Abstract:
The present invention relates to a method to determine a position of at least one mark provided on a substrate, comprising the actions of: detecting a first mark on said substrate by using a first detector, detecting a first set of marks comprising at least a second mark on said substrate by using a second detector, computing a first list of relative distance(s) between said first mark and mark(s) in said first set of marks, detecting the second mark on said substrate by using one of said first or said second detectors, detecting a second set of marks comprising at least said first mark on said substrate by using an available detector, computing a second list of relative distance(s) between said second mark and mark(s) in said second set of marks, determining the position of at least one mark by using the information in said first and said second lists of relative distance(s).

Description:
TECHNICAL FIELD 
   The present invention relates to a registration measurement method and an apparatus therefore and, more particularly, it relates to a registration measurement method suitable for registration or positioning of substrates in mask/reticle writers and/or positioning of substrates in stepper/aligners and an apparatus therefor. 
   BACKGROUND OF THE INVENTION 
   When a large display or part of a display, color filter or another 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 dimensions up to 1800 mm times 2400 mm. The exposure system includes an aligner, or stepper, that emits light through the glass plate and onto the substrate. 
   It is very important that registration of masks, i.e., the absolute placement in a coordinate system, is good enough to permit masks from different systems to fit together, e.g., the color filter and a TFT-array. Furthermore, large TFT-substrates may use one, two or more masks stitched together to cover a large exposure area. 
   A requirement for good alignment mark positioning determination in conventional registration system is inter alia a stable coordinate system, for instance a tradiational XY-coordinate system. A potential problem may arise when many positions have to be measured and the time to perform said measurement is significant, i.e., during a long measuring period drifts of any type may arise. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an object of the present invention to provide a method for measuring alignment mark positions with reduced process time. 
   This object, among others, is according to a first aspect of the invention attained by a method to determine a position of at least one mark provided on a substrate, comprising the actions of detecting a first mark on said substrate by using a first detector, detecting a first set of marks comprising at least a second mark on said substrate by using a second detector, computing a first list of relative distance(s) between said first mark and mark(s) in said first set of marks, detecting the second mark on said substrate by using one of said first or said second detectors, detecting a second set of marks comprising at least said first mark on said substrate by using an available detector, computing a second list of relative distance(s) between said second mark and mark(s) in said second set of marks, determining the position of at least one mark by using the information in said first and said second lists of relative distance(s). 
   The invention also relates to an apparatus to determine a position of at least one mark provided on a substrate, comprising a rotatable substrate holder, a guiding rail movable in a first direction, at least a first and a second detector provided on said guiding rail and movable in a second direction essentially perpendicular to said first direction, at least one interferometer capable to measure a relative distance between said first and said second detector. 
   Further characteristics of the invention, and advantages thereof, will be evident from the detailed description of preferred embodiments of the present invention given hereinafter and the accompanying  FIGS. 1-11 , which are given by way of illustration only, and thus are not limitative of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrate schematically a prior art measurement concept when determining positions of alignment marks in a coordinate system. 
       FIG. 2  illustrates schematically an example embodiment according to the present invention of a measurement concept when determining positions of alignment marks. 
       FIG. 3  illustrates a view from above of an example embodiment of a measurement apparatus according to the present invention. 
       FIG. 4   a - d  illustrate example embodiments of how different alignments marks may be measured by the apparatus according to  FIG. 3 . 
       FIG. 5   a - b  illustrate an example embodiment of a two mark reference method according to the present invention. 
       FIG. 6   a  illustrates two reference marks arranged in an imaginary coordinate system. 
       FIG. 7-8  illustrates en example embodiment according to the present invention of how another mark may be found. 
       FIG. 9  illustrates an example embodiment according to the present invention of how yet another mark may be found. 
       FIG. 10  illustrates an example embodiment according to the present invention of how valid mark positions may be determined. 
       FIG. 11  illustrates an example embodiment of transmission detection of mark according to the present invention. 
       FIG. 12  illustrates an example embodiment of a rotatable stage and supporting mechanism. 
   

   DETAILED DESCRIPTION 
   The following detailed description is made with reference to the figures. Preferred embodiments are described to illustrate the present invention, not to limit its scope, which is defined by the claims. Those of ordinary skill in the art will recognize a variety of equivalent variations on the description that follows. 
   Further, the preferred embodiments are described with reference to a substrate. It will be obvious to one ordinary skill in the art that said substrate may be a reflective substrate or a transmissive substrate. 
     FIG. 1  illustrates a classic measurement concept, i.e., a coordinate measurement system, for instance a XY coordinate system, where registration of marks  120  arranged on a substrate  100  are calculated from measurement of absolute positions relative an origo  130 . 
     FIG. 2  illustrates a measurement method according to an example embodiment of the present invention. In the inventive method measurement is based on measuring distance between marks  220  provide on a substrate  200 , i.e., mutual distances instead of absolute positions relative an origo as in the prior art method. 
     FIG. 3  illustrates a view from above of an example embodiment of a measurement apparatus  300  according to the present invention. Said measuring apparatus  300  comprising a distance measuring device  305 , a support frame  310 , a first detector  315 , a second detector  320 , a rotatable stage  325 , optional stage alignment marks  330 . The distance measuring device may for instance be an interferometer laser. In the illustrated example embodiment in  FIG. 3 , said distance measuring device  305  is arranged fixed on said support frame  310 . Said distance measuring device keeps control of the distance between said first detector  315  and said second detector  320 . This is accomplished by measuring the distance between to said first detector  315  and said second detector  320  at each moment in time. The distance between said first detector  315  and said second detector  320  is provided by computing the difference between said distances. In an alternative embodiment a distance measuring device is provided on one of said first detector  315  or said second detector  320 . In such an embodiment one will be provided with the absolute distance between said first and said second detector immediately without any calculation as in the previous example. 
   The support frame supports in this example embodiment the first detector  315 , the second detector  320  and said distance measuring device  305 . The support frame is provided to move in a first direction  340  over a substrate  350 . Said first detector and said second detector are movable in a second direction  335 , said second direction  335  may in an example embodiment be essentially perpendicular to said first direction  340 . 
   The stage  325 , upon which said substrate  350  may be provided, may be rotatable, denoted in  FIG. 3  by reference numeral  360 , around its central axis. Said stage may optionally be provided with alignment marks  330 . Said alignment marks together with alignment marks provided on said substrate  350  may be used in order to align said substrate  350  on said stage  325 , or for calibration purposes. 
     FIG. 4   a  illustrates a view from above of a plate  400  to be measured. Said plate  400  comprises 9 alignment marks denoted a-i in  FIG. 4   a.    
     FIG. 4   b - d  illustrates how a relative distance between alignment mark a and some other alignment marks are measured. To start with one of the detectors, the first detector  315  or the second detector  320 , detects alignment mark a. The detector, who has detected alignment mark a, is fixed to said alignment mark while the other detector is locating another alignment mark. In  FIG. 4   b  the other detector, first or second depending on which detector is fixed on alignment mark a, is detecting alignment mark b. The first and second detectors may be moved relative to each other in order to located said alignment mark b. It may also be that the support frame  310  is moved during said location of said alignment mark b as well as a possible rotation of the support upon which said plate is arranged which is currently measured. So, it may be a cooperation of three movements in order to find an alignment mark while fixing one of the detectors on another alignment marks, these movements are 1) the relative movements of the first detector  325  to the second detector  320 ; 2) the stage rotation; and 3) the movement of the support frame  310 . It is to be noted that the first and second detectors may be moved relative to each other so that its relative distance may be enlarged or reduced. The stage may be rotated in a clockwise fashion or in an ant-clockwise fashion. The support frame may be moved in a positive first direction or in a negative first direction. 
   In  FIG. 4   c  one of the detectors are still fixed on alignment mark a while the other detector is detecting alignment mark c. Compared to  FIG. 4   b , the distance between the first detector and the second detector has been changed, enlarged distance, the stage has been rotated clockwise and the support frame has been moved slightly to the right, i.e., in the positive direction. 
   In  FIG. 4   d  one of the detectors are still fixed on alignment mark a, and the other detector is detecting alignment mark i. A movement of the support frame  310 , a rotation of the stage and movement of the first and second detectors relative to each other may detect any mutual distance between any two alignment marks. The only restrictions are the minimum distance between two alignment marks which have to be larger than the possible minimum distance between said first and said second detector and the maximum distance between two alignment marks, which is defined as the maximum distance between said first and second detectors, i.e., the width of the support frame  310 . 
     FIGS. 5   a  and  5   b  depict an example embodiment of a two mark reference method. In this method the distance is firstly measured between a first alignment mark and all other alignment marks and secondly the distance is measured between a second alignment marks and all other alignment marks. The invention is not limited to the use of only two detectors. A plurality of detectors for measuring distance may be used to optimize throughput and/or accuracy. In the illustrated embodiment said first alignment mark is alignment mark a, and said second alignment mark is alignment mark e. Note that it is just the distance and not vectors in  FIG. 5   b . In  FIG. 5   a  a list is provided with two columns. A first column represent the distance from alignment mark a to all other alignment marks, each distance to respective alignment mark arranged in separate lines. In a second column the distance from alignment mark e to all other alignment marks, each distance to respective alignment marks in separate lines. From this list of distances from two alignment marks it is possible to determine all other relative distances between all alignment marks, the following illustrations will show how this may be accomplished. 
     FIG. 6   a  illustrates that alignment marks a end e may be placed in an imaginary coordinate system where rotation of said alignment marks are not important. This is because we only measure the mutual distance and not the distance relative to a fixed origin.  FIG. 6   a  also illustrates that the distance between a and e should be equal to the distance between e and a. Due to misperfection in any measurement system, a mean value of two measurement may better represent the reality than a single measurement. 
     FIG. 7  illustrates how a position of a third alignment mark may be found out of the list of measurement from two alignment marks. The alignment mark to be found its position of is alignment mark b. A first circle  710  with a radius equal to the distance from alignment mark a to alignment mark b is made with its center coinciding with alignment mark a. A second circle  720  with a radius equal to the distance from alignment mark e to alignment mark b is made with its center coinciding with alignment mark e. The first and second circles  710 ,  720  intersect with each other in this embodiment at two points.  FIG. 8  depicts that alignment mark b may be found at any of these intersection points b′, b″ of said first circle  810  with sad second circle  820 , however one of the intersection points b′, b″ represent a false position of the alignment mark b. 
     FIG. 9  illustrates how a position of alignment mark c may be determined. A third circle  930  having a radius equal to the distance from alignment mark a to alignment mark c is made with a center coinciding with alignment mark a. A fourth circle  940  having a radius equal to the distance from alignment mark e to alignment mark c is made with a center of said circle coinciding with alignment mark e. The third circle  930  and the fourth circle  940  intersect with each other at two points c′ and c″. One of these intersections c′, c″ represent the true position of alignment mark c. In the same figure the first circle  910  and the second circle  920  are drawn and the intersection points b′ and b″ of the first circle  910  with the second circle  920 . 
     FIG. 10  illustrates how the true positions of the alignment marks may be found. Alignment marks are provided on a substrate in predetermined positions. If two positions are known, as in this case, one may determine which of the two alternatives b′ and b″ will represent the true and false position respectively. In the leftmost picture in  FIG. 10  five alignment marks are illustrated, a, e, b′, b″, c′, c″, f′ and f″. By comparing the known predetermined positions of alignment mark, i.e., a sort of map where an alignment mark should appear with respect to substrate edges and other alignment marks, with the measured alternatives of the same alignment marks, one may determine which of the alternatives is true or false. In the leftmost picture in  FIG. 10   b″, c ″ and  f ″ represent a false position and they are therefore over marked with a cross. In the rightmost picture in  FIG. 10   b′, c ′ and  f ′, together with alignment mark a and e represent true alignment mark positions. The dashed alignment marks in the right most figures represent the positions of the rest of the alignment marks on the substrate. 
     FIG. 11  illustrates a side view of an example embodiment of a measuring apparatus  1100  according to the present invention. Said measuring apparatus  1100  comprising a first measuring detector  1110 , a second measuring detector  1120 , a support frame  1130 , a first illumination source  1140 , a second illumination source  1150 , a substrate  1160 , and a transparent support  1170 . The first measuring device  1110  and the second measuring device  1120  are movable along a positive and a negative first direction. The support frame  1130  is movable in a second direction, which is essentially perpendicular to said first direction. The first illumination source  1140  and the second illumination source  1150  are movable in the positive and negative first direction. The motion of said first and second illumination sources correlated to the motion of the first and second measuring detectors respectively. The illumination sources illuminate the substrate  1160  from beneath, i.e. in a transmission mode. The transparent support is rotatable, for instance by rotating the transparent substrate support  1170 . This rotation can be made according to  FIG. 12  which comprises no central shaft. In the inventive example embodiment illustrated in  FIG. 12  a stage  1210  is driven by means of perimeter drive, i.e., rotatable wheels  1220 ,  1222 ,  1224  contacting an outer rim of the stage  1210 . Rotating said wheels  1220 ,  1222 ,  1224  in a clockwise fashion will rotate said stage  1210  in a anti-clockwise fashion and vice versa. A light source  1230  is provided beneath/under the stage  1210 . Said rotatable wheels may not only have the functionality of contacting and rotating said stage. Said Wheels  1220 ,  1222 ,  1224  may also function as to support said stage, i.e., there is no need of a central shaft. 
   In yet an alternative embodiment according to the present invention said detectors  315  and  320  may be provided on a rotatable gantry, i.e., said detectors may not only be capable of moving back and forth from each other but they may also be rotatable around a point above said stage  325 . In such an embodiment said stage  325  may be fixed or rotatable. 
   In the illustrated embodiment only two measurement detectors are used. In alternative embodiment more than two measurement detectors may be used to speed up the measurement time. 
   While the present invention is disclosed by reference to the preferred embodiments and examples detailed above, it is understood that these examples are intended in an illustrative rather than in a limiting sense. It is contemplated that modifications and combinations will readily occur to those skilled in the art, which modifications and combinations will be within the spirit of the invention and the scope of the following claims.