Abstract:
The present invention relates to a method for writing a pattern on a surface intended for use in exposure equipment, including the steps of: arranging an object having a thickness (T) provided with a surface on a stage of a pattern generating apparatus, dividing the surface into a number of measurement points, where two adjacent measurement points being spaced a distance (P) apart not exceeding a predetermined maximum distance, determining the gradient of the surface at each measurement point, calculating a 2-dimensional local offset (d) in the x-y plane for each measurement point as a function of the gradient, and the thickness (T) of object, and correcting the pattern to be written on said surface by using the 2-dimensional local offset (d). The invention also relates to a method for measuring the physical properties of a surface.

Description:
[0001]     A method for writing a pattern on a surface intended for use in exposure equipment and for measuring the physical properties of the surface.  
       TECHNICAL FIELD OF THE INVENTION  
       [0002]     The present invention relates to a method for writing a pattern on a surface, preferably on a glass plate made from quartz, for use in exposure equipment, as defined in claim  1 . The invention also relates to a method for measuring the physical properties of the surface to determine the shape of the surface of a plate as defined in claim  10 .  
       BACKGROUND TO THE INVENTION  
       [0003]     When a large display or part of a display, colour filter or an other similar application, is produced, an exposure system transfer an image from a glass plate, preferably made from high quality quarts, onto a rather large substrate, which may have a dimension up to 1100 mm times 1300 mm or even more. The exposure system includes an aligner, or stepper, that emits light through the glass plate and onto the substrate, see  FIG. 1 . The glass plate is held in place by two rulers, or alternatively by a frame, and therefore the shape of the glass plate is deformed and the aligner, or stepper, compensates for this calculated deformation. The front side of the glass plate that carries the pattern of the image is arranged on the rulers, and a perfect reproduced image by the system on a substrate is dependent on that the front side of the glass plate is absolutely flat.  
         [0004]     It is very important that the registration of masks, i.e. the absolute placement in a Cartesian coordinate system, is good enough to permit masks from different systems to fit together, e.g. the colour filter and the TFT-array. Furthermore, large TFT substrates may use two or more masks stitched together to cover a large exposure area.  
         [0005]     In pattern generating systems for small plates, a three-foot device is used to support the plate during pattern generation and measurement, but the weight of a glass plate, with a thickness of 10 mm and a size of 1000×1000 mm, is approximately 40 kg, which will not be suitable to place on three pins. An alternative solution is to use an air cushion for plate support, but this introduces other problems like determining the exact position of the plate during exposure of the pattern. Another alternative is to handle the consequences that will arise when placing the plate directly on the stage (i.e. the support) of a pattern generating apparatus, although the plate will be deformed.  
       SUMMARY OF THE INVENTION  
       [0006]     The object of the invention is to provide a method for writing a pattern on a glass plate that is independent of any physical deformations that will occur when writing the pattern.  
         [0007]     This object is achieved by the method as defined in claim  1 .  
         [0008]     A further object with the invention is to provide a method for measuring a glass plate being independent of any physical deformations that will occur when measuring the plate.  
         [0009]     This object is achieved by the method as defined in claim  10 .  
         [0010]     An advantage with the present invention is that unevenness in the support of the pattern generating apparatus (or measuring apparatus) will not introduce any error in the pattern or the measurement.  
         [0011]     A further advantage is that any unevenness of the back surface and/or the front surface of the glass plate will not introduce any errors in the pattern or the measurement.  
         [0012]     Still a further advantage with the present invention is that contamination in form of particles and/or air trapped between the plate and the support can be compensated for, and therefore will not introduce any error in the pattern or measurement.  
         [0013]     Still another advantage is that it is possible to even correct the deformation that will occur in the exposure equipment together with the deformation generated during the pattern writing process, provided that information regarding deformation in the exposure equipment is known when manufacturing the plate, as is disclosed in the published international patent application WO 00/72090 by the same applicant. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  shows an exposure system according to prior art.  
         [0015]      FIG. 2  shows a pattern generating apparatus according to prior art.  
         [0016]      FIG. 3  illustrates the plate bending effect for calculating an offset according to the present invention.  
         [0017]      FIGS. 4   a  and  4   b  illustrate the plate bending effect a glass plate with a flat top and a shaped bottom and the introduction of a reference surface when arranged on a flat support.  
         [0018]      FIGS. 5   a  and  5   b  illustrate the plate bending effect a glass plate with a shaped top and a flat bottom and the introduction of a reference surface when arranged on a flat support.  
         [0019]      FIGS. 6   a  and  6   b  illustrate the plate bending effect a glass plate with a flat top and a flat bottom and the introduction of a reference surface when arranged on a shaped support.  
         [0020]      FIGS. 7   a  and  7   b  show measured x-y coordinates of a glass plate and compensated x-y coordinates of the same glass plate using the correction function, and  FIG. 7   c  shows the difference between the measurements without compensation and the measurements with compensation.  
         [0021]      FIG. 8  shows a three-dimensional measurement of a glass plate with particles distorting the shape of the plate.  
         [0022]      FIGS. 9   a  and  9   b  show measured x-y coordinates of the glass plate illustrated in  FIG. 8 , and the compensated x-y coordinates of the same glass plate using the correction function. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0023]      FIG. 1  shows an exposure system  10  which uses a glass plate  11  resting on two rulers  12 . The weight of the glass plate will cause the glass plate  11  to bend when placed on the rulers  12 . The deformation of the glass plate caused by the weight is easy to calculate and can be corrected for. The glass plate  11  is provided with a pattern arranged on the downwards pointing surface  13  resting on the rulers  12 . A light source  14  emits light  15  onto the glass plate  11  and the pattern arranged on the surface  13  of the glass plate  11  will produce a copy of the pattern on a substrate  16 . The substrate  16  could be a TFT intended for a TV monitor. Normally, the pattern is transferred to the substrate  16  in a one-to-one relationship.  
         [0024]     Other necessary optics is not shown in  FIG. 1 , since the purpose of the figure is to describe the function principals, rather than a complete exposure system.  
         [0025]      FIG. 2  shows a pattern generating apparatus  20 , which also could be used as a measuring apparatus, including means to write a pattern  21 , e.g. mirrors directing a laser beam from a laser, and means  22  to measure the height H z  between the apparatus  20  and a glass plate  11  with the surface  13  on which the pattern is to be written is placed upwards on a support  23 , so called stage. The pattern writing means  21  may be translated over the entire surface of the stage, which movement may be implemented in a number of ways.  FIG. 2  illustrates one way where the stage is provided with means to move it in relation to the pattern writing means  21  in the x direction, and where the pattern writing means  21  is attached to a sliding support  24  arranged on a beam  25  to move the pattern writing means in the y direction. Other possible ways to implement the translation of the pattern writing means is to provide the means to move the stage in both x and y direction with a non-moving pattern writing means, or the pattern writing means could be provided with means to move in both x and y direction with a non-moving stage.  
         [0026]     The apparatus  20  is also provided with an angled foot plate  26  arranged a constant distance above the surface  13  of the glass plate  11  by means of an air cushion  27 . The foot plate  26  and the pattern writing means  21  are attached to the sliding support  24  via a flexible attachment  28 , to allow the distance between the sliding support  24  and the pattern writing means/foot plate to vary dependent on the roughness of the surface  13  of the glass plate  11 . The varying distance in the z direction, i.e. the height H z , may be measured to calculate the roughness of the surface  13  in the z direction. The size of the foot plate that is parallel to the surface  13  of the glass plate  11  has an opening for a laser beam from the pattern writing means  21  and is preferably rather large, e.g. 5 mm on each side, since the purpose of the measurement is to detect deviations in height over a relatively large distance. The air cushion beneath foot plate will act as an auto focus device for the pattern generating apparatus due to the constant distance between the foot plate and the glass plate. The invention should however not be limited to this kind of pattern generating apparatus using an air cushion as an auto focus device, but other types of systems that will provide focus for the system could be used. The essential part is that the apparatus  20  is provided with means to measure the height H z  between the apparatus and the surface  13  of the glass plate  11  and thereby the variation in height when the pattern writing means  21  is moved in relationship to the stage  23 , and thus the surface  13 .  
         [0027]     An essential part of the invention is to determine a reference surface against which the difference in height H z  is calculated. This difference is denoted H, as is illustrated in connection with  FIG. 3 . The reference surface could have any desired shape as long as the shape of the reference surface is maintained unchanged. Preferably, the shape of the reference surface is a flat plane.  
         [0028]     If it were possible, it may have been desirable to use the “free” (non gravity) form, i.e. the centre line of the plate as a reference surface, which is rather difficult to achieve in practise. The bottom surface of the plate is not a good alternative for a reference surface since a stepper or an aligner use the top surface as a reference.  
         [0029]     On the other hand if the top surface would be used as a reference surface, there is an additional need to know the bottom shape of the plate and the shape of the support. The shape of the support may be obtained, but it is very difficult to achieve knowledge of the bottom surface in practice. The top surface may however be measured without the knowledge of the bottom surface. A large glass plate that is placed on a three-foot will be deformed due to the weight of the plate, but a deformation function for a perfect plate may be calculated if the thickness of the plate, the material of the plate and the configuration of the three-foot are known. A measurement of the non-perfect glass plate, when placed on the three-foot, will generate a measurement of the deformed plate. The shape of top surface is then calculated by subtraction the calculated deformation function for a perfect plate from the measurement of the deformed plate.  
         [0030]     The top surface of a glass plate is normally much more even, i.e. less variation in height in relation to the centre line, compared to the bottom surface, and the best compromise should therefore be to make the top surface of the plate to be the reference surface. It should however be noted that it is not evident that the top surface is the best choice due to the deformation of the glass plate during the following step in the exposure system, as shown in  FIG. 1 . If the top surface  13  of the glass plate exhibits variations close to the position where it rests on the rulers  12 , the pattern on the surface  13  will be distorted in a vicinity of the rulers  12 .  
         [0031]     It should however be noted that any surface may be used as reference surface, although the top side is preferred.  
         [0032]      FIG. 3  illustrates the plate bending effect for a glass plate  11  having a thickness T. A reference surface  30  is determined, in this example the reference surface is flat, and the glass plate is divided into several measurement points  31  and the height H z  is measured at each measurement point by the means  22  shown in  FIG. 2 . The height H between the reference plane  30  and the deformed surface  13  of the glass plane  11  can easily be calculated by subtracting the height of the reference surface  30  at the measurement point from the height H z  measured for the surface  13  of the glass plate  11  by the apparatus  20 .  
         [0033]     A local offset d (as a function of x and y) is thereafter calculated for each measurement point and depends on three variables: the thickness of the glass plate (T), the distance between adjacent measurement points (P) and the measured height (H) between the reference surface  30  and the surface  13  of the glass plate  11 . The local offset should be interpreted as the position deviation from the position where a pattern should be written in relationship to the reference surface, as described in connection with  FIGS. 4-6 . The pitch P on the surface of the plate differs from the nominal pitch P nom  on the reference surface.  
         [0034]     The distance between adjacent measurement points should not exceed a predetermined distance, which is dependent on the required accuracy for the measurement to get a reasonable good result from the measurement. An example of maximum distance between adjacent measurement points is 50 mm if the thickness of the glass plate  11  is around 10 mm and the glass plate material is quartz. The distance between adjacent measurement points also vary dependent on the thickness of the glass plate to obtain the same measurement accuracy. The variations in thickness of the glass plate is may be around 10-15 μm, but could be larger. The measurement points could be randomly distributed across the surface  13 , but are preferably arranged in a grid structure with a predetermined distance between each point, i.e. pitch, that is not necessarily the same in the x and y direction.  
         [0035]     The local offset is a function of the gradient in x and y direction at each measurement point and could be calculated using very simple expressions.  
         [0036]     An angle α may be calculated from the measured height H provided the distance P between two adjacent measurement points  31   a  is known.  
         [0037]     For small angles α:  
       α   =     H   P         
 
         [0038]     Furthermore the local offset d may be calculated provided a is small using the formula:  
       d   =         T   2     *   α     =       H   *   T       2   *   P             
 
         [0039]     It should however be noted that the formula for calculating the local offset d above, only is a non-limiting example of a calculation to determine the offset d. The gradient in each measurement point could be directly measured by the system and the local offset is proportional to the gradient and the thickness of the plate.  
         [0040]     As previously mentioned above,  FIG. 3  illustrates the bending effect in one dimension, but the local offset d is a 2-dimensional function of the derivative in each measurement point (dx and dy).  
         [0041]     As a non-limiting example we assume that the distance between two adjacent points  31  is 40 mm, the thickness of the glass plate is 10 mm, and that the measured height H is 1 μm, which will result in a one-dimensional local offset d of 125 nm.  
         [0042]      FIGS. 4   a  and  4   b  illustrate the plate bending effect a glass plate  41  with a flat top surface  43  and a shaped bottom surface  42  and the introduction of a reference surface  44 , which is flat in this example, when supported by a flat support  45 .  
         [0043]     When the glass plate  41  is arranged on the flat support  45 , the shape of the top surface  43  is changed and the bottom surface  42  will generally follow the flat support  45 . The result of this is that the pattern generated, illustrated by the dots  46  on the top surface, has to be expanded to obtain a correct reference surface.  
         [0044]      FIGS. 5   a  and  5   b  illustrate the plate bending effect a glass plate  51  with a shaped top surface  53  and a flat bottom surface  52  and the introduction of a reference surface  44 , which is flat in this example, when arranged on a flat support  45 .  
         [0045]     When the glass plate  51  is arranged on the flat support  45 , the shape of the top surface  43  is unchanged and the bottom surface  42  will follow the flat support  45 . The pattern generated, illustrated by the dots  55  on the top surface, has to be expanded to obtain a correct reference surface, since the top surface will be flattened out when positioned in the exposure equipment as described in  FIG. 1 , at least in the vicinity of the rulers  12 . The part of the glass plate positioned right between the rulers  12  will be deformed. Furthermore the rulers will deform the pattern on the glass plate unless the shape of the rulers  12  is in accordance with the shape of the reference surface.  
         [0046]      FIGS. 6   a  and  6   b  illustrate the plate bending effect a glass plate  61  with a flat top surface  43  and a flat bottom surface  52  and the introduction of a reference surface  44 , which is flat in this example, when arranged on a shaped support  62 .  
         [0047]     When the glass plate  61  is arranged on the shaped support  62 , the shape of the top surface  43  is changed and the bottom surface  42  will generally follow the shaped support  62 . The pattern generated, illustrated by the dots  64  on the top surface, has to be expanded to obtain a correct reference surface, since the top surface will be flattened out when positioned in the exposure equipment as described in  FIG. 1 .  
         [0048]      FIGS. 4   a - 4   b ,  5   a - 5   b  and  6   a - 6   b  illustrate extreme conditions and in reality all three variations are present during the process of writing a pattern on a glass plate.  
         [0049]     The overall error is however much smaller since all errors from the bottom surface, support surface and contamination, see  FIGS. 8, 9   a  and  9   b , are eliminated or at least reduced.  
         [0050]      FIG. 7   a  shows measured x-y coordinates of a reference glass plate and compensated x-y coordinates of the same reference glass plate using a calculated correction function according to the present invention.  FIG. 7   b  shows the measured height H (z correction data) obtained at the same time as the x and y coordinates for marks depicted on the surface of the reference glass plate.  FIG. 7   c  shows the difference between the measurements without compensation and the measurements with compensation.  
         [0051]     The size of the glass plate is in this example 800×800 mm, and the distance between each dashed line  70  in  FIG. 7   a  is 50 mm, and the scale of the deviation of the two plotted charts are 500 nm between each dashed line  70 . The grey lines  71  correspond to the measured deviation of the x and y coordinate on the reference glass plate. The black lines  72  correspond to the compensated x and y coordinates of the same reference glass plate using the Z correction effect based on the measured height H shown in  FIG. 7   b . The minimum height is −20.705 μm and the maximum height is +16.664 μm compared to the determined reference surface and the height H is depicted as a function  73 . The distance between the lines in x and y direction is the same as in  FIG. 7   a , i.e. 50 mm, and the distance between the lines in z direction is 2 μm.  
         [0052]      FIG. 7   c  clearly illustrates the deviations between the two functions in  FIG. 7   a . When comparing the measured height H in  FIG. 7   b  with the deviation in  FIG. 7   c  it is easy to see the relationship between the derivative of the height and the local offset. When the derivative of the height is zero, as in position  74 , then the local offset d is zero. When the derivative of the height is high, as in position  75 , then the local offset d is large.  
         [0053]     A transition from a low H value to high H value corresponds to that the glass plate has a “negative” bend, as illustrated in  FIG. 3 , and vice versa. The calculated local offset, i.e. the difference between the grey and the black lines is largest when the change of the derivative of the height H in x and y direction is the highest.  
         [0054]      FIG. 8  shows a three-dimensional measurement  80  of a glass plate with two present particles, placed between the plate and the support, having a height of 16 μm and 6 μm, respectively. The measurement was performed using a grid structure and the distance between the measurement points was set to 50 mm and the thickness of the plate was 10 mm. The scale in z direction was set to 2 μm per division. The presence of the large particle causes the x and y measurement illustrated in  FIG. 9   a  to deviate more than 500 nm.  
         [0055]      FIG. 9   a  shows measured x-y coordinates of the glass plate illustrated in  FIG. 8 , and  FIG. 9   b  shows the compensated x-y coordinates of the same glass plate using the correction function calculated from the measured deviating height measurement in  FIG. 8 . The effect of particles will be greatly reduced on the final image generated on the glass plate as is illustrated in  FIG. 9   b.    
         [0056]     Although a glass plate has been used as an illustrative example in the patent application, the scope of the claims should not be limited to a plate made of glass.  
         [0057]     Furthermore, the pattern generating apparatus could of course include correction functions for any repeatable error, e.g. errors present in substrates for the manufacturing of TFT-arrays that are introduced in the substrates during the manufacture of the substrates, as well as repeatable errors introduced in the manufacturing process in the aligner, or stepper as previously mentioned.  
         [0058]     The method may naturally be implemented into a computer program for performing the measurements, and calculating the local offset for each measurement point.