 .  
           [0011]    In one refinement, the radius is chosen depending on the surroundings. In the case of a light-absorbing structure which is arranged around a light-transmissive structure, an inner corner is rounded with a smaller radius than an outer corner of the light-absorbing structure.  
           [0012]    In a further refinement, in accordance with a rule, a constriction value is determined for an elongate reference structure at a position of the original layout. Depending on the constriction value, the structure located at the same position in the new layout is then constricted at least in sections transversely with respect to the longitudinal direction in comparison with the reference structure. This measure takes account of the so-called “peanuts” effect because constrictions of structures are simulated which can be attributed to the absence of adjacent structures during the production of the mask. The “peanuts” effect is explained below with reference to FIG. 7.  
           [0013]    In a second aspect, the invention relates to a method for generating optimized mask layout data for photomasks. In the method in accordance with the second aspect, original data which define an original layout for the simulation of a lithography method are again prescribed. Proceeding from the original data, new data are calculated automatically or in another way. The new data define a new layout which is more similar, with regard to the geometry, to a mask that is produced or can be produced using the original data than to the original layout.  
           [0014]    By way of example, the new data can be calculated by simulation of the method steps during the production of the mask. As an alternative, however, the method in accordance with the first aspect or in accordance with one of the abovementioned developments and refinements can also be used in order to define the new data.  
           [0015]    It is an object of the second aspect of the invention to specify a simple method for generating optimized mask layout data for photomasks. Furthermore, the intention is to specify an associated apparatus and an associated program.  
           [0016]    The invention in accordance with the second aspect proceeds from the consideration that taking account of the influence of the production process of the mask is only a partial step on the way to defining final mask layout data which are then actually used for the mask production. This is because the changes that occur through the production process in turn require a change of the original layout in a corrected layout. These changes have hitherto been carried out manually, but can also be automated.  
           [0017]    In the invention&#39;s method in accordance with the second aspect, corrected data are automatically defined proceeding from the new data in such a way that a corrected mask that is produced or can be produced using the corrected data is more similar, with regard to the geometry, to the original layout than to a mask that is produced or can be produced using the original data. Consequently, the original layout is considered as the aim of the production process of the mask. The reference to the original layout allows the definition of simple criteria for the automatic correction.  
           [0018]    Alternatively or cumulatively, in the method in accordance with the second aspect, the corrected data are designed in such a way that the mask that is produced or can be produced using the corrected data has better lithographic imaging properties than a mask that is produced or can be produced using the original data. The lithographic imaging properties are of fundamental importance for the structure widths that can be achieved during the wafer exposure. The chip production yield can be considerably increased by virtue of the improved imaging with a mask produced from the optimized mask layout data. Moreover, it is thus possible to produce chips having a greater electrical performance, e.g. with regard to a higher clock frequency or a lower current consumption.  
           [0019]    One criterion for the automatic correction is, in one development, the ratio of the dark areas and of the bright areas of the original layout. In the corrected layout, this ratio is to be preserved or changed by a prescribed value. And this is because the area ratio is initially changed in the new layout on the basis of the effects that are taken into account.  
           [0020]    In one development, the corrected layout data are calculated on the basis of correction rules which are based on deviations in the geometry of a layout from a mask that is produced according to this layout or a mask that is modeled proceeding from this layout with simulation of the production process. The fact that the deviations in the geometry are taken into account already means that it is possible to define so many correction rules that the correction can be completely or almost completely automated.  
           [0021]    In one refinement, the method discussed above in connection with the shortening value is used for defining the new data. In accordance with a correction rule, depending on the shortening value, a lengthening value is then determined for a reference structure at a position of the original layout. Depending on the lengthening value, the structure located at the same position in the corrected layout is subsequently lengthened in the longitudinal direction in comparison with the structure located at the same position in the new layout. The aim here is to approximate to the structure prescribed by the original layout at the same position. This is done in an iteration method, for example. However, it is also possible to use approximation specifications. A correction method is explained below with reference to FIG. 8.  
           [0022]    In a further refinement, use is made of the method discussed above in connection with the radius or curvature value. In accordance with a further correction rule, depending on the radius or curvature value, a lengthening value is then determined for a reference structure at a position of the original layout. Depending on the lengthening value, the structure located at the same position in the corrected layout is subsequently lengthened in the longitudinal direction and/or in the transverse direction in comparison with the structure located at the same position in the new layout. In this refinement, instances of shortening which are brought about by the rounding of the corners are compensated for again.  
           [0023]    In a next refinement, in accordance with a further correction rule, depending on the constriction value, a widening value is determined for a reference structure at a position of the original layout. The abovementioned development of the method in accordance with the first aspect of the invention is used for determining the constriction value. Depending on the widening value, the structure located at the same position in the corrected layout is then widened at least in sections transversely with respect to the longitudinal direction in comparison with the structure located at the same position in the new layout. What is achieved by this measure, despite the “peanuts” effect, is that the corrected mask contains a structure having a constant or intended width.  
           [0024]    In addition to the abovementioned refinements, other correction rules are also used for correcting the consequences of other effects. In this case, simple geometric relationships are utilized each time.  
           [0025]    In one development, the lengthening and/or the widening is implemented whilst maintaining the form of the structure in the new mask. As an alternative, however, simple structures can also be attached in the course of the lengthening or widening. By way of example, if the intention is to correct instances of rounding, then small squares are attached to the structure of the original mask to the left and right of a central axis, in order to obtain the corrected structure. The structure thus obtained a serif-shaped configuration, as known previously from OPC methods (Optical Proximity Correction—correction of proximity-induced diffraction effects). Known OPC methods take account, in particular, of the exposure process of the wafer. By contrast, the method according to the invention essentially takes account of the effects which are brought about by the mask writer and the mask production process.  
           [0026]    The invention additionally relates to an apparatus, in particular a data processing system, for generating mask layout data for lithography simulation or for generating optimized mask layout data for photomasks. However, use is also made of circuit arrangements or special hardware in a data processing system. The apparatus is constructed in such a way that the method steps according to one of the methods in accordance with the first aspect or in accordance with the second aspect or in accordance with the developments thereof are implemented during operation. Thus, the technical effects mentioned above also apply to the apparatus.  
           [0027]    Furthermore, the invention relates to a program having a command sequence that can be executed by a data processing system. The method steps in accordance with the first aspect or in accordance with the second aspect or in accordance with a development of one of these aspects are implemented during the execution of the command sequence. The program is held for example in a RAM module (Random Access Memory) in a programmable memory module, on a floppy disc or on a compact disc, abbreviated to CD. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0028]    Exemplary embodiments of the invention are explained below with reference to the accompanying drawings, in which:  
         [0029]    [0029]FIG. 1 shows a basic illustration of wafer lithography and lithography simulation and also of method steps for generating the layout data for a lithography mask;  
         [0030]    [0030]FIG. 2 shows the effects of “line shortening” and “corner rounding” using structures in the center of an original layout;  
         [0031]    [0031]FIG. 3 shows the effect of intensified “line shortening” at the edge of a structure of an original layout;  
         [0032]    [0032]FIG. 4 shows the effect of reduced “line shortening” on account of an adjacent structure;  
         [0033]    [0033]FIG. 5 shows the effect of “corner rounding” in the case of structures having irregular borders,  
         [0034]    [0034]FIG. 6 shows the effect of “corner rounding” in the case of a dark structure which surrounds a bright structure;  
         [0035]    [0035]FIG. 7 shows the “peanuts” effect; and  
         [0036]    [0036]FIG. 8 shows an original mask and also a mask that is calculated therefrom and corrected with regard to the effect of line shortening. 
     
    
     DETAILED DESCRIPTION  
       [0037]    [0037]FIG. 1 shows a basic illustration of the wafer lithography and lithography simulation and also the method steps for generating the layout data for a photomask that is utilized during the production of integrated circuits. It is assumed that an original layout  10  and original-layout data  12  defining this original layout  10  have been prescribed. The data format is suitable for inputting into a program for simulation of the lithography process proceeding from the original layout data. Examples of such programs are the “Solid-C” program from Sigma-C or the “Prolith” program from Finle. The original-layout data  12  are prescribed, for example, by an upstream development department depending on the basis of the electrical properties of the circuit. An integrated circuit produced in an ideal production process in accordance with the original layout  10  would thus satisfy the electrical requirements. However, during a real production process, deviations begin to occur even as early as during the production of a mask proceeding from the original layout  10 .  
         [0038]    An exemplary original layout  10 , shown in FIG. 1, contains three rectangular dark structures  14 ,  16  and  18 . An arrow symbolizes the production process  20  of a mask proceeding from the original-layout data  12  of the original layout  10 . A mask  22  having three dark structures  24 ,  26  and  28  is produced. These dark structures  24 ,  26  and  28  are dimensioned, for example, on the order of 100 nanometers. The structures  24 ,  26  and  28  are located in this order in the mask  22  at the same position as the structures  14 ,  16  and  18  in the original layout  10 . By way of example, the top left corner  30  of the original layout  10  and the top left corner  32  of the mask  22  can be used as a reference point.  
         [0039]    The original layout  10  still has to be corrected. However, to reduce costs, however, no production process  20  is implemented. Instead, a geometry-change method  34  is implemented. This geometry-change method  34  accounts for the influences of the production process  20  on the mask  22 . The geometry-change method  34  is stored, for example, in a program for a data processing system. The inputs  36  for the geometry-change method  34  include original-layout data  12  of the original layout  10 . In addition, during the creation of the program for realizing the geometry-change method  34 , rules were stored in the program. These rules account for general deviations between the geometry of the the mask  22  and that of the original layout  10  that occur during the production of a mask. Such deviations can include: the rounding of the corners during the production of the mask  22 , (see arrows  38  and  40 ), line shortening, constrictions of an elongate structure in specific sections, (referred to as the “peanuts” effect), rounding of corners, and CD (critical dimension) linearity.  
         [0040]    During the implementation of the geometry-change method  34 , the deviations are calculated on the basis of the prescribed rules. These rules account for the extent and the area of the respective processed structure  14 ,  16  or  18  and also the influence of adjacent structures  16  to  18 .  
         [0041]    The result of the implementation of the geometry-change method  34  is layout data of a new mask layout  42  that includes dark structures  44 ,  46  and  48  corresponding to the structures  14 ,  16  and  18 . In this context, “corresponding” means that a structure in the original layout  10  is located at the same position as the corresponding structure in the new mask layout  42 . The structures  44 ,  46  and  48  have, for example, rounded corners and are shortened in comparison with the structures  14 ,  16  and  18 , respectively. This line shortening will be explained in more detail below with reference to FIGS.  2  to  4 . The structures  44 ,  46  and  48  have forms very similar to the structures  24 ,  26  and  28  of the mask  22 .  
         [0042]    In the first exemplary embodiment, the new mask layout  42  is corrected manually to better meet the requirements of the upstream development department. Corrected layout data  50 ,  52  are subsequently used instead of the original-layout data  12  of the original layout  10 . By means of single or multiple iteration, one or more new mask layouts are produced instead of the new mask layout  42 . The data of the last new mask layout will be used for the simulation of the exposure and development during the processing of a wafer (see arrow  54  and method step  56 ). By way of example, one of the programs “Solid-C” and “Prolith” already mentioned is used for the simulation.  
         [0043]    Prior to the simulation, parameters  58  that characterize an exposure process  60  and a development process  61  of the wafer processing must be input into the program. The exposure process  60  would typically be carried out by an exposure apparatus  62 , which may still be in the development stage at the time of the simulation  56 . In this case, the parameters set as development aims should be input instead of actual parameters. The exposure apparatus  62  contains, for example, a laser unit  64  for generating a laser beam  66  that images the structures on the mask, by means of an optical arrangement, usually in a manner reduced in size, onto a silicon wafer  72  coated with a photoresist layer  70 . During the simulation  56 , the method steps of the exposure process  60  and of the development process  61  of the photoresist are simulated by calculations made with the aid of equations that describe the physical processes taking place.  
         [0044]    At the end of the simulation  56 , results data  74  are output (see arrow  76 ). The results data  74  represent a resist pattern that essentially corresponds to a resist pattern  78  of the kind that might actually be produced with the aid of the exposure apparatus  62  and the development process  61  (see arrow  80 ). Elongate photoresist structures  82 ,  84  and  86 , corresponding in this order to the structures  14 ,  16  and  18 , remain on the silicon wafer  72 .  
         [0045]    Proceeding from the result data  74 , further corrections of the layout data are often necessary (see arrows  88  and  52 ). After one or more iterations, result data suitable for the simulation of further method steps during the production of an integrated circuit are generated (see arrow  90 ). By way of example, the etching of the silicon wafer  72  is subsequently simulated.  
         [0046]    In a second exemplary embodiment, the correction is also carried out automatically in the context of the geometry-change method  34 . By way of example, during the correction: instances of line shortening are eliminated by lengthening the relevant structures; instances of corner-rounding are avoided by “attaching” correction areas, and constrictions on account of the “peanuts” effect are widened.  
         [0047]    During the implementation of the geometry-change method  34 , one or more iterations are automatically carried out in the second exemplary embodiment (see arrows  50  to  52 ). The automatic correction is explained in more detail below with reference to FIG. 8.  
         [0048]    [0048]FIG. 2 shows the effects of “line shortening” and “corner rounding” using structures  100 ,  102  and  104  of an original layout  110  and, corresponding to those structures, structures  120 ,  122  and  124  of a new mask layout  130 . A broken line shows the value x=0 at which the mutually parallel structures  100 ,  102  and  104  begin. Points  132  and  134  indicate that the structures  100 ,  102  and  104  shown are adjoined above and below by further parallel structures (not shown). Points  136  and  138  illustrate the same facts for the structures  120 ,  122  and  124 . The new mask layout  130  is generated from the layout data of the original layout  110  by having the geometry-change method  34  account for influences of the production process for real masks. Consequently, the new mask layout  130  is similar to a mask produced with the aid of the original layout  110 . Lines  140 ,  142  and  144  show the original profile of the structures  100 ,  102  and  104  in the new mask layout  130 . A broken line again shows the value x=0.  
         [0049]    The line shortening is identical for the three structures  120 ,  122  and  124  that are arranged parallel to one another at the same distance so that only the shortening and the corner rounding for the structure  122  are explained below. In comparison with the structure  102 , the structure  122  is shortened at one end by a difference value D 1  to which a distance  150  corresponds. The shortening takes account of the influence of the production process during the production of a mask from the layout data of the original layout  110 . Moreover, the ends of the structures  120 ,  122  and  124  shown in FIG. 2 have been rounded to account for the influence of the production process. A radius-of-curvature for the rounding is independently determined during the implementation of the geometry-change method  34 . This radius-of-curvature corresponds to half the structure width  152  in this exemplary embodiment.  
         [0050]    [0050]FIG. 3 shows the effect of intensified “line shortening” at the edge of a structure of an original layout  170  in comparison with a new mask layout  190 . The effect is explained using structures  160 ,  162  and  164  of the original layout  170  and corresponding to those structures, structures  180 ,  182  and  184  of the new mask layout  190 . A broken line shows the value x=0 at which the mutually parallel structures  160 ,  162  and  164  begin. Points  192  indicate that the structures  160 ,  162  and  164  shown are adjoined by further parallel structures below (not shown). Points  194  illustrate the same facts for the structures  180 ,  182  and  184 . The structures  160 ,  162  and  164 , and  180 ,  182  and  184 , are not adjoined by any structures above. The new mask layout  190  is generated from the layout data of the original layout  170  by having the geometry-change method  34  account for influences of the production process on real masks. Consequently, the new mask layout  190  is similar to a mask produced with the aid of the original layout  170 . Lines  200 ,  202  and  204  show the original profile of the structures  100 ,  102  and  104  in the new mask layout  190 . A broken line again shows the value x=O.  
         [0051]    The greatest degree of line shortening is that for the structure  180 . For the two structures  182  and  184 , the line shortening is approximately identical. In comparison with the structure  162  or  164 , the structure  182  or  184 , respectively, is shortened at one end by a difference value D 2  to which a distance  210  corresponds. The difference value D 2  has approximately the same value as the difference value D 1 . By contrast, in comparison with the structure  160 , the structure  180  is shortened by a difference value D 3  greater than the difference value D 1 , see distance  214 . The shortening accounts for the influence of the production process during the production of a mask from the layout data of the original layout  110 . In particular, the greater shortening of the structure  180  shows that there are no further structures arranged above this structure. Moreover, the ends of the structures  180 ,  182  and  184  shown in FIG. 3 have been rounded to account for the influence of the production process. A radius-of-curvature for the rounding is determined independently during the implementation of the geometry-change method  34 . This radius-of-curvature corresponds to half the structure width  212  in this exemplary embodiment.  
         [0052]    [0052]FIG. 4 shows the effect of reduced “line shortening” on account of an adjacent structure  360  and  362 , respectively, using structures  300 ,  302  and  304  of an original layout  310  and corresponding to those structures, structures  320 ,  322  and  324  of a new mask layout  330 . A broken line shows the value x=0 at which the mutually parallel structures  300 ,  302  and  304  begin. Points  332  and  334  indicate that the structures  300 ,  302  and  304  shown are adjoined by further parallel structures (not shown) above and below those structures. Points  336  and  338  illustrate the same facts for the structures  320 ,  322  and  324 . The structure  360  lies transversely with respect to the structures  300 ,  302  and  304  at a distance from the beginning of these structures that corresponds to the distance between adjacent structures  300 ,  302  and  304 . The structure  362  lies transversely with respect to the structures  320 ,  322  and  324  at a distance from the beginning of these structures that corresponds to the distance between adjacent structures  320 ,  322  and  324 .  
         [0053]    The geometry-change method  34  generates the new mask layout  330  from the layout data of the original layout  310  by accounting for influences of the production process for real masks. Consequently, the new mask layout  330  is similar to a mask produced with the aid of the original layout  310 . Lines  340 ,  342  and  344  show the original profile of the structures  300 ,  302  and  304  in the new mask layout  330 . A broken line again shows the value x=0.  
         [0054]    The line shortening is identical for the three structures  320 ,  322  and  324  that are arranged parallel to one another at the same distance, so that only the shortening and the corner rounding for the structure  322  are explained below. In comparison with the structure  302 , the structure  322  is shortened at one end by a difference value D 4  to which a distance  350  corresponds. The shortening accounts for deviations introduced by the production process during the production of a mask from the layout data of the original layout  110 . On account of the structure  360  or  362 , respectively, the difference value D 4  is less than the difference values D 1  or D 2  shown in FIGS. 2 and 3, respectively.  
         [0055]    Moreover, the ends of the structures  320 ,  322  and  324  shown in FIG. 4 have been rounded in order to take account of the influence of the production process. A radius-of-curvature for the rounding is determined independently during the implementation of the geometry-change method  34 . This radius corresponds to half the structure width  352  in this exemplary embodiment.  
         [0056]    [0056]FIG. 5 shows the effect of “corner rounding” in the case of structures  400  to  406  of an original layout  410  having borders that are irregular compared to those of structures  420  to  426  of a new layout  430  calculated with the aid of the geometry-change method  34 . The structures  400  to  406  have serifs  440  at the corners. The serifs  440  were attached to originally rectangular structures in an OPC method. In the structures  420  to  426 , these serifs  440  have been rounded.  
         [0057]    [0057]FIG. 6 shows the effect of “corner rounding” in the case of a dark structure  450  surrounding a bright rectangular structure  452 . Both structures belong to an original layout  460 . A new mask layout  470  containing structures  480  and  482 , corresponding to the structures  450  and  452  respectively, was calculated on the original layout  460  with the aid of the geometry-change method  34 . The outer corners of the dark structure  480  are rounded with a radius R 1  that is greater than a radius R 2  that rounds the inner corners of the structure  480 .  
         [0058]    [0058]FIG. 7 shows the so-called “peanuts” effect using rectangular structures  490  to  496  of an original layout  500  in comparison with structures  510  to  516  of a new mask layout  520  which has been calculated from the original layout  500  with the aid of the geometry-change method  34 . The structures  510  to  516  display constrictions  522  in their central sections. The constrictions  522  can be attributed to the absence of closely adjacent structures.  
         [0059]    [0059]FIG. 8 shows an original layout  600  and a new mask layout  602  whose mask data have been calculated by iterative application of the geometry-change method  34 . In the course of this calculation, the change of structures of the original layout  600 , e.g. of the structure  604 , on account of the production method are first calculated. These changes led to a first new mask layout (not illustrated). The layout data was subsequently corrected proceeding from the first new mask layout. The new mask layout  602  was then calculated from the corrected layout data by repeated implementation of the geometry-change method  34 .  
         [0060]    The layout data were corrected in such a way that the areas of the dark structures  604  and  606  are identical. This is possible if the rounded portions of the structure  606  project beyond a frame  608  that illustrates the original position of the structure  604  corresponding to the structure  606 .  
         [0061]    The corrected data are used to produce a mask. The corners of the structures in the corrected layout are not rounded.  
         [0062]    Having described the invention, and a preferred embodiment thereof, what we claim as new and secured by letters patent is: