Patent Publication Number: US-11397380-B2

Title: Critical dimension measurement system and method of measuring critical dimensions using same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 U.S.C. § 119 to Korean Patent Application No. 10-2017-0122302, filed on Sep. 22, 2017 in the Korean Intellectual Property Office and U.S. patent application Ser. No. 15/973,912 filed May 8, 2018, the disclosure of which is incorporated by reference herein in its entirety. 
     TECHNICAL FIELD 
     Exemplary embodiments of the present inventive concept relate to a critical dimension measurement system and a critical dimension measurement method, and more particularly, to a critical dimension measurement system which may measure critical dimensions of mask patterns, and a method of measuring critical dimensions using the same. 
     DISCUSSION OF RELATED ART 
     With increases in degrees of integration and performance of semiconductor devices, the number of design rules with regard thereto is decreasing, and the widths of patterns forming semiconductor devices are being narrowed. The size of mask patterns of photomasks used in photolithography processes, as well as the size of photoresist patterns for forming the patterns to form semiconductor devices, as described above, is being reduced. As the size of mask patterns decreases, an amount of time required to measure the critical dimensions of mask patterns is increased. 
     SUMMARY 
     According to an exemplary embodiment of the present inventive concept, a critical dimension measurement system includes a voltage measurement circuit, a control circuit, and a critical dimension measurement circuit. The voltage measurement circuit may measure potentials of mask patterns of a photomask. The control circuit may include an information storage circuit for storing distribution information on the potentials of the mask patterns, measured by the voltage measurement circuit, and information on layout patterns corresponding to the mask patterns of the photomask. The critical dimension measurement circuit may be operated, by the control circuit, in a first measurement mode and a second measurement mode running for a shorter time than the first measurement mode, and measure critical dimensions of the mask patterns. 
     According to an exemplary embodiment of the present inventive concept, a critical dimension measurement system includes a surface potential measurement circuit, a control circuit, and a critical dimension measurement circuit. The surface potential measurement circuit may measure potentials of mask patterns of a photomask. The control circuit may include an information storage part configured to store distribution information on the potentials of the mask patterns measured by the surface potential measurement circuit. The critical dimension measurement circuit may exchange an electrical signal with the control circuit, and measure critical dimensions of the mask patterns of the photomask, transferred from the surface potential measurement circuit, in a default measurement mode and a simple measurement mode running for a shorter time than the default measurement mode, using the distribution information on the potentials of the mask patterns. 
     According to an exemplary embodiment of the present inventive concept, a critical dimension measurement system includes a surface potential measurement circuit and a critical dimension measurement circuit. The surface potential measurement circuit may measure potentials of mask patterns. The critical dimension measurement circuit may measure critical dimensions of the mask patterns, transferred from the surface potential measurement circuit, in a default measurement mode and a simple measurement mode running for a shorter time than the default measurement mode, using distribution information on the measured potentials of the mask patterns. 
     According to an exemplary embodiment of the present inventive concept, a method of measuring critical dimensions includes forming a layout including layout patterns, forming a photomask including mask patterns corresponding to the layout patterns, storing information on the layout patterns, classifying measurement sites of the photomask as a default measurement site and a simple measurement site, using the information on the layout patterns, measuring critical dimensions of a first mask pattern within the default measurement site in a default measurement mode, and measuring critical dimensions of a second mask pattern within the simple measurement site in a simple measurement mode. The simple measurement mode may run for a shorter time than the default measurement mode. 
     According to an exemplary embodiment of the present inventive concept, a method of measuring critical dimensions includes forming a layout including layout patterns, forming mask patterns corresponding to the layout patterns, setting, with a control circuit of a critical dimension measurement apparatus, a default measurement mode and a simple measurement mode running for a shorter time than the default measurement mode, using information on the layout patterns, and measuring critical dimensions of the mask patterns with the critical dimension measurement apparatus. Critical dimensions of first mask patterns of the mask patterns may be measured in the default measurement mode, and critical dimensions of second mask patterns of the mask patterns may be measured in the simple measurement mode. 
     According to an exemplary embodiment of the present inventive concept, a method of measuring critical dimensions includes forming a layout including layout patterns, using a computer system, forming a photomask including mask patterns corresponding to the layout patterns, using a mask manufacturing apparatus, storing information on a default measurement site and a simple measurement site of the photomask with a critical dimension measurement apparatus, measuring, with the critical dimension measurement apparatus, critical dimensions of a first mask pattern within the default measurement site in a default measurement mode, and measuring, with the critical dimension measurement apparatus, critical dimensions of a second mask pattern within the simple measurement site in a simple measurement mode running for a shorter time than the default measurement mode. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The above and other aspects and features of the present inventive concept will be more clearly understood by describing in detail exemplary embodiments thereof with reference to the accompanying drawings. 
         FIG. 1  is a block diagram schematically illustrating a critical dimension measurement system, according to an exemplary embodiment of the present inventive concept. 
         FIG. 2  is a conceptual plan view illustrating a layout, according to an exemplary embodiment of the present inventive concept. 
         FIG. 3  is a plan view illustrating measurement regions of interest of layout patterns, according to an exemplary embodiment of the present inventive concept. 
         FIG. 4  is a cross-sectional view illustrating a photomask, according to an exemplary embodiment of the present inventive concept. 
         FIG. 5  is a schematic cross-sectional view illustrating a surface potential measurement device measuring the potential of mask patterns of a photomask, according to an exemplary embodiment of the present inventive concept. 
         FIG. 6  is a graph illustrating the relationship between the length of measurement regions of interest and reproducibility according to an exemplary embodiment of the present inventive concept. 
         FIG. 7A  is a flowchart illustrating a method of measuring critical dimensions, according to an exemplary embodiment of the present inventive concept. 
         FIG. 7B  is a flowchart illustrating a method of measuring critical dimensions, according to an exemplary embodiment of the present inventive concept. 
         FIGS. 8A and 8B  are flowcharts illustrating a method of measuring critical dimensions, according to exemplary embodiments of the present inventive concept. 
         FIGS. 9A, 9B, and 9C  are flowcharts illustrating a method of measuring critical dimensions, according to exemplary embodiments of the present inventive concept. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Exemplary embodiments of the present inventive concept provide a critical dimension measurement system, which may reduce an amount of time required to measure critical dimensions. 
     Exemplary embodiments of the present inventive concept also provide a method of measuring critical dimensions, which may reduce an amount of time required to measure critical dimensions. 
     Exemplary embodiments of the present inventive concept will be described more fully hereinafter with reference to the accompanying drawings. Like reference numerals may refer to like elements throughout this application. 
     An example of a critical dimension measurement system, according to an exemplary embodiment of the present inventive concept, will be described below.  FIG. 1  is a block diagram schematically illustrating a critical dimension measurement system, according to an exemplary embodiment of the present inventive concept. 
     Referring to  FIG. 1 , the critical dimension measurement system may include a computer system  10 , a mask manufacturing apparatus  15 , and a critical dimension measurement apparatus  100 . 
     The computer system  10  may include a workstation  3 , and a terminal or a display device  6  connected to the workstation  3 . The computer system  10  may include a design tool able to design semiconductor integrated circuits (ICs) and/or a layout tool able to draw layouts of designed ICs. Thus, the critical dimension measurement system may form a layout including layout patterns, using the computer system  10 . 
     The critical dimension measurement system may form a photomask  20 , using a layout formed using the computer system  10 . The photomask  20  may be formed by the mask manufacturing apparatus  15 . The photomask  20  may include mask patterns corresponding to the layout patterns of the layout. 
     The critical dimensions of the mask patterns of the photomask  20  may be measured with the critical dimension measurement apparatus  100 . The critical dimension measurement apparatus  100  may be a critical dimension scanning electron microscope (CD-SEM). 
     In an exemplary embodiment of the present inventive concept, the critical dimension measurement apparatus  100  may include a surface potential measurement unit  110 , a critical dimension measurement unit  130 , a signal processing unit  155 , a display unit  170 , and a control unit  160 . 
     The photomask  20 , formed by the mask manufacturing apparatus  15 , may be moved onto a first stage  113  within the surface potential measurement unit  110 . The surface potential measurement unit  110  may include a surface potential measurement device  116  able to measure the potential of the mask patterns of the photomask  20  placed on the first stage  113 . For example, the surface potential measurement unit  110  may be a voltage measurement unit and the surface potential measurement device  116  may be a voltage measurement device. 
     The critical dimension measurement unit may include a second stage  131  on which the photomask  20 , including the mask patterns of which the potential has been measured by the surface potential measurement unit  110 , may be moved, placed, and aligned. 
     The critical dimension measurement unit  130  may include an electron beam irradiation device irradiating, with an electron beam  145 , the surface of the photomask  20  placed on the second stage  131 , and a detector  151  detecting secondary electrons  148  generated by irradiating the surface of the photomask  20  with the electron beam  145 . The critical dimension measurement unit  130  may operate in a default measurement mode or in a simple measurement mode running for a shorter time than the default measurement mode. 
     The electron beam irradiation device may include an electron gun  133 , a condenser lens  136 , a deflecting coil  139 , and an object lens  142 . The electron gun  133  may emit electrons. Electrons, emitted from the electron gun  133 , may be converted into the electron beam  145 , while sequentially passing through the condenser lens  136 , the deflecting coil  139 , and the object lens  142 , to be radiated onto the surface of the photomask  20  aligned on the second stage  131 . 
     The amount of the secondary electrons  148 , detected by the detector  151 , may be converted into image data by the signal processing unit  155 , and such image data may be displayed as an image on the display unit  170 . 
     The control unit  160  may include an information storage part  163 . The information storage part  163  may store programs and information for measuring the critical dimensions of the mask patterns of the photomask  20 . The information storage part  163  may also store information on the layout patterns of the layout formed by the computer system  10 . The information storage part  163  may further store information on the surface potential distribution of the photomask  20  measured by the surface potential measurement unit  110 . The information on the surface potential distribution of the photomask  20  may include information on the potential of the mask patterns. 
     The control unit  160  may exchange an electrical signal with the critical dimension measurement unit  130 . The control unit  160  may control the critical dimension measurement unit  130  to operate in the default measurement mode or in the simple measurement mode, using the information on the layout patterns and the information on the surface potential distribution of the photomask  20  stored in the information storage part  163 . The control unit  160  may calculate the critical dimensions of the mask patterns, using information on the mask patterns of the photomask  20 , for example, images of the mask patterns of the photomask  20 , obtained according to operations of the critical dimension measurement unit  130 . The information storage part  163 , included in the control unit  160 , may store the images and the critical dimensions of the mask patterns of the photomask  20 . 
     According to an exemplary embodiment of the present inventive concept, the surface potential measurement unit  110 , the critical dimension measurement unit  130 , the signal processing unit  155 , the display unit  170 , the control unit  160 , and the information storage part  163  may be circuits. 
     An example of the layout that may be formed by the computer system  10  will be described with reference to  FIG. 2 .  FIG. 2  is a conceptual plan view of a layout, according to an exemplary embodiment of the present inventive concept. 
     Referring to  FIG. 2 , a layout  30  that may be formed by the computer system  10  of  FIG. 1  may have layout areas  30   a  to  30   f . The layout areas  30   a  to  30   f  may include layout patterns. 
     In an example, the layout areas  30   a  to  30   f  may include a first layout area  30   a , a second layout area  30   b , a third layout area  30   c , a fourth layout area  30   d , a fifth layout area  30   e , and a sixth layout area  30   f.    
     In an example, the layout areas  30   a  to  30   f  may include various shapes and sizes of layout patterns. For example, the first to third layout areas  30   a  to  30   c  may be areas including layout patterns  33 ,  36 , and  39  having a line shape, such as a wiring or the like, the fourth and fifth layout areas  30   d  and  30   e  may be areas including layout patterns  42  and  45  having an isolated shape, such as a contact or pad, and the sixth layout area  30   f  may be an area including layout patterns  48   a  and  48   b  having a line shape as well as a layout pattern  48   c  having an isolated shape. 
     In an example, a portion of the layout areas  30   a  to  30   f  may include the same shape and size of layout patterns. For example, the first layout area  30   a  may be provided as a plurality of first layout areas  30   a , which may include the same shape and size of layout patterns  33 . Similarly, the fourth layout area  30   d  may be provided as a plurality of fourth layout areas  30   d , which may include the same shape and size of layout patterns  42 . 
     In an example, the layout patterns  33  within the first layout area  30   a  may have a higher arrangement density than the layout patterns  36  within the second layout area  30   b.    
     In an example, the layout patterns  39  within the third layout area  30   c  may have a greater width than the layout patterns  33  within the first layout area  30   a  and/or the layout patterns  36  within the second layout area  30   b.    
     In an example, the layout patterns  42  within the fourth layout area  30   d  may have a lower arrangement density than the layout patterns  45  within the fifth layout area  30   e.    
     In  FIG. 2 , the layout patterns  33 ,  36 ,  39 ,  42 ,  45 ,  48   a ,  48   b , and  48   c  within the first to sixth layout areas  30   a  to  30   f  are intended to illustrate exemplary embodiments of the present inventive concept, and may be modified to have various shapes and sizes. 
     In an exemplary embodiment of the present inventive concept, the layout patterns  33 ,  36 ,  39 ,  42 ,  45 ,  48   a ,  48   b , and  48   c  of the layout  30  may correspond to the mask patterns of the photomask  20 , as described above with reference to  FIG. 1 . Thus, the layout  30  of  FIG. 2  may be replaced with the photomask  20 , for the purpose of description. Further, the layout patterns  33 ,  36 ,  39 ,  42 ,  45 ,  48   a ,  48   b , and  48   c  of the layout  30  of  FIG. 2  may be replaced with the mask patterns of the photomask  20 , for the purpose of description. Moreover, planar shapes of the layout patterns  33 ,  36 ,  39 ,  42 ,  45 ,  48   a ,  48   b , and  48   c  of the layout  30  of  FIG. 2  may be those of the mask patterns of the photomask  20 . 
     In an example, the first to sixth layout areas  30   a  to  30   f  of the layout  30  may correspond to first to sixth measurement sites  20   a  to  20   f  of the photomask  20 . The first to sixth measurement sites  20   a  to  20   f  of the photomask  20  of  FIG. 1  may be set by allowing semiconductor designers, developers, or engineers to designate areas of a photomask in which defects may be highly likely to occur, or areas having patterns of which the critical dimensions are required to be measured. 
     In an exemplary embodiment of the present inventive concept, the layout patterns of each of the layout areas  30   a  to  30   f  may have measurement regions of interest. The measurement regions of interest may be areas which are required to be measured. Here, the measurement regions of interest of the layout patterns may also be areas for measuring the critical dimensions of the mask patterns within the measurement sites of the photomask  20 . For example, the measurement regions of interest may be areas of layout patterns corresponding to areas of mask patterns, of which the critical dimensions are required to be measured, of the photomask  20  of  FIG. 1 . 
     An example of the above-mentioned measurement regions of interest of the layout patterns will be described with reference to  FIG. 3 .  FIG. 3  is a plan view illustrating measurement regions of interest of layout patterns, according to an exemplary embodiment of the present inventive concept. Planar shapes and sizes of layout patterns  48   a ,  48   b , and  48   c , as illustrated in  FIG. 3 , are only examples, and may be modified in various manners. 
     Referring to  FIG. 3 , the layout  30  may include a first line layout pattern  48   a , a second line layout pattern  48   b , and a third isolated layout pattern  48   c.    
     The first line layout pattern  48   a  may include a first measurement region of interest ROI_ 1 , the second line layout pattern  48   b  may include a second measurement region of interest ROI_ 2 , and the third isolated layout pattern  48   c  may include a third measurement region of interest ROI_ 3 . 
     In an example, the lengths of the first and second measurement regions of interest ROI_ 1  and ROI_ 2  may be greater than that of the third measurement region of interest ROI_ 3 . In an example, the length of the first measurement region of interest ROI_ 1  may be greater than that of the second measurement region of interest ROI_ 2 . 
     In an example, the first line layout pattern  48   a , the second line layout pattern  48   b , and the third isolated layout pattern  48   c  may be the layout patterns of the sixth layout area  30   f  described above with reference to  FIG. 2 . Thus, information on the first to third measurement regions of interest ROI_ 1  to ROI_ 3  of the first line layout pattern  48   a , the second line layout pattern  48   b , and the third isolated layout pattern  48   c  of the sixth layout area  30   f  may be stored in the information storage part  163  of the control unit  160  of  FIG. 1 . The information on the first to third measurement regions of interest ROI_ 1  to ROI_ 3  may include information on the lengths and widths Wa, Wb, and Wc of the first to third measurement regions of interest ROI_ 1  to ROI_ 3 . 
     The information on the first to third measurement regions of interest ROI_ 1  to ROI_ 3  may be used to measure the critical dimensions of mask patterns within the sixth measurement site  20   f  of the photomask  20 . To measure the critical dimensions of the mask patterns, the electron beam  145 , described above with reference to  FIG. 1 , may be radiated to areas of the mask patterns corresponding to the first to third measurement regions of interest ROI_ 1  to ROI_ 3  of the first line layout pattern  48   a , the second line layout pattern  48   b , and the third isolated layout pattern  48   c  of the sixth layout area  30   f  in a width direction D W , perpendicular to a length direction D L  of the first to third measurement regions of interest ROI_ 1  to ROI_ 3 . 
     Since the layout  30  may be formed using the computer system  10  of  FIG. 1 , the lengths and the widths Wa, Wb, and Wc of the first to third measurement regions of interest ROI_ 1  to ROI_ 3  of the layout patterns  48   a ,  48   b , and  48   c  of the layout  30  may be obtained by software. 
     An example of the photomask  20 , including the mask patterns, will be described with reference to  FIG. 4 .  FIG. 4  is a cross-sectional view illustrating a photomask, according to an exemplary embodiment of the present inventive concept. 
     Referring to  FIG. 4 , the photomask  20  may include a substrate  22  and mask patterns  24  disposed on the substrate  22 . In an example, the photomask  20  may be a phase inversion mask. However, the present inventive concept is not limited thereto. For example, the photomask  20  may be a binary mask. Hereinafter, the phase inversion mask will be illustrated as an example, for convenience of description. 
     The photomask  20  may include an outer mask pattern  25  surrounding the mask patterns  24 , and a light blocking pattern  28  covering the outer mask pattern  25 . 
     The substrate  22  may be a transparent substrate. For example, the substrate  22  may be formed of glass or quartz. Each of the mask patterns  24  and the outer mask pattern  25  may be formed of a compound of molybdenum (Mo) and silicon (Si). For example, the mask patterns  24  and the outer mask pattern  25  may be formed of a MoSi compound, or a MoSi compound containing impurities, for example, nitrogen (N) and/or oxygen (O). The light blocking pattern  28  may be formed of a material able to block light. For example, the light blocking pattern  28  may be formed of a metal material, such as chromium (Cr) or the like, or an inorganic material. 
     In an example, the mask patterns  24  may have a planar shape corresponding to those of the layout patterns  33 ,  36 ,  39 ,  42 ,  45 ,  48   a ,  48   b , and  48   c  of the layout  30 . 
     As described above with reference to  FIG. 1 , the surface potential measurement unit  110  may include the surface potential measurement device  116 . An example of the surface potential measurement device  116  will be described with reference to  FIG. 5 .  FIG. 5  is a schematic cross-sectional view illustrating a surface potential measurement device measuring the potential of mask patterns of a photomask, according to an exemplary embodiment of the present inventive concept. 
     Referring to  FIG. 5 , the surface potential measurement device  116  of the surface potential measurement unit  110 , described above with reference to  FIG. 1 , may include at least two probes  116   a  and  116   b . For example, the potential of a mask pattern  24   a  to be measured may be measured by moving the at least two probes  116   a  and  116   b  of the surface potential measurement device  116  up and down, such that the at least two probes  116   a  and  116   b  may be in contact with the mask pattern  24   a  to be measured, of the mask patterns  24  of the photomask  20 . The mask pattern  24   a  to be measured may be a mask pattern having a shape corresponding to that of any one of the layout patterns  33 ,  36 ,  39 ,  42 ,  45 ,  48   a ,  48   b , and  48   c  described above with reference to  FIG. 2 . 
       FIG. 6  is a graph illustrating the relationship between the length of measurement regions of interest and reproducibility according to an exemplary embodiment of the inventive concept. In  FIG. 6 , the horizontal axis X represents the length of measurement regions of interest, and the vertical axis Y represents reproducibility. The vertical axis Y may be the difference between the actual width of a mask pattern, and the width of the mask pattern measured with the critical dimension measurement apparatus, or may be an error caused by an instrument. The graph, illustrated in  FIG. 6 , may be obtained while the critical dimension measurement apparatus is set up using a reference photomask. 
     In  FIG. 6 , the length of a measurement region of interest (ROI_L) may increase in the horizontal axis X direction, and a difference between the actual width of the mask pattern and the width of the mask pattern measured with the critical dimension measurement apparatus may increase in the vertical axis Y direction. In the graph of  FIG. 6 , a length A of a reference measurement region of interest on the horizontal axis X is displayed, and a reference difference B on the vertical axis Y as described above may be understood as a significantly increased error of an allowable mask pattern width. 
     It can be seen from the graph of  FIG. 6  that, when the length of the measurement region of interest is greater than the length A of the reference measurement region of interest, the reliability of critical dimensions measured with the critical dimension measurement apparatus may be high, and when the length of the measurement region of interest is less than the length A of the reference measurement region of interest, the reliability of critical dimensions measured with the critical dimension measurement apparatus may be low. 
     Examples of a method of measuring critical dimensions of mask patterns of a photomask, according to exemplary embodiments of the present inventive concept, will be described below.  FIG. 7A  is a flowchart illustrating a method of measuring critical dimensions, according to an exemplary embodiment of the present inventive concept, and  FIG. 7B  is a flowchart illustrating a method of measuring critical dimensions, according to an exemplary embodiment of the present inventive concept. 
     Referring to  FIGS. 1, 6, and 7A , the layout  30  including the layout patterns  33 ,  36 ,  39 ,  42 ,  45 ,  48   a ,  48   b , and  48   c  may be formed (S 10 ). The layout  30  may be formed using the computer system  10 . The photomask  20  may be formed (S 20 ). The photomask  20  may be formed by the mask manufacturing apparatus  15 . 
     The information on the layout patterns  33 ,  36 ,  39 ,  42 ,  45 ,  48   a ,  48   b , and  48   c  may be stored (S 30 ). The information on the layout patterns  33 ,  36 ,  39 ,  42 ,  45 ,  48   a ,  48   b , and  48   c  may be stored in the information storage part  163  of the control unit  160  included in the critical dimension measurement apparatus  100 . The information on the layout patterns  33 ,  36 ,  39 ,  42 ,  45 ,  48   a ,  48   b , and  48   c  may include information on the shape (or image), arrangement density, and width of each of the layout patterns  33 ,  36 ,  39 ,  42 ,  45 ,  48   a ,  48   b , and  48   c , and on the lengths of measurement regions of interest of the layout patterns  33 ,  36 ,  39 ,  42 ,  45 ,  48   a ,  48   b , and  48   c.    
     The measurement sites of the photomask  20  may be classified as a default measurement site and a simple measurement site (S 40 ). The control unit  160  of  FIG. 1  may classify the measurement sites of the photomask  20  as the default measurement site and the simple measurement site, and the information storage part  163  of the control unit  160  of  FIG. 1  may store information on the default measurement site and the simple measurement site. Classifying the measurement sites of the photomask  20  as the default measurement site and the simple measurement site may include using the information on the layout patterns  33 ,  36 ,  39 ,  42 ,  45 ,  48   a ,  48   b , and  48   c . The information on the layout patterns  33 ,  36 ,  39 ,  42 ,  45 ,  48   a ,  48   b , and  48   c  may include the information on the arrangement density and width of each of the layout patterns  33 ,  36 ,  39 ,  42 ,  45 ,  48   a ,  48   b , and  48   c , and on the lengths of measurement regions of interest of the layout patterns  33 ,  36 ,  39 ,  42 ,  45 ,  48   a ,  48   b , and  48   c.    
     In  FIG. 7B , the potential of the mask patterns of the photomask may be measured, and information on the potential may be stored (S 25 ). Thus, classifying the measurement sites of the photomask  20  as the default measurement site and the simple measurement site may include using the potential information on the mask patterns, described above with reference to  FIG. 7A , together with the information on the layout patterns  33 ,  36 ,  39 ,  42 ,  45 ,  48   a ,  48   b , and  48   c.    
     Referring to  FIGS. 1 through 6, and 7A or 7B , the critical dimensions of the mask patterns within the measurement sites of the photomask  20  may be measured (S 50 ). The critical dimension measurement unit may be moved to the default measurement site (S 60 ). Subsequently, the critical dimensions of a first mask pattern within the default measurement site may be measured in the default measurement mode (S 70 ). 
     The critical dimension measurement unit may be moved to the simple measurement site (S 80 ). Subsequently, the critical dimensions of a second mask pattern within the simple measurement site may be measured in the simple measurement site running for a shorter time than the default measurement mode (S 90 ). 
     In an example, measuring the critical dimensions of the mask patterns within the measurement sites of the photomask  20  may include moving to the default measurement site, measuring the critical dimensions of the first mask pattern within the default measurement site in the default measurement mode, moving to the simple measurement site, and measuring the critical dimensions of the second mask pattern within the simple measurement site in the simple measurement mode. In a different manner, measuring the critical dimensions of the mask patterns within the measurement sites of the photomask  20  may include moving to the simple measurement site, measuring the critical dimensions of the second mask pattern within the simple measurement site in the simple measurement mode, moving to the default measurement site, and measuring the critical dimensions of the first mask pattern within the default measurement site in the default measurement mode. 
     With reference to  FIG. 2 , since the photomask  20  is formed using the layout  30 , the first to sixth layout areas  30   a  to  30   f  of the layout  30  may be replaced with the first to sixth measurement sites  20   a  to  20   f  of the photomask  20 , and the layout patterns  33 ,  36 ,  39 ,  42 ,  45 ,  48   a ,  48   b , and  48   c  may be replaced with the mask patterns of the photomask  20 , for the purpose of description. 
     In an exemplary embodiment of the present inventive concept, among the first to sixth measurement sites  20   a  to  20   f  of the photomask  20 , the default measurement site may be a site with a mask pattern having a high degree of difficulty of critical dimension measurement, and the simple measurement site may be a site with a mask pattern having a low degree of difficulty of critical dimension measurement. 
     In an example, the first to sixth measurement sites  20   a  to  20   f  of the photomask  20  may be classified as a preceding measurement site of which the critical dimension measurement is performed, and a subsequent measurement site of which the critical dimension measurement is performed immediately after the critical dimension measurement of the preceding measurement site. The preceding measurement site and the subsequent measurement site may be classified as a default measurement site or a simple measurement site, using information on a layout pattern corresponding to a mask pattern within the preceding measurement site and on a layout pattern corresponding to a mask pattern within the subsequent measurement site. 
     In an example, when a measurement site that may be the preceding measurement site among the first to sixth measurement sites  20   a  to  20   f  of the photomask  20  is the default measurement site, a measurement site that may be the subsequent measurement site thereamong may be the default measurement site or the simple measurement site. For example, when a degree of difficulty of critical dimension measurement of a mask pattern within the subsequent measurement site is lower than a degree of difficulty of critical dimension measurement of a mask pattern within the preceding measurement site that may be the default measurement site, the subsequent measurement site may be set or classified as the simple measurement site. In a different manner, when a degree of difficulty of critical dimension measurement of a mask pattern within the subsequent measurement site is higher than or equal to a degree of difficulty of critical dimension measurement of a mask pattern within the preceding measurement site that may be the default measurement site, the subsequent measurement site may be set or classified as the default measurement site. 
     In an example, when the shapes and sizes of a mask pattern within the subsequent measurement site and a mask pattern within the preceding measurement site are substantially the same, the subsequent measurement site may be set or classified as the simple measurement site. For example, when the first measurement sites  20   a  corresponding to the first layout areas  30   a , described above with reference to  FIG. 2 , are set as the preceding measurement site and the subsequent measurement site, one first measurement site  20   a  that may be the preceding measurement site of which the critical dimension measurement is performed first, among the first measurement sites  20   a  including mask patterns having substantially the same shape and size, may be set as the default measurement site, and another first measurement site  20   a , of which the critical dimension measurement is performed after the critical dimension measurement of the one first measurement site  20   a , may be set as the simple measurement site. 
     In exemplary embodiments of the present inventive concept, a mask pattern within the default measurement site, among the first measurement sites  20   a , may be designated as a first mask pattern or a default mask pattern, and a mask pattern within the simple measurement site thereamong may be designated as a second mask pattern or a simple mask pattern. 
     In an example, when the critical dimension measurement is sequentially performed on the first and second measurement sites  20   a  and  20   b  corresponding to the first and second layout areas  30   a  and  30   b , described above with reference to  FIG. 2 , the arrangement density of mask patterns within the second measurement site  20   b  may be lower than that of mask patterns within the first measurement site  20   a . Thus, the first measurement site  20   a  that may be the preceding measurement site may be set as the default measurement site, and the second measurement site  20   b  that may be the subsequent measurement site may be set as the simple measurement site. 
     In an example, when the critical dimension measurement is sequentially performed on the first and third measurement sites  20   a  and  20   c  corresponding to the first and third layout areas  30   a  and  30   c , described above with reference to  FIG. 2 , the size or width of mask patterns within the third measurement site  20   c  may be greater than that of mask patterns within the first measurement site  20   a . Thus, the first measurement site  20   a  that may be the preceding measurement site may be set as the default measurement site, and the third measurement site  20   c  that may be the subsequent measurement site may be set as the simple measurement site. 
     In an example, when the critical dimension measurement is sequentially performed on the first and sixth measurement sites  20   a  and  20   f  corresponding to the first and sixth layout areas  30   a  and  30   f , described above with reference to  FIG. 2 , a degree of difficulty of critical dimension measurement of mask patterns within the first measurement site  20   a  may be similar to a degree of difficulty of critical dimension measurement of mask patterns within the sixth measurement site  20   f  Thus, the first and second measurement sites  20   a  and  20   b  may be set as default measurement sites. 
     Within measurement sites of which the critical dimension measurement is performed sequentially, as the arrangement density of a mask pattern decreases and the size or width of the mask pattern increases, a degree of difficulty of critical dimension measurement of the mask pattern may decrease. Similarly, as the arrangement density of a mask pattern increases and the size or width of the mask pattern decreases, a degree of difficulty of critical dimension measurement of the mask pattern may increase. 
     When the first to sixth measurement sites  20   a  to  20   f  of the photomask  20  are provided as a plurality of measurement sites, the subsequent measurement site, described above, may be a measurement site preceding another measurement site. Thus, the preceding measurement site may be the simple measurement site. 
     In an example, when a measurement site that may be the preceding measurement site among the first to sixth measurement sites  20   a  to  20   f  of the photomask  20  is the simple measurement site, a measurement site that may be the subsequent measurement site thereamong may be the default measurement site or the simple measurement site. For example, when a degree of difficulty of critical dimension measurement of a mask pattern within the subsequent measurement site is higher than a degree of difficulty of critical dimension measurement of a mask pattern within the preceding measurement site that may be the simple measurement site, the subsequent measurement site may be set or classified as the default measurement site, and when a degree of difficulty of critical dimension measurement of a mask pattern within the subsequent measurement site is lower than or equal to a degree of difficulty of critical dimension measurement of a mask pattern within the preceding measurement site, the subsequent measurement site may be set or classified as the simple measurement site. 
     In an example, when the fourth measurement site  20   d  corresponding to the fourth layout area  30   d , described above with reference to  FIG. 2 , is the simple measurement site and the preceding measurement site, and the fifth measurement site  20   e  corresponding to the fifth layout area  30   e  is the subsequent measurement site, a mask pattern within the fifth measurement site  20   e  may have a higher arrangement density than a mask pattern within the fourth measurement site  20   d . Thus, the fifth measurement site  20   e  may be set or classified as the default measurement site. 
     In an exemplary embodiment of the present inventive concept, the first to sixth measurement sites  20   a  to  20   f  may be set or classified as the default measurement site and the simple measurement site, using surface potential information on mask patterns obtained from the surface potential measurement unit  110  of the critical dimension measurement apparatus  100 . For example, when a mask pattern within the subsequent measurement site has substantially the same potential as a mask pattern within the preceding measurement site, the subsequent measurement site may be set or classified as the simple measurement site. For example, when a mask pattern within the subsequent measurement site has a different surface potential from a mask pattern within the preceding measurement site, the subsequent measurement site may be set or classified as the default measurement site. 
     In an exemplary embodiment of the present inventive concept, when the sum of lengths of measurement regions of interest of mask patterns within any one measurement site is greater than the length A of the reference measurement region of interest described above with reference to  FIG. 6 , the one measurement site may be set or classified as the simple measurement site, and when the sum of lengths of measurement regions of interest of mask patterns within any one measurement site is less than the length A of the reference measurement region of interest described above with reference to  FIG. 6 , the one measurement site may be set or classified as the default measurement site. 
     Examples of a method of measuring the critical dimensions of the first mask pattern within the default measurement site in the default measurement mode and a method of measuring the critical dimensions of the second mask pattern within the simple measurement site in the simple measurement mode described above with reference to  FIG. 7A or 7B , will be described with reference to  FIGS. 8A and 8B . 
       FIGS. 8A and 8B  are flowcharts illustrating a method of measuring critical dimensions, according to exemplary embodiments of the present inventive concept. 
     Referring to  FIG. 7A or 7B, and 8A , the critical dimensions of the first mask pattern within the default measurement site may be measured in the default measurement mode (S 70   a ). Measuring the critical dimensions of the first mask pattern within the default measurement site in the default measurement mode may include autofocusing (S 72 ) for the first mask pattern, pattern matching (S 74 ) for the first mask pattern, and measuring the critical dimensions of the first mask pattern (S 76   a ). 
     Autofocusing (S 72 ) may refer to focusing the electron beam  145  of  FIG. 1  on the surface of the photomask  20  with the object lens  142  of  FIG. 1 . 
     Pattern matching (S 74 ) may include matching a reference pattern previously stored in the information storage part  163  of the control unit  160  with the first mask pattern. The reference pattern may be a pattern that may be obtained from the layout  30 , or a pattern that may be obtained from image information on a mask pattern of the photomask  20 . Thus, pattern matching (S 74 ) may allow the critical dimensions of a measurement region of interest of the first mask pattern corresponding to the measurement region of interest described above with reference to  FIG. 6  to be accurately measured. 
     Referring to  FIG. 7A or 7B, and 8B , the critical dimensions of the second mask pattern within the simple measurement site may be measured in the simple measurement mode (S 90   a ). Measuring the critical dimensions of the second mask pattern within the simple measurement site in the simple measurement mode may include performing one or neither of autofocusing (S 72 ) and pattern matching (S 74 ) for the second mask pattern (S 91 ), and measuring the critical dimensions of the second mask pattern (S 96   a ). 
     Since a degree of difficulty of critical dimension measurement of the second mask pattern within the simple measurement site is lower than a degree of difficulty of critical dimension measurement of the first mask pattern within the default measurement site, even when one or neither of autofocusing (S 72 ) and pattern matching (S 74 ) for the second mask pattern is performed, the second mask pattern may be measured and the critical dimensions of the measurement region of interest of the second mask pattern may be produced. 
     Thus, the method of measuring the critical dimensions of the second mask pattern within the simple measurement site in the simple measurement mode may include performing one or neither of autofocusing (S 72 ) and pattern matching (S 74 ) for the second mask pattern, as compared to the method of measuring the critical dimensions of the first mask pattern within the default measurement site in the default measurement mode, thus reducing an amount of time required to measure the critical dimensions of the mask patterns of the photomask  20 . 
     In an exemplary embodiment of the present inventive concept, when a measurement site among the measurement sites of the photomask  20  is set or classified as the simple measurement site, pattern matching (S 74 ) for the measurement site may not be performed and critical dimension measurement thereof may be performed. 
     In an exemplary embodiment of the present inventive concept, when the subsequent measurement site that may be the simple measurement site, among the measurement sites of the photomask  20 , is adjacent to the preceding measurement site thereamong, autofocusing (S 72 ) for the subsequent measurement site may not be performed and critical dimension measurement thereof may be performed. 
     In an exemplary embodiment of the present inventive concept, using potential information on mask patterns obtained from the surface potential measurement unit  110  of the critical dimension measurement apparatus  100 , as described above with reference to  FIG. 1 , any one of autofocusing (S 72 ) and pattern matching (S 74 ) for the mask patterns may not be performed and critical dimension measurement thereof may be performed. For example, when a mask pattern within the subsequent measurement site has substantially the same potential as a mask pattern within the preceding measurement site, autofocusing (S 72 ) and pattern matching (S 74 ) for the mask pattern within the subsequent measurement site may not be performed and critical dimension measurement thereof may be performed. 
     In an exemplary embodiment of the present inventive concept, as described above with reference to  FIG. 6 , when the length of a measurement region of interest of a measurement site is greater than the length A of the reference measurement region of interest, pattern matching (S 74 ) for the measurement site may not be performed and critical dimension measurement thereof may be performed. The length of the measurement region of interest of the measurement site may be the sum of lengths of measurement regions of interest of all of mask patterns to be measured within any one measurement site. For example, a plurality of mask patterns to be measured may be within any one measurement site and each of the mask patterns to be measured may have a measurement region of interest, so that the measurement regions of interest having a plurality of lengths may be present within the one measurement site. Thus, when the sum of lengths of measurement regions of interest is greater than the length A of the reference measurement region of interest of  FIG. 6  within the any one measurement site, the one measurement site may be set as the simple measurement site, and pattern matching (S 74 ) for the one measurement site may not be performed and the critical dimension measurement thereof may be performed. 
     In an exemplary embodiment of the present inventive concept, the number of pixels and the number of scanning times may be adjusted to reduce an amount of time required to measure the critical dimensions of mask patterns of the photomask  20 . Similar to as described above, examples of a method of measuring critical dimensions, which may reduce an amount of time required to measure critical dimensions by adjusting the number of pixels and the number of scanning times, will be described with reference to  FIGS. 9A, 9B, and 9C .  FIGS. 9A, 9B, and 9C  are flowcharts illustrating a method of measuring critical dimensions, according to exemplary embodiments of the present inventive concept. 
     Referring to  FIG. 7A or 7B, and 9A , the critical dimensions of the first mask pattern within the default measurement site may be measured in the default measurement mode (S 70   b ). Measuring the critical dimensions of the first mask pattern within the default measurement site in the default measurement mode may include autofocusing (S 72 ) for the first mask pattern, pattern matching (S 74 ) for the first mask pattern, and measuring the critical dimensions of the first mask pattern (S 76   b ). Measuring the critical dimensions of the first mask pattern (S 76   b ) may be performed using a default number of pixels and a default number of scanning times. The default measurement mode may include measuring the critical dimensions of the first mask pattern, using the default number of pixels and the default number of scanning times. 
     Referring to  FIG. 7A or 7B, and 9B , the critical dimensions of the second mask pattern within the simple measurement site may be measured in the simple measurement mode (S 90   b ). Measuring the critical dimensions of the second mask pattern within the simple measurement site in the simple measurement mode may include autofocusing (S 72 ) for the second mask pattern, pattern matching (S 74 ) for the second mask pattern, and measuring the critical dimensions of the second mask pattern (S 96   a ). Measuring the critical dimensions of the second mask pattern (S 96   b ) may be performed using a number of pixels less than the default number of pixels and/or a number of scanning times less than the default number of scanning times. The simple measurement mode may include measuring the critical dimensions of the second mask pattern, using the number of pixels less than the default number of pixels or the number of scanning times less than the default number of scanning times. 
     In an example, the default number of pixels may be about 1,024, and the number of pixels less than the default number of pixels may be about 512. In an example, the default number of scanning times may be about 48, and the number of scanning times less than the default number of scanning times may be about 16 or 32. The numbers, associated with the pixels and the scanning times, are only examples, and may be modified. 
     Referring to  FIG. 9C , the critical dimensions of the second mask pattern within the simple measurement site may be measured in the simple measurement mode (S 90   c ). Measuring the critical dimensions of the second mask pattern within the simple measurement site in the simple measurement mode may include performing one or neither of autofocusing (S 72 ) and pattern matching (S 74 ) for the second mask pattern (S 91 ) and measuring the critical dimensions of the second mask pattern (S 96   c ). Measuring the critical dimensions of the second mask pattern (S 96   c ) may be performed using the number of pixels less than the default number of pixels and/or the number of scanning times less than the default number of scanning times. Thus, an amount of time required to measure the critical dimensions of mask patterns may be reduced. 
     The above-mentioned exemplary embodiments of the critical dimension measurement system and the method of measuring critical dimensions may be applied to measure the critical dimensions of mask patterns of a photomask. However, the present inventive concept is not limited thereto. For example, the above-mentioned exemplary embodiments of the critical dimension measurement system and the method of measuring critical dimensions may be applied to measure the critical dimensions of various patterns, as well as mask patterns of a photomask. For example, the above-mentioned exemplary embodiments of the critical dimension measurement system and the method of measuring critical dimensions may also be applied to measure the critical dimensions of various patterns formed on a semiconductor wafer. For example, the above-stated photomask may be replaced with a wafer, and the previously mentioned mask patterns may be replaced with photoresist patterns or circuit patterns. 
     The various operations of methods described above may be performed by any suitable means capable of performing the operations, such as various hardware and/or software component(s), circuits, and/or module(s). 
     The software may comprise an ordered listing of executable instructions for implementing logical functions, and can be embodied in any “processor-readable medium” for use by or in connection with an instruction execution system, apparatus, or device, such as a single or multiple-core processor or processor-containing system. 
     The blocks or steps of a method or algorithm and functions described in connection with the embodiments disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. If implemented in software, the functions may be stored on or transmitted over as one or more instructions or code on a tangible, non-transitory computer-readable medium. A software module may reside in Random Access Memory (RAM), flash memory, Read Only Memory (ROM), Electrically Programmable ROM (EPROM), Electrically Erasable Programmable ROM (EEPROM), registers, hard disk, a removable disk, a CD ROM, or any other form of storage medium known in the art. 
     As set forth above, according to exemplary embodiments of the present inventive concept, a critical dimension measurement method and a critical dimension measurement system, which may reduce an amount of time required to measure critical dimensions of mask patterns of a photomask, may be provided. Thus, a critical dimension measurement apparatus may measure the critical dimensions of a larger number of photomasks. Further, the critical dimension measurement method and the critical dimension measurement system may reduce the amount of time required to measure the critical dimensions of the mask patterns, thus enabling the photomask to be formed within a shorter period of time and using the photomask in a photolithography process. Accordingly, the critical dimension measurement method and the critical dimension measurement system may reduce an amount of time required to fabricate a semiconductor device from a layout, which may increase productivity. 
     While the present inventive concept has been shown and described above with reference to exemplary embodiments thereof, it will be apparent to those of ordinary skill in the art that modifications and variations in form and details could be made thereto without departing from the spirit and scope of the present inventive concept, as set forth by the following claims.