Abstract:
A method of measuring a gap between a mask and a substrate by providing a mask and a substrate facing each other. The mask includes an array of patterns, and a at least one window disposed between two of the patterns. Each of the patterns corresponds to a display device. The method also includes projecting an incident laser beam onto the substrate through the window of the mask and determining a gap between the mask and the substrate in a middle region of the substrate in response to first and second reflected beams. The first reflected beam is generated by the reflection of the incident laser beam by the mask, and the second reflected beam is generated by the reflection of the incident laser beam by the substrate. Determining the gap between the mask and the substrate in the middle region allows for the correction of any undesirable deflection of the mask.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of measuring a gap between a mask and a substrate of flat panel displays, such as plasma display panels and liquid crystal displays. 
     2. Description of the Related Art 
     A photolithography process for providing patterns on substrates of flat panel displays is a part of the fabrication of the flat panel displays. The exposure phase of the photolithographic process is achieved by aligners. 
     Aligners for fabrication of flat panel displays often adopt a proximity exposure method. The proximity exposure method involves maintaining a small gap, 50 to 250 microns wide, between the substrate and the mask during exposure. This gap minimizes mask damage. 
     A conventional aligner for proximity exposure is disclosed in Japanese Unexamined Patent Application No. Jp-A 2001-12905.  FIG. 1  shows a schematic of a conventional aligner and is designated by reference numeral  150 . The aligner  150  includes a substrate stage  106  that has an upper surface  106 S. A transparent substrate  104 , such as a glass substrate, is disposed on the upper surface  106  and is secured by vacuum clamping. The upper surface  106 S is square and has a side-length of L6. 
     The substrate stage  106  is connected to stage drivers  143 , which are respectively controlled by controllers  140 . The positional control of the substrate stage  106 , including the vertical control and leveling, is achieved by controllers  140  and stage drivers  143 . 
     As shown in  FIG. 2 , the substrate  104  is square and has a side-length of L4. Except for the reflective square regions  105  near the corners of the main surface of the substrate  104 , the main surface of the substrate  104  is covered with a photo resist  104   a  ( FIG. 1 ). That is, the substrate  104  is exposed at the square regions  105 . The exposed square regions are referred to as gap measuring reflector regions  105 , hereinafter. The gap measuring reflector regions  105  have a side-length of L5. 
     As shown in  FIG. 1 , the aligner  150  includes a frame-structured mask stage  103  onto which a square mask  101  having a side-length of L1 is secured. The mask  101  has a main surface, disposed opposite substrate  104 , on which a transfer pattern is formed. As shown in  FIG. 3 , transparent gap measuring marks  102  are disposed near the respective corners of the mask  101 . The gap measuring windows  102  are square and have a side-length of L2. 
     As shown in  FIG. 1 , the aligner  150  also includes laser beam emitters  107 , such as laser diodes, and laser beam detectors  108 , such as photo diodes. The laser beam emitters  107  and the laser beam detectors  108  are disposed over the mask  101 . The laser beam emitters  107  project laser beams  109  onto the gap measuring windows  102  at an angle of 45 degree with respect to the mask  101 . A part of each laser beam  109  is reflected by the mask  101  to generate a reflected beam  110  while the other part of the each laser beam  109  passes through the gap measuring windows  102  to generate a reflected beam  111 . Each of the laser beam detectors  108  receives the reflected beam  110  from the mask  101  and the reflected beam  111  from the substrate  104 . 
     The exposure process by the aligner  150  begins with positioning the mask  101  and the substrate  104  so that the centers of the windows  102  and the reflector regions  105  are aligned. 
     Then, the gaps between the mask  101  and the substrate  104  are measured at the corners with the laser beam emitters  107  and the laser beam detectors  108 . The laser beam emitters  107  respectively project the laser beams  109  onto the gap measuring windows  102  at an angle of incident of 45 degrees. The laser beam detectors  108  receive the reflected beams  110  from the mask  101  and the reflected beams  111  from the substrate  104 , and the laser beam detectors  108  generate spot position data representative of the positions of the spots where the laser beam detectors  108  receive the reflected beams  110  and  111 . The spot position data may be representative of the distance between the spots of the reflected beams  110  and  111  provided on the laser beam detectors  108 . Controllers  140  calculate the associated gaps between the mask  101  and the substrate  104 , located near the corners, on the basis of the spot position data received from the receivers  108 . 
     Controllers  140  then operate drivers  143  to control the position of the substrate stage  106  so that the gaps becomes equal. 
     After positioning substrate stage  106 , the photo resist disposed on the substrate  104  is exposed with an ultraviolet light, which goes through the pattern on the mask  101 . 
     However, the conventional aligner thus described suffers from a problem in that the pattern on the substrate requires reflective gap measuring marks. This undesirably reduces flexibility of the design of the pattern on the substrate. 
     An aligner that solves this problem is disclosed in Japanese Unexamined Patent Application No. Jp-A-Heisei 11-194501. This aligner is equipped with a substrate holder, a thickness measuring unit, a gap sensor and a controller. The substrate holder has an upper surface on which a substrate is secured. The thickness measuring unit measures the thickness of the substrate. The gap sensor determines the gap between the mask and the upper surface of the substrate holder. The controller calculates the gap between the mask and the substrate from the gap between the mask and the upper surface of the substrate holder and the thickness of the substrate, and the controller regulates the gap between the mask and the substrate in response to the calculated gap. This eliminates the need for providing reflective gap measuring marks on the substrate. 
     Another aligning method to achieve accurate alignment of the mask and the substrate is disclosed in Japanese Unexamined Patent Application No. Jp-A-Heisei 7-260424. This aligning method provides first alignment marks, consisting of diffraction gratings, on the mask at predetermined intervals, and also provides second alignment marks of diffraction gratings on the substrate. A laser beam emitted from a He—Ne laser is projected onto the mask and the substrate and is diffracted by the first and second alignment marks respectively disposed on the mask and the substrate. The relative position of the mask and the substrate is determined on the basis of the diffracted beams from the first and second alignment marks. Using diffracted beams enables accurate determination of the relative position. The mask and the substrate are then aligned in response to the determined relative position. An accurate determination of the relative position allows the mask and the substrate to be accurately aligned. 
     Recently, the size of flat display panel substrates have been enlarged to improve production efficiency. Substrates having a length of more than one meter, for example, are commercially available. Enlarging the substrates allows for a plurality of display device to be fabricated on a single substrate, thus, decreasing the number of required steps. For example, a large substrate, on which a plurality of display device have been fabricated, reduces the number of exposure processes necessary for fabricating the same number of display devices. This effectively reduces the fabrication cost of the display devices. 
     Enlarging the substrate, however, produces an undesirable deflection of the mask because enlargement of the substrate is inevitably accompanied by the enlargement of the mask used in the exposure process. Any deflection of the mask prevents the gap between the mask and the substrate from being consistently regulated to a desired gap, and thus, enlarges the difference in the dimension of the pattern transferred to the substrate. In a region where the gap is larger than the desired gap, for example, the width of lines transferred to the substrate are undesirably larger than the desired width, and vice versa. As a result, the width of lines undesirably varies widely on the substrate. 
     To correct for the undesirable deflection of the mask, the deflection of the mask needs to be measured or determined. Accordingly, a need exists to provide a way for determining the deflection of the mask. 
     SUMMARY OF THE INVENTION 
     In summary, the present invention addresses determining and correcting the deflection of masks used for proximity exposure on enlarged substrates. Determining and correcting the deflection of a mask allows for consistently regulating the gap between the mask and the substrate. 
     In an aspect of the present invention, a method comprises: 
     providing a mask which includes: 
     an array of patterns, each of which corresponds to a display device, 
     a window disposed between two of the patterns, 
     placing a substrate to face the mask; 
     projecting an incident laser beam onto the substrate through the window of the mask; and 
     determining a gap between the mask and the substrate in a middle region of the substrate in response to first and second reflected beams, the first reflected beam being generated by the incident laser beam reflected by the mask, and the second reflected beam being generated by the incident laser beam being reflected by the substrate. 
     Determining the gap between the mask and the substrate in the middle region advantageously provides for correcting the undesirable deflection of the mask. 
     The array of patterns may be arranged in a row or in rows and columns. 
     When the mask includes other windows disposed around the array of the patterns, the method preferably includes: 
     projecting other incident laser beams onto the substrate through the other windows; 
     determining gaps between the mask and the substrate near corners of the substrate in response to third and fourth laser beams, the third laser beams being generated by the other incident laser beams being reflected by the mask, and the fourth laser beams being generated by the other incident laser beams being reflected by the substrate, and 
     determining a deflection of the mask based on the determined gap in the middle region and the gaps near the corners. 
     When the substrate is covered with a photo resist, it is advantageous that a portion of the main surface of the substrate is exposed, and a second reflected laser beam is generated by the incident laser beam being reflected by the exposed portion. 
     In an another aspect of the present invention, a proximity exposure method comprising: 
     providing a mask which includes: 
     an array of patterns, each of which respectively corresponds to a display device, 
     a window disposed between adjacent two of the patterns, 
     placing a substrate on a substrate stage opposed to the mask; 
     projecting an incident laser beam onto the substrate through the window of the mask; and 
     determining a gap between the mask and the substrate in a middle region of the substrate in response to first and second reflected beams, the first reflected beam being generated by the incident laser beam reflected by the mask, and the second reflected beam being generated by the incident laser beam being reflected by the substrate; and
         removing a deflection of the mask in response to the determined gap in the middle region.       

     The removing preferably includes: 
     securing the mask and a glass plate to form a sealed space between the mask and the glass plate; and 
     inflating or evacuating the sealed space in response to the determined deflection. 
     The determination of the gap in the middle region may be executed every time the substrate is exchanged or every time the mask is exchanged. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of a conventional aligner  150 ; 
         FIG. 2  shows a plan view of the substrate  104 ; 
         FIG. 3  shows a plan view of the mask  101 ; 
         FIG. 4  shows a plan view illustrating an alignment of the mask  101  and the substrate  104 ; 
         FIG. 5  shows a plan view of a mask used in an embodiment of the present invention; 
         FIGS. 6 and 7  are schematics of an aligner used in the embodiment of the present invention; 
         FIG. 8  is a block diagram of the aligner; 
         FIG. 9  shows a deflection remover used in the embodiment; 
         FIG. 10  shows a plan view of a substrate with reflective regions; 
         FIGS. 11 and 12  show a method of determining gaps using the reflective regions provided for the substrate; 
         FIG. 13  shows a plan view of a substrate in an alternative embodiment; and 
         FIG. 14  shows a plan view of a substrate in another alternative embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention are described below in detail with reference to the attached drawings. 
     In one embodiment, a glass mask  51  shown in  FIG. 5  is used in the exposure process. The glass mask  51  includes an array of patterns  52  and  53 , each of which corresponds to a complete display device (not to a portion of a display device). Patterns  52  and  53  are transferred to a substrate using a photolithography technique. 
     The glass mask  51  includes gap measuring windows  2  around patterns  52 , and  53 . The gap measuring windows  2  are transparent regions that allow light to pass through. The gap measuring windows  2  are positioned at the corners of mask  51 . 
     A gap measuring window  2   a  is additionally disposed in a non-patterned region (or blank space) between patterns  52  and  53 . 
       FIG. 6  shows an aligner  50  that is consistent with this embodiment and is used in a proximity exposure process. The aligner  50  includes a frame-structured mask stage  3 , a substrate stage  6 , laser beam emitters  7  and laser beam detectors  8 . The substrate stage  6  has an upper surface  6 S on which a substrate  4  covered with a photo resist  4   a  is secured by vacuum clamping. The mask stage  3  supports the mask  51  so that the main surface of the mask  51  is opposite the main surface of the substrate  4 . 
     The laser beam emitters  7  and the laser beam detectors  8  are used to determine gaps between the mask  51  and the substrate  4  near the corners thereof. The laser beam emitters  7  project laser beams  9  onto the gap measuring windows  2 . A part of each laser beam  9  is reflected by the mask  51  while the other part of each laser beam  9  passes through the mask  51  and is reflected by the substrate  4 . Each laser beam detector  8  receives the reflected laser beam  10  from the mask  51  and the reflected laser beam  11  from the substrate  4  and generates spot position data representative of the positions of the spots where each laser beam detector  8  receives the reflected beams  10  and  11 . The spot position data may be representative of the distance between the spots of the reflected beams  10  and  11 . The gaps between the mask  51  and the substrate  4  near the corners thereof are calculated on the basis of the spot position data developed by the laser beam detectors  8 . 
     As shown in  FIG. 7 , the aligner  50  additionally includes a laser beam emitter  13  and a laser beam detector  14  to measure or determine a gap between the mask  51  and the substrate  4  in the middle region thereof. The laser beam emitter  13  projects a laser beam  15  onto the gap measuring window  2   a . A part of the laser beam  15  is reflected by the mask  51  while the other part of the laser beam  15  passes through the mask  51  and is reflected by the substrate  4 . The laser beam detector  14  receives the reflected laser beam  16  from the mask  51  and the reflected laser beam  17  from the substrate  4 , and laser beam detector  14  generates spot position data representative of the positions of the spots where the laser beam detector  14  receives the reflected beams  16  and  17 . The spot position data may be representative of the distance between the spots of the reflected beams  16  and  17 . The gap between the mask  51  and the substrate  4  in the middle region thereof is calculated on the basis of the spot position data from the laser beam detector  14 . 
     As shown in  FIG. 8 , the laser beam detectors  8  and  14  respectively provide the spot position data for a controller  40  to determine the gaps between the mask  51  and the substrate  4 . In response to the spot position data from the laser beam detectors  8 , the controller  40  determines the gaps between the mask  51  and the substrate  4  near the corner thereof. Furthermore, the controller  40  determines the gap between the mask  51  and the substrate  4  in the middle region thereof in response to the spot position data from the laser beam detector  14 . The controller  40 , in response to the determined gaps (including both near the corners and in the middle region), operates the stage driver  43  to control the position of the substrate stage  6 . 
     In addition, controller  40  calculates the deflection of the mask  51  on the basis of the gaps near the corners and in the middle region. Controller  40  displays the calculated deflection of the mask  51  on the screen of the display  44 . 
     The determination of the gaps between the mask  51  and the substrate  4  on the substrate stage  6 , and the calculation of the deflection of the mask  51  may be periodically executed. For example, the determination of the gaps and the calculation of the deflection may be executed every other week or month. When a predetermined number of substrates go through the exposure process by the aligner  50 , the periodic determination of the gaps helps regulate the gaps between the mask  51  and the substrate  6  to a desired value. 
     When the number of substrates going through the exposure process using the aligner  50  in a given day is varied, the determination of the gaps between the mask and the substrate and the calculation of the deflection of the mask is preferably executed every time the substrate  4 , which is placed on the substrate stage  6 , is exchanged or every time the mask  51  is exchanged. 
     The calculation of the deflection of the mask  51  is preferably followed by correcting the deflection from the mask  51 . In order to correct the deflection from the mask  51 , the aligner  50  preferably includes a deflection remover  60  as shown in  FIG. 9 . 
     The deflection remover  60  includes a transparent glass plate  61  and a mask holder  62 . The glass plate  61  is the same size as the mask  51 . The mask holder  62  fixes the mask  51  so that the mask  51  is disposed opposite the glass plate  61  and provides a sealed space  63  therebetween. The transparent glass plate  61  allows laser beams  9  and  15  emitted from laser beam emitters  7  and  13  ( FIGS. 6 and 7 ) to be projected onto the mask  51  and the substrate  4  therethrough. 
     The mask holder  62  is provided with a gas inlet  62   a  and a gas outlet  62   b . The gas inlet  62   a  is coupled to a tank  64  filled with high pressure air, and the gas outlet  62   b  is coupled to a vacuum pump  65 . The tank  64  and the vacuum pump  65  are operated in response to the calculated deflection of the mask  51 . 
     In the event that the mask  51  is convex, toward the substrate  4 , the vacuum pump  65  is operated to evacuate the sealed space  63 . The evacuation of the sealed space  63  exerts a force on the mask  51  toward the glass plate  61  to remove the deflection of the mask  51 . 
     In the event that the mask  51  is concave, toward the glass plate  61 , the tank  65  is operated to inflate the sealed space  63 . The inflation by the tank  65  exerts a force on the mask  51  toward the substrate  4  to remove the deflection of the mask  51 . 
     The pressure of the sealed space  63  is regulated by the tank  64  and the vacuum pump  65  in response to the deflection of the mask  51 , i.e., the gap between the mask  51  and the substrate  4  in the middle region thereof. Accordingly, the deflection of the mask  51  is appropriately removed. 
     As shown in  FIG. 10 , it is advantageous if square portions of the main surface of the substrate  4  are exposed (that is, not covered with the photo resist  4   a ( FIG. 6 )) in order to improve the reflection coefficient of the substrate  4 . The exposed square portions in the corners of the substrate  4  are referred to as reflective regions  5 , and the exposed square portion in the middle region of the substrate  4  is referred to as a reflective region  5   a . The reflective regions  5  are positioned so that the reflective regions  5  face the gap measuring windows  2  disposed near the corners of the mask  51  when the substrate  4  is aligned to the mask  51 . Correspondingly, the reflective regions  5   a  faces the gap measuring windows  2   a  in the middle region of the mask  51  when the substrate  4  is aligned to the mask  51 . Preferably, the photo resist  4   a  is applied by printing onto the substrate  4  in order to facilitate the formation of the reflective regions  5  and  5   a.    
     When the reflective regions  5  and  5   a  are provided on the substrate  4 , as shown in  FIG. 11 , laser beams  9  emitted by laser beam emitters  7  are projected onto the reflective regions  5  through the gap measuring windows  2 , and the laser beam  15  emitted by the laser beam emitter  13  is projected onto the reflective regions  5   a  through the gap measuring windows  2   a  as shown in  FIG. 12 . The reflective regions  5  and  5   a  increase the intensity of the reflected laser beams  11  and  17  from the substrate  4 , which effectively improves the accuracy of the determination of the gaps between the mask  51  and the substrate  4 . 
     In an alternative embodiment, with reference to  FIG. 13 , a mask  71  is used in place of the mask  51  in the exposure process. The mask  71  includes an array of the same patterns  72 ,  73 , and  74  arranged in a row. Each of the patterns  72  to  74  corresponds to a complete display device (not to a portion of a display device). The patterns  72  to  74  are transferred to the substrate  4  by a photolithography technique. 
     The glass mask  71  includes gap measuring windows  2  near the corners thereof around the array of the patterns  72  to  74 . The windows  2  are transparent regions that allow the laser beams  7  to pass through. 
     A gap measuring windows  2   b  and  2   c  are additionally disposed on the mask  71  to allow laser beams to pass through. The gap measuring window  2   b  is disposed in a non-patterned region between patterns  72  and  73 , and the gap measuring window  2   c  is disposed in a non-patterned region between patterns  73  and  74 . The gap measuring window  2   b  is positioned at a distance L/3 from the left edge of the mask  71 , and the measuring window  2   c  is positioned a distance 2L/3 from the left edge of the mask  71 , where L is the length of the mask  71 . 
     In order to determine the gaps between the mask  71  and the substrate  4  near the corners thereof, laser beams are projected by the laser beam emitters  7  onto the substrate  4  through the gap measuring windows  2 , and reflected laser beams are received by the laser beam detectors  8  from the mask  71  and the substrate  4 . The gaps between the mask  71  and the substrate  4  near the corners thereof are determined on the basis of the positions of the spots of the reflected laser beams on the laser beam detectors  8 . 
     Correspondingly, in order to determine gaps between the mask  71  and the substrate  4  in the middle region thereof, laser beams are projected onto the substrate  4  through the gap measuring windows  2   b  and  2   c , and reflected laser beams are received by laser beam detectors from the mask  71  and the substrate  4 . The gaps between the mask  71  and the substrate  4  in the middle regions thereof are determined on the basis of the positions of the spots of the reflected laser beams on the laser beam detectors. The reflected laser beams associated with the gap measuring window  2   b  provide information on the gap at the position L/3 from the left edge of the mask  71 . Correspondingly, the reflected laser beams associated with the gap measuring window  2   c  provide information on the gap at the position 2L/3 from the left edge of the mask  71 . 
     The deflection of the mask  71  is calculated on the basis of the gaps between the mask  71  and the substrate  4  near the corners thereof and the gaps in the middle region thereof. In response to the calculated deflection of the mask  71 , the deflection remover  60  is operated to correct any deflection of the mask  71 . 
     In another alternative embodiment, as shown in  FIG. 14 , a mask  81  is used to achieve exposure in place of the mask  51 . 
     The mask  81  includes an array of the same patterns  82  to  85  arranged in rows and columns. Each of the patterns  82  to  85  corresponds to a complete display device (not to a portion of a display device). The patterns  82  to  85  are transferred to the substrate  4  by a photolithography technique. 
     The mask  81  includes gap measuring windows  2  near the corners thereof around the array of the patterns  82  to  85 . The windows  2  are transparent regions that allow the laser beams  7  to pass therethrough to determine the gaps between the mask  81  and the substrate  4  near the corners thereof. 
     A gap measuring windows  2   d  through  2   h  are additionally disposed on the mask  81  to allow laser beams to pass therethrough to determine the gaps between the mask  81  and the substrate  4  in the middle region thereof. The gap measuring window  2   d  is disposed in a non-patterned region between patterns  82  and  83 , and the gap measuring window  2   e  is disposed in a non-patterned region between patterns  84  and  85 . The gap measuring window  2   f  is disposed in a non-patterned region between patterns  82  and  84 , and the gap measuring window  2   g  is disposed in a non-pattern region between patterns  83  and  85 . The gap measuring window  2   h  is disposed at the center of the mask  81 . 
     The determination of the gaps between the mask  81  and the substrate  4  is achieved by the aforementioned method. In order to determine gaps between the mask  81  and the substrate  4  near the corners thereof, laser beams are projected by laser beam emitters  7  onto the substrate  4  through the gap measuring windows  2 , and reflected laser beams are received by laser beam detectors  8  from the mask  81  and the substrate  4 . The gaps between the mask  81  and the substrate  4  near the corners thereof are determined on the basis of the positions of the spots of the reflected laser beams on the laser beam detectors  8 . 
     Correspondingly, in order to determine gaps between the mask  81  and the substrate  4  in the middle region thereof, laser beams are projected onto the substrate  4  through the gap measuring windows  2   d  to  2   h , and reflected laser beams are received by laser beam detectors from the mask  71  and the substrate  4 . The gaps between the mask  81  and the substrate  4  in the middle regions thereof are determined on the basis of the positions of the spots of the reflected laser beams on the laser beam detectors. The reflected laser beams associated with the gap measuring window  2   d ,  2   e , and  2   h  provide information on the gap at the position L/2 from the left edge of the mask  81 . The reflected laser beams associated with the gap measuring window  2   f  provide information on the gap at the position L/4 from the left edge of the mask  81 . The reflected laser beams associated with the gap measuring window  2   g  provide information on the gap at the position 3L/4 from the left edge of the mask  81 . 
     The deflection of the mask  81  is calculated on the basis of the determined gaps between the mask  81  and the substrate  4  near the corners thereof and in those in the middle region thereof. In response to the calculated deflection of the mask  81 , the deflection remover  60  is operated to correct any deflection of the mask  81 . 
     One skilled in the art would appreciate that laser beams are not required to be projected through all the gap measuring windows  2   d  to  2   h . Preferable combinations of the gap measuring windows  2   d  to  2   h  used to determined the gaps in the middle region are as follows: 
     (1) the gap measuring window  2   h,    
     (2) the gap measuring windows  2   h ,  2   f , and  2   g,    
     (3) the gap measuring windows  2   d  (or  2   e ),  2   f , and  2   g,    
     (4) the gap measuring windows  2   f , and  2   g,    
     (5) the gap measuring windows  2   h ,  2   d  (or  2   e ),  2   f , and  2   g , and 
     (6) the gap measuring windows  2   h ,  2   d ,  2   e ,  2   f , and  2   g.    
     Those who are skilled in the art would also appreciate that the number of the rows and columns in which patterns are arranged may be three or more. 
     Although the invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form may be changed in the details of construction and the combination and arrangement of the parts may be changed without departing from the scope of the invention as hereinafter claimed.