BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    Embodiments of the invention relate to a wafer stage. In particular, embodiments of the invention relate to a wafer stage adapted to hold a wafer, and a method for attaching a wafer to a wafer stage. 
         [0003]    This application claims priority to Korean Patent Application No. 10-2006-0000652, filed on Jan. 3, 2006, the subject matter of which is hereby incorporated by reference in its entirety. 
         [0004]    2. Description of Related Art 
         [0005]    In general, semiconductor devices are fabricated by application of a complex sequence of fabrication processes (e.g., material deposition, etching, ashing, diffusion, photolithography, etc.) to a wafer. By means of these selectively and repeatedly applied fabrication processes, various material layers (e.g., conductive and insulating layers) are formed and patterned on the wafer. 
         [0006]    Among the fabrication processes commonly used to manufacture semiconductor devices, photolithography processes generally comprise a coating process wherein a wafer is coated with a photoresist; an exposure process during which the coated wafer is selectively exposed to light through a reticle defining a circuit-pattern; and, a developing process during which a developing solution is applied to remove the portions of the photoresist from the wafer. 
         [0007]    Conventional photolithography equipment adapted for use in photolithography processes may be generally divided into coating/developing equipment (e.g., a spinner) and exposure equipment (e.g., a scanner or stepper). Spinners and scanners are typically associated with one another in a fabrication line to perform photolithography processes. 
         [0008]    Conventional exposure equipment includes a wafer stage adapted to hold a wafer. The wafer stage typically includes a wafer chuck adapted to directly hold the wafer, and a driver adapted to move (i.e., transfer) the wafer chuck. 
         [0009]    The wafer chuck includes lift pins on a central portion of the wafer chuck, and the lift pins may be raised and lowered so that the wafer may be placed on the wafer chuck. The wafer chuck also includes a plurality of vacuum holes adapted to draw the wafer onto the wafer chuck once the wafer is positioned by operation of the lift pins. 
         [0010]    That is, to expose a wafer coated with photoresist, for example, the wafer is first positioned over the wafer chuck by a transfer robot. The lift pins of the wafer chuck are raised to support a bottom surface of the wafer. The wafer, which is supported by the lift pins, is then positioned on a top surface of the wafer chuck as the lift pins are lowered. As soon as the lift pins have been completely lowered, the wafer is held to the wafer chuck by a vacuum suction force provided through the vacuum holes of the wafer chuck. The wafer, which is held to the wafer chuck by the vacuum suction force, is held (i.e., fixed) to the wafer chuck with a predetermined amount of pressure so that the wafer will not shake during the exposure process for forming a predetermined pattern on the wafer. 
         [0011]    However, as illustrated in figures (FIGS.)  1 A and  1 B, a conventional wafer stage  10  comprises lift pins  30  disposed in a central portion of a wafer chuck  20 , as described above. Thus, a wafer W, and in particular wafer W having a diameter of 300 mm, which is placed on wafer chuck  20  by lift pins  30  and held to wafer chuck  20  using vacuum holes (not shown), sags at its edge of when it is placed on wafer chuck  20  to be held by wafer chuck  20 . As illustrated in  FIG. 1B , wafer chuck  20  collides with the edge of wafer W when wafer W is placed on wafer chuck  20 . Thus, placing wafer W on and attaching wafer W to conventional wafer stage  10  may damage the edge of wafer W. Further, when the exposure process is performed for a relatively long amount of time, a portion of wafer chuck  20  may be worn down by periodic collisions between wafer W and wafer chuck  20 . 
       SUMMARY OF THE INVENTION 
       [0012]    Embodiments of the invention provide a wafer stage, wherein a wafer may be attached to the wafer stage for an exposure process without substantially damaging the wafer. Embodiments of the invention also provide a method for attaching a wafer to a wafer stage for an exposure process without substantially damaging the wafer. 
         [0013]    In one embodiment, the invention provides a wafer stage comprising a wafer chuck adapted to hold a wafer; lift pins adapted to pass through the wafer chuck, move vertically, and support the wafer; and an air expulsion unit adapted to expel air towards an edge of the wafer. 
         [0014]    In another embodiment, the invention provides a wafer stage comprising a wafer chuck; lift pins adapted to pass through the wafer chuck, move vertically, and support the wafer; an air expulsion unit adapted to expel air towards an edge of the wafer; and a controller adapted to control when the air expulsion unit expels air towards the edge of the wafer. The wafer chuck comprises a body, a plurality of chucking protrusions disposed on an upper surface of the body and adapted to contact the wafer when the wafer is attached to the wafer chuck, and an edge ring disposed on an edge of the upper surface of the body and adapted to contact the edge of the wafer. 
         [0015]    In yet another embodiment, the invention provides a method for attaching a wafer to a wafer chuck comprising lowering lift pins supporting the wafer, and expelling air towards an edge of the wafer using an air expulsion unit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    Embodiments of the invention will be described herein with reference to the accompanying drawings, in which like reference symbols indicate like or similar elements. For purposes of clarity, elements in the drawings may not be drawn to scale. In the drawings: 
           [0017]      FIGS. 1A and 1B  are cross-sectional views illustrating a conventional wafer chuck; 
           [0018]      FIG. 2  is a schematic view illustrating exposure equipment comprising a wafer stage in accordance with an embodiment of the invention; 
           [0019]      FIG. 3  is a perspective view illustrating a wafer stage in accordance with an embodiment of the invention; 
           [0020]      FIG. 4  is a top plan view illustrating a wafer stage in accordance with an embodiment of the invention; 
           [0021]      FIG. 5  shows a wafer stage in accordance with an embodiment of the invention; 
           [0022]      FIG. 6  is a magnified view of a circle A of  FIG. 5 ; 
           [0023]      FIG. 7  illustrates the operation of the vacuum unit and the air expulsion unit of a wafer stage in accordance with another embodiment of the invention; 
           [0024]      FIG. 8  is a flowchart illustrating a method for attaching a wafer to a wafer stage in accordance with an embodiment of the invention; and, 
           [0025]      FIG. 9  is a flowchart illustrating a method for attaching a wafer to a wafer stage in accordance with another embodiment of the invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0026]      FIG. 2  is a schematic view illustrating exposure equipment comprising a wafer stage in accordance with an embodiment of the invention. 
         [0027]    Referring to  FIG. 2 , exposure equipment  100  generally comprises a light source  110 , a reticle R defining a circuit pattern, an optical system adapted to convey light emitted from light source  110  onto a wafer W, and a wafer stage  200  on which wafer W is attached (i.e., chucked) and which moves wafer W during the exposure process. 
         [0028]    Light source  110  emits light having a predetermined wavelength. For example, light source  110  may emit ultraviolet light or extreme ultraviolet light. 
         [0029]    The optical system is adapted to guide the light emitted from light source  110  towards wafer W to irradiate light onto wafer W, which is attached to wafer stage  200 . In addition, the optical system is subdivided into an illumination optical system  130 , which is interposed between light source  110  and reticle R, and a projection optical system  140 , which is interposed between reticle R and wafer stage  200 . 
         [0030]    Illumination optical system  130  comprises a filter (not shown) that allows only a predetermined band of light emitted from light source  110  to pass through. Illumination optical system  130  also comprises a condenser lens (not shown) that condenses and irradiates onto reticle R the light filtered by the filter. Projection optical system  140  comprises a plurality of reduction projection lenses (not shown) adapted to focus light received through reticle R via illumination optical system  130  onto a predetermined portion of the wafer W. 
         [0031]    In addition, while not shown, the optical system may be used as a reflective optical system comprising multipurpose optical lenses and reflectors. 
         [0032]      FIG. 3  is a perspective view illustrating a wafer stage in accordance with an embodiment of the invention,  FIG. 4  is a top plan view illustrating a wafer stage in accordance with an embodiment of the invention,  FIG. 5  shows a wafer stage in accordance with an embodiment of the invention, and  FIG. 6  is a magnified view of a circle A of  FIG. 5 . 
         [0033]    An embodiment of a wafer stage in accordance with the invention will be described below with reference to  FIGS. 3 through 6 . 
         [0034]    Wafer stage  200  comprises a wafer chuck  210  to which wafer W (of FIG.  2 ) is attached, and a drive unit  260  adapted to move wafer W during the exposure process. Wafer stage  200  further comprises a lift pin unit  220  disposed at a central portion of wafer chuck  210 , a vacuum unit  230  adapted to pull wafer W towards wafer chuck  210  when wafer W as wafer W is placed on wafer chuck  210  and when wafer W is attached to wafer chuck  210 , and an air expulsion unit  240  adapted to expel air towards wafer W, and particularly towards an edge of wafer W, when wafer W is placed on and attached to wafer chuck  210  by lift pin unit  220  and vacuum unit  230 . 
         [0035]    Additionally, wafer chuck  210  comprises a body  212 , a plurality of chucking protrusions  214  disposed on an upper surface of body  212  and adapted to make contact with a bottom surface of wafer W when wafer W is placed on and attached to wafer chuck  210 , and an edge ring  216  disposed along an edge of the upper surface of body  212  and adapted to make contact with an edge of the bottom surface of wafer W. 
         [0036]    In the embodiment illustrated in  FIGS. 3 through 6 , body  212  has a circular disc shape, and lift pin holes  215  are disposed in a central portion of body  212  and formed such that lift pins  222  of lift pin unit  220  will fit into lift pin holes  215 , respectively. 
         [0037]    As mentioned previously, the plurality of chucking protrusions  214  are disposed on the upper surface of body  212  and are adapted to make contact with the bottom surface of wafer W when wafer W is placed on and attached to wafer chuck  210 . In the embodiment illustrated in  FIGS. 3 through 6 , chucking protrusions  214  protrude from body  212  to a predetermined height and are integrally formed with body  212 . 
         [0038]    In addition, edge ring  216  is disposed on the edge of the upper surface of body  212  and is adapted to make contact with the edge of the bottom surface of wafer W when wafer W is placed on and attached to wafer chuck  210 . In embodiment illustrated in  FIGS. 3 through 6 , edge ring  216  comprises of a first edge ring  218  and a second edge ring  217 . 
         [0039]    Drive unit  260  comprises a base  261  disposed beneath body  212 , and also comprises drive blocks  263  and  265  adapted to move body  212 , which is disposed over  261 , along a two-dimensional plane (e.g., an X-Y plane). In the illustrated embodiment of  FIGS. 3 through 6 , a bottom side of body  212  is disposed on a mirror block  262  adapted to sense a position of body  212  disposed over base  261 . Drive blocks  263  and  265  are each disposed on a respective side of body  212 . Driver  263  is adapted to function as a Y-axis driver  263  adapted to move wafer W, which is attached to body  212 , along a first axis (e.g., a Y-axis), and driver  265  is adapted to function as an X-axis driver  265  that is connected to Y-axial driver  263  and is adapted to move wafer W along a second axis orthogonal to the first axis (e.g., an X-axis). 
         [0040]    Although it is not shown, coils are disposed on both sides of Y-axis driver  263 , and magnets adapted to react with the coils disposed on Y-axis driver  263  are disposed on body  212  and X-axis driver  265 . Accordingly, wafer W, which is attached to body  212 , may be moved over base  261  in accordance with reactions between the coils and the magnets. 
         [0041]    Lift pin unit  220  is adapted to guide wafer W onto body  212 , to which wafer W will be attached. In the embodiment illustrated in  FIGS. 3 through 6 , lift pin unit  220  comprises lift pins  222  adapted to fit into lift pin holes  215  formed in a central region of body  212 , and a lifting mechanism  224  adapted to move lift pins  222  up and down (i.e., vertically). Lifting mechanism  224  may be implemented using a hydraulic or pneumatic cylinder, or a mechanical assembly, such as a ball screw driven by a separate motor. In any practical form, lifting mechanism  224  is adapted to move wafer W up and down. 
         [0042]    Vacuum unit  230  is adapted to pull wafer W, which is guided to body  212  by lift pins  222 , using the suction generated by the vacuum (i.e., vacuum suction). Vacuum unit  230  comprises a vacuum channel  231  formed in body  212  and a plurality of vacuum holes  232  adapted to communicate with vacuum channel  231  and exposed through top surfaces of chucking protrusions  214 . 
         [0043]    In the illustrated embodiment of  FIGS. 3 through 6 , each of the plurality of vacuum holes  232  is exposed through the top surface of a respective chucking protrusion  214 , as mentioned above, or through a top surface of first edge ring  218 , which is an outer ring of edge ring  216 . 
         [0044]    Vacuum unit  230  further comprises a vacuum line  233 , which is disposed partially outside of body  212  and is connected to vacuum channel  231 , and a vacuum pump  235  that is connected to vacuum line  233 . 
         [0045]    The edge of wafer W, particularly when wafer W has a diameter of 300 mm, may sag when wafer W is lowered and pulled onto body  212  by lift pins  222  and vacuum unit  230 . Air expulsion unit  240  is adapted to prevent the edge of wafer W from colliding with body  212  when the edge of wafer W sags. 
         [0046]    Air expulsion unit  240  comprises an air expulsion channel  241  formed in body  212 , a plurality of air expulsion holes  242  adapted to communicate with air expulsion channel  241  and exposed through an upper surface of second edge ring  217 , which is the inner edge ring of edge ring  216 , and an air expulsion line  243 , which is disposed partially outside of body  212  and is connected to air expulsion channel  241 . Air expulsion unit  240  further comprises an air expulsion pump  245  connected to air expulsion line  243 . 
         [0047]    In another embodiment, vacuum holes  232  may be disposed in second edge ring  217  rather than first edge ring  218 , and air expulsion holes  242  may be disposed in first edge ring  218  rather than second edge ring  217 . 
         [0048]    Vacuum unit  230  further comprises a vacuum sensor  237  adapted to sense a vacuum suction force applied to wafer W when wafer W is being pulled towards body  212  (i.e., the force applied to wafer W by body  212 ). 
         [0049]    In addition, air expulsion unit  240  further comprises a pressure sensor  247  adapted to measure an amount of air pressure applied to the edge of wafer W by air expulsion unit  240 . 
         [0050]    Wafer stage  200  further comprises a controller  250  adapted to control the process of attaching wafer W to wafer chuck  210 . Controller  250  is electrically connected to vacuum pump  235 , air expulsion pump  245 , vacuum sensor  237 , and pressure sensor  247 . Controller  250  operates vacuum pump  235  to pull wafer W towards wafer chuck  210  when wafer W is being placed on wafer chuck  210 . Further, controller  250  operates air expulsion pump  245  to prevent the edge of the wafer W from colliding with body  212  of wafer chuck  210  when wafer W is being placed on wafer chuck  210 . 
         [0051]    Controller  250  receives a measurement value from pressure sensor  247 , which is adapted to measure the amount of air pressure applied through air expulsion holes  242 , air expulsion channel  241 , and air expulsion line  243 , so that controller  250  can determine whether or not the air pressure is equivalent to a preset air pressure. If the measured value received from pressure sensor  247  is greater than (i.e., beyond) a preset value, controller  250  can generate an interlock to stop the process of attaching wafer W to wafer chuck  210 . 
         [0052]      FIG. 7  illustrates another embodiment of a wafer stage in accordance with the invention. Referring to  FIG. 7 , a position sensor  280  is disposed at an upper portion of wafer chuck  210  and is adapted to sense when wafer W has reached wafer chuck  210 . In the embodiment illustrated in  FIG. 7 , the upper surface of position sensor  280  is preferably lower than the upper surfaces of chucking protrusions  214  and lower than the upper portion of edge ring  216  so that position sensor  280  does not hinder wafer W from being attached to wafer chuck  210 . Although it is not shown, a plurality of position sensors  280  may be disposed on one side of the upper portion of wafer chuck  210 . 
         [0053]    Position sensor  280  may be an optical sensor comprising a light emitting element adapted to emit light towards wafer W, which is lowered by lift pins  222 , and a light receiving element adapted to receive light reflected by wafer W. Alternatively, position sensor  280  may be a proximity sensor. 
         [0054]    In addition, position sensor  280  is electrically connected to controller  250 . Thus, controller  250  can determine when air should be expelled towards the edge of wafer W in accordance with the position of wafer W as sensed by position sensor  280 . 
         [0055]    A method for attaching a wafer to a wafer stage, in accordance with an embodiment of the invention, wherein the method uses a wafer stage in accordance with an embodiment of the invention, will now be described with reference to the accompanying drawings. 
         [0056]    Referring to  FIG. 8 , to attach wafer W to wafer stage  200  for the exposure process, wafer W, which is coated with a photoresist layer, is first positioned (i.e., transferred) over body  212  by a transfer robot (not shown). When the positioning of wafer W is completed (i.e., when wafer W has been moved over body  212 ), lift pins  222  are raised to support the bottom surface of wafer W. Then, lift pins  222  are lowered to guide wafer W to body  212  (S 10 ). 
         [0057]    When lifting mechanism  224  begins to lower lift pins  222 , controller  250  operates vacuum unit  230 . Specifically, controller  250  operates vacuum pump  235  to generate a vacuum suction force and exert the vacuum suction force on wafer W through body  212 , and more specifically, through vacuum line  233 , vacuum channel  231 , and vacuum holes  232 . Thus, wafer W, which is guided to body  212  by lift pins  222 , is pulled towards the upper surface of body  212  in accordance with the vacuum suction force exerted by vacuum unit  230  (S 20 ). 
         [0058]    Controller  250  then continues to sense the vacuum suction force, which is exerted on wafer W by vacuum unit  230 , using vacuum sensor  237  (S 30 ). 
         [0059]    Vacuum sensor  237  then checks whether or not the vacuum suction force by which wafer W is being pulled towards body  212  has reached a first preset value, that is, whether or not wafer W has reached a first preset position (S 40 ). The first preset position is a position at which the vacuum suction force generated by vacuum unit  230  and applied to wafer W has a first preset value. 
         [0060]    If the vacuum suction force by which wafer W is being pulled towards body  212  by vacuum unit  230  has reached the first preset value, air expulsion unit  240  expels air towards the edge of wafer W (S 50 ). That is, if wafer W has reached the first preset position, vacuum sensor  237  sends a signal to controller  250 , and controller  250  then operates air expulsion pump  245  to expel air towards the edge of wafer W. In contrast, when wafer W has not reached the first preset position, controller  250  continuously checks whether wafer W has reached the first preset position. By expelling air towards the edge of wafer W as described above, wafer W is substantially prevented from sagging when wafer W is guided by lift pins  222 . 
         [0061]    Then, controller  250  checks whether or not the vacuum suction force by which wafer W is being pulled by vacuum unit  230  has reached a second preset value, that is, whether or not wafer W has reached a second preset position (S 60 ). The second preset position is a position at which the vacuum suction force generated by vacuum unit  230  and applied to wafer W has a second preset value. 
         [0062]    If the vacuum suction force has reached the second preset value, air expulsion unit  240  stops expelling air towards the edge of wafer W (S 70 ). In other words, if wafer W has reached the second preset position, vacuum sensor  237  sends a signal to controller  250 , and controller  250  then stops operating air expulsion pump  245 . In contrast, if the vacuum suction force by which wafer W is pulled has not reached the second preset value, air expulsion unit  240  continues to expel towards the edge of wafer W. Air expulsion unit  240  stops expelling air towards the edge of wafer W when the vacuum suction force has reached the second preset value because expelling air towards wafer W has no effect on wafer W as wafer W comes near to body  212 . 
         [0063]    Thereafter, wafer W is placed on and attached to body  212  by continuously lowering lift pins  222  (S 80 ). 
         [0064]    As shown in  FIG. 9 , the expulsion of air towards the edge of wafer W can alternatively be controlled in accordance with the output of position sensor  280 , i.e., the position of wafer W. 
         [0065]      FIG. 9  is a flowchart illustrating a method for attaching a wafer to a wafer stage in accordance with another embodiment of the invention. 
         [0066]    Referring to  FIG. 9 , to attach wafer W to wafer stage  200  to perform the exposure process, wafer W, which is coated with a photoresist layer, is first positioned (i.e., transferred) over body  212  by a transfer robot (not shown). When the positioning of wafer W is completed, lift pins  222  are raised to support the bottom surface of wafer W. Then, lift pins  222  are lowered to guide wafer W to body  212  (S 110 ). 
         [0067]    When lifting mechanism  224  begins to lower lift pins  222 , controller  250  operates vacuum unit  230 . Specifically, controller  250  operates vacuum pump  235  to generate a vacuum suction force and exert the vacuum suction force on wafer W through body  212 , and more specifically, through vacuum line  233 , vacuum channel  231 , and vacuum holes  232 . Thus, wafer W, which is guided to body  212  by lift pins  222 , is pulled towards the upper surface of body  212  in accordance with the vacuum suction force exerted by vacuum unit  230  (S 120 ). 
         [0068]    Then, while wafer W is lowered by lift pins  222 , position sensor  280  continually senses a position of wafer W (S 130 ). Position sensor  280  continues to sense the position of wafer W until wafer W is placed on and attached to body  212 . 
         [0069]    Position sensor  280  senses whether or not wafer W, which is being lowered, has reached a preset position (S 140 ). If wafer W has reached the preset position, position sensor  280  sends an electrical signal to controller  250 . In response, controller  250  operates air expulsion pump  245  such that it begins expelling air towards the edge of wafer W (S 150 ). 
         [0070]    While wafer W is being lowered by lift pins  222  as air is being expelled towards the edge of wafer W, controller  250  gradually decreases the pressure with which air is expelled towards the edge of wafer W in accordance with the position of wafer W, which is measured by position sensor  280  (S 160 ). 
         [0071]    Thereafter, wafer W is placed on and attached to body  212  by continuously lowering lift pins  222  (S 170 ). The pressure with which the air is expelled by air expulsion unit  240  decreases in accordance with the position of wafer W, which is measured by position sensor  280 , and air expulsion unit  240  is adapted to stop expelling air once wafer W is disposed on body  212 . 
         [0072]    As stated above, the wafer stage, in accordance with embodiments of the invention, is adapted to expel air towards the edge of a wafer during the process of attaching the wafer to the wafer stage in order to prevent the wafer from being damaged by a collision between the edge of the wafer and the wafer chuck. In addition, the method for attaching the wafer to the wafer stage, in accordance with embodiments of the invention, comprises expelling air towards the edge of a wafer during the process of attaching the wafer to the wafer stage in order to prevent the wafer from being damaged by a collision between the edge of the wafer and the wafer chuck. 
         [0073]    While embodiments of the invention have been shown and described herein, it will be understood by those of ordinary skill in the art that various changes in form and detail may be made to the embodiments without departing from the scope of the invention as defined by the accompanying claims.
A wafer stage and related methods are disclosed. The wafer stage comprises a wafer chuck adapted to hold a wafer; lift pins adapted to pass through the wafer chuck, move vertically, and support the wafer; and an air expulsion unit adapted to expel air towards an edge of the wafer. The method for attaching a wafer to a wafer chuck comprises lowering lift pins supporting the wafer; and expelling air towards an edge of the wafer using an air expulsion unit.