Abstract:
A method is composed of providing a mask and placing a substrate to face the mask. The mask includes an array of patterns, and a window disposed between two of the patterns. Each of the patterns corresponds to a display device. The method 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 incident laser beam reflected by the mask, and the second reflected beam is 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 a step for removing or releasing undesirable deflection of the mask.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention is related to a method for measuring a gap between a mask and a substrate of flat panel displays, such as plasma display panels and liquid crystal displays.  
           [0003]    2. Description of the Related Art  
           [0004]    Fabrication of flat panel displays includes photolithography for providing patterns on substrates of flat panel displays. The photolithographic process includes exposure, which is achieved by aligners.  
           [0005]    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.  
           [0006]    A conventional aligner for proximity exposure is disclosed in Japanese Unexamined Patent Application No. Jp-A 2001-12905. FIG. 1 shows a schematic of the conventional aligner, which is denoted by a numeral  150 . The aligner  150  includes a substrate stage  106  having an upper surface  106 S. A transparent substrate  104 , such as a glass substrate, is secured by vacuum clamping on the upper surface  106 . The upper surface  106 S is square, having a side-length of L 6 .  
           [0007]    The substrate stage  106  is drivingly connected to the stage drivers  143 , which are respectively controlled by controllers  140 . The posture control of the substrate stage  106 , including the height control and leveling, is achieved by the controllers  140  and the stage drivers  143 .  
           [0008]    As shown in FIG. 2, the substrate  104  is square, having a side-length of L 4 . The main surface of the substrate  104  is covered with a photo resist  104 a other than reflective square regions near the corners of the main surface of the substrate  104 . That is, the substrate  104  is exposed at the square regions. 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 L 5 .  
           [0009]    As shown in FIG. 1, the aligner  150  includes a frame-structured mask stage  3  onto which a square mask  101  having a side-length of L 1  is secured. The mask  101  has a main surface opposed to the substrate  104 , on which a pattern to be transferred 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, having a side-length of L 2 .  
           [0010]    As shown in FIG. 1, the aligner  150  further includes laser beam emitters  107  such as laser diodes, and laser beam detectors  108 , such as photo diodes, both being 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 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 .  
           [0011]    Exposure 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.  
           [0012]    Then, the gaps between the mask  101  and the substrate  104  near the corners thereof are measured with the laser beam emitters  107  and the laser beam detectors  108 . The laser b am 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 generate spot position data representative of the positions of the spots where the laser beam detectors  108  receives 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 . The controllers  140  calculate the associated gaps between the mask  101  and the substrate  104  near the corners thereof on the basis of the spot position data received from the receivers  108 .  
           [0013]    The controllers  140  then operate the drivers  143  to control the posture of the substrate stage  106  so that the gaps becomes equal.  
           [0014]    After the control of the posture of the substrate stage  106 , the photo resist disposed on the substrate  104  is exposed through the pattern on the mask  101  with an ultraviolet light.  
           [0015]    The conventional aligner thus described suffers from a problem that the pattern on the substrate is required to include reflective gap measuring marks. This undesirably reduces flexibility of the design of the pattern on the substrate.  
           [0016]    An aligner for solving this problem is disclosed in Japanese Unexamined Patent Application No. Jp-A-Heisei 11-194501. The disclosed aligner includes The 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 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.  
           [0017]    Another aligning method is disclosed to achieve accurate alignment of the mask and the substrate in Japanese Unexamined Patent Application No. Jp-A-Heisei 7-260424. The aligning method involves providing first alignment marks consisting of diffraction gratings on the mask at predetermined intervals, and providing 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 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. The use of the diffracted beams enables accurate determination of the relative position. The mask and the substrate are then aligned in response to the determined relative position. The accurate determination of the relative position allows the mask and the substrate to be accurately aligned.  
           [0018]    Recently, sizes of substrates of flat display panels have been enlarged to improve production efficiency. Substrates having a length more than one meter, for example, are commercially available. Enlargement of the substrates allows a plurality of display device to be fabricated on a single substrate, and thus decreases the number of required steps. For example, a large substrate on which a plurality of display device is fabricated reduces the number of exposure processes necessary for fabricating the same number of the display device. This effectively reduces fabrication cost of display devices.  
           [0019]    Enlargement of the substrate, however, raises a problem of an undesirable deflection of the mask, because the enlargement of the substrate is inevitably accompanied by the enlargement of the mask to achieve exposure onto the enlarged substrate. The deflection of the mask prevents the gap between the mask and the substrate from being homogeneously 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.  
           [0020]    To remove or release undesirable deflection of the mask, the deflection of the mask is desirably measured or determined. A need exists to provide a technology for determining the deflection of the mask.  
         SUMMARY OF THE INVENTION  
         [0021]    In summary, the present invention addresses determining and removing deflection of masks used for proximity exposure onto enlarged substrates. Determining and removing deflection of a mask provides a step for homogeneously regulating the gap between the mask and the substrate.  
           [0022]    In an aspect of the present invention, a method is composed of:  
           [0023]    providing a mask which includes:  
           [0024]    an array of patterns, each of which corresponds to a display device,  
           [0025]    a window disposed between two of the patterns,  
           [0026]    placing a substrate to face the mask;  
           [0027]    projecting an incident laser beam onto the substrate through the window of the mask; and  
           [0028]    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.  
           [0029]    Determining the gap between the mask and the substrate in the middle region advantageously provides a step for removing or releasing undesirable deflection of the mask.  
           [0030]    The patterns disposed may be arranged in a row or in rows and columns.  
           [0031]    When the mask includes other windows disposed around the array of the patterns, the method preferably includes:  
           [0032]    projecting other incident laser beams onto the substrate through the other windows;  
           [0033]    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  
           [0034]    determining a deflection of the mask based on the determined gap in the middle region and the gaps near the corners.  
           [0035]    When the substrate is covered with a photo resist, it is advantageous that-a portion of a main surface of the substrate is exposed, and the second reflected laser beam is generated by the incident laser beam being reflected by the exposed portion.  
           [0036]    In an another aspect of the present invention, an proximity exposure method comprising:  
           [0037]    providing a mask which includes:  
           [0038]    an array of patterns, each of which respectively corresponds to a display device,  
           [0039]    a window disposed between adjacent two of the patterns,  
           [0040]    placing a substrate on a substrate stage opposed to the mask;  
           [0041]    projecting an incident laser beam onto the substrate through the window of the mask; and  
           [0042]    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  
           [0043]    removing a deflection of the mask in response to the determined gap in the middle region.  
           [0044]    the removing preferably includes:  
           [0045]    securing the mask and a glass plate to form a sealed space between the mask and the glass plate; and  
           [0046]    inflating or evacuating the sealed space in response to the determined deflection.  
           [0047]    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  
       [0048]    [0048]FIG. 1 is a schematic of the conventional aligner  150 ;  
         [0049]    [0049]FIG. 2 shows a plan view of the substrate  104 ;  
         [0050]    [0050]FIG. 3 shows a plan-view of the mask  101 ;  
         [0051]    [0051]FIG. 4 shows a plan view illustrating an alignment of the mask  101  and the substrate  104 ;  
         [0052]    [0052]FIG. 5 shows a plan view of a mask used in an embodiment of the present invention;  
         [0053]    [0053]FIGS. 6 and 7 are schematics of an aligner used in the embodiment of the present invention;  
         [0054]    [0054]FIG. 8 is a block diagram of the aligner;  
         [0055]    [0055]FIG. 9 shows a deflection remover used in the embodiment;  
         [0056]    [0056]FIG. 10 shows a plan view of a substrate with reflective regions;  
         [0057]    [0057]FIGS. 11 and 12 show a method of determining gaps using the reflective regions provided for the substrate;  
         [0058]    [0058]FIG. 13 shows a plan view of a substrate in an alternativ embodiment; and  
         [0059]    [0059]FIG. 14 shows a plan view of a substrate in another alternative embodiment. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0060]    Preferred embodiments of the present invention are described below in detail with reference to the attached drawings.  
         [0061]    In one embodiment, a glass mask  51  shown in FIG. 5 is used to achieve exposure. The glass mask  51  includes an array of the patterns  52  and  53 , each of which corresponds to a complete display device (not to a portion of a display device). The patterns  52  and  53  are transferred to a substrate by a photolithography technique.  
         [0062]    The glass mask  51  includes gap measuring windows  2  around the patterns  52 , and  53 . The gap measuring windows  2  are transparent regions to allow light to pass through. The gap measuring windows  2  are positioned in the corners of the mask  51 .  
         [0063]    A gap measuring window  2   a  is additionally disposed in a non-patterned region (or blank space) between the patterns  52  and  53 .  
         [0064]    [0064]FIG. 6 shows an aligner  50  used to achieve proximity exposure in this embodiment. 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 opposed to the main surface of the substrate  4 .  
         [0065]    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 the each laser beam  9  is reflected by the mask  51 , while the other part of the each laser beam  9  passes through the mask  51 , and is reflected by the substrate  4 . The 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 the 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 th laser beam detectors  8 .  
         [0066]    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 each 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 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 .  
         [0067]    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  is responsive to the determined gaps (including both near the corners and in the middle region) for operating the stage driver  43  to control the posture of the substrate stage  6 .  
         [0068]    In addition, the controller  40  calculates the deflection of the mask  51  on the basis of the gaps near the corners and in the middle region. The controller  40  displays the calculated deflection of the mask  51  on the screen of the display  44 .  
         [0069]    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. The periodic determination of the gaps helps regulate the gaps between the mask  51  and the substrate to a desired value, when a predetermined numb r of substrates go through exposure by the aligner  50 .  
         [0070]    When the number of substrates going through exposure by the aligner  50  in a day is variable, 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  is exchanged to be placed on the substrate stage  6 , or every time the mask  51  is exchanged.  
         [0071]    The calculation of the deflection of the mask  51  is preferably followed by removing the deflection from the mask  51 . In order to removing the deflection from the mask  51 , the aligner  50  preferably includes a deflection remover  60  as shown in FIG. 9.  
         [0072]    The deflection remover  60  includes a transparent glass plate  61 , and a mask holder  62 . The glass plate  61  has the same size of the mask  51 . The mask holder  62  fixes the mask  51  so that the mask  51  is opposed to the glass plate  61  to provide a sealed space  63  therebetween. The transparent glass plate  61  allows the laser beams  9  and  15  emitted from the laser beam emitters  7  and  13  to be projected onto the mask  51  and the substrate  4  therethrough.  
         [0073]    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  is operated in response to the calculated deflection of the mask  51 .  
         [0074]    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 stress on the mask  51  toward the glass plate  61  to remove the deflection of the mask  51 .  
         [0075]    In the event that the mask  51  is convex toward the glass plate  61 , on the other hand, the tank  65  is operated to inflate the sealed space  63 . The inflation by the tank  65  exerts a stress on the mask  51  toward the substrate  4  to remove the deflection of the mask  51 .  
         [0076]    The pressure of the sealed space  63  is regulated by the tank  64  and the vacuum pump  65  in response to the gap between the mask  51  and the substrate  4  in the middle region thereof, that is, the deflection of the mask  51 . This results in that the deflection of the mask  51  is appropriately removed.  
         [0077]    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  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 . Application of the photo resist  4   a  by printing preferably facilitates the provision of the reflective regions  5  and  5   a  onto the substrate  4 .  
         [0078]    When the reflective regions  5  and  5   a  are provided on the substrate  4 , as shown in FIG. 11, the laser beams  9  emitted by the 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 r gions  5   a  through the gap measuring windows  2   a  as shown in FIG. 12. The reflective regions  5  and  5   a  increases the intensity of the reflected laser beams  11  and  17  from the substrate  4 , and effectively improves the accuracy of the determination of the gaps between the mask  51  and the substrate  4 .  
         [0079]    In an alternative embodiment, with reference to FIG. 13, a mask  71  is used to achieve exposure in place of the mask  51 . 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.  
         [0080]    The glass mask  71  includes gap measuring windows  2  near the corners thereof around the array of the patterns  72  to  74 , which are transparent regions to allow the laser beams  7  to pass through.  
         [0081]    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 the patterns  72  and  73 , and the gap measuring window  2   b  is disposed in a non-patterned region between the patterns  73  and  74 . The gap measuring window  2   b  is positioned L/3 apart from the left edge of the mask  71 , and the measuring window  2   c  is positioned 2L/3 apart from the left edge of the mask  71 , where L is the length of the mask  71 .  
         [0082]    In order to determine 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 .  
         [0083]    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 apart 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 apart from the left edge of the mask  71 .  
         [0084]    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 in those in the middle region thereof. In response to the calculated deflection of the mask  71 , the deflection remover  60  is operated to remove the deflection of the mask  71 .  
         [0085]    In another alternative embodiment, as shown in FIG. 14, a mask  81  is used to achieve exposure in place of the mask  51 .  
         [0086]    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.  
         [0087]    The mask  81  includes gap measuring windows  2  near the corners thereof around the array of the patterns  82  to  85 , which are transparent regions to allow the laser beams  7  to pass therethrough to determine the gaps between the mask  81  and the substrate  4  near the corners thereof.  
         [0088]    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 the patterns  82  and  83 , and the gap measuring window  2   e  is disposed in a non-patterned region between the patterns  84  and  85 . The gap measuring window  2   f  is disposed in a non-patterned region between the patterns  82  and  84 , and the gap measuring window  2   g  is disposed in a non-pattern region between the patterns  83  and  85 . The gap measuring window  2   h  is disposed at the center of the mask  81 .  
         [0089]    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 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  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 .  
         [0090]    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 apart 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 apart 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 apart from the left edge of the mask  81 .  
         [0091]    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 remove the deflection of the mask  81 .  
         [0092]    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:  
         [0093]    (1) the gap measuring window  2   h,    
         [0094]    (2) the gap measuring windows  2   h ,  2   f , and  2   g,    
         [0095]    (3) the gap measuring windows  2   d  (or  2   e ),  2   f , and  2   g,    
         [0096]    (4) the gap measuring windows  2   f , and  2   g,    
         [0097]    (5) the gap measuring windows  2   h ,  2   d  (or  2   e ),  2   f , and  2   g , and  
         [0098]    (6) the gap measuring windows  2   h ,  2   d ,  2   e ,  2   f , and  2   g.    
         [0099]    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.  
         [0100]    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 has been changed in the details of construction and the combination and arrangement of parts may be resorted to without departing from the scope of the invention as hereinafter claimed.