Patent Publication Number: US-2018053673-A1

Title: Stage leveling device for high performance mask aligner

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of and priority to Korean Patent Application Serial No. 10-2016-0103573, filed on Aug. 16, 2016, entitled (translation), “STAGE DEVICE FOR HIGH PERFORMANCE MASK ALIGNER,” which is hereby incorporated by reference in its entirety into this application. 
     BACKGROUND 
     Field of the Invention 
     Embodiments of the invention relate to a stage leveling device for preventing leveling misalignment by firmly fixing a leveling-completed wafer stage and improving performance by increasing a maximum capacity (weight) of a wafer to be leveled. 
     Description of the Related Art 
     Recently, low cost and high performance highly integrated semiconductor devices have been required to secure market competitiveness. For high integration of semiconductor devices, scaling-down processes including a process of thinning and shortening gate oxide films and channels of transistor devices, and the like are involved and, accordingly, technologies and systems for manufacturing semiconductors are being developed into various types. 
     Photolithography, which is one semiconductor manufacturing process, includes: a wafer washing process of removing impurities from a surface of a wafer; a surface treatment process of surface-treating the wafer so that a photosensitive film is satisfactorily adhered to the wafer; a photosensitive film coating process of uniformly coating the photosensitive film on the wafer to a desired thickness; alignment and light exposure processes of aligning a mask or a reticle (hereinafter, referred to as a mask) on the wafer with the photosensitive film coated thereon and exposing the mask to light so as to form circuit patterns of the mask on the wafer; and a developing process of removing the photosensitive film deformed by light exposure using a washing solution. In particular, a core semiconductor device for the alignment/light exposure processes is called a mask aligner. 
     A general mask aligner includes a mask stage on which a mask with predetermined patterns is mounted, a wafer stage on which a wafer etched according to the patterns of the mask is mounted, and a projection optical system installed therebetween. 
     A mask with circuit patterns is mounted on the mask stage, a vacuum-sucked wafer is mounted on the wafer stage, and then a position to be exposed to light is selected by horizontal and vertical movements thereof. 
     As related art, Korean Patent Registration No. 1360954 (title of the invention: Stage Leveling Device for Mask Aligner having Improved Leveling Performance), which was previously registered by the applicant of the present application, is disclosed. 
     According to the above related art, leveling is performed by freely moving a wafer stage upward and downward using a plurality of leveling air shock absorbers, centering is maintained by supporting the center of the wafer stage at one point using a single centering module, and then the leveling is maintained such that a stage fixing cylinder fixes side surfaces of a leveling rod by applying pressure thereto. 
     Even though such leveling is precisely performed, the wafer is no longer level when fine movement of a wafer stage occurs after completion of leveling. 
     According to the above related art, a leveling-completed wafer stage is fixed by partially pressing side surfaces of the leveling rod and thus the wafer stage is not level by deviations (errors) occurring while the leveling rod moves upward and downward and leftward and rightward, and slip occurs due to a weak fixing strength of the leveling rod and thus the weight of the wafer (sample) that can be leveled to the maximum extent is decreased. 
     In addition, a general air cylinder is used in the air shock absorber of the above related art and thus flexibility according to movement is reduced due to a high initial setting value and fine pressure adjustment is also difficult. 
     In addition, centering performance of the centering module for one-point supporting the center of the wafer stage varies according to processing and assembly degrees and thus a high level of skill and ultraprecision are required in manufacturing processes and, when such requirements are not satisfied, the centering process is not accurately performed. 
     SUMMARY 
     According to at least one embodiment, there is provided is a stage leveling device for performing locking, such that, when a locking ring inserted at an outer side of a leveling rod maintains an inclined state, an inner circumferential surface of the locking ring fixes opposite sides of the leveling rod by applying pressure thereto and for performing unlocking such that, when the locking ring maintains a horizontal state, the inner circumferential surface of the locking ring is spaced apart from the leveling rod. 
     According to another embodiment, there is provided a stage leveling device that, in a process of performing leveling of a wafer in a state in which a locking ring is unlocked by moving a locking control cylinder forward, the locking ring may lock a leveling rod by receiving feedback of a pressure value of a leveling air shock absorber including a low friction cylinder and moving the locking control cylinder backward at an accurate time at which leveling has been completed, based on the pressure value. 
     According to another embodiment, there is provided a stage leveling device characterized in that three centering modules installed at a circumference of a wafer stage may precisely maintain centering by supporting an outer circumferential surface of the wafer stage at three points. 
     According to at least one embodiment, there is provided a stage leveling device for a high-performance mask aligner, having enhanced leveling maintenance, in which the stage leveling device includes a plurality of guide blocks spaced apart from each other on an upper end of a base plate and each provided, at a center thereof, with a rod guide hole penetrating upward and downward. The stage level device further includes a plurality of leveling air shock absorbers attached to a bottom surface of the base plate and each including a piston rod penetrating the base plate and inserted into a rod guide hole, a plurality of leveling rods each inserted into the rod guide hole to be mounted on an upper end of the piston rod and each including a ground pin at an upper portion thereof to be exposed to the outside, and a wafer stage mounted on upper ends of the leveling rods. Further, the stage leveling device includes a plurality of guide bolts each having a lower side coupled to the guide block in an integrated manner and an upper side penetratively coupled to upper and lower portions of the wafer stage, a ring housing fixedly installed on an upper end of the guide block in a state of being penetrated by the piston rod of the leveling air shock absorber, and a locking ring inserted into the ring housing in a state of being penetrated by the piston rod and provided, at one side thereof, with a rotatory support sphere supported by and inserted into a support hole formed in an inner circumferential surface of the ring housing. Additionally, the stage leveling device includes a locking control cylinder inserted into a cylinder groove recessed in an upper end of the guide block and provided, at an upper portion thereof, with a piston rod grounded to a bottom surface of the locking ring. 
     According to another embodiment, there is provided a stage leveling device for a high-performance mask aligner, having enhanced leveling maintenance, in which the stage leveling device includes a plurality of guide blocks spaced apart from each other on an upper end of a base plate and each provided, at a center thereof, with a rod guide hole penetrating upward and downward, a plurality of leveling air shock absorbers attached to a bottom surface of the base plate and each including a piston rod penetrating the base plate and inserted into a rod guide hole, and a plurality of leveling rods each inserted into the rod guide hole to be mounted on an upper end of the piston rod and each including a ground pin at an upper portion thereof to be exposed to the outside. Further, the stage leveling device includes a wafer stage provided with a plurality of supports attached to a bottom surface thereof to be positioned on upper ends of the leveling rods, a plurality of guide bolts each having a lower side coupled to the guide block in an integrated manner and an upper side penetratively coupled to upper and lower portions of each support, and a ring housing fixedly installed on an upper end of the guide block in a state of being penetrated by the piston rod of the leveling air shock absorber. Additionally, the stage leveling device includes a locking ring inserted into the ring housing in a state of being penetrated by the piston rod and provided, at one side thereof, with a rotatory support sphere is supported by and inserted into a support hole formed in an inner circumferential surface of the ring housing, and a locking control cylinder inserted into a cylinder groove recessed in an upper end of the guide block and provided, at an upper portion thereof, with a piston rod grounded to a bottom surface of the locking ring. 
     According to at least one embodiment, the leveling air shock absorber includes a low friction cylinder. 
     According to at least one embodiment, when the locking control cylinder is moved backward, locking is performed such that the locking ring maintains an inclined state while rotating downward about the rotatory support sphere and opposite sides of an inner circumferential surface of the locking ring fixes opposite sides of an outer circumferential surface of the leveling rod by applying pressure thereto, whereas, when the locking control cylinder is moved forward, unlocking is performed such that the locking ring maintains a horizontal state while rotating upward about the rotatory support sphere and the inner circumferential surface of the locking ring is spaced apart from the outer circumferential surface of the leveling rod. 
     According to at least one embodiment, the locking ring is provided with a compression ring inserted into a ring groove formed in a center of the inner circumferential surface, and an interference preventing part is recessed in each of upper and lower edge portions of the inner circumferential surface. 
     According to at least one embodiment, the rotatory support spheres are screw-coupled in an integrated form with intervals therebetween at a coupling protrusion formed at one side of the locking ring, and a portion thereof inserted into the support hole has a spherical shape. 
     According to at least one embodiment, the locking ring is provided with a spring support groove recessed in an upper surface of one side thereof, the ring housing is provided with the spring support groove recessed in an inner surface of an upper side thereof, and the spring support grooves are provided with a spring supported thereby and inserted thereinto to elastically bias the locking ring downward. 
     According to at least one embodiment, the ring housing is provided, on one side thereof, with a stopper screw-coupled to be exposed to the inside, wherein the stopper restricts excessive rotation so that the locking ring maintains a horizontal state while the locking ring rotates upward. 
     According to at least one embodiment, when the locking control cylinder is moved forward, the locking ring unlocks the leveling rod to perform normal leveling of a wafer and, in this process, a programmable logic controller (PLC) receives feedback of a pressure applied to the leveling rod via a pressure sensor installed at the leveling air shock absorber and compares the pressure with a set pressure value and, as a result of determination, the PLC controls the locking ring to lock the leveling rod while moving the locking control cylinder backward at a time at which the leveling is completed. 
     According to at least one embodiment, the guide blocks include three centering modules and to maintain centering of the wafer stage, wherein two centering modules of the three centering modules and are configured such that a fixing centering bar is attached in an integrated manner to an outer surface of the guide block, and a ball plunger coupled to an upper portion of the fixing centering bar closely contacts an outer surface of the support, and wherein the one centering module (of the three centering modules and is configured such that a lower side of a rotatory centering bar, through which an connection pin penetrates, is rotatably installed at an outer surface of the guide block, a ball plunger coupled to an upper portion of the rotatory centering bar closely supports the outer surface of the support, a spring support bolt penetrating the rotatory centering bar is connected to one side of a tensile spring, and the other side of the tensile spring is connected to a spring fixing part in a state of being inserted into a spring inserting part formed in the guide block. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the features and advantages of the invention, as well as others which will become apparent, may be understood in more detail, a more particular description of the invention briefly summarized above may be had by reference to the embodiments thereof which are illustrated in the appended drawings, which form a part of this specification. It is to be noted, however, that the drawings illustrate only various embodiments of the invention and are therefore not to be considered limiting of the invention&#39;s scope as it may include other effective embodiments as well. 
         FIG. 1  is a plan view of a stage leveling device according to an embodiment of the invention. 
         FIG. 2  is a front view of the stage leveling device according to an embodiment of the invention. 
         FIG. 3  is a plan view of a base plate according to an embodiment of the invention. 
         FIG. 4  is a partial enlarged cross-sectional view taken along line A-A of  FIG. 1  according to an embodiment of the invention. 
         FIG. 5  is a front cross-sectional view illustrating installation states of a leveling rod, a ring housing, a locking ring, and a locking control cylinder according to an embodiment of the invention. 
         FIG. 6  is a plan cross-sectional view illustrating installation states of the leveling rod, the ring housing, and the locking ring according to an embodiment of the invention. 
         FIG. 7  illustrates plan and front cross-sectional views of the locking ring according to an embodiment of the invention. 
         FIG. 8  is a front cross-sectional view illustrating a state in which the locking ring is unlocked according to an embodiment of the invention. 
         FIG. 9  is a front cross-sectional view illustrating the locking ring in a locked state according to an embodiment of the invention. 
         FIG. 10  is a front cross-sectional view illustrating an installation state of a stopper according to an embodiment of the invention. 
         FIG. 11  is a front view of a centering module provided with a fixing centering bar according to an embodiment of the invention. 
         FIG. 12  is a front cross-sectional view of a centering module provided with a rotary centering bar according to an embodiment of the invention. 
         FIG. 13  is a plan cross-sectional view of the centering module provided with a rotary centering bar according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Although the following detailed description contains many specific details for purposes of illustration, it is understood that one of ordinary skill in the relevant art will appreciate that many examples, variations, and alterations to the following details are within the scope and spirit of the invention. Accordingly, the exemplary embodiments of the invention described herein are set forth without any loss of generality, and without imposing limitations, relating to the claimed invention. Like numbers refer to like elements throughout. Prime notation, if used, indicates similar elements in alternative embodiments. 
     Referring to the drawings, a plurality of guide blocks  20 , a plurality of leveling air shock absorbers  30 , a plurality of leveling rods  40 , a wafer stage  50 , a plurality of guide bolts  60 , a ring housing  70 , a locking ring  80 , and a locking control cylinder  90  are included as elements of the various embodiments of the invention. 
     According to at least one embodiment, a base plate  10  is movably installed upward and downward by a known vertical moving device and is provided, on an upper end thereof, with a plurality of guide blocks  20  spaced apart from each other by an interval of  120 ° in a circumferential direction, and the guide block  20  is provided, at the center thereof, with a rod guide hole  21  penetrating upward and downward to guide the leveling rod  40  upward and downward. 
     A plurality of leveling air shock absorbers  30  is attached to a bottom surface of the base plate  10  to correspond to the guide blocks  20 , and a piston rod  31  of each leveling air shock absorber  30  passes through a rod path  11  formed in the base plate  10  and is inserted through a lower side of the rod guide hole  21 . 
     According to at least one embodiment, the leveling air shock absorber  30  includes a low friction cylinder that smoothly operates even at a low pressure due to the design of a piston having a low sliding resistance, instead of a general air cylinder, and, accordingly, the leveling air shock absorber  30  has fine pressure adjustment and contact force control effects. 
     According to at least one embodiment, the leveling rod  40  mounted on an upper end of the piston rod  31  is inserted through an upper portion of the rod guide hole  21 , and a ground pin  41  protruding from an upper portion of the leveling rod  40  is exposed to the outside so that a wafer stage  50  is mounted on upper ends of the ground pins  41 . 
     In this regard, a magnet  42  is embedded below the leveling rod  40  and the piston rod  31  of the leveling air shock absorber  30  is closely grounded by a magnetic force of the magnet  42  and, accordingly, the leveling rod  40  and the piston rod  31  mutually move in an integrated form, whereby leveling performance is significantly improved. 
     In the guide bolts  60  inserted into bolt insertion grooves  22  formed at bottom surfaces of the guide blocks  20 , a lower side of each guide bolt  60  penetrates an upper portion of the guide block  20  and is screw-coupled in an integrated manner and an upper side thereof penetrates upper and lower portions of the wafer stage  50  and is screw-coupled with a nut  23  on the upper side. Accordingly, the wafer stage  50  and the guide blocks  20  maintain an integrated form without separation and the guide blocks  20  may smoothly guide downward movement of the wafer stage  50  when leveling is performed. 
     Thus, even though impact is transferred to the wafer stage  50 , the leveling air shock absorber  20  acts as a buffer smoothly absorbing impact. In particular, in at least one embodiment of the invention, leveling air shock absorbers  30  are installed at three points and thus may smoothly perform leveling by most stably supporting the wafer stage  50  at three points. 
     According to at least one embodiment, the wafer stage  50  may be directly seated on the upper ends of the leveling rods  40 . In this case, to prevent the size of the wafer stage  50  from increasing more than as required, the wafer stage  50  is provided, at a bottom surface thereof, with a plurality of supports  52  protruding to the outside, the supports  52  are mounted on the upper ends of the leveling rods  40 , and the guide bolts  60  may be screw-coupled with the nuts  23  closely contacting upper surfaces of the supports  52 , in a state of being penetratively coupled with upper and lower portions of the supports  52 . 
     In addition, after the leveling process is completed, the leveling air shock absorbers  30  are locked by simultaneously fixing opposite sides of the leveling rods  40  by applying pressure thereto, thereby fixing the wafer stage  50  without movement, which is a core technology of various embodiments of the invention. 
     To achieve such configuration, the ring housing  70  with an opening at a bottom portion thereof is fixedly installed on an upper end of the guide block  20 , and the piston rod  31  of the leveling air shock absorber  30  penetrates the ring housing  70  to be exposed to the outside. 
     According to at least one embodiment, the locking ring  80  in a state of penetrating the piston rod  31  is inserted inside the ring housing  70 , and a rotatory support sphere  81  formed at one side of the locking ring  80  is supported by and inserted into a support hole  71  formed at an inner circumferential surface of the ring housing  70  and, accordingly, the locking ring  80  may rotate upward and downward about the rotatory support sphere  81 . 
     In this regard, the locking ring  80  has a disk shape and is provided with a coupling protrusion  82  at one side thereof, and the rotatory support spheres  81  respectively supported by and inserted into the support holes  71  formed in an inner surface of the ring housing  70  are screw-coupled in an integrated form with intervals therebetween at the coupling protrusion  82 . In addition, a portion of the rotatory support sphere  81  which is inserted into the support hole  71  has a spherical shape and thus a contact area between the rotatory support sphere  81  and the support hole  71  is small and, accordingly, friction therebetween is decreased during rotation, thereby enabling smooth operation. 
     According to at least one embodiment, the guide block  20  is provided, at an upper end thereof, with a cylinder groove  24  recessed adjacent to the rod guide hole  21 , the locking control cylinder  90  for selectively controlling locking/unlocking of the locking ring  80  is inserted into the cylinder groove  24 , and a piston rod  91  disposed at the locking control cylinder  90  is exposed to the outside and grounded to a bottom surface of the locking ring  80  so that the locking ring  80  can rotate upward and downward about the rotatory support sphere  81  according to an upward or downward linear movement of the locking control cylinder  90 . 
     According to at least one embodiment, a leveling process of maintaining a degree of parallelization with high accuracy between a mask and a wafer mounted on a wafer chuck of the wafer stage  50  is performed and then a leveling process for preventing the degree of parallelization between the mask and the wafer from being decreased by temporarily fixing the leveling rod  40  that has supported the wafer stage  50  so as to freely move upward and downward may be maintained. 
     In this regard, as illustrated in  FIG. 8 , the locking ring  80  maintains a horizontal state when the leveling process is performed and thus a gap is formed between an inner circumferential surface  83  of the locking ring  80  and an outer circumferential surface of the leveling rod  40  and, accordingly, the leveling rod  40  freely moves upward and downward, which enables normal leveling and, after the leveling process is completed, the leveling rod  40  is fixed by applying pressure thereto. 
     In detail, as illustrated in  FIG. 9 , the leveling rod  40  is fixed such that, when the locking control cylinder  90  is moved backward, the locking ring  80  maintains an inclined state while rotating downward about the rotatory support sphere  81  and opposite sides of the inner circumferential surface  83  of the locking ring  80  fixes opposite sides of the outer circumferential surface of the leveling rod  40  by applying pressure thereto, thereby maintaining locking. 
     Thus, the locking ring  80  fixes the opposite sides of the outer circumferential surface of the leveling rod  40  by applying pressure thereto and fixes the wafer stage at the same time without fine movement, tilt, and torsion of the locking rod  40  and, accordingly, precise leveling is maintained and working quality is significantly improved, during light exposure. 
     In addition, the fixed state of the leveling rod  40  may be unlocked such that, when the locking control cylinder  90  is moved forward, the locking ring maintains its position in a horizontal state while rotating upward about the rotatory support sphere  81  and the inner circumferential surface  83  of the locking ring  80  is spaced apart from the outer circumferential surface of the leveling rod  40  to thus form a gap therebetween, thereby automatically performing unlocking. 
     According to at least one embodiment, the locking ring  80  is provided with a ring groove  84  recessed in the center of the inner circumferential surface  83  thereof, and a compression ring  85  inserted into the ring groove  84  further compression-fixes the outer circumferential surface of the leveling rod  40  and, accordingly, the fixing power of the locking ring  80  may be significantly enhanced. In addition, an interference preventing part  86  is recessed in each of upper and lower edge portions of the inner circumferential surface  83  and thus, when the locking ring  80  maintains an inclined state, an interference phenomenon, in which the edge portions are stuck by the outer circumferential surface of the leveling rod  40 , may be effectively prevented by the interference preventing parts  86 . 
     In addition, as illustrated in  FIGS. 7 and 8 , the locking ring  80  is provided with a spring support groove  87  recessed in an upper surface of one side thereof, and the spring support groove  87  is recessed in an inner surface of an upper side of the ring housing  70  to correspond to the spring support groove  87  of the locking ring  80 . A spring  88  is supported by and inserted into the spring support grooves  87  so as to elastically bias the locking ring  80  downward and thus, when the piston rod  91  of the locking control cylinder  90  is moved backward in a downward direction, the spring  88  applies elasticity, thereby further facilitating a downward rotation of the locking ring  80 . 
     In addition, when the locking ring  80  rotates upward, as illustrated in  FIG. 10 , a stopper  100  is screw-coupled to be exposed to the inside at one side of the ring housing  70  so as to prevent the locking ring  80  from escaping from the horizontal state due to excessive rotation. In addition, the stopper  100  restricts excessive rotation of the locking ring  80  that rotates upward, thereby enabling the locking ring  80  to perform unlocking while maintaining an accurate horizontal state. 
     In addition, in the leveling process, the locking of the locking ring  80  may be controlled at a more accurate time by receiving feedback of a pressure applied to the leveling rod  40 . 
     For this configuration, when the locking control cylinder  90  is moved forward, the locking ring  80  unlocks the leveling rod  40 , thereby performing normal leveling of a wafer. In this process, a programmable logic controller (PLC) receives feedback of a pressure applied to the leveling rod  40  via a pressure sensor  110  installed at the leveling air shock absorber  30  and compares the feedback with a set pressure value. As a result of determination, the PLC controls the locking ring  80  to lock the leveling rod  40  while moving the locking control cylinder  90  backward and thus locking of the locking ring  80  is rapidly performed at an accurate time at which leveling of the wafer is completed, whereby leveling maintenance may be enhanced. 
     In addition, configuration of three centering modules  130  and  140  is further applied to at least one embodiment of the invention. In this regard, the outer circumferential surface of the wafer stage  50  is supported at three points by three centering modules  130  and  140  installed at a circumference of the wafer stage  50  and also centering may be more accurately maintained. 
     Each of the three centering modules  130  and  140  is positioned at the guide block  20  to maintain centering of the wafer stage  50 . As illustrated in  FIG. 11 , the two centering modules  130  of the three centering modules  130  and  140  perform two-point support in a state in which a fixing centering bar  131  is attached in an integrated manner to an outer surface of the guide block  20 , and a ball plunger  133  coupled to an upper portion of the fixing centering bar  131  closely contacts an outer surface of the support  52 . In addition, the two centering modules  130  maintain a fixed state at all times to provide a base point for centering. 
     In addition, as for the remaining centering module  140  of the three centering modules  130  and  140 , as illustrated in  FIGS. 12 and 13 , a lower side of a rotatory centering bar  141 , through which a connection pin  142  penetrates, is rotatably installed at a connection bracket  20 a protruding from the outer surface of the guide block  20 , a ball plunger  143  coupled to an upper portion of the rotatory centering bar  141  closely supports the outer surface of the support  52 , a spring support bolt  144  penetrating the rotatory centering bar  141  is connected to one side of a tensile spring  145  inserted into a spring inserting part  26  disposed in the guide block  20 , and the other side of the tensile spring  145  is connected to a spring fixing part  146  penetrating the spring inserting part  26  in a perpendicular state and screw-coupled with the guide block  20 . 
     Thus, the rotatory centering bar  141  is supported by and presses the wafer stage  50  towards the two fixing centering bars  131  that provide a base point by a tensile force of the tensile spring  145  and, accordingly, accurate centering may be more precisely maintained at all times. 
     DESCRIPTION OF REFERENCE NUMERALS 
       
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 10: base plate 
                 20: guide block 
               
               
                   
                 21: guide hole 
                 30: leveling air shock absorber 
               
               
                   
                 31, 91: piston rod 
                 40: leveling rod 
               
               
                   
                 41: ground pin 
                 50: wafer stage 
               
               
                   
                 52: support 
                 60: guide bolt 
               
               
                   
                 70: ring housing 
                 71: support hole 
               
               
                   
                 80: locking ring 
                 81: rotatory support sphere 
               
               
                   
                 82: coupling protrusion 
                 83: inner circumferential surface 
               
               
                   
                 85: compression ring 
                 86: interference preventing part 
               
               
                   
                 87: spring support groove 
                 88: spring 
               
               
                   
                 90: locking control cylinder 
                 100: stopper 
               
               
                   
                 110: pressure sensor 
                 130, 140: centering module 
               
               
                   
                 131: fixing centering bar 
                 133, 143: ball plunger 
               
               
                   
                 141: rotatory centering bar 
                 144: spring support bolt 
               
               
                   
                 145: tensile spring 
                 146: spring fixing sphere 
               
               
                   
                   
               
            
           
         
       
     
     Embodiments of the invention provide non-obvious advantages over the conventional art. For example, embodiments of the invention provide an inner circumferential surface of a locking ring inserted at an outer side of a leveling rod fixes opposite sides of the leveling rod by applying pressure thereto and a leveling air shock absorber is locked without movement of the leveling rod, thereby fixing a wafer stage and thus, during light exposure, precise leveling is maintained and working quality is significantly improved, and an effect of significantly improving performance of a leveling device is obtained by increasing a maximum capacity (weight) of a wafer to be leveled, by enhancing a fixing power of the wafer stage. 
     In addition, embodiments of the invention provide that opposite sides of the leveling rod are fixed at a constant pressure and thus leftward and rightward movements of the leveling rod are prevented and tilt, twist or torsion of the leveling rod is also prevented and, accordingly, the leveling rod has significantly enhanced durability. 
     In addition, embodiments of the invention provide that a low friction cylinder is applied to the leveling air shock absorber that performs leveling and thus fine pressure adjustment and contact force control are possible and, in a process of performing leveling of a wafer using a pressure sensor installed at the leveling air shock absorber, locking is performed by moving a locking control cylinder at an accurate time at which leveling has been completed, by controlling locking of the leveling air shock absorber by receiving feedback of a pressure value. 
     In addition, embodiments of the invention provide three centering modules installed at a circumference of the wafer stage maintain centering in a way of supporting an outer circumferential surface of the wafer stage at three points and thus errors according to processing and assembly degrees of the centering modules barely occur and, accordingly, a high level of skill and accuracy are not required in manufacturing of the centering modules, which enables easy fabrication, reduced manufacturing costs, and very accurate centering performance 
     Embodiments of the invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step. 
     Unless defined otherwise, all technical and scientific terms used have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. 
     The singular forms “a,” “an,” and “the” include plural referents, unless the context clearly dictates otherwise. 
     As used herein and in the appended claims, the words “comprise,” “has,” and “include” and all grammatical variations thereof are each intended to have an open, non-limiting meaning that does not exclude additional elements or steps. 
     “Optionally” means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur. As used herein, terms such as “first” and “second” are arbitrarily assigned and are merely intended to differentiate between two or more components of an apparatus. It is to be understood that the words “first” and “second” serve no other purpose and are not part of the name or description of the component, nor do they necessarily define a relative location or position of the component. Furthermore, it is to be understood that the mere use of the term “first” and “second” does not require that there be any “third” component, although that possibility is contemplated under the scope of the embodiments of the invention. 
     Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range. 
     All publications mentioned are incorporated by reference to disclose and describe the methods or materials, or both, in connection with which the publications are cited. The publications discussed are provided solely for their disclosure prior to the filing date of the present application. Nothing is to be construed as an admission that the invention is not entitled to antedate such publication by virtue of prior invention. Further, the dates of publication provided may be different from the actual publication dates, which may need to be independently confirmed. 
     Although embodiments of the invention have been described in detail, it should be understood that various changes, substitutions, and alterations can be made hereupon without departing from the principle and scope of embodiments of the invention. Accordingly, the scope of embodiments of the invention should be determined by the following claims and their appropriate legal equivalents.