Abstract:
According to a first aspect of the present invention, there is provided a method of correcting a mask for a data program of a read only memory, comprising selecting an optional data from a data map comprising first data and second data, the optional data being one of the first data, and inspecting neighboring data around the optional data and, where all the neighboring data surrounding the optional data are the second data, correcting a shape of the mask in a position corresponding to the optional data.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-367357, filed Dec. 1, 2000, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of correcting a mask used in a data program of a read only memory and a mask manufactured by the particular method. 
     2. Description of the Related Art 
     In recent years, a mask ROM (Read Only Memory) of a large capacity is used in accordance with an increase in demands for a stationary data memory. Also, a fine process technology is employed in accordance with an increase in the capacity of the mask ROM. 
     A mask in which the ROM data are patterned, hereinafter referred to as “ROM mask”, is used in the data program of the mask ROM. Patterns are arranged to conform with the ROM data in the ROM mask. The pattern is generally shaped square or oblong. 
     In the conventional technology described above, the problem described below is generated with reduction in the design rule. To be more specific, it was possible for the size of the resist in the PEP (Photo Engraving process) step to fail to form the optimum value depending on combination of the ROM data. The difficulty is generally considered to be caused by the optical proximity effect. 
     In order to avoid the problem noted above, and OPC (Optical Proximity Correction) technology is employedin the general mask. The OPC technology noted above represents a technology for performing the optimum correction by computer processing. 
     However, in the computer processing by the OPC technology, correction is applied to the figure data of the ROM mask, giving rise to the problem that a considerably long time is required for the correction. Therefore, in the mask ROM requiring a short delivery time, the ROM mask is required to be manufactured in a short time, with the result that it was impossible to make corrections by computer processing as in the general mask. 
     As described above, in the prior art, it was difficult to make the optimum correction in the ROM mask in a short time. 
     BRIEF SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, there is provided a method of correcting a mask for a data program of a read only memory, comprising: selecting an optional data from a data map comprising first data and second data, the optional data being one of the first data; and inspecting neighboring data around the optional data and, where all the neighboring data surrounding the optional data are the second data, correcting a shape of the mask in a position corresponding to the optional data. 
     According to a second aspect of the present invention, there is provided a mask for a data program of a read only memory, comprising holes formed in positions corresponding to first data of a data map comprising the first data and second data, the holes including a hole of first shape and a hole of second shape, the hole of first shape corresponding to the first data surrounded by the second data from within the first data, the hole of second shape corresponding to the first data be adjacent to the first data from within the first data. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     FIG. 1 shows a ROM data map according to a first embodiment of the present invention; 
     FIG. 2 shows a ROM mask before correction according to the first embodiment of the present invention; 
     FIG. 3 is a flow chart showing the mask correcting method according to the first embodiment of the present invention; 
     FIG. 4 shows the ROM data map according to the first embodiment of the present invention; 
     FIG. 5 shows a ROM mask after correction according to the first embodiment of the present invention; 
     FIG. 6 shows a ROM data map according to a second embodiment of the present invention; 
     FIG. 7 shows the ROM mask before correction according to the second embodiment of the present invention; 
     FIG. 8 is a flow chart showing the mask correcting method according to the second embodiment of the present invention; 
     FIG. 9 shows the ROM data map according to the second embodiment of the present invention; 
     FIG. 10 shows the ROM data map according to the second embodiment of the present invention; 
     FIG. 11 shows the ROM mask after correction according to the second embodiment of the present invention; 
     FIG. 12 is a flow chart showing a mask correcting method according to a third embodiment of the present invention; 
     FIG. 13 shows a ROM data map according to the third embodiment of the present invention; and 
     FIG. 14 shows the ROM mask after correction according to the third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A mask for a data program in which the ROM (Read Only Memory) data “1” and “0” are patterned, hereinafter referred to as “ROM mask”, is used in the mask ROM for the read only memory. In general, square or oblong holes are formed in the ROM mask corresponding to the ROM data “1”. 
     In embodiments of the present invention, the data correction is performed on a ROM data map in the computer processing for preparing a mask for the read only mask. The embodiments of the present invention will now be described with reference to the accompanying drawings. 
     [First Embodiment] 
     In the ROM mask, where another hole is not present around an optional hole, the optional hole is put in an insulated state and is diminished, with the result that an ion implantation cannot be applied sufficiently to the hole of the insulated state. Therefore, in the first embodiment, a positive correction is applied to the hole for the insulated “1” data on the ROM data map. 
     FIG. 1 shows the ROM data map according to the first embodiment present invention, and FIG. 2 shows the ROM mask before correction according to the first embodiment of the present invention. 
     In the first embodiment, used is a ROM data map  12  in which ROM data  11  of “1” and “0” are irregularly arranged as shown in FIG.  1 . Also, as shown in FIG. 2, used is a ROM mask  13  corresponding to the ROM data map  12  shown in FIG.  1 . Incidentally, the first embodiment is applied to the case where adjacent holes  14   a  of the ROM mask  13  are positioned apart from each other. 
     FIG. 3 is a flow chart showing the mask correcting method according to the first embodiment of the present invention. FIG. 4 shows the ROM data map according to the first embodiment of the present invention. Further, FIG. 5 shows the ROM mask after correction according to the first embodiment of the present invention. The mask correcting method according to the first embodiment of the present invention will now be described in detail. 
     First of all, an optional “1” data  15  on the ROM data map  12  is selected as shown in FIG. 4 (step ST 1 ). Then, neighboring data  16  around the optional “1” data  15  are inspected, and it is judged whether or not all the neighboring data  16  are “0”, as shown in FIG. 4 (step ST 2 ). The neighboring data  16  is defined to include, for example, four data positioned upward, downward, rightward and leftward of the optional “1”, data  15 . 
     Where it has been found that all the neighboring data  16  around the optional “1” data  15  are not “0”, the ROM mask  13  is not corrected (step ST 3 ). On the other hand, where it has been found that all the neighboring data  16  around the optional “1” data  15  are “0”, it is necessary to apply a positive correction to the ROM mask  13  and, thus, the amount of correction is studied (step ST 4 ). In the study of the correcting amount, the optimum correcting amount is obtained by the comparison with the neighboring holes that are not corrected. 
     In the next step, a positive correction is applied to the hole  14   b  of the ROM mask  13  corresponding to the optional “1” data  15  in accordance with the correcting amount noted above, as shown in FIG. 5 (step ST 5 ) so as to manufacture the ROM mask  13 . 
     As described above, in the ROM mask  13  to which is applied the optimum correction according to the first embodiment of the present invention, the insulated hole  14   b  is made larger than the neighboring hole. 
     According to the first embodiment of the present invention, the ROM mask  13  is prepared by applying a suitable correction at the stage of “1”, “0” data, not by applying correction to the figure data after preparation of the ROM mask  13  by studying the “1”, “0” pattern of the ROM data  11  providing the basis of the ROM mask  13 . 
     To be more specific, in the computer calculation, a very troublesome processing was required for adding a correction to the figure data. However, if correction is applied in the stage of the “1”, “0” data, a suitable correction can be achieved relatively simply by determining the rule of the correction. It follows that, in the manufacture of the ROM mask  13 , the time for the computer processing can be shortened so as to achieve the delivery of the mask ROM in a short time. 
     It has been found that, when a test was applied to the actual pattern data, the method of the present invention permits shortening the time for the computer calculation to {fraction (1/10)} or less, compared with the conventional method in which a correcting treatment is applied to the figure data. 
     In the first embodiment of the present invention, the range of the neighboring data  16  is defined to include four data positioned upward, downward, rightward and leftward of the optional “1” data  15 . However, the neighboring data are not limited to these four data. For example, it is possible for the range of the neighboring data  16  to include 8 data positioned upward, downward, rightward, leftward, and additional obliquely positioned four data. 
     It is possible to change the shape of the hole to the shape differing from that of another hole in place of changing the size of the hole of the ROM mask  13  corresponding to the optional “1” data  15 . In other words, it is possible to change the hole of the ROM mask  13  corresponding to the “1” data in the case where all the neighboring data are “0” to an oblong hole, with the hole of the ROM mask  13  corresponding to the ordinary “1” data being made square. 
     [Second Embodiment] 
     The first embodiment is directed to the case where the adjacent holes in the ROM mask are positioned apart from each other. However, the second embodiment is directed to the case where the adjacent holes in the ROM mask are contiguous to each other. 
     In the ROM mask, where the adjacent holes are contiguous to each other, the size of the hole is increased with an increase in the number of holes that are contiguous to each other. Therefore, in the second embodiment, a negative correction is applied to the holes of the ROM mask corresponding to the contiguous “1” data on the basis of the number of “1” data contiguous to each other in the ROM data map. 
     FIG. 6 shows the ROM data map according to the second embodiment of the present invention, and FIG. 7 shows the ROM mask before correction according to the second embodiment of the present invention. 
     In the second embodiment, used is a ROM data map  12  in which the ROM data of “1” and “0” are irregularly arranged as shown in FIG.  6 . Also, as shown in FIG. 7, used is a ROM mask  13  corresponding to the ROM data map  12  shown in FIG.  6 . Incidentally, the second embodiment is applied to the case where the holes of the ROM mask  13  are joined if the “1” data on the ROM data map  12  are contiguous. 
     FIG. 8 is a flow chart showing the mask correction method according to the second embodiment of the present invention. Each of FIGS. 9 and 10 shows a ROM data map according to the second embodiment of the present invention. Also, FIG. 11 shows the ROM mask after correction according to the second embodiment of the present invention. The mask correcting method according to the second embodiment of the present invention will now be described in detail. 
     First of all, an optional first “1” data  21  on the ROM data map  12  is selected, as shown in FIG. 9 (step ST 1 ). Then, first neighboring data  22  around the first “1” data  21  are inspected so as to judge whether there are “ 1 ” data within the first neighboring data  22  (step ST 2 ). In this case, the range of the neighboring data  22  is defined to include four data positioned upward, downward, rightward and leftward of the first “1” data  21 . Where it has been found that “1” data is not included in the first neighboring data  22 , the ROM mask  13  is not corrected (step ST 3 ). 
     On the other hand, where “1” data are included in the first neighboring data  22 , it is judged whether there are “1” data contiguous to a second “1” data  23 , as shown in FIG. 10 (step ST 4 ). In other words, a second neighboring data  24  of the second “1” data  23  is inspected. The second neighboring data  24  is defined to include the range of four data positioned upward, downward, rightward and leftward of, for example, the second “1” data  23 . 
     Where “1” data is included in the second neighboring data  24 , the judgment is continued until “1” data contiguous to the “1” data ceases to be present. As a result, where “1” data ceased to be included in the second neighboring data  24 , the correcting amount is studied (step ST 5 ). The study of the correcting amount is performed while making comparison with the hole that is not corrected, and an optimum correcting amount is obtained in accordance with the number of consecutive “1” data. 
     In the next step, a negative correction is applied to the hole of the ROM mask  13  in accordance with the correction amount so as to prepare the ROM mask  13 , as shown in FIG.  11 . 
     As described above, in the ROM mask  13  to which is applied the optimum correction in accordance with the second embodiment of the present invention, the hole comprising of a plurality of consecutive holes is made smaller than the sum of a plurality of discrete holes. 
     The second embodiment described above permits producing the effect similar to that produced in the first embodiment. 
     According to the second embodiment of the present invention described above, the range of the first neighboring data  22  includes the four data positioned upward, downward, rightward and leftward of the first “1” data  21 . Likewise, the range of the second neighboring data  24  includes the four data positioned upward, downward, rightward and leftward of the first “1” data  23 . However, the range of each of the first and second neighboring data  22  and  24  is not limited to the four data noted above. For example, it is possible for the range of the first neighboring data  22  to include 8 data positioned upward, downward, rightward, and leftward of the first “1” data  21 , as well as obliquely positioned additional four data. Likewise, it is possible for the range of the second neighboring data  24  to include 8 data positioned upward, downward, rightward, and leftward of the first “1” data  23 , as well as obliquely positioned additional four data. 
     [Third Embodiment] 
     In the ROM mask, the size of the hole is made smaller in the region having a low hole density, with the result that it is difficult to perform the ion implantation through the hole. Therefore, in the third embodiment, a positive correction is applied to the ROM data in the case where the “1” data density per unit area on the ROM data map is lower than a predetermined value. 
     FIG. 12 is a flow chart showing the mask correction method according to the third embodiment of the present invention. FIG. 13 shows the ROM data map according to the third embodiment of the present invention. Further, FIG. 14 shows the ROM mask after correction according to the third embodiment of the present invention. The mask correction method according to the third embodiment of the present invention will now be described in detail. 
     First of all, an optional region  31  is selected on the ROM data map  12 , as shown in FIG. 13 (step ST 1 ). Then, the number of “1” data present within the optional region  31  is detected so as to calculate the density of the “1” data within the optional region  31 . Further, it is judged whether or not the density of the “1” data within the optional region  31  is lower than a predetermined value (step ST 2 ). The predetermined value is set at, for example, 10%. 
     Where it has been found that the density of the “1” data is not lower than the predetermined value, the ROM mask  13  is not corrected (step ST 3 ). On the other hand, where it has been found that the density of the “1” data is lower than the predetermined value, it is necessary to apply a positive correction to the ROM mask  13  and, thus, the correcting amount is studied (step ST 4 ). The correction amount is studied by comparison with the neighboring hole that is not corrected so as to obtain an optimum correcting amount. 
     In the next step, a positive correction is applied to the hole  14   b  of the ROM mask  13  in accordance with the correcting amount, as shown in FIG. 14 (step ST 5 ), so as to manufacture the ROM mask  13 . 
     As described above, in the ROM mask  13  to which is applied the optimum correction according to the third embodiment of the present invention, the holes in the low density region are made larger than the holes in the high density region. Incidentally, the term “low density region” noted above denotes the region where the density of the “1” data in the optional region is not higher than the predetermined value, e.g., 10%. 
     The third embodiment described above permits producing the effect similar to that produced by the first embodiment. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.