Patent Publication Number: US-9841668-B2

Title: Photomasks for reducing thermal stress generated by heat

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 14/742,394 filed on Jun. 17, 2015, entitled PHOTOMASKS FOR REDUCING THERMAL STRESS GENERATED BY HEAT, which claims priority under 35 U.S.C 119(a) to Korean Application No. 10-2015-0025835, filed on Feb. 24, 2015, in the Korean intellectual property Office, the disclosures of which are incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     1. Technical Field 
     Various embodiments of the present disclosure relate to photomasks used in lithography processes and, more particularly, to photomasks for reducing thermal stress generated by heat. 
     2. Related Art 
     In general, a semiconductor device may include a plurality of patterns disposed over a semiconductor substrate. The patterns may be formed using a photolithography process and an etch process to realize active elements and/or passive elements. The photolithography process may be used to form photoresist patterns. More specifically, the photolithography process may be performed by coating a photoresist material on a target layer to form a photoresist layer, by selectively exposing portions of the photoresist layer to light with a photomask, and by developing the exposed photoresist layer to form the photoresist patterns. The photoresist patterns may be used as etch masks for patterning the target layer. As such, the photomask may be used to transfer predetermined patterns onto the photoresist layer and may be generally comprised of a light transmission substrate and a plurality of transfer patterns disposed on the light transmission substrate. 
     SUMMARY 
     Various embodiments are directed to photomasks for reducing thermal stress generated by heat. 
     According to an embodiment, a photomask includes a light transmission substrate having a transfer region and a frame region, a light-transmitting region exposing a portion of the light transmission substrate in the transfer region corresponding to a transfer pattern, a phase shift region surrounding the light-transmitting region in the transfer region. The phase shift region includes a first phase shift region surrounding the light-transmitting region and a second phase shift region surrounding the first phase shift region. A first phase shift pattern is disposed on the light transmission substrate in the first phase shift region, and a plurality of second phase shift patterns are disposed on the light transmission substrate in the second phase shift region. 
     According to an embodiment, a photomask includes a light transmission substrate having a transfer region and a frame region, a transfer pattern disposed on a portion of the light transmission substrate in the transfer region, and a light-transmitting region surrounding the transfer pattern in the transfer region. The light-transmitting region includes a first light-transmitting region surrounding the transfer pattern and a second light-transmitting region surrounding the first light-transmitting region. The light transmission substrate in the first light-transmitting region is exposed. A plurality of light-blocking patterns are disposed on the light transmission substrate in the second light-transmitting region. And each of the plurality of light-blocking patterns has a closed loop shape. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments of the present disclosure will become more apparent in view of the attached drawings and accompanying detailed description, in which: 
         FIG. 1  is a plan view illustrating a binary photomask according to an embodiment; 
         FIG. 2  is an enlarged view illustrating a portion of a transfer region of the binary photomask shown in  FIG. 1 ; 
         FIG. 3  is a cross-sectional view taken along a line I-I′ of  FIG. 2 ; 
         FIG. 4  is an enlarged view illustrating a portion of a frame region of the binary photomask shown in  FIG. 1 ; 
         FIG. 5  is a cross-sectional view taken along a line II-II′ of  FIG. 4 ; 
         FIG. 6  is a plan view illustrating a light-blocking region in a transfer region and a frame region included in the binary photomask shown in  FIG. 1  according to an embodiment; 
         FIG. 7  is a plan view illustrating a light-blocking region in a transfer region and a frame region included in the binary photomask shown in  FIG. 1  according to another embodiment; 
         FIG. 8  is a plan view illustrating a light-blocking region in a transfer region and a frame region included in the binary photomask shown in  FIG. 1  according to still another embodiment; 
         FIG. 9  is a plan view illustrating a phase shift photomask according to an embodiment; 
         FIG. 10  is an enlarged view illustrating a portion of a transfer region of the phase shift photomask shown in  FIG. 9 ; 
         FIG. 11  is a cross-sectional view taken along a line III-III′ of  FIG. 10 ; 
         FIG. 12  is an enlarged view illustrating a portion of a frame region of the phase shift photomask shown in  FIG. 9 ; 
         FIG. 13  is a cross-sectional view taken along a line IV-IV′ of  FIG. 12 ; 
         FIG. 14  is a plan view illustrating a phase shift region in a transfer region and a frame region included in the phase shift photomask shown in  FIG. 9  according to an embodiment; 
         FIG. 15  is a plan view illustrating a phase shift region in a transfer region and a frame region included in the phase shift photomask shown in  FIG. 9  according to another embodiment; 
         FIG. 16  is a plan view illustrating a phase shift region in a transfer region and a frame region included in the phase shift photomask shown in  FIG. 9  according to still another embodiment; 
         FIG. 17  is a plan view illustrating a photomask according to an embodiment; 
         FIG. 18  is an enlarged view illustrating a portion of a transfer region of the photomask shown in  FIG. 17 ; 
         FIG. 19  is a cross-sectional view taken along a line V-V′ of  FIG. 18 ; and 
         FIG. 20  is a schematic view illustrating an exposure system in which the photomask of  FIG. 17  is loaded. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     In a photolithography process, light having a specific wavelength may be emitted from a light source and may irradiate a photoresist layer formed on a wafer through a photomask. Light-blocking regions of the photomask may prevent the light from irradiating the photoresist layer on the wafer, and only light-transmitting regions of the photomask may allow the light to reach the wafer. The light-blocking regions may be regions on which light-blocking patterns are disposed. During the photolithography process, the light-blocking patterns may absorb a large amount of optical energy of the light irradiating the photomask, thereby generating heat in the photomask. The heat may be conducted to a light transmission substrate of the photomask, and thus the light transmission substrate may be expanded and deformed due to the heat. The thermal deformation of the light transmission substrate may change position coordinates of patterns of the photomask, and thus cause an overlay error between the photomask and the wafer in an exposure step. 
     In addition, the light penetrating the photomask may pass through a plurality of lenses constituting a lens module of an exposure system to reach the wafer. Thus, the plurality of lenses may also absorb the optical energy of the light emitted from the light source to generate heat therein. As a result, the plurality of lenses may be expanded, and thus distort a phase of the light passing through the lenses. This may lead to an abnormal lithography process. 
     The following embodiments may provide photomasks which are capable of substantially preventing the light emitted from the light source from being absorbed into transfer patterns such as light-blocking patterns or phase shift patterns of the photomasks during a photolithography process. Moreover, the following embodiments may provide photomasks which may substantially prevent generation of thermal stress of lenses in an exposure system by reducing an amount of the light which irradiates the lenses through light-transmitting regions of the photomasks. 
     It will be understood that although the terms first, second, third, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another element. Thus, a first element in some embodiments could be termed a second element in other embodiments without departing from the teachings of the present disclosure. 
     It will also be understood that when an element is referred to as being located “on”, “over”, “above”, “under”, “beneath” or “below” another element, it may directly contact the other element, or at least one intervening element may be present therebetween. Accordingly, the terms such as “on”, “over”, “above”, “under”, “beneath”, “below” and the like that are used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the present disclosure. 
     It will be further understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. 
       FIG. 1  is a plan view illustrating a binary photomask  100  according to an embodiment. In  FIG. 1 , a configuration for relieving thermal stress of the binary photomask  100  according to the present embodiment is not illustrated in order to reduce the complexity of the drawing. The configuration for relieving the thermal stress of the binary photomask  100  according to the present embodiment will be described in detail with reference to  FIGS. 2 to 5 . In addition, various configurations for relieving thermal stress of photomasks according to other embodiments will be described in detail with reference to  FIGS. 6 to 8 . 
     Referring to  FIG. 1 , the binary photomask  100  may have a transfer region  110  and a frame region  120  surrounding the transfer region  110 . The transfer region  110  may correspond to a region in which patterns configured to be transferred onto a wafer are disposed. The frame region  120  may correspond to a marginal region which is provided to prevent process errors that are due to double exposures between two adjacent shot areas defined in an exposure step. The two adjacent shot areas may be two adjacent chip areas. 
     A plurality of transfer patterns  112  may be disposed in the transfer region  110 . The plurality of transfer patterns  112  may be two-dimensionally arrayed in rows and columns and spaced apart from each other. In the present embodiment, the plurality of transfer patterns  112  may have a uniform size and may be uniformly spaced apart from each other. However, in some embodiments, sizes of the plurality of transfer patterns  112  may be nonuniform and/or distances between the plurality of transfer patterns  112  may be nonuniform. In either embodiment, the configuration for relieving thermal stress of the binary photomask  100  according to the present embodiment may be equally applicable. 
     As illustrated in  FIG. 1 , each of the transfer patterns  112  may be a hole-shaped pattern. However, the type of the transfer patterns  112  illustrated in  FIG. 1  is merely exemplary. For example, the transfer patterns  112  can be line patterns spaced apart from each other instead of hole-shaped patterns. Although  FIG. 1  illustrates the transfer patterns  112  each having a rectangular shape, embodiments are not limited thereto. In some embodiments, the transfer patterns  112  may have non-rectangular shapes. Each of the transfer patterns  112  may correspond to a light-transmitting region  114  which is comprised of a portion of a light transmission substrate exposed by an opening in a light-blocking region  116 . That is, the transfer region  110  may include the light-transmitting regions  114  corresponding to the transfer patterns  112  and the light-blocking region  116  surrounding the light-transmitting regions  114  in a plan view. 
     A configuration of the light-blocking region  116  will be described more fully with reference to  FIGS. 2 and 3 , which illustrate in detail a portion  150  of the transfer region  110  included in the binary photomask  100 . A light-blocking pattern such as a chromium (Cr) pattern may be disposed in the frame region  120 . Thus, the frame region  120  may substantially block light during an exposure step. 
     The transfer patterns  112  disposed in the transfer region  110  may be transferred onto a wafer by an exposure step. In particular, the transfer patterns  112  corresponding to the light-transmitting regions  114  may be transferred onto a positive tone resist layer formed on the wafer. Specifically, if an exposure step is performed with the binary photomask  100 , portions of the positive tone resist layer that correspond to the transfer patterns  112  may be exposed to light passing through the transfer patterns  112  disposed in the transfer region  110  of the binary photomask  100 . Light may not irradiate the remaining portion of the positive tone resist layer that corresponds to the light-blocking region  116  of the transfer region  110 . As a result of the exposure step, a chemical structure of the exposed portions of the positive tone resist layer may change. Thus, the exposed portions of the positive tone resist layer may be selectively dissolved by a developer, and the transfer patterns  112  of the binary photomask  100  are transferred onto the positive tone resist layer. 
       FIG. 2  is an enlarged view illustrating the portion  150  of the transfer region  110  of the binary photomask  100  shown in  FIG. 1 , and  FIG. 3  is a cross-sectional view taken along a line I-I′ of  FIG. 2 . In  FIGS. 2 and 3 , the same reference numerals as used in  FIG. 1  denote the same elements. Referring to  FIGS. 1, 2 and 3 , the transfer patterns  112  may correspond to the light-transmitting regions  114 , which are comprised of portions of a light transmission substrate  102  that are exposed in the transfer region  110 . The light-transmitting regions  114  may be surrounded by the light-blocking region  116 . 
     A top surface  102   a  of the light transmission substrate  102  in each light-transmitting region  114  may be exposed. Thus, during the exposure step, the light irradiating a bottom surface  102   b  of the light transmission substrate  102  may penetrate the light transmission substrate  102  in the light-transmitting region  114  to reach a wafer through the top surface  102   a  of the light transmission substrate  102  and a lens module (not shown) of an exposure system. 
     The light-blocking region  116  surrounding the light-transmitting regions  114  may include a plurality of first light-blocking region  116   a  and a second light-blocking region  116   b . Each of the first light-blocking regions  116   a  may have a uniform width W 1  along a perimeter of a corresponding one of the light-transmitting regions  114  and surround the corresponding light-transmitting region  114 . In each of the first light-blocking regions  116   a , a first light-blocking pattern  117  may be disposed on the top surface  102   a  of the light transmission substrate  102 . 
     The second light-blocking region  116   b  may correspond to a region between the first light-blocking regions  116   a  in the light-blocking region  116 . That is, the transfer region  110  may include the light-transmitting regions  114  corresponding to the transfer patterns  112 , the first light-blocking regions  116   a  surrounding the light-transmitting regions  114 , respectively, and the second light-blocking region  116   b  disposed between the first light-blocking regions  116   a  to surround the first light-blocking regions  116   a.    
     A plurality of second light-blocking patterns  118  may be two-dimensionally arrayed along rows and columns in the second light-blocking region  116   b  and spaced apart from each other in a plan view. The top surface  102   a  of the light transmission substrate  102  may be partially exposed by the second light-blocking patterns  118 . In the present embodiment, the exposed portion of the light transmission substrate  102  between the second light blocking patterns  118  may be a first exposure region  119 - 1 . A distance D 1  between the second light-blocking patterns  118  arrayed in each row may be substantially equal to a distance D 2  between the second light-blocking patterns  118  arrayed in each column. 
     Second light-blocking patterns  118 ′ may be second light-blocking patterns  118  may include second light-blocking patterns  118 ′ disposed directly adjacent to each of the first light-blocking patterns  117 . The term “directly adjacent” refers to a member of a group that is the closest member of the group to another structure. A first structure that is “directly adjacent” to a second structure does not necessarily touch the second structure. A distance D 3  between the first light-blocking pattern  117  and any one of the second light-blocking patterns  118 ′ may be substantially equal to the distance D 1  between the second light-blocking patterns  118  arrayed in each row and the distance D 2  between the second light-blocking patterns  118  arrayed in each column. The second light-blocking patterns  118  may be located at cross points of the rows and the columns. That is, the second light-blocking patterns  118  may be two-dimensionally arrayed in a matrix form. 
     Each of the second light-blocking patterns  118  may have a rectangular closed loop shape. That is, each of the second light-blocking patterns  118  may have an opening that penetrates a central portion thereof. Accordingly, portions of the top surface  102   a  of the light transmission substrate  102  may be exposed by the openings of the second light-blocking patterns  118 . In the present embodiment, the portions of the light transmission substrate  102  exposed by the openings of the second light-blocking patterns  118  may be second exposure regions  119 - 2 . Thus, the light-blocking region  116  in the transfer region  110  may include light-blocking regions covered with the first and second light-blocking patterns  117  and  118  and light-transmitting regions comprised of the first and second exposure regions  119 - 1  and  119 - 2 . 
     If the exposure step is performed with an exposure system in which the binary photomask  100  is loaded, light  201  irradiating the bottom surface  102   b  of the light transmission substrate  102  may penetrate the light transmission substrate  102  and may exit out of the light transmission substrate  102  through the top surface  102   a  of the light transmission substrate  102 . The light  201  exiting out of the light transmission substrate  102  may irradiate the positive tone resist layer formed on the wafer through the lens module of the exposure system. Since the light  201  penetrating the light-transmitting regions  114  irradiates the positive tone resist layer formed on the wafer, the transfer patterns  112  may be transferred onto the positive tone resist layer. Most of the light  201  irradiating the first and second light-blocking patterns  117  and  118  in the light-blocking region  116  may be blocked from reaching the positive tone resist layer formed on the wafer. 
     Meanwhile, the light  201  irradiating the first and second exposure regions  119 - 1  and  119 - 2  in the light-blocking region  116  may penetrate the light transmission substrate  102  and may exit out of the light transmission substrate  102  through the top surface  102   a  of the light transmission substrate  102 . However, in an embodiment, the first and second exposure regions  119 - 1  and  119 - 2  may have widths that are narrower than a resolution limit of the exposure system. Thus, undesired patterns corresponding to the first and second exposure regions  119 - 1  and  119 - 2  may not be transferred onto the positive tone resist layer formed on the wafer, and the light  201  exiting out of the first and second exposure regions  119 - 1  and  119 - 2  may not influence the transfer of the transfer patterns  112  corresponding to the light-transmitting regions  114 . 
     According to the above embodiment, an amount of the light absorbed into the binary photomask  100  may be reduced by an amount of the light penetrating the light transmission substrate  102  in the first and second exposure regions  119 - 1  and  119 - 2 , as compared to a case in which an entire portion of the light-blocking region  116  is fully covered with a light-blocking layer. This may reduce thermal stress of the binary photomask  100  that is due to heat generated by the light absorbed into the binary photomask  100  during the exposure step. As a result, deformation of the binary photomask  100  may be suppressed, thus substantially preventing positions or shapes of the overlay patterns in the binary photomask  100  from being changed. 
       FIG. 4  is an enlarged view illustrating a portion  160  of the frame region  120  of the binary photomask  100  shown in  FIG. 1 , and  FIG. 5  is a cross-sectional view taken along a line II-II′ of  FIG. 4 . In  FIGS. 4 and 5 , the same reference numerals as used in  FIG. 1  denote the same elements. Referring to  FIGS. 1, 4 and 5 , a plurality of frame light-blocking patterns  128  may be two-dimensionally arrayed on the top surface  102   a  of the light transmission substrate  102  and spaced apart from each other. That is, the plurality of frame light-blocking patterns  128  may be disposed in rows and columns in a plan view. The top surface  102   a  of the light transmission substrate  102  may be exposed between the frame light-blocking patterns  128 . In the present embodiment, the exposed portion of the light transmission substrate  102  between the frame light blocking patterns  128  may be a first frame exposure region  129 - 1 . 
     A distance D 4  between the frame light-blocking patterns  128  arrayed in each row may be substantially equal to a distance D 5  between the frame light-blocking patterns  128  arrayed in each column. These distances D 4  and D 5  may be substantially equal to the distance D 1  between the second light-blocking patterns  118  arrayed in each row and the distance D 2  between the second light-blocking patterns  118  arrayed in each column which are described with reference to  FIGS. 2 and 3 . The frame light-blocking patterns  128  may be located at cross points of the rows and the columns. That is, the frame light-blocking patterns  128  may be two-dimensionally arrayed in a matrix form. 
     Each of the frame light-blocking patterns  128  may have a rectangular closed loop shape. That is, each of the frame light-blocking patterns  128  may have an opening that penetrates a central portion thereof. Accordingly, portions of the top surface  102   a  of the light transmission substrate  102  may be exposed by the openings of the frame light-blocking patterns  128 . In the present embodiment, the portions of the light transmission substrate  102  exposed by the openings of the frame light-blocking patterns  128  may be second frame exposure regions  129 - 2 . Thus, the frame region  120  may include light-blocking regions covered with the frame light-blocking patterns  128  and light-transmitting regions comprised of the first and second frame exposure regions  129 - 1  and  129 - 2 . 
     If an exposure step is performed with the exposure system in which the binary photomask  100  is loaded, most of light  202  irradiating the frame light-blocking patterns  128  in the frame region  120  may be blocked from reaching the positive tone resist layer formed on the wafer. Meanwhile, the light  202  irradiating the first and second frame exposure regions  129 - 1  and  129 - 2  in the frame region  120  may penetrate the light transmission substrate  102  and may exit out of the light transmission substrate  102  through the top surface  102   a  of the light transmission substrate  102 . The light  202  exiting out of the first and second frame exposure regions  129 - 1  and  129 - 2  may or may not irradiate the positive tone resist layer formed on the wafer. 
     For example, if the first and second frame exposure regions  129 - 1  and  129 - 2  are designed to have widths that are narrower than a certain value, such that the light exiting out of the first and second frame exposure regions  129 - 1  and  129 - 2  has a low intensity, the light exiting out of the first and second frame exposure regions  129 - 1  and  129 - 2  does not change a chemical structure of the exposed portions of the positive tone resist layer. Accordingly, exposed portions of the positive tone resist layer may not be dissolved by a developer even though the light  202  exiting out of the first and second frame exposure regions  129 - 1  and  129 - 2  irradiates the positive tone resist layer. Alternatively, if the first and second frame exposure regions  129 - 1  and  129 - 2  may each have a predetermined width, such that the light exiting out of the first and second frame exposure regions  129 - 1  and  129 - 2  may be diffracted, the diffracted light may not irradiate the positive tone resist layer. In either case, undesired patterns corresponding to the first and second exposure regions  129 - 1  and  129 - 2  may not be transferred onto the positive tone resist layer formed on the wafer. 
     As compared with a case in which an entire portion of the frame region  120  is fully covered with a light-blocking layer, in this embodiment, an amount of the light absorbed into the binary photomask  100  may be reduced by at least an amount of the light penetrating the light transmission substrate  102  in the first and second frame exposure regions  129 - 1  and  129 - 2 . This may reduce thermal stress of the binary photomask  100  that is due to heat generated by the light absorbed into the binary photomask  100 . As a result, deformation of the binary photomask  100  may be suppressed, thus substantially preventing positions or shapes of the overlay patterns in the binary photomask  100  from being changed. In particular, the overlay patterns are generally disposed in the frame region. Thus, if the frame region  120  illustrated in  FIGS. 4 and 5  is employed in photomasks, an overlay accuracy may be improved because deformation of the overlay patterns in the photomasks may be suppressed. 
       FIGS. 6, 7 and 8  are plan views illustrating the light-blocking region  116  and the frame region  120  included in the binary photomask  100  shown in  FIG. 1  according to various embodiments. As illustrated in  FIG. 6 , each of second light-blocking patterns  138  disposed in the light-blocking region  116  and each of frame light-blocking patterns  148  disposed in the frame region  120  may have a circular closed loop shape (i.e., an annular shape). That is, each of the second light-blocking patterns  138  and the frame light-blocking patterns  148  may have an opening that penetrates a central portion thereof. Accordingly, portions of the top surface ( 102   a  of  FIG. 3 ) of the light transmission substrate ( 102  of  FIG. 3 ) may be exposed by the openings of the second light-blocking patterns  138  and the openings of the frame light-blocking patterns  148 . An exposed portion of the light transmission substrate  102  between the second light-blocking patterns  138  may be a first exposure region  139 - 1 , and portions of the light transmission substrate  102  exposed by the openings of the second light-blocking patterns  138  may be second exposure regions  139 - 2 . Similarly, an exposed portion of the light transmission substrate  102  between the frame light-blocking patterns  148  may be a first frame exposure region  149 - 1 , and portions of the light transmission substrate  102  exposed by the openings of the frame light-blocking patterns  148  may be second frame exposure regions  149 - 2 . 
     As illustrated in  FIG. 7 , each of second light-blocking patterns  158  disposed in the light-blocking region  116  and each of frame light-blocking patterns  168  disposed in the frame region  120  may have a rectangular shape. An exposed portion of the light transmission substrate  102  between the second light-blocking patterns  158  may be a first exposure region  159 , and an exposed portion of the light transmission substrate  102  between the frame light-blocking patterns  168  may be a first frame exposure region  169 . 
     As illustrated in  FIG. 8 , each of second light-blocking patterns  178  disposed in the light-blocking region  116  and each of frame light-blocking patterns  188  disposed in the frame region  120  may have a circular shape. In order to minimize a planar area of a space between the second light-blocking patterns  178 , the second light-blocking patterns  178  may be arrayed on the first surface  102   a  of the light transmission substrate  102  such that each of the second light-blocking patterns  178  is disposed in a space surrounded by six of the second light-blocking patterns  178 . Similarly, the frame light-blocking patterns  188  may be arrayed on the first surface  102   a  of the light transmission substrate  102  such that each of the frame light-blocking patterns  188  is disposed in a space surrounded by six of the frame light-blocking patterns  188 . That is, the second light-blocking patterns  178  and the frame light-blocking patterns  188  may be located at central points and vertices of a plurality of hexagons constituting a honeycomb structure, as illustrated in  FIG. 8 . An exposed portion of the light transmission substrate  102  between the second light-blocking patterns  178  may be a first exposure region  179 , and another exposed portion of the light transmission substrate  102  between the frame light-blocking patterns  188  may be a first frame exposure region  189 . In some embodiments, each of the second light-blocking patterns  178  and the frame light-blocking patterns  188  may have a hexagonal shape. The second light-blocking patterns  178  and the frame light-blocking patterns  188  having the hexagonal shape may be disposed in the same array structure as illustrated in  FIG. 8 . 
       FIG. 9  is a plan view illustrating a phase shift photomask  300  according to an embodiment. In  FIG. 9 , a configuration for relieving thermal stress of the phase shift photomask  300  according to the present embodiment is not illustrated in order to reduce the complexity of the drawing. The configuration for relieving the thermal stress of the phase shift photomask  300  according to the present embodiment will be described in detail with reference to  FIGS. 10 to 13 . In addition, various configurations for relieving thermal stress of photomasks according to other embodiments will be described in detail with reference to  FIGS. 14 to 16 . 
     Referring to  FIG. 9 , the phase shift photomask  300  may have a transfer region  310  and a frame region  320  surrounding the transfer region  310 . The transfer region  310  may correspond to a region in which patterns configured to be transferred onto a wafer are disposed. The frame region  320  may correspond to a marginal region which is provided to prevent process errors that are due to double exposures between two adjacent shot areas (e.g., two adjacent chip areas) defined in an exposure step. A plurality of transfer patterns  312  may be disposed in the transfer region  310 . The plurality of transfer patterns  312  may be two-dimensionally arrayed in rows and columns and spaced apart from each other. In the present embodiment, the plurality of transfer patterns  312  may have a uniform size and may be uniformly spaced apart from each other. However, in some embodiments, sizes of the plurality of transfer patterns  312  may be nonuniform and/or distances between the plurality of transfer patterns  312  may be nonuniform. In either embodiment, the configuration for relieving the thermal stress of the phase shift photomask  300  according to the present embodiment may be equally applicable. 
     As illustrated in  FIG. 9 , each of the transfer patterns  312  may be a hole-shaped pattern. However, the type of the transfer patterns  312  illustrated in  FIG. 9  is merely exemplary. For example, the transfer patterns  312  can be line patterns spaced apart from each other instead of hole-shaped patterns. Although  FIG. 9  illustrates an example in which each of the transfer patterns  312  has a rectangular shape, embodiments are not limited thereto. In some embodiments, the transfer patterns  312  may have non-rectangular shapes. Each of the transfer patterns  312  may correspond to a light-transmitting region  314  which is comprised of a portion of a light transmission substrate exposed by an opening in a phase shift region  316 . That is, the transfer region  310  may include the light-transmitting regions  314  corresponding to the transfer patterns  312  and the phase shift region  316  surrounding the light-transmitting regions  314  in a plan view. A configuration of the phase shift region  316  will be described more fully with reference to  FIGS. 10 and 11 , which illustrate in detail a portion  350  of the transfer region  310  included in the phase shift photomask  300 . A light-blocking pattern such as a chromium (Cr) pattern may be disposed in the frame region  320 . Thus, the frame region  320  may substantially block light during an exposure step. 
     The transfer patterns  312  disposed in the transfer region  310  may be transferred onto a wafer by an exposure step. In particular, the transfer patterns  312  corresponding to the light-transmitting regions  314  may be transferred onto a positive tone resist layer formed on the wafer. Specifically, if the exposure step is performed with the phase shift photomask  300 , portions of the positive tone resist layer that correspond to the transfer patterns  312  may be exposed to light passing through the transfer patterns  312  disposed in the transfer region  310  of the phase shift photomask  300 . Light passing through the phase shift region  316  may have an intensity of about 5% to 8% of the intensity of the light passing through the transfer patterns  312 , and the light passing through the phase shift region  316  may irradiate a portion of the positive tone resist layer that corresponds to the phase shift region  316 . The light passing through the phase shift region  316  may have a phase difference of about 180 degrees, as compared with the light passing through the transfer patterns  312 . As a result, only the portions of the positive tone resist layer that correspond to the transfer patterns  312  may be chemically changed and may be selectively dissolved by a developer. 
       FIG. 10  is an enlarged view illustrating a portion  350  of the transfer region  310  of the phase shift photomask  300  shown in  FIG. 9 , and  FIG. 11  is a cross-sectional view taken along a line III-III′ of  FIG. 10 . In  FIGS. 10 and 11 , the same reference numerals as used in  FIG. 9  denote the same elements. Referring to  FIGS. 9, 10 and 11 , the transfer patterns  312  may correspond to the light-transmitting regions  314 , which are comprised of portions of a light transmission substrate  302  that are exposed in the transfer region  310 . The light-transmitting regions  314  may be surrounded by the phase shift region  316 . A top surface  302   a  of the light transmission substrate  302  in each light-transmitting region  314  may be exposed. Thus, during the exposure step, the light irradiating a bottom surface  302   b  of the light transmission substrate  302  may penetrate the light transmission substrate  302  in the light-transmitting region  314  to reach a wafer through the top surface  302   a  of the light transmission substrate  302  and a lens module of an exposure system. 
     The phase shift region  316  surrounding the light-transmitting regions  314  may include a plurality of first phase shift regions  316   a  and a second phase shift region  316   b . Each of the first phase shift regions  316   a  may have a uniform width W 4  along a perimeter of a corresponding one of the light-transmitting regions  314  and surround the corresponding light-transmitting region  314 . In each of the first phase shift regions  316   a , a first phase shift pattern  317  may be disposed on the top surface  302   a  of the light transmission substrate  302 . The second phase shift region  316   b  may correspond to a region between the first phase shift regions  316   a  in the phase shift region  316 . That is, the transfer region  310  may include the light-transmitting regions  314  corresponding to the transfer patterns  312 , the first phase shift regions  316   a  surrounding the light-transmitting regions  314 , and the second phase shift region  316   b  surrounding the first phase shift regions  316   a.    
     A plurality of second phase shift patterns  318  may be two-dimensionally arrayed along rows and columns in the second phase shift region  316   b  and spaced apart from each other in a plan view. The top surface  302   a  of the light transmission substrate  302  may be exposed between the second phase shift patterns  318 . In the present embodiment, the exposed portion of the light transmission substrate  302  between the second phase shift patterns  318  may be a first exposure region  319 - 1 . A distance D 6  between the second phase shift patterns  318  arrayed in each row may be substantially equal to a distance D 7  between the second phase shift patterns  318  arrayed in each column. The second phase shift patterns  318  may include second phase shift patterns  318 ′ disposed directly adjacent to each of the first phase shift patterns  317 . A distance D 8  between the first phase shift pattern  317  and any one of the second phase shift patterns  318 ′ may be substantially equal to the distance D 6  between the second phase shift patterns  318  arrayed in each row and the distance D 7  between the second phase shift patterns  318  arrayed in each column. The second phase shift patterns  318  may be located at cross points of the rows and the columns. That is, the second phase shift patterns  318  may be two-dimensionally arrayed in a matrix form. 
     Each of the second phase shift patterns  318  may have a rectangular closed loop shape. That is, each of the second phase shift patterns  318  may have an opening that penetrates a central portion thereof. Accordingly, portions of the top surface  302   a  of the light transmission substrate  302  may be exposed by the openings of the second phase shift patterns  318 . In the present embodiment, the portions of the light transmission substrate  302  exposed by the openings of the second phase shift patterns  318  may be second exposure regions  319 - 2 . Thus, the phase shift region  316  in the transfer region  310  may include light-blocking regions covered with the first and second phase shift patterns  317  and  318  and light-transmitting regions comprised of the first and second exposure regions  319 - 1  and  319 - 2 . 
     If an exposure step is performed with an exposure system in which the phase shift photomask  300  is loaded, light  401  irradiating the bottom surface  302   b  of the light transmission substrate  302  may penetrate the light transmission substrate  302  and may exit out of the light transmission substrate  302  through the top surface  302   a  of the light transmission substrate  302 . The light  401  exiting out of the light transmission substrate  302  may irradiate a positive tone resist layer formed on a wafer through a lens module of the exposure system. Since the light  401  penetrating the light-transmitting regions  314  irradiates the positive tone resist layer formed on the wafer, the transfer patterns  312  may be transferred onto the positive tone resist layer. The light passing through the first and second phase shift patterns  317  and  318  may have an intensity of about 5% to 8% of the intensity of the light passing through the transfer patterns  312 , and the light passing through the first and second phase shift patterns  317  and  318  may have a phase difference of about 180 degrees, as compared with the light passing through the transfer patterns  312 . 
     Meanwhile, most of the light  401  irradiating the first and second exposure regions  319 - 1  and  319 - 2  in the phase shift region  316  may penetrate the light transmission substrate  302  and may exit out of the light transmission substrate  302  through the top surface  302   a  of the light transmission substrate  302 . However, the first and second exposure regions  319 - 1  and  319 - 2  may have widths that are narrower than a resolution limit of the exposure system. Thus, undesired patterns corresponding to the first and second exposure regions  319 - 1  and  319 - 2  may not be transferred to the positive tone resist layer formed on the wafer, and the light  401  exiting out of the first and second exposure regions  319 - 1  and  319 - 2  may not influence the transfer of the transfer patterns  312  corresponding to the light-transmitting regions  314 . 
     According to the above embodiment, an amount of the light absorbed into the phase shift photomask  300  may be reduced by at least an amount of the light penetrating the light transmission substrate  302  in the first and second exposure regions  319 - 1  and  319 - 2 , as compared to a case in which an entire portion of the phase shift region  316  is fully covered with a phase shift layer. This may reduce thermal stress of the phase shift photomask  300  that is due to heat generated by the light absorbed into the phase shift photomask  300  during an exposure step. As a result, deformation of the phase shift photomask  300  may be suppressed, thus substantially preventing positions or shapes of overlay patterns in the phase shift photomask  300  from being changed. 
       FIG. 12  is an enlarged view illustrating a portion  360  of the frame region  320  of the phase shift photomask  300  shown in  FIG. 9 , and  FIG. 13  is a cross-sectional view taken along a line IV-IV′ of  FIG. 12 . In  FIGS. 12 and 13 , the same reference numerals as used in  FIG. 9  denote the same elements. Referring to  FIGS. 9, 12 and 13 , a plurality of frame light-blocking patterns  328  may be two-dimensionally arrayed on the top surface  302   a  of the light transmission substrate  302  and spaced apart from each other. That is, the plurality of frame light-blocking patterns  328  may be disposed in rows and columns in a plan view. Each of the frame light-blocking patterns  328  may include a phase shift pattern  328 - 1  and a light-blocking pattern  328 - 2  which are sequentially stacked on the top surface  302   a  of the light transmission substrate  302 . The top surface  302   a  of the light transmission substrate  302  may be exposed between the frame light-blocking patterns  328 . In the present embodiment, the exposed portion of the light transmission substrate  302  between the frame light blocking patterns  328  may be a first frame exposure region  329 - 1 . A distance D 9  between the frame light-blocking patterns  328  arrayed in each row may be substantially equal to a distance D 10  between the frame light-blocking patterns  328  arrayed in each column. These distances D 9  and D 10  may be substantially equal to the distance D 6  between the second phase shift patterns  318  arrayed in each row and the distance D 7  between the second light-blocking patterns  318  arrayed in each column which are described with reference to  FIGS. 10 and 11 . The frame light-blocking patterns  328  may be located at cross points of the rows and the columns. That is, the frame light-blocking patterns  328  may be two-dimensionally arrayed in a matrix form. 
     Each of the frame light-blocking patterns  328  may have a rectangular closed loop shape. That is, each of the frame light-blocking patterns  328  may have an opening that penetrates a central portion thereof. Accordingly, portions of the top surface  302   a  of the light transmission substrate  302  may be exposed by the openings of the frame light-blocking patterns  328 . In the present embodiment, the portions of the light transmission substrate  302  exposed by the openings of the frame light-blocking patterns  328  may be second frame exposure regions  329 - 2 . Thus, the frame region  320  may include light-blocking regions covered with the frame light-blocking patterns  328  and light-transmitting regions comprised of the first and second frame exposure regions  329 - 1  and  329 - 2 . 
     If an exposure step is performed with the exposure system in which the phase shift photomask  300  is loaded, most of light  402  irradiating the frame light-blocking patterns  328  in the frame region  320  may be blocked from reaching the positive tone resist layer formed on the wafer. Meanwhile, the light  402  irradiating the first and second frame exposure regions  329 - 1  and  329 - 2  in the frame region  320  may penetrate the light transmission substrate  302  and may exit out of the light transmission substrate  302  through the top surface  302   a  of the light transmission substrate  302 . The light  402  exiting out of the first and second frame exposure regions  329 - 1  and  329 - 2  may or may not irradiate the positive tone resist layer formed on the wafer. 
     For example, if the first and second frame exposure regions  329 - 1  and  329 - 2  are designed to have widths that are narrower than a certain value, such that the light exiting out of the first and second frame exposure regions  329 - 1  and  329 - 2  has a low intensity, the light exiting out of the first and second frame exposure regions  329 - 1  and  329 - 2  does not change a chemical structure of exposed portions of the positive tone resist layer. Accordingly, the exposed portions of the positive tone resist layer may not be dissolved by a developer even though the light  402  exiting out of the first and second frame exposure regions  329 - 1  and  329 - 2  irradiates the positive tone resist layer. 
     Alternatively, if the first and second frame exposure regions  329 - 1  and  329 - 2  may have a predetermined width, so that the light exiting out of the first and second frame exposure regions  329 - 1  and  329 - 2  may be diffracted, the diffracted light may not irradiate the positive tone resist layer. In either embodiment, undesired patterns corresponding to the first and second exposure regions  329 - 1  and  329 - 2  may not be transferred onto the positive tone resist layer formed on the wafer. 
     As compared to a case in which an entire portion of the frame region  320  is fully covered with a light-blocking layer, in this embodiment, an amount of the light absorbed into the phase shift photomask  300  may be reduced by at least an amount of the light penetrating the light transmission substrate  302  in the first and second frame exposure regions  329 - 1  and  329 - 2 . This may reduce thermal stress of the phase shift photomask  300  that is due to heat generated by the light absorbed into the phase shift photomask  300 . As a result, deformation of the phase shift photomask  300  may be suppressed, thus substantially preventing positions or shapes of the overlay patterns in the phase shift photomask  300  from being changed. In particular, the overlay patterns are generally disposed in the frame region. Thus, if the frame region  320  illustrated in  FIGS. 12 and 13  is employed in photomasks, an overlay accuracy may be improved because deformation of the overlay patterns in the photomasks may be suppressed. 
       FIGS. 14, 15 and 16  are plan views illustrating the phase shift region  316  and the frame region  320  included in the phase shift photomask  300  shown in  FIG. 9  according to various embodiments. As illustrated in  FIG. 14 , each of second phase shift patterns  338  disposed in the phase shift region  316  and each of frame light-blocking patterns  348  disposed in the frame region  320  may have a circular closed loop shape (i.e., an annular shape). That is, each of the second phase shift patterns  338  and the frame light-blocking patterns  348  may have an opening that penetrates a central portion thereof. Accordingly, portions of the top surface ( 302   a  of  FIG. 11 ) of the light transmission substrate ( 302  of  FIG. 11 ) may be exposed by the openings of the second phase shift patterns  338  and the frame light-blocking patterns  348 . An exposed portion of the light transmission substrate  302  between the second phase shift patterns  338  may be a first exposure region  339 - 1 , and portions of the light transmission substrate  302  exposed by the openings of the second phase shift patterns  338  may be second exposure regions  339 - 2 . Similarly, an exposed portion of the light transmission substrate  302  between the frame light-blocking patterns  348  may be a first frame exposure region  349 - 1 , and portions of the light transmission substrate  302  exposed by the openings of the frame light-blocking patterns  348  may be second frame exposure regions  349 - 2 . 
     As illustrated in  FIG. 15 , each of second phase shift patterns  358  disposed in the phase shift region  316  and each of frame light-blocking patterns  368  disposed in the frame region  320  may have a rectangular shape. An exposed portion of the light transmission substrate  302  between the second phase shift patterns  358  may be a first exposure region  359 , and an exposed portion of the light transmission substrate  302  between the frame light-blocking patterns  368  may be a first frame exposure region  369 . 
     As illustrated in  FIG. 16 , each of second phase shift patterns  378  disposed in the phase shift region  316  and each of frame light-blocking patterns  388  disposed in the frame region  320  may have a circular shape. In order to minimize a planar area of a space between the second phase shift patterns  378 , the second phase shift patterns  378  may be arrayed on the first surface  302   a  of the light transmission substrate  302  such that each of the second phase shift patterns  378  is disposed in a space surrounded by six of the second phase shift patterns  378 . Similarly, the frame light-blocking patterns  388  may be arrayed on the first surface  302   a  of the light transmission substrate  302  such that each of the frame light-blocking patterns  388  is disposed in a space surrounded by six of the frame light-blocking patterns  388 . That is, the second phase shift patterns  378  and the frame light-blocking patterns  388  may be located at central points and vertices of a plurality of hexagons constituting a honeycomb structure, as illustrated in  FIG. 16 . An exposed portion of the light transmission substrate  302  between the second phase shift patterns  378  may be a first exposure region  379 , and another exposed portion of the light transmission substrate  302  between the frame light-blocking patterns  388  may be a first frame exposure region  389 . In some embodiments, each of the second phase shift patterns  378  and the frame light-blocking patterns  388  may have a hexagonal shape. The second phase shift patterns  378  and the frame light-blocking patterns  388  having the hexagonal shape may be disposed in the same array structure as illustrated in  FIG. 16 . 
       FIG. 17  is a plan view illustrating a photomask  500  according to an embodiment. In  FIG. 17 , a configuration of the photomask  500  for preventing a lens (or lens module) of an exposure system from being heated is not illustrated in order to reduce the complexity of the drawing. The configuration of the photomask  500  for preventing the lens (or lens module) of the exposure system from being heated will be described in detail with reference to  FIGS. 18 and 19 . 
     Referring to  FIG. 17 , the photomask  500  may have a transfer region  510  and a frame region  520  surrounding the transfer region  510 . The transfer region  510  may correspond to a region in which patterns configured to be transferred onto a wafer are disposed. The frame region  520  may correspond to a marginal region which is provided to prevent process errors that are due to double exposures between two adjacent shot areas (e.g., two adjacent chip areas) defined in an exposure step. A plurality of transfer patterns  514  may be disposed in the transfer region  510 . The plurality of transfer patterns  514  may be two-dimensionally arrayed in rows and columns and spaced apart from each other. In the present embodiment, the plurality of transfer patterns  514  may have a uniform size and may be uniformly spaced apart from each other. However, in some embodiments, sizes of the plurality of transfer patterns  514  may be nonuniform and/or distances between the plurality of transfer patterns  514  may be nonuniform. In either embodiment, the configuration of the photomask  500  for preventing the lens (or lens module) of the exposure system from being heated may be equally applicable. 
     As illustrated in  FIG. 17 , each of the transfer patterns  514  may be a hole-shaped pattern. However, the type of the transfer patterns  514  illustrated in  FIG. 17  is merely exemplary. For example, the transfer patterns  514  can be line patterns spaced apart from each other instead of hole-shaped patterns. Although  FIG. 17  illustrates an embodiment in which each of the transfer patterns  514  has a rectangular shape, embodiments are not limited thereto. In some embodiments, the transfer patterns  514  may have non-rectangular shapes. Each of the transfer patterns  514  may correspond to a light-blocking pattern or a phase shift pattern. That is, the transfer region  510  may include the transfer patterns  514  such as light-blocking patterns or phase shift patterns and a light-transmitting region  516  surrounding the transfer patterns  514 . The light-transmitting region  516  will be described more fully with reference to  FIGS. 18 and 19 , which illustrate in detail a portion  550  of the transfer region  510  included in the photomask  500 . A light-blocking pattern such as a chromium (Cr) pattern may be disposed in the frame region  520 . Thus, the frame region  520  may substantially block light during an exposure step. 
     The transfer patterns  514  disposed in the transfer region  510  may be transferred onto a wafer by an exposure step. In particular, the transfer patterns  514  corresponding to light-blocking patterns or phase shift patterns may be transferred onto a negative tone resist layer formed on the wafer. Specifically, if the exposure step is performed with the photomask  500 , no light irradiates portions of the negative tone resist layer that correspond to the transfer patterns  514 . In contrast, a portion of the negative tone resist layer, which corresponds to the light-transmitting region  516 , may be exposed to light passing through the light-transmitting region  516 . As a result of the exposure step, the exposed portions of the negative tone resist layer may be cross-linked and polymerized to have a chemical structure that is not dissolved by a developer. Thus, if the negative tone resist layer is developed after the exposure step is performed, only the non-exposed portions of the negative tone resist layer that correspond to the transfer patterns  514  may be selectively removed. 
       FIG. 18  is an enlarged view illustrating the portion  550  of the transfer region  510  of the photomask  500  shown in  FIG. 17 , and  FIG. 19  is a cross-sectional view taken along a line V-V′ of  FIG. 18 . In  FIGS. 18 and 19 , the same reference numerals as used in  FIG. 17  denote the same elements. Referring to  FIGS. 17, 18 and 19 , the transfer patterns  514  may be disposed on a top surface  502   a  of a light transmission substrate  502  in the transfer region  510 . The transfer patterns  514  may be surrounded by the light-transmitting region  516 . The light-transmitting region  516  may correspond to a region in which the top surface  502   a  of the light transmission substrate  502  is exposed. Thus, during the exposure step, the light irradiating a bottom surface  502   b  of the light transmission substrate  502  may penetrate the light transmission substrate  502  in the light-transmitting region  516  to reach the wafer through the top surface  502   a  of the light transmission substrate  502  and a lens module of an exposure system. 
     The light-transmitting region  516  surrounding the transfer patterns  514  may include a first light-transmitting region  516   a  and a second light-transmitting region  516   b . The first light-transmitting region  516   a  may have a uniform width W 3  along a perimeter of the transfer pattern  514  and surround the transfer pattern  514 . In the first light-transmitting region  516   a , an entire portion of the top surface  502   a  of the light transmission substrate  502  may be fully exposed. The second light-transmitting region  516   b  may surround the first light-transmitting region  516   a . Thus, the transfer region  510  may include the transfer patterns  514 , the first light-transmitting regions  516   a  surrounding the transfer patterns  514 , and the second light-transmitting regions  516   b  surrounding the first light-transmitting regions  516   a.    
     A plurality of light-blocking patterns  518  may be two-dimensionally arrayed along rows and columns in the second light-transmitting region  516   b  and spaced apart from each other in a plan view. The top surface  502   a  of the light transmission substrate  502  may be exposed between the light-blocking patterns  518  in the second light-transmitting region  516   b . A distance D 11  between the light-blocking patterns  518  arrayed in each row may be substantially equal to a distance D 12  between the light-blocking patterns  518  arrayed in each column. The light-blocking patterns  518  may be spaced apart from the first light-transmitting region  516   a  by a distance D 13 , and the distance D 13  may be less than the distance D 11  between the light-blocking patterns  518  arrayed in each row and the distance D 12  between the light-blocking patterns  518  arrayed in each column. In some embodiments, the distance D 13  between the first light-transmitting region  516   a  and the light-blocking patterns  518  may be about half the distance D 11  between the light-blocking patterns  518  arrayed in each row or about half the distance D 12  between the light-blocking patterns  518  arrayed in each column. The light-blocking patterns  518  may be two-dimensionally arrayed in the second light-transmitting region  516   b  to have a matrix form in a plan view. 
     Each of the light-blocking patterns  518  may have a rectangular closed loop shape. That is, each of the light-blocking patterns  518  may have an opening that penetrates a central portion thereof. A width of each side of the light-blocking patterns  518  may be narrower than a resolution limit of the exposure system. Thus, the light-blocking patterns  518  may not be transferred onto the wafer. Portions of the top surface  502   a  of the light transmission substrate  502  may be exposed by the openings of the light-blocking patterns  518 . Thus, the light-transmitting region  516  may include the plurality of first light-transmitting regions  516   a  and the second light-transmitting regions  516   b . The second light-transmitting region  516   b  may include light-transmitting regions exposed by the openings of the light-blocking patterns  518 , light-blocking regions covered by the light-blocking patterns  518 , and a light-transmitting region between the light-blocking patterns  518 . 
       FIG. 20  is a schematic view illustrating an exposure system  600  in which the photomask  500  of  FIG. 17  is loaded. Referring to  FIG. 20 , the photomask  500  may have the transfer patterns  514  corresponding to light-blocking patterns or phase shift patterns and the light-transmitting region  516  surrounding the transfer patterns  514 , as described with reference to  FIGS. 17, 18 and 19 . The light-blocking patterns  518  having a smaller size than a resolution limit of the exposure system  600  may be disposed in the light-transmitting region  516 . The photomask  500  having the aforementioned configuration may be loaded into the exposure system  600 , and light  601  generated from a light source (not shown) may irradiate the photomask  500 . The light  601  irradiating the photomask  500  may pass through the photomask  500  or may be blocked by the photomask  500  according to structures of patterns disposed on the photomask  500 . Light passing through the photomask  500  may irradiate a lens  610  (or lens module) of the exposure system  600 , as indicated by arrows  602 . Light passing through the lens  610  may travel along appropriate optical paths and may reach a negative tone resist layer  622  formed on a wafer  620 , as indicated by arrows  603 . In general, this exposure step may be repeatedly performed to expose a plurality of chip regions included in the wafer  620 . Thus, the light may be repeatedly irradiating the lens  610  while the exposure step is repeatedly performed to expose the plurality of chip regions included in the wafer  620 . 
     Since the negative tone resist layer  622  is used as a resist layer, the light-transmitting region  516  of the photomask  500  may have a relatively large area. During a single exposure step, an amount of the light irradiating the lens  610  may increase as compared with the embodiments illustrated in  FIGS. 1 to 16 . As a result, thermal stress of the lens  610  may occur when it is heated, and thus the lens  610  may be deformed. As a result, an aberration of the lens  610  may be changed to degrade the quality of pattern images transferred onto the wafer  620 . However, according to the present embodiments, the plurality of light-blocking patterns  518  each having a width smaller than the resolution limit of the exposure system  600  may be disposed in the light-transmitting region  516 . Thus, an amount of the light irradiating the lens  610  may be reduced without transfer of patterns corresponding to the plurality of light-blocking patterns  518 . Accordingly, the thermal stress of the lens  610  may be alleviated. 
     The embodiments of the present disclosure have been disclosed above for illustrative purposes. Those of ordinary skill in the art will appreciate that various modifications, additions, and substitutions are possible, without departing from the scope and spirit of the present disclosure as claimed below.