Patent Number: 
Section: description

Now, embodiments of the present invention will be described with reference to the drawing. (1st Embodiment) According to the present invention, in an electron beam exposure method of character projection (CP) system in which an electron beam is shaped into a desired form by means of an aperture having a permeation window of a desired form and in which a specimen is irradiated and exposed, when characters as a unit for performing exposure are extracted from the design data of exposure patterns, in particular, of minute patterns of submicrometer or less, the number of characters required for exposure to patterns is calculated and the number of shots of exposure is estimated. In the above-described calculation method, the number of characters required for exposure to a figure in each level in the pattern data and the number of shots of exposure are calculated, characters are selected therefrom within the number of characters which can be employed for exposure so that the total number of shots may be the smallest, and the exposure to the other characters excluded from the selection is performed by means of the conventional variable shaping beam (VSB) system. In order to perform an exposure of CP system, it is necessary to dispose a beam shaping mask (character aperture) for shaping an electron beam into a character form required for the exposure in the electronic optical system. Permeation bores having the forms of the characters for CP exposure extracted by means of the above-described method are formed on a character aperture and one or more rectangular permeation windows for VSB beam shaping are disposed, thereby permitting an exposure of CP system as described above. As a result thereof, the number of characters required for exposure and the number of shots thereof can be reduced, thereby to permit an improved throughput in the electron beam exposure to be achieved. Now, the embodiment of the present invention will be described specifically. The present inventor calculated the number of kinds of characters and the number of shots required when the exposure of a gate layer (gate electrodes, gate wires) of 128 k bit memory cell was performed by means of the CP exposure, calculated the number of shots required when the VSB exposure was performed, and compared the CP exposure with the VSB exposure. The above-described calculation was performed using the pattern data of an actual memory cell. Here, the calculation was performed assuming that the maximum size of electron beams which could be irradiated onto the specimen was 10 xcexcmxe2x80xa2. FIG. 1A is a schematic view showing the patterns used. The cell array 1 is a line and space pattern, that is, a typical pattern having a draw-out portion 2 of a characteristic form on both ends thereof. Here, as shown in FIG. 1B, an enlarged view shows what kinds of patterns are used, and it seems easy to judge which pattern is to be extracted as characters for CP exposure. However, actually, even if the patterns are simple as shown in the figure, it is relatively difficult to calculate how a pattern is to be cut into characters after having taken the forms and arrangement positions of all the patterns into consideration, and the amount of calculations is large even in calculations on the computer. This embodiment shows that the calculations can be performed by means of an optimal method without taking the form of an arranged pattern, the arrangement position and the correlation with the other patterns into consideration. As design data (pattern data) of a device pattern for exposure, generally used ones of GDSII STREAM format are used. In the pattern data of this form, several figure cells are named and defined, any number of single reference arrangements locating only one of these figure cells at a specified position and of array reference arrangements locating a specified number of figure cells in an array form are arranged to create a tree structure of figure cells hierarchically. Each figure cell includes, in addition to the above-described reference arrangements, apex coordinates of polygons, line segments and line patterns connecting the line segments, as actual figure data. The figure cell itself is called a STRUCTURE, the single reference arrangement a SREF, the array reference arrangement an AREF, the polygon a BOUNDARY, and the line pattern of a PATH. FIG. 2 shows an example of the arrangement of the STRUCTURES defined in the pattern data. The STRUCTURES shown in the figure are arranged on the STRUCTURE of the top level (Cell Top) by means of the SREF and the AREF, and cell0, cell1 and the like can be called STRUCTURES of one level below. Of course, several STRUCTURES and BOUNDARIES are arranged on these STRUCTURES to create a hierarchy of STRUCTURES thereby. FIG. 2 shows the external form of each STRUCTURE in a rectangle, wherein the names given to the STRUCTURES and the numbers of repeated arrangements by means of the AREF in xe2x80x9c(columnxc3x97row)xe2x80x9d are shown respectively. The STRUCTURES without the number of arrangements are the ones arranged by means of the SREF. Moreover, FIG. 3 shows the hierarchy of STRUCTURES of xe2x80x9ccell5xe2x80x9d and below occupying a large part of the central cell array portion. Among the values entered behind the STRUCTURES, the ones not enclosed in xe2x80x9c( )xe2x80x9d show the external form sizes of the figure cells in xcexcm respectively. The cell4, as shown in FIG. 2, is arranged also at other positions. Now, a method of extracting characters using the pattern data of FIG. 2 will be described. [Step 1] Firstly, with regard to all figure cells (STRUCTURES) included in the pattern data shown in FIG. 2, the external form sizes, the numbers of kinds of characters in CP exposure, as well as the numbers of shots in CP exposure and VSB exposure are found respectively by means of calculations. FIGS. 4A and 4B show STRUCTUREs composition of the pattern data shown in FIG. 2, and numbers of kinds of characters in CP exposure, as well as the external form sizes (widthxc3x97height), the numbers of kinds of characters, angle of arrangements, reversed arrangements, and further the number of shots in CP exposure and VSB exposure in the form of xe2x80x9cb: number of CP shots, number of VSB shotsxe2x80x9d with BOUNDARIES. Since no flattening of the levels of the STRUCTURES (development of the STRUCTURES arranged by means of the SREF and the AREF) is performed, the above-described calculations of the external form sizes and the like can be carried out relatively simply and in a short time. It is found that the STRUCTURES (Cell Top) are composed of combinations of 8 kinds of STRUCTURES xe2x80x9cAxe2x80x9d to xe2x80x9cHxe2x80x9d composed of only BUNDARIES and of 9 kinds of STRUCTURES including BOUNDARIES together with reference arrangements. Since these STRUCTURES are designed in very small sizes and are all smaller than the beam sizes, 17 kinds of characters are sufficient for CP exposure. However, when the number of shots by means of the VSB and the number of shots by means of the CP exposure using the above-described method are found respectively, the results of: VSB 280,292 CP (17 kinds) 138,311 are obtained, and these results show that the number of shots can be reduced only to about half as large as that in case of the VSB even if the CP exposure is performed. [Step 2] Using the hierarchy of the STRUCTURE (Cell Top) in the Step 1, all levels are observed sequentially from the top level to judge which STRUCTURE is to be extracted as characters for the CP exposure. Here, it is assumed that the relation between different STRUCTURES arranged on the same STRUCTURE is unknown, and STRUCTURES having different names or those having the same name, but rotated or reversed are treated as different characters. Accordingly, in this case, all the STRUCTURES arranged on the xe2x80x9cCell Topxe2x80x9d are judged to have quite different forms and are allocated to other characters. Since all the STRUCTURES have a size of 10 xcexcmxe2x80xa2 or less except xe2x80x9ccell5xe2x80x9d, they are extracted as one kind of character respectively. For example, in case of cell0, a pattern composed of a BOUNDARY included in cell0 and a BOUNDARY of xe2x80x9cAxe2x80x9d arranged for reference and xe2x80x9cBxe2x80x9d arranged in two ways is treated as one character. [Step 3] Now, it is determined how cell5 which cannot be extracted as one character is used for the CP exposure. From FIG. 3 and the result of Step 1 it is found that cell5 is composed of an array arrangement of 4 rowsxc3x974 columns of cell51. Since cell51 has a size of 38.25 xcexcmxc3x9738.4 xcexcm, 16 shots of 4 rowsxc3x974 columns are found by dividing this size by a beam size of 10 xcexcmxe2x80xa2. At this stage, since the kinds of characters required for the CP exposure to cell51 are unknown, a maximum of 16 kinds of characters are necessary. When cell51 is employed as characters for the CP exposure, the number of shots required for the exposure to cell5 is 16xc3x97(4xc3x974)=256 shots. [Step 4] Further, in a level below cell51, cell52 has a size of 19.05 xcexcmxc3x9719.2 xcexcm, which is equivalent to 4 shots, and therefore, when cell52 is extracted as characters for the CP exposure, the number of kinds of characters (CPs) required for the exposure to cell5 are a maximum of 4, and the number of shots is equal to 4xc3x97(2xc3x972)xc3x97(4xc3x974)=256 shots. When cell4 below cell52 is employed as characters for the CP exposure, the kinds of Characters and the number of shots are both 1, however, since cell4 is arranged 16xc3x9732xc3x97512 times on cell52, 1xc3x97512xc3x974xc3x9716xc3x9732,768 shots are required for the exposure to cell5, and hence, it has a larger effect of reducing the number of shots to extract cell52 as characters for the CP exposure. [Step 5] Here, since cell52 is composed of only an array arrangement of cell4s and references are made even times both in the longitudinal and transverse directions (references are made non-prime times), a maximum of 4 kinds of characters judged only from the size of cell52 means that one kind of character is sufficient. [Step 6] Moreover, for pattern data of GDSII STREAM format, in case of performing an array arrangement by means of AREF, the external form size after the array arrangement as well as the number of longitudinal and transverse arrangements are specified respectively to be able to find the longitudinal and transverse spaces (pitches) of the arrangements easily. Using this information, for example, with regard to cell1, since the external form size thereof when AREF is specified is 1.8 xcexcmxc3x97156.6 xcexcm and cell1 is arranged 1xc3x97261 times, it is found that the longitudinal pitch is 0.6 xcexcm. Here, there are provided three methods of allocation to characters for the CP exposure as described below: 1. A pattern repeated about several times of the number of references is selected as characters so that there may be no remainder even if the CP exposure is repeated; 2. When a maximum number of STRUCTURES falling within the maximum beam size are extracted as one character and there is any remainder which cannot be used for the CP exposure, the remainder is used as new (other) characters for the CP exposure; and 3. In case of No. 2, the exposure to the remainder is performed by means of the VSB. With regard to these cases, the number of kinds of CP characters and the number of shots required for the exposure to the pattern of cells 1 arranged in an array will be estimated. Firstly, in case of No. 1, since the number of times of repetition is equal to 261xc3x979xc3x9729, a pattern in which 9 or 29 cell1s are connected to each other in a pitch of 0.6 xcexcm can be employed as characters for the CP exposure. Accordingly, since the beam has a size of 10 xcexcmxe2x80xa2, a pattern in which 9 cell1s are connected to each other is employed as characters. At this time, the number of CP characters is 1 and the number of shots is 29. Now, in case of No. 2, a maximum of 16 cell1s can be included in the beam of 10 xcexcm. The number of cell1s which can be used for the CP exposure is a maximum of 16xc3x9716xc3x97256, and the remaining 5 cell1s are in excess. Accordingly, the number of character is 2 and the number of shots is 17. Finally, in case of No. 3, when the exposure to cell1 is performed by means of the VSB system, the number of shots is 8, and therefore, it is found that 16+8xc3x975xc3x9756 shots are required. In this case, the number of characters is 1. Accordingly, it is found preferable to select the first method for preventing the number of characters from increasing and to select the second method for keeping the number of shots as small as possible. [Step 7] Also with regard to other 9 STRUCTURES arranged by means of AREF, calculations are performed similarly as in Step 6. Since there is no remainder, for example, in hkds2, the CP exposure can be performed in 16 shots using one kind of character. The results of the calculations performed as described above will be shown in Table 1. Two kinds of values are shown in cases of No. 1 and No. 2 of Step 6. From these results it is found that in order to perform an electron beam exposure to the 128 k memory cell pattern used at this time by means of the CP system 19 and 21 kinds of characters are required for the CP exposure, that the numbers of shots of the CP exposure are 450 and 426 respectively, and that the number of shots can be reduced to {fraction (1/620)} through {fraction (1/660 )} in comparison to 280,292 in case of the exposure being performed by means of the VSB system. In the method of extracting CP exposure characters used in this embodiment, since characters can be extracted without developing the form of a pattern, the mutual relations among a plurality of patterns and the figure cells arranged for reference in the figure cells in the pattern data, a simplified processing on the computer performing this work and a shortened data processing time can be achieved. In other words, by using this extraction method, characters for the CP exposure effectively can be extracted by means of a simple algorithm and that with a small load on the computer. Moreover, since the hierarchy of the figure cells is basically composed of reference arrangements, it is easy to pick out the extracted character patterns as independent patterns or to create pattern data from which the extracted portion has been removed, and therefore, for example, when the exposure to the patterns remaining after having extracted the patterns for the CP exposure is performed by means of VSB, VSB pattern data must be created, however, the processing for the work can be simplified substantially. Moreover, no patterns in the figure cells are divided or integrated with other patterns, and therefore, even if recalculations are performed under changed conditions, for example, in case of another beam size which can be irradiated, optimized extractions of the patterns and calculations of the number of shots can be achieved more easily by using the results of the previous calculations. Now, another embodiment of the present invention will be described with emphasis on the differences from this embodiment, and the matters described in this embodiment such as apparatus used, effects and advantages will be omitted if not needed. (2nd Embodiment) In the above-described embodiment 1, there is described the method of pattern extraction permitting a reduced number of shots by extracting characters for the CP exposure from the pattern data of a device without examining the forms and arrangement information of the pattern in detail. In this embodiment there will be described a method of pattern extraction permitting reduced numbers of kinds of characters and shots from the design pattern data of a device without examining the forms and arrangement information of the pattern in detail. That is, in this embodiment, there will be described a method of reducing the number of characters for the CP exposure and the number of shots by arranging the extracted characters into groups using only the reference arrangement information (arrangement position coordinates in the single arrangement, as well as external form size and number of repetitions after array arrangement in the array arrangement) described in the pattern data and the external form size of each arranged figure found by means of calculations. [Step 1] Characters for the CP exposure are extracted by means of the techniques of Steps 1 to 7 in Embodiment 1. Here, in cell1 and cell8, either case 1 or case 2 may be applied. [Step 2] By referring only to the arrangement information of the STRUCTURES composing the characters extracted in Step, it is examined if the plurality of extracted characters can be grouped together into one character. Although the hierarchy of the STRUCTURES shown in FIG. 2 is complicated, the arrangement of the characters extracted therefrom is relatively simplified. So, for example, hkds0 and cell0 are both arranged only once by means of SREF and they are arranged in contact with the top level. The external form sizes of these two STRUCTURES are found to be equal to or smaller than the maximum beam size. Accordingly, when these are defined again as one character, the number of characters of 2 and the number of shots of 2 are reduced to the half and become 1 respectively. In order to perform such an examination, since the coordinates arranged by means of SREF are described in the data of GDSII STREAM format, candidates for the characters to be easily grouped together can be obtained by sorting and arranging the STRUCTURES according to the arrangement coordinates thereof. By means of this method it can be examined if the STRUCTURES arranged by means of SREF can be grouped together into a character for the CP exposure. As a result thereof, cell0 and hkds 0, cell2 and hkds 1, cell7 and hkds 6, as well as cell9 and hkds 7 are defined again as one character respectively, and in comparison to the result of Embodiment 1 the number of characters can be reduced by 4, and the number of shots can be also reduced by the number of 4 times. [Step 3] Now, the characters composed of the STRUCTURES arranged in an array by means of AREF in a pattern of GDSII STREAM format are examined. In this case, by sorting the STRUCTURES according to the arrangement coordinates thereof similarly as in case of SREF in Step 2, STRUCTURES capable of being grouped together into a character are searched. At the same time, STRUCTURES of which areas arranged in an array have the same width or the same height are extracted as candidates. As described in first Embodiment, since the areas arranged by means of AREF are described in the pattern data, this work can be also performed easily. 1. Cell1 and cell8 are arranged longitudinally in an array of 261 pieces and have the same Y coordinate in the areas where they are arranged, however, since they are transversely spaced from each other, a portion of them cannot be grouped together for the CP exposure. 2. The portions of hkds 2, hkds 3, cell3, cell4, cell5 (cell52s arranged in an array of 8xc3x978), cell4, cell6, hkds 4 and hkds 5 arranged in an array have the same X coordinate in the areas where they are arranged, and at the same time they have the same width. 3. Among these STRUCTURES, cell4s are arranged above and below the portion of cell5, and cell5 is composed of cell4s arranged in an array of 128xc3x97256, however, when the calculations are developed into the STRUCTURES of a lower level as described above, the amount of calculations increases and a large load is applied to the processing, and therefore, such a development shall not be employed. Accordingly, the portion of cell5 is regarded as further more undividable, and as a result thereof, it is to be judged that the portion of cell5 cannot be grouped together with cell4s above and below for the CP exposure using the same character. 4. Among the STRUCTURES having the same width and transverse position in the array arrangements described above, since it is found that the portion of cell5 cannot be grouped together into a character for the CP exposure, a group of hkds 2, hkds 3, cell3 and cell4 as well as a group of cell4, cell6, hkds4 and hkds5 above and below the portion of cell5 remain as candidates. These groups are spaced from each other by more than the maximum beam size at the portion of cell5, what shows that it is to be examined if the characters of each of these two groups can be grouped together. 5. When looking at the external form size of each of the groups, the height thereof in the longitudinal direction in which no repeated arrangements are performed is equal to or smaller than the maximum beam size, what shows that the heights of these two groups may be grouped together respectively in the longitudinal direction for the CP exposure repeatedly in the transverse direction using the same character. 6. If the height of the external form size composed of 4 STRUCUTRES is larger than the maximum beam size, these STRUCTURES may be further divided into a plurality of groups. 7. With regard to each of the groups above and below cell5 composed of 4 STRUCTURES in this case, the number of transverse repetitions of each of the 4 STRUCTURES is 128, what shows that each STRUCTURE arranged in the longitudinal direction is to be defined again as one character and that the exposure is to be repeated 128 times in the transverse direction. In Steps 6 and 7 of the first Embodiment, since 8 STRUCTURES are converted as one character into an array arrangement of 16xc3x971, the number of kinds of characters and the number of shots of 16 (shots)xc3x974 kinds are required, while in this case the number of kinds of characters and the number of shots can be reduced to 16 shotsxc3x971 kind. 8. In this case the numbers of repetitions are all the same, however, in order to judge if grouping between different AREFs is possible, a common divisor of each number of repetitions is to be searched. If there is any common divisor, the STRUCTURES can be grouped together and converted into the shots of characters of the number of times of the common divisor. The effect of reducing the number of kinds of characters (CPs) for the CP exposure and the number of shots obtained with this embodiment will be shown in Table 2. Table 2 shows that this embodiment can reduce the number of kinds of characters for the CP exposure by 10 and the number of shots by 100. That is, Number of VSB shots: 280,292 xe2x86x92Case 1 9 kinds 350 shots Case 2 11 kinds 326 shots indicates that the number of shots can be reduced to {fraction (1/800)} or less of the VSB exposure. In this embodiment, only by adding the information concerning the reference arrangement described in advance in the pattern data to the method of extracting characters of the first embodiment, the number of characters and the number of shots can be further reduced. Since also the method of extracting characters of this embodiment can unify characters into groups by calculating only the external form size of a figure and define the figure again effectively by repeating the repetition unit of array arrangements a plurality of times, the number of characters and the number of shots can be reduced very effectively with the load applied to data processing being kept to a minimum. Further, also in this method of extracting characters, since the figures grouped together into groups can be recomposed only by defining the reference arrangement information of the figure cells defined respectively in the design pattern data, the pattern data can be generated simply and effectively. (Embodiment 3) In this embodiment, there will be described a method of extracting characters using a method of determining which patterns are to be used as characters for the CP exposure and which patterns are to be exposed by means of VSB when the number of characters which can be extracted as characters for the CP exposure is limited due to the limits of an exposure apparatus on the assumption that patterns unsuitable for the CP exposure are used for the exposure by means of the VSB system. The design patterns used are the same as in first and second embodiments. [Step 1] According to the methods of the first and second embodiments, candidates for the characters for the CP exposure are extracted. The relation between the number of characters and the number of shots extracted in the first embodiment and optimized in the second embodiment will be shown in Table 3. [Step 2] With regard to each STRUCTURE of Table 3, xe2x80x9cCP effectivenessxe2x80x9d defined in the equation below is calculated. CP effectiveness={(Number of VSB shots)xe2x88x92(Number of CP shots)}/{(Number of CP shots)xc3x97(Number of kinds of Characters)} Here, the number of kinds of Characters is the number of characters required for the exposure of CP system to the STRUCTURE, and the number of CP shots and the number of VSB shots are the numbers of shots required for the exposure to the STRUCTURE respectively. For example, when calculating the CP effectiveness Ecell1,1 and Ecell1,2 of cell1 in case 1 and case 2, the results of: Ecell 1,1=(2,088xe2x88x9229)/(29xc3x971)=77.0  Ecell 1,2=(2,088xe2x88x9217)/(17xc3x972)=60.9  are obtained, and these results indicate that a larger CP effectiveness is achieved when characters are extracted so that a smaller number of kinds of Characters may be achieved in stead of a smaller number of shots. [Step 3] When the characters capable of being used for the CP exposure are extracted in the order of the magnitude of the CP effectiveness calculated in Step 2 among all the characters, even in an exposure apparatus having a limit to the number of characters for the CP exposure, the number of shots can be reduced efficiently in comparison to an electron beam exposure method in which the VSB exposure is performed using the patterns unsuitable for the CP exposure. In Table 4 there will be shown the total number of characters as well as the total number of shots of CP exposure and VSB exposure required when the CP effectiveness of each STRUCTURE in Table 3 is found, the STRUCTURES are arranged from above in the order of the magnitude of the CP effectiveness and each STRUCTURE is extracted as a character for the CP exposure in the order of the magnitude of the CP effectiveness. Table 4 shows that the total number of shots required for exposing the entire 128 k memory cell is 474 when the Characters extraction of case2 is performed in an electron beam exposure apparatus in which 5 characters can be used for the CP exposure. In contrast thereto, when the Characters extraction of case1 is performed, Table 4 shows that the total number of shots increases substantially to 2,521 in comparison to case1 even if the CP exposure using the same 5 characters is performed. As described above, when the number of selectable CP characters is limited and all the patterns cannot be formed by means of the CP exposure, it is very effective for reducing the number of shots to use the CP effectiveness shown in this embodiment as a parameter and to extract characters suitable for an optimal CP exposure. Moreover, by forming the characters extracted according to this method by means of the CP exposure, it becomes possible to expect the largest throughput of an electron beam exposure apparatus used which is required in the specifications. Further, as shown in Step 6 of the first embodiment, when there are a lot of choices of character selection, an optimal combination of characters can be determined easily and correctly using this CP effectiveness. Having described the embodiments of the present, the present invention is not limited to these embodiments. For example, in the above-described embodiments, as design pattern data of device patterns for exposure (pattern data), ones of GDSII STREAM format are used, however, the design pattern data are not limited thereto. The method of extracting characters described above may be performed using a computer-readable recording medium (for example, CD-ROM, DVD) having recorded a program for causing the computer to execute the specified procedures. In addition thereto, various modifications can be made without departing from the summary of the present invention. 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.