Patent Publication Number: US-2016238951-A1

Title: Roll to roll mask-less lithography with active alignment

Description:
BACKGROUND 
     1. Field 
     Embodiments of the present invention generally relate to apparatus and methods for lithographic patterning. More particularly, embodiments of the present invention relates to apparatus and methods for lithographic patterning on flexible substrates. 
     2. Description of the Related Art 
     Flexible substrates, such as polymer/plastic substrates, metal foil substrates, are used to fabricate flexible circuits for various applications, such as display, organic light-emitting diodes (OLEDs), and solar cells. However, there are problems in creating a precisely located pattern image on top of an existing patterned layer where there is unavoidable and unpredictable distortion in the pre-existing pattern formed on the flexible substrates. The distortion in the existing pattern is due to inherent inhomogeneity and instability in the flexible substrate material. Such material stretches and shrinks in non-repeatable, non-uniform ways during sequences of deposition, exposure/patterning and etching. As a result, maintaining layer to layer overlay accuracy using conventional lithography is difficult and fine geometry control is almost impossible. 
     To date there have been several means of patterning on plastic substrates which achieve greater and lesser degrees of success, but always suffering from some sort of compromised performance. One means of patterning multiple layers on a plastic roll material substrate is to use “SAIL” (Self-Aligned Imprint Lithography) technology. SAIL not only compromises the design rules somewhat, but moreover, the patterns that can be created are also limited and force the user to limit the designs of the end products. Another means is to cut the rolls into individual plastic sheets and laminate them to stable substrate material, e.g., glass or metal; then process the laminated materials and delaminate the them when processing is fully completed. However, cutting, laminating, processing and delaminating have increased cost and the inefficiency of the extra steps and the yield losses associated with delaminating. Another means is to simply compromise design rules and accept a lower-grade display quality with large overlay margins. However, comprising design rules cannot satisfy the increased demand for high resolutions. 
     Therefore, there is a need for improved apparatus and methods for lithographic patterning on flexible substrates. 
     SUMMARY 
     Embodiments of the present invention relates to apparatus and methods for a maskless lithography on a flexible substrate with active alignment. 
     One embodiment of the present invention provides a lithography apparatus. The lithography apparatus includes a substrate transfer assembly comprising a cylindrical roller rotatable about a central axis and configured to transfer a flexible substrate on a cylindrical substrate supporting surface, and an image printing assembly comprising a plurality of printing units. Each of the plurality of printing units is positioned facing the substrate supporting surface and the plurality of printing units form an arc concentric to the substrate supporting surface. 
     Another embodiment of the present invention provides an apparatus for lithographic patterning. The apparatus includes a substrate transfer assembly for moving a flexible substrate continuously on a substrate supporting surface, an image printing assembly comprising a plurality of printing units disposed over the printing region and a controller connected with the image printing assembly. Each of the plurality of image units includes an image sensing device, and an image printing device. The controller is configured to perform, for each of the plurality of printing units, receiving and analyzing an image of an upcoming printing area captured by the image sensing device, determining one or more characteristics of the upcoming printing region, generating an exposure pattern from a target pattern and the one or more characteristics of the upcoming printing region, and sending the exposure pattern to the image printing device of the printing unit. 
     Yet another embodiment of the present invention provides a method for performing maskless lithography. The method includes moving a flexible substrate continuously on a cylindrical substrate supporting surface relative to a plurality of printing units disposed over the cylindrical substrate supporting surface, capturing an image of an upcoming printing region on the flexible substrate for each of the plurality of printing units, determining one or more characteristics of the upcoming printing area from the captured image, generating an exposure pattern from a target pattern and the one or more characteristics of the upcoming printing area, printing the exposure pattern on the upcoming printing region using the corresponding printing unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1A  is a schematic sectional view of a lithography apparatus according to one embodiment of the present invention. 
         FIG. 1B  is a schematic front view of the lithography apparatus of  FIG. 1A . 
         FIG. 1C  is a partial flattened side view of the lithography apparatus of  FIG. 1A . 
         FIG. 1D  is an enlarged portion of  FIG. 1C . 
         FIG. 2A  is a schematic partial perspective view of the lithography apparatus of  FIG. 1A  showing operation of a first row of printing units. 
         FIG. 2B  is a schematic partial perspective view of the lithography apparatus of  FIG. 1A  showing operation of a second row of printing units. 
         FIG. 2C  is a schematic partial perspective view of the lithography apparatus of  FIG. 1A  showing operation of a last row of printing units. 
         FIG. 3  is a schematic view of a printing unit according to one embodiment of the present invention. 
         FIG. 4  is a flow chart showing a method according to one embodiment of the present invention. 
         FIG. 5A  is a schematic sectional view of a lithography apparatus according to one embodiment of the present invention. 
         FIG. 5B  is a schematic front view of the lithography apparatus of  FIG. 5A . 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one embodiment may be beneficially utilized on other embodiments without specific recitation. 
     DETAILED DESCRIPTION 
     Embodiments of the present invention relates to apparatus and methods for a maskless lithography on a flexible substrate with active alignment. The lithography apparatus according embodiments of the present invention allows for very local (within a few millimeters or less) adjustments in patterning, compensating for variations/distortions in pre-existing patterns on a substrate. 
     In one embodiment, a lithography apparatus includes a cylindrical roller rotatable about a central axis and configured to transfer a flexible substrate on a cylindrical substrate supporting surface. A plurality of printing units, each includes an image sensing device and an imaging printing device, may be positioned facing the substrate supporting surface. The plurality of printing units may capture images of pre-existing patterns and/or markers on the flexible substrate as the flexible substrate is being transferred continuously and exposure patterns for each printing unit may be adjusted “on-the-fly” according to the captured image, thus achieving active alignment. 
       FIG. 1A  is a schematic sectional view of a lithography apparatus  100  according to one embodiment of the present invention.  FIG. 1B  is a schematic front view of the lithography apparatus  100 . The lithography apparatus  100  applies mask data patterns on flexible substrates used to forming flexible circuits, such as display, organic light-emitting diodes (OLEDs), solar cells, and the like. The lithography apparatus  100  includes a substrate transfer assembly  110  and an image printing assembly  120 . The substrate transfer assembly  110  moves a flexible substrate  102  relative to the imaging printing assembly  120  while the imaging printing assembly  120  prints a pattern on the flexible substrate  102 . A system controller  140  may be connected to the image printing assembly  120  and the substrate transfer assembly  110  to facilitate printing one or more layer of patterns on the substrate  102 . 
     The substrate transfer assembly  110  may include a cylindrical roller  112  and a driving unit  114  configured to rotate the cylindrical roller  112  about a central axis  116 . An outer surface of the cylindrical roller  112  forms a substrate supporting surface  118 . During operation, the flexible substrate  102  contacts and is supported by the substrate supporting surface  118 . Particularly, the portion of the flexible substrate  102  being printed by the imaging printing assembly  110  is supported by the substrate supporting surface  118 . 
     The imaging printing assembly  120  includes a plurality of printing units  122  each positioned facing the substrate supporting surface  118 . Each of the plurality of printing units  122  is configured to detect an image of a corresponding area of the flexible substrate  102  supported on the substrate supporting surface  118  and print on the flexible substrate  102  a pattern generated according to the detected image. Each of the plurality of printing units  122  is positioned at a fixed distance  126  to the substrate supporting surface  118 . Because the flexible substrate  102  contacts the substrate supporting surface  118  while being printed, any distortion of the flexible substrate  102  will not result in distortion of the distance between the flexible substrate  102  and each of the plurality of printing unit  122 , thus, improving quality of the printed images by reducing errors in field of depth. 
     In one embodiment, the fixed distance  126  to the corresponding area on the substrate supporting surface  118  is substantially the same for the plurality of printing units  122  so that the plurality of printing units  122  are arranged in a cylindrical plane concentric to the substrate supporting surface  118 . 
     In one embodiment, the plurality of printing units  122  form a plurality of rows  124   1 - 124   n . Each row  124   1 - 124   n  includes multiple printing units  122  linearly aligned along the direction parallel to the central axis  116  of the cylindrical roller  112 . The plurality of rows  124   1 - 124   n  are disposed parallel to one another. The multiple printing units  122  in each row  124   1 - 124   n  may be positioned at equal distance. The number of imagining units  122  in each row  124   1 - 124   n  may be the same. The printing units  122  among the plurality of rows  124   1 - 124   n  may be aligned in a staggered manner along the axial direction parallel to the central axis  116 . The staggered alignment of the plurality of rows  124   1 - 124   n  allows each printing unit  122  to print on a different area on the flexible substrate  102  and the plurality of rows  124   1 - 124   n  of printing units  122  cover an entire strip traversing the flexible substrate  102 . 
     In one embodiment, the plurality of printing units  122  may be mounted on guide bars  130  attached to a frame  132 . Each guide bar  130  may be parallel to the central axis  116  and support one row of printing units  122 . Locations of the printing units  122  along each guide bar  130  may be adjusted together or individually to achieve desired alignment of the imagining units  122 . 
     Each of the plurality of printing units  122  are connected to the system controller  122 . During operation, the cylindrical roller  112  rotates at a substantially constant rate to transfer the flexible substrate  102  relative to the printing assembly  120 . Each printing unit  122  may periodically capture an image of the corresponding surface area of the flexible substrate  102  being transferred by the cylindrical roller  112 . The captured image, including markers, patterns, or other surface features, may be transferred to the system controller  140 . The system controller  140  analyzes the captured image to determine characteristics of an upcoming printing area for the particular image unit  122 . For example, the system controller  140  may determine a coordinate, amount of distortion, amount of wandering, or other characteristics of the upcoming printing area. Based on the determined characteristics, the system controller  140  generates an exposure pattern for the printing unit  122 , and sends the exposure pattern to the printing unit  122 . The printing unit  122  prints the printing pattern upon receiving the exposure pattern. 
       FIG. 1C  is a partial flattened side view of the lithography apparatus  100  showing the arrangement of the plurality of printing units  122  and the relative positions of the plurality of printing unit  122  and the substrate supporting surface  118 .  FIG. 1D  is an enlarged portion of  FIG. 1C . In  FIG. 1C , the cylindrical substrate supporting surface  118  is flattened on an x-y plane for clarity. The x axis parallels to the central axis  116  of the cylindrical roller  112 . The y axis traverses the x axis and represents the direction the flexible  102  is transferred by the cylindrical roller  112  during operation. 
     Each of the plurality of printing units  122  has a footprint  144  and a printing region  142 .  FIG. 1C  schematically shows that the printing region  142  is located in one corner of the footprint  144  for clarity of illustration. The printing region  142  may be located in other positions. Because the printing unit  122  generally scales down the printed image to achieve high resolution, the printing region  142  is generally smaller than the footprint  144  for each printing unit  122 . Each printing region  142  may have a printing width  150  along the x direction and a printing length  152  along the y direction. For a substrate having a substrate width  146 , at least a number N of printing units  122  may be used to print a band  254  having a printing length  152  traversing the flexible substrate  102  while the flexible substrate  102  moves mono-directionally along the y direction, where N may be calculated by: 
     
       
         
           
             
               
                 
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                         substrate 
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                         width 
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                         146 
                       
                       
                         printing 
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                         width 
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                         150 
                       
                     
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     Because the footprint  144  of the printing units  122  is larger than the printing region  142 , the at least N printing units  122  may be arranged in a plurality of parellel rows  124   1 - 124   n  for printing on the entire region in the band  254 . Each row  124   1 - 124   n  may include multiple printing units  122  distributed along the x direction. The printing areas  142  of the multiple printing units  122  in each row  124  may have the same y coordinate and different x coordinate in the x-y plane. The printing units  122  in different rows  124   1 - 124   n  being staggered so that printing units  122  in different rows  124  do not print on the same areas when the flexible substrate  102  passes by. Each of the plurality of printing units  122  may have a printing area  142  starting at a unique x coordinate. 
     In one embodiment, the printing area  142  of one printing unit  122  may overlap with the printing area(s)  142  of printing unit(s)  122  designated to print neighboring area(s) to ensure that the entire width of the flexible substrate  102  is covered by the printing units  122 . In one embodiment, the plurality of printing unit  122  may be arranged so that neighboring printing regions  142  overlap with one another at between about ______ % to ______ % of the printing width  150 . 
     In one embodiment, a total printing width  148  covered by the plurality of printing units  122  may be greater than the substrate width  146  to tolerate any wanderings of the flexible substrate  102  during operation. Wandering refers to lateral shifting of the flexible substrate  102  in the x direction when the flexible substrate  102  is being transferred by the cylindrical roller  112 . 
     As shown in  FIG. 1C , the multiple printing units  122  in every row  124   1 - 124   n  are arranged in equal unit spacing  158 , and the plurality of rows  124   1 - 124   n  are positioned in equal row spacing  156 . Each of the plurality of rows  124   1 - 124   n  is shifted towards the right compared to the row  124   1 - 124   n  upstream. The amount of shifting between neighboring rows may be about the printing width  150  minus an overlapping width. The number m of printing units  122  in each row  124  may be determined by: 
     
       
         
           
             
               
                 
                   N 
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                         total 
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                         printing 
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                         width 
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                         148 
                       
                       
                         unit 
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                         spacing 
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                         158 
                       
                     
                     . 
                   
                 
               
               
                 
                   Equation 
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     The number n of rows  124  may be determined by: 
     
       
         
           
             
               
                 
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                         unit 
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                         spacing 
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                         158 
                       
                       
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                         width 
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                         150 
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                               percentatge 
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     Thus, the total n times m of printing unit  122  may be used in the image printing assembly  120 . 
     The amount of row spacing  156  may be arranged to be multiple times of the printing length  152  of each printing unit  122  for the imaging printing assembly  120  to efficiently cover the entire length (along the y direction) of the flexible substrate  102 . 
     During operation, the cylindrical roller  112  rotates at a substantially constant angular speed to transfer the flexible substrate  102  at a substantially constant linear speed relative to the image printing assembly  120 . The image printing assembly  120  of the lithography apparatus  100  may remain stationary during operation. The linear speed of the flexible substrate  102  relative to the imaging printing assembly  120  may be selected according to process speed of the printing unit  122  and the system controller  140  so that the flexible substrate  102  can be printed while moving at a constant linear speed relative to the imaging printing assembly  120 . In one embodiment, the flexible substrate  102  may travel at a speed of about 200-300 mm/minute relative to the imaging printing assembly  120 . While the flexible substrate  102  is being transferred, the band  154  on the flexible substrate  102  travels along the y direction sequentially aligning with each row  124   1 - 124   n  When aligned with a row  124  of printing units  122 , a portion of the band  154  is printed by the row  124  of printing units  122 . When the band  154  travels passed the last row  124   n , the band  154  across the entire substrate width  146  is printed. The imaging printing assembly  120  prints patterns on the flexible substrate  102  band by band as the flexible substrate  102  passes by. In one embodiment, the imaging printing assembly  120  may be configured to print band by band at a slightly overlapped manner to ensure the flexible substrate  102  receives continuous coverage length wise. 
       FIGS. 2A-2C  schematically illustrates a sequence of the band  154  of flexible substrate being printed by the lithography apparatus  100  of  FIGS. 1A-1C .  FIG. 2A  is a schematic partial perspective view of the lithography apparatus  100  showing the band  154  of the flexible substrate  102  being printed by the first row  124   1  of the printing units  122 . Only the first row  124   1  of the printing units  122  are shown in  FIG. 2A  for clarity. While the band  154  aligns with the first row  124   1  of the printing units  122 , each printing unit  122  in the first row  124   1  prints in a first region  204  in the band  154 . The multiple first regions  204  are spaced apart by the unit spacing  158  of the printing units  122  in the first row  124   1 . 
     In one embodiment, the flexible substrate  102  may include optional markers  202 . The markers  202  may be used by the lithography apparatus  100  to determine characteristics of the flexible substrate  102 . For example, the lithography apparatus  100  may determine position, distortion, and/or wandering of the flexible substrate  102  near the band  154  from a captured image of the markers  202 . Alternatively, the lithography apparatus  100  may determine characteristics of the flexible substrate  102  near the band  154  from a captured image of the existing image/pattern on the flexible substrate  102 . 
       FIG. 2B  is a schematic partial perspective view of the lithography apparatus  100  showing the band  154  of the flexible substrate  102  being printed by the second row  124   2  of the printing units  122 . Only the second row  124   2  of the printing units  122  are shown in  FIG. 2B  for clarity. While the band  154  aligns with the second row  124   2  of the printing units  122 , each printing unit  122  in the second row  124   2  prints in a second region  206  in the band  154 . The multiple second regions  206  are spaced apart by the unit spacing  158  of the printing units  122  in the second row  124   2 . Each second region  206  may overlap with a corresponding first region  204  by an overlapping strip  208 . Each second region  206  joins with the corresponding first region  204  to form a joined region  207 . After the second row  124   2  finishes printing, the band  154  have multiple joined region  207  that are printed and are at unit spacing  158  apart. 
     While the band  154  travels from the position aligning with the first row  124   1  to the position aligning with the second row  124   2 , additional bands  154 ′,  154 ″ may sequentially align with and be printed by the first row  124   1  when the row spacing  156  is larger than the printing width  150  of each printing unit  122 .  FIG. 2B  illustrates that bands  154 ,  154 ′,  154 ″ are spaced apart. Alternatively, the bands  154 ,  154 ′,  154 ′ may be joined together with overlapping regions to satisfy process requirements, for example to print patterns larger than the printing length  152  of one printing unit  122 . 
       FIG. 2C  is a schematic partial perspective view of the lithography apparatus  100  showing the band  154  of the flexible substrate  102  being printed by the nth row and last row  124   n  of the printing units  122 . Only the last row  124   n  of the printing units  122  are shown in  FIG. 2C  for clarity. Before arriving at the position that aligns with the last row  124   n  , the band  154  has been printed by the previous n-1 rows  124   1 ,  124   2 , . . . .  124   n-1  with multiple joined regions  210  spaced at equal distance apart. While the band  154  aligns with the last row  124   n  of the printing units  122 , each printing unit  122  in the last row  124   n  prints in a last region  212  in the band  154 . Each last region  210  overlaps with adjacent joined region(s)  210  closing any unprinted gaps in the band  154 . After the last row  124   n  finishes printing, the entire band  154  on the flexible substrate  102  has been printed. 
     Each printing unit  122  in the lithography apparatus  100  is configured to capture images of or near a printing region and to print a generated pattern without using a mask.  FIG. 3  is a schematic view of the printing unit  122  according to one embodiment of the present invention. 
     The printing unit  122  includes an image sensing device  302  and an image printing device  303 . The image sensing device is directed towards a printing area to capture an image of a portion of a printing plane  312 . The image printing device  303  is positioned to print a pattern on a portion of the printing plane  312 . The image sensing device  302  may be a CCD (charged-coupled device) camera. The image sensing device  302  is connected to a printing image controller  304 . The printing image controller  304  receives and analysis the captured images from the image sensing device  302 . The printing image controller  304  may be connected to and provide control to the plurality of printing units  122 . In one embodiment, the printing image controller  304  may be part of a system controller of a lithography apparatus, such as the system controller  140 . 
     The image printing device includes a DMD (digital mirror device)  306 , and one or more light sources  308  directed to the DMD  306 . The DMD  306  may include an array of micro mirrors. Each micro mirror may be switched between an ON position and an OFF position. At the ON position, the micro mirror reflects the light from the light source  308  while at the OFF position, the micro mirror does not reflect the light from the light source  308 . Each micro mirror may represent one pixel in a binary image. By switching individual micro mirrors between ON and OFF positions, the DMD  306  may project a pattern of binary image towards the printing plane  312  so that the pattern may be printed on a substrate  301  positioned on the printing plane  313 . In one embodiment, optics  310  may be positioned between the DMD  306  and the printing plane  312  to reduce the size of the binary image and increase resolution of the printed pattern. 
       FIG. 4  is a flow chart showing a method  400  for printing a maskless pattern on a substrate using the printing unit  122  according to one embodiment of the present invention. 
     In Box  410 , the camera  304  of the printing unit  122  may capture an image of upcoming printing region on the substrate to be printed and send the captured the image to the printing image controller  304 . The upcoming printing region may have one or more features such as markers and pre-existing pattern. 
     In Box  420 , the captured image may be analyzed to determine one or more characteristics of the upcoming printing region on the substrate by the printing image controller  304 . In one embodiment, analyzing the captured image may include indentify a location of the upcoming printing region on the substrate with respect to a target pattern to be printed by the printing unit  122 . The relative location may be determined by one or more marks and/or the pre-existing pattern on the upcoming printing region captured in the image. In one embodiment, additional features characteristics, such as amount of wandering, degree of distortions along different directions, may be determined from the captured image. 
     In Box  430 , an exposure pattern may be generated from the target pattern and the determined one or more characteristics of the upcoming printing region. Box  430  may be performed by the printing image controller  304 . In one embodiment, generating the exposure pattern may include cropping a portion of the target pattern that would fit in the upcoming printing region based on the determined relative location. In another embodiment, generating the exposure pattern may further include modifying the target pattern according to the amount of wandering and/or distortion. 
     In Box  440 , the exposure pattern is sent to the DMD  306  for printing. The array of micro mirrors in the DMD  306  may be switched to ON or OFF position according to the exposure pattern. The light source  308  may be powered and the light reflected by DMD  306  projected to the printing region to print the exposure pattern thereon. 
     Box  410  to Box  440  may be repeated when the substrate  301  moves continuously relative to the printing unit  122 . For example, in the lithography apparatus  100 , each of the plurality of printing units  122  may repeatedly perform Box  410  to Box  440  to print a pattern on the flexible substrate  102  which continuously moves relative to the printing units  122 . 
       FIG. 5A  is a schematic sectional view of a lithography apparatus  500  according to another embodiment of the present invention.  FIG. 5B  is a schematic front view of the lithography apparatus  500 . The lithography apparatus  500  is similar to the lithography apparatus  100  except that the lithography apparatus  100  includes enough number of printing units that allow a full continuous operation while the lithography apparatus  500  includes shifting mechanism and reverse substrate motion to allow a step by step operation using fewer printing units. 
     The lithography apparatus  500  includes the substrate transfer assembly  110  as described with  FIGS. 1A-1B . A system controller  540  may send control signal to the driving unit  114  to rotate the cylindrical roller  112  back and forth so that the flexible substrate  102  may be transferred both forwards and backwards. 
     The lithography apparatus  500  includes an imaging printing assembly  520  having a plurality of printing units  122  each positioned facing the substrate supporting surface  118 . The plurality of printing units  122  may be arranged in at least one row  524  along the direction of the central axis  116  of the cylindrical roller  112 . In the imaging printing assembly  520 , the total number of printing units  122 , total rows of printing units  122  and/or number of printing units  122  in each row may be less than the minimum numbers calculated according to Equations 1-3. The reduced numbers may be selected to reduce costs of a large number of printing units, limited by the space available for the minimum numbers of printing units due to factors such as smaller diameter of the cylindrical roller or higher resolution requirement. Three rows  524   1 ,  524   2 ,  524   3  of printing units  122  are shown in the example of  FIGS. 5A-5B . However, the number of rows may be varied depending on one or more factors, such as cost, space available, dimension of the cylindrical roller, or resolution requirements. 
     Each row  524   1 ,  524   2 ,  524   3  is similar to the rows  124  of printing units  122  of the lithography apparatus  100  described above except that the printing units  122  in each row  524   1 ,  524   2 ,  524   3  may be shifted along the direction parallel to the central axis  116  during operation. In one embodiment, the printing units  122  in each row  524   1 ,  524   2 ,  524   3  may be arranged in equal spacing and being shifted in unison to maintain the equal spacing. Alternatively, spacings between neighboring printing units  122  in each row  524   1 ,  524   2 ,  524   3  may be variously arranged according to processing requirement and each printing unit  122  may be shifted individually. 
     Each row  524   1 ,  524   2 ,  524   3  of printing units  122  may be mounted on a perspective guide bar  530  attached to a frame  532 . The guide bars  530  are positioned substantially parellel to the central axis  116 . In one embodiment, each printing units  122  may be attached to the guide bar  530  by a linear bearing. A shifting actuator  534  may be attached to each guide bar  530  to move the plurality units  122  along the guide bar  530  to shift positions of the plurality units  122 . In the embodiment of  FIG. 5B , the shifting actuator  534  is configured to move the printing units  122  in unison. Alternatively, each printing unit  122  may be attached to one shifting actuator and being shifted individually. 
     In addition to analyzing captured images of the flexible substrate  102  from the printing units  122  and generating exposure patterns according to the captured images as the system controller  140  does, the system controller  540  also controls and synchronizes the rotating direction and/or speed of the cylindrical roller  122  and the shifting of the plurality of rows  524   1 ,  524   2 ,  524   3 . 
     During operation, the cylindrical roller  112  first rotates forward at a substantially constant rate to transfer the flexible substrate  102  relative to the image printing assembly  520  so that a band on the flexible substrate  102  may be printed by all of the rows  524   1 ,  524   2 ,  524   3 of the printing unit  122 . After the band being printed passes all the rows  524   1 ,  524   2 ,  524   3  for the first time, the band is only partially printed because the number of printing units  122  in the image printing assembly  520  is less than the number of printing units  122  required to cover the entire width of the flexible substrate  102 . 
     The cylindrical roller  112  then rotates backward to transfer the band to be upstream to the first row  524   1  once more. The rows  524   1 ,  524   2 ,  524   3 of printing units  122  may be shifted so that each printing unit  122  aligns with a region in the band that is not previously printed. 
     Next, the cylindrical roller  112  rotates forward again to transfer the band of flexible substrate  102  at a substantially constant linear speed to so that the band can pass the rows  524   1 ,  524   2 ,  524   3  and being printed again. 
     The flexible substrate  102  may be transferred back and forth multiple times until the entire width of the band is printed. The cylindrical roller  112  may then rotate forward again to start the same printing process for next band of flexible substrate. 
     Even though apparatus and methods for maskless lithography are discussed in the examples above, embodiments of the present invention may be used in any application which requires similar “on-the-fly” exposure control. 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.