Abstract:
A maskless exposure system that has multiple maskless optical engines arranged in an (N×M) matrix that form and project a pattern onto a substrate. A first stage system is capable of driving the maskless optical engines in a first direction, a second stage system capable of holding and moving the substrate in a second direction perpendicular to the first direction. A control system that processes data and synchronizing movement of the first and second stage systems and a vision system that detects the positions of the second stage system to synchronize movements with the multiple optical engines.

Description:
BACKGROUND 
     The present system and method relates to maskless photolithography also called direct-write digital image technology for an ultra-large size flat panel display (FPD) patterning, and more particularly, it relates to an exposure apparatus for projecting a pattern directly onto an ultra-large substrate from a computer system so that the pattern is aligned with a previously formed substrate pattern in the computer system to produce an ultra-large flat panel display and the like. 
     Flat panel display (FPD) substrates have widely been used as display elements for personal computers, television sets and the like. Typically, a liquid crystal display (LCD) substrate is manufactured by forming transparent thin film electrodes on a photosensitive substrate (glass substrate) by photolithography. To carry out the photolithography, projection exposure apparatuses project a mask pattern onto a photoresist layer formed on a glass substrate through a projection optical system. 
     Recently, it has been desired that the area of flat panel display substrate be increased, and, accordingly, to increase an exposure area of the projection exposure apparatus. 
     In manufacturing large thin-film transistor LCDs, mass-producing 6 or 8 panels on a single glass substrate is typically most efficient. As demand for larger and larger LCDs continues to grow, manufacturers have increased mother glass dimensions from 680×880 mm 10 years ago to up to 2880×3080 mm now in mass production. There are several companies who are building the 10th Generation factory, which uses 2880×3080-mm glass. 
     In order to increase the exposure area, there has been proposed an exposure apparatus of so-called step-and-scan. In step and scan, after an initial exposure, the mask and the photosensitive substrate are shifted by a predetermined distance in the direction perpendicular to the scanning direction and then another scanning-type exposure is achieved. 
     The pixel cell array and color filter patterning processes in LCD manufacturing create some of the greatest challenges in scaling to Gen 10, in terms of both technology requirements and manufacturing costs. Typical alpha silicon (a-Si) thin-film transistors have critical dimensions around 3.5 μm and require alignment accuracy of ±1 μm. In color filter manufacturing, only the black matrix step (a black screen like pattern formed on the color filter that prevents light leakage, improves contrast and separates RGB sub-pixels) requires less than 10 μm resolution and alignment accuracy of less than ±3 μm, RCSB pixels, spacers and vertical alignment protrusions typically do not necessitate resolution precision of less than 20 μm. Even though LCD exposure specifications are large compared with those of semiconductors, the challenges in exposing very large areas and maintaining throughput presents serious issues to be overcome. 
     The primary method of maintaining productivity as substrates have grown has been to increase the size of the mask and exposure field. The largest masks used in production today for Gen 8 are 1220×1400×13 mm. With a pellicle (a thin, transparent membrane that prevents particles from contaminating the mask surface) attached, these easily can cost more than $350,000 for a single binary mask. To maintain throughput at Gen 10 and expose 2880×3080 mm substrates in four scans, photolithography and mask companies are developing even larger masks in the range of 1600×1800×17 mm. Initially, these very heavy quartz masks may cost more than $1 million apiece. 
     In the case of array exposure, average Gen-10 machine prices are expected to be nearly six times higher than those of Gen-4 machines, white the average increase for other tool types likely will be around twice as high. In 2000, photolithography costs accounted for only 14 percent of total array equipment spending, but when Gen-10 tools begin shipping, the costs are expected to account for up to 29 percent. For these reasons, exposure is a prime target of cost-cutting strategies. 
     In conventional photolithography, the patterned masks or films for high resolution application are typically very expensive and have a short lifetime. In addition, the photomasks are characterized as requiring a very long lead time. The long mask lead time is a problem when a short product development cycle is desired. Further, if a particular mask design is found to require a design change, no matter how small the change, then mask modification cost and lead time to implement the required change can cause serious manufacturing problems. 
     At present there is a need for a viable alternative to conventional photolithography for mass production that can meet all of the requirements of the pixel cell array process in LCD manufacturing. 
     SUMMARY 
     The present system and method has solved these previously stated problems and describes a system for mass production of ultra large flat panel displays that meets production registration requirements and allows immediate correction of any design issue. 
     An object of the present system and method is to provide a maskless exposure apparatus which can realize full scanning exposure of an ultra-large exposure area with excellent imaging performance and low running cost, short cycle time and without lowering the manufacturing throughput. One of the technical advances is achieved by a novel maskless optical engine and method for photolithography which provides a digital image from a spatial light modulator (SLM) writing directly onto specific sites on an ultra-large size substrate with vision systems which detect alignment marks and errors between the maskless engine and the site of the ultra-large substrate. The spatial light modulator acts to form a pixel image on the surface of a substrate. The system may also be designed with two conjugate points, one of which is coincident with said spatial light modulator and another conjugate point coincident on the substrate. The end result overcomes the disadvantages of the conventional exposure systems which are long, mask lead time and high cost of the photomasks. 
     In the present system and method, the maskless scanning exposure apparatus performs an ultra-large maskless exposure that utilizes UV light sources, spatial light modulators (SLM), maskless optical engines, motion stage systems, reference position sensors, vision systems, a control system, data conversion and data processing software utilized in a computer system. It is envisioned that the exposure apparatus light source may include ultraviolet, infrared, visible light, electron beam, ion beam, and X-ray sources. 
     The general method is to generate pattern data from the computer and expose photoresist on the surface of an ultra-large size substrate through spatial light modulators (SLM) that are imaged by a maskless optical system (maskless optical engine) over a moving stage. Spatial light modulators may include DMD, LCOS, LCD and other 2D display panels. 
     The maskless composite engine may include to position detector which ma be a CCD camera, or laser position detector or other position detector to read the position of a reference line or marks which are fixed on the stage or the substrate. The maskless composite engine may also include real-time image generation to form an image on the substrate front the designed pattern, at the position where the control system reads the stage encoders and position detectors. The maskless composite engine may also include an auto-focus system which detects the focus distance to the surface of the ultra-large substrate and adjusts the maskless optical engine position to get the best focus at anytime. 
     In order to achieve the objects described above, an ultra-large size FPD maskless exposure system of a first aspect of the present system and method includes an ultra-large substrate with photo sensitive material, a maskless composite engine which is set above the substrate and includes a maskless optical engine with an exposure light source (maskless optical engines can share one light source by a beamsplitter). The system may also include a vision system with a light source, the wavelength of which is different from light used to expose photoresist on the substrate. The vision system may be mounted with each of the maskless optical engines and aligned to the optical axis with the maskless optical engine axis that a beamsplitter nearly transparent for exposure light and partially reflective for the vision system light. The vision system may monitor the position of the optical engine at the start position. The function of the vision system is to check optical engine position and also align the exposure pattern with the ultra-large substrate position. The maskless optical engine may have an individual controlled Z axis stage to change the distance between the maskless optical engine and substrate by auto-focus detect function. The maskless optical engine may have a Y axis motion stage which is controlled by a control system and synchronized with maskless optical engines, the stave can make relative moving between maskless optical engines and the substrate. In one embodiment, the system includes one row of the above maskless composite engines for generating the pattern and for creating a plurality of pixel elements by SLM on the ultra-large substrates. 
     The maskless composite engines art mounted on an X stage which is controlled by the control system. There may be two reference position sensors which are fixed above the edges of the Y stage along the scanning direction, during operation. There may be a reference line or mark under the each reference position sensor which is located on the stage or on the substrate. The system may also include position marks on the stage under the maskless composite engines at the start points of the scans. The position marks may also indicate the end point and line of movement. In general, the exposure region of the maskless optical engine is a rectangle shape and the rectangle is tilted an angle relative to the scanning direction about 1˜10 degree according to exposure parameter selection. 
     If the ultra-large substrate surface has no pattern, the maskless composite engines can directly read the reference marks on the stage from the vision system to ascertain the correct position. The two reference position sensors may read the position and calculate the errors in a direction perpendicular to the scanning direction for the stage position relative to the reference lines. The errors may include Y stage yaw, straight and pitch errors during translation, temperature variation and vibration induced errors. 
     The computer system may generate the image data based on the position information, then turn on the exposure light source and start the scan. When the stage is scanning, the two reference position sensors read the stage position so that the position error may be calculated corrected for by updating the image data sent to the maskless optical engines. If a prior pattern exists on the substrate surface, the vision systems may read the mark positions in the pattern and save the position data of the substrate in the computer and correct exposure data. 
     If a multi-scan is to be performed on the entire substrate, the X stage translates a distance of the width of the exposure region and the vision system reads the marks at the each start point of the scan for each maskless engine. In this case, the exposure regions through the respective maskless optical engines are formed on the ultra-large substrate so that a sum of the lengths along the scanning direction is constant over the direction perpendicular to the scanning direction. The amount (dose) of exposure light is set to be constant over the entire surface of the substrate. 
     At the intersection of two scans the overlay is referred to as the stitch area. Due to the tilt, of the substrate, the stitch area is smoothed from one scan to the next, so that multi-scan exposure can achieve a large exposure area with accurate and smooth exposure even with compact maskless optical engines and small exposure regions. Also, if each maskless optical engine is compact, scanning exposure can be made while minimizing occurrence of aberrations and keeping excellent imaging performance. To increase the exposure speed, several rows of the maskless engines also can be added in a staggered arrangement. 
     Another aspect of the present system and method includes a substrate with photo sensitive, material on the both sides, a matrix of maskless composite engines set above the substrate. The M×N matrix of maskless composite engines are on a same plane above the XY stage and are aligned in M rows and N columns in X and Y direction respectively. An XY motion stage is controlled by a control system and synchronized with the maskless composite engines. The XY stage translates the substrate relative to the maskless composite engines. A reference mark plate or mask plate with the stage or the substrate is transparent to the exposure light source and reflect the vision system light. The reference mask includes lines and marks for the maskless composite engines and is located on vision system focus plane. In general, the exposure region of the maskless optical engine is rectangular and is tilted an angle relative to the scanning direction about 1˜10 degree according to exposure parameter selection. The first and second stages may move in a dimensional plane parallel to the substrate. 
     In the exposure process, the exposure light sources are first turned off and the vision system light sources are turned on. If the ultra-large substrate surface has no pattern, the vision system of the maskless composite engine can directly read the mark on the reference mark plate to correct the stage position. The vision system also may include a reference position sensor to read the position for position error calculation of the stage relative to the reference marks or lines. The computer system generates image data based on the position information for each maskless composite engine and then turns on the exposure light source to scan in the Y or X direction. 
     During scans, the errors of the stage position are fed back to the computer system from the vision systems and the corrected image data are send to each maskless optical engines for exposing. If there is a prior pattern on the substrate surface, the vision systems can read the mark positions in the pattern and then save the substrate position data in the computer and correct exposure data. 
     If a multi-scan for the entire substrate is to be performed, the stage is translated a distance of the width of the exposure region at X direction (or Y direction) and the vision systems reads the marks at the each start point of the scans for each maskless engine, in this case, the exposure regions through the respective maskless optical engines are formed on the ultra-large substrate so that a sum of lengths along the scanning direction is constant over the direction perpendicular to the scanning direction. The amount of exposure light is set to be constant over the entire surface of the substrate. 
     The region between the overlay of two scans is referred to as the stitch area. Due to the tilt of the substrate, the stitch area is smoothed from one scan to the next. There is also a stitch area between the exposure areas of maskless optical engines at the start or end of each scan. To smooth this stitch area, a grey scale exposure mode or as control the exposure light intensity to get smooth transition from one engine to the next engine is used, so that multi-scan exposure can achieve a large exposure area with accurate and smooth exposure on the whole substrate. 
     The present system and method aims to eliminate the above-mentioned drawbacks in a conventional exposure system, and an object of the present system and method is to provide a maskless exposure apparatus having alignment mark detection system with each maskless optical engine which can greatly improve accuracy in alignment of patterns with a simple arrangement. 
     It is further object of the present system to provide a maskless exposure system that has multiple maskless optical engines arranged in an (N×M) matrix that form and project a pattern onto a substrate. A first stage system is capable of driving the maskless optical engines in a first direction, a second stage system capable of holding and moving the substrate in a second direction perpendicular to the first direction. A control system processes data and synchronizes movement of the first and second stage systems and a vision system that detects the positions of the second stage system to synchronize movements with the optical engines. 
     One method of projecting a pixel-mask pattern onto a plate has a first side coated with a first photosensitive material and a second side coated with a second photosensitive material. The method uses a plurality of maskless optical engines, forming and projecting a pattern onto a substrate, driving the maskless optical engines in a first direction, moving the substrate in a second direction perpendicular to the first direction, synchronizing the driving of the first stage system and the moving of the second stage system, and detecting positions of the second stage system to synchronize movements with optical engines. 
     The present system and method will become more fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present system and method. 
     The scope of applicability of the present system and method will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the system and method, are given by way of illustration only, since various changes and modifications within the spirit and scope of the system and method will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and other advantages of the present system and method will be more clearly understood horn the following detailed description when taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a basic diagram illustrating a maskless optical engine; 
         FIG. 2  illustrates a maskless optical engine with a vision system; 
         FIG. 3  illustrates a maskless optical engine with a co-axial vision system; 
         FIG. 4  illustrates another embodiment of  FIG. 3 ; 
         FIG. 5  illustrates an ultra-large size maskless exposure system with two maskless optical engines and two reference position sensors by multi-scans; 
         FIG. 6  illustrates a stitch area by two scans; 
         FIG. 7  illustrates stitch areas with two stagger rows of maskless optical engines for a whole area exposure by one scan; 
         FIG. 8  illustrates a matrix maskless exposure system of the present system and method; 
         FIG. 9  illustrates a stitch area vs. a light intensity change for adjacent two maskless engines in the scanning direction; 
         FIG. 10  illustrates reference mark plate layout; 
         FIG. 11  illustrates reference mark plate installation for a matrix maskless exposure system; 
         FIG. 12  illustrates reference mark plate installation on top of the system for a matrix maskless exposure system; 
         FIG. 13  illustrates reference mark plate installation on the top of the stage for a matrix maskless exposure system; 
         FIG. 14  illustrates block diagram of a maskless exposure system; 
         FIG. 15  illustrates flow chart for a maskless exposure of the present system and method; 
         FIG. 16  illustrates a perspective view of a system design of the present system and method; 
         FIG. 17  illustrates a top view of  FIG. 16 ; 
         FIG. 18  illustrates a perspective view of a system design of the present system and method; 
         FIG. 19  illustrates a front view of  FIG. 18 ; 
         FIG. 20  illustrates a top view of  FIG. 18 ; 
         FIG. 21  illustrates a perspective view of an ultra-large system design of the present system and method; 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure relates to an ultra-large size maskless exposure system, such as can be used in PCB, LCD, and other photolithographic processing. It is understood, however, that the following disclosure provides many different embodiments, or examples, for implementing different features of one or more systems and methods. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to limit the present disclosure from that described in the claims. 
     Reference will now be made in greater detail to an exemplary embodiment of the system and method, an example of which is illustrated in the accompanying drawings. Wherever possible, the same reference numerals may be used throughout the drawings and the description to refer to the same parts for easy description and understanding. 
     With reference now to  FIG. 1 , a simple maskless exposure system includes a light source  101  with a optical fiber  116  and light collimator and homogenizer  102  the output light is reflected by mirror  104  to a spatial light modulator (SLM)  103 , a optical lens  105  images SLM  103  to a subject  107 . A resist layer or coating may be disposed on the surface of the subject  107 . The light collimator and homogenizer  102  provide a uniform tight beam onto the SLM  103 , a stage surface plate  108  is to hold the substrate  107 . The stage surface plate  108  can move XY direction by a control system. The SLM  103  creates a desired pixel pattern (the pixel-mask pattern). The pixel-mask pattern may be available and resident at the SLM  103  for a desired, specific duration which is synchronized with the stage surface plate movement. Light emanating from for through) the pixel-mask pattern of the SLM  103  then passes through the optical system  105 . The light from optical system  105  focuses onto the top surface of the substrate  107 . The substrate  107  may be a LCD glass plate, PCB board, or semiconductor wafer. It is understood, however, that many different substrates can benefit from the present system and method, including for further example, a non-flat substrate. It is desired to project a plurality of patterns on the substrate  107  using the maskless exposure system. There are Z axis moving mechanisms for focus adjustment of maskless optical engine  100 . 
       FIG. 2  shows a maskless exposure system with a vision system. A vision system consist of a camera  203 , an image lens  204 , a light source  201  which does not expose the photo sensitive material on the substrate  107 , such as yellow or red light. A collimation lens  202  is to collect the light from the light source  201  and illuminates the substrate  107  thru a beamsplitter  206  which generally has approximately half light reflection and half transmission. The light is reflected back from the substrate  107  and goes to camera lens  204  and camera  203 . The camera reads the pattern on the substrate if a pattern is present. In this example, this vision system reads the mark, the vision system optical axis is not aligned with maskless optical system axis  106 . If the maskless optical system axis  106  shifts due to temperature, vibration or other factors, the vision system lacks inline feedback to correct for the error. To check the optical axis alignment exposure testing and measure of the optical axis  205 , 106  positions is required. 
       FIG. 3  shows an approach for maskless optical system alignment by utilizing a co-axis vision system. In this example a beamsplitter  301  is added to the front of the optical system  105 . The purpose of the beamsplitter  301  is to combine the vision system axis and maskless optical system axis  106 . The beamsplitter  301  is almost transparent for the light from the light source  101  and semi-reflects the light from light source  201 . The optical system  105  requires optimization of beamsplitter  301  to improve image quality. 
       FIG. 4  shows another approach having a function similar to  FIG. 3 . In this example the vision system and beamsplitter  301  are set proximate to SLM. Additionally, the camera lens  204  may be removed if the working distance of the optical system  105  allows. 
     The maskless optical engines in  FIG. 1 ,  FIG. 2 ,  FIG. 3 , and  FIG. 4  are simple maskless engine  100  in  FIG. 1  which is called Gen-1 maskless system. There are Gen-2 and Gen-3 of the maskless systems which can replace the maskless optical engines for higher performance. 
       FIG. 5  shows a maskless system of the present system and method that includes two of the maskless composite engines in  FIG. 3  or  FIG. 4  as regions  507 ,  514  at a row for generating the pattern and for creating a plurality of pixel elements on the ultra-large substrate  517 . The substrate  517  is held in place by a vacuum table  504  which can move along the  511  direction (Y) and be synchronized with the maskless composite engines by a control system. The regions  507 ,  514  are images projected from the two maskless composite engines on the substrate  517 . The maskless composite engines are mounted on an X stage (not show here) which is controlled by the control system also. There are two reference position sensors  515 , 508  which are fixed on the X stage bridge above the edges of the table  504  along the scanning direction Y. 
     A reference line  516 , 505  (or mark) is located under the each reference position sensor  515 , 508  which is located on the stage  504  or on the substrate  517 . Position marks (cross marks)  519  are located on the stage  504  under the maskless composite engines at the start points of each scan. In general, the exposure regions  514 , 507  of the maskless optical engines rectangular shaped  514 , 507  and are tilted at an angle relative to the scanning direction Y about 1˜10 degree according to exposure parameter selection. 
     The Y stage  504  is supported by two linear bearings  501 , 503  which sit on a granite base  502  with vibration isolators (not show here). Due to the size of the Y stage, there are two linear motors  512 , 510  on the both sides of the stage  504  and two linear encoders  513 , 509  for Y stage position feedback. During exposure, at first, the exposure light sources are turned off and the vision system light sources of the maskless optical engines are turned on. If the substrate surface  517  is unpatterned, the vision systems can directly read the marks  519  on the stage  504  to correct the position. The two reference position sensors  515 , 508  read the position for error calculation in the direction perpendicular to the scanning direction Y. Y stage  504  position errors may include yaw, straight and pitch errors during translation. The computer system generates the image data based on the position information and then turns on the exposure light source to start the scan. When the Y stage  504  is scanning, the two reference position sensors  515 , 508  read the Y stage position errors in the direction perpendicular to the scanning direction, and then the computer system corrects the image data and sends the data to the two maskless composite engines. If there is a pattern on the substrate surface, the vision systems of the maskless composite engines reads the alignment mark positions on the substrate and saves the position data in the computer. The corrected the image data is matched to the substrate position for exposure. If a multi-scan in  FIG. 5  is required for the entire substrate, the X stage translates a distance equal to the width of the exposure region  514 , 507  and the vision systems reads the marks  519  at the each start point of the scan for each maskless engine. 
     If during the scan, the two encoders  513 , 509  do not match, the two sides of the Y stage are not at same position, as an example; 
     assume there is a difference dy=Y 1 −Y 0 , 
     where Y 0  is data from encoder  513  and Y 1  is data from encoder  509 , 
     so that based on each maskless optical engine X position, the Y position of the maskless engine can be calculated by,
 
 Y=dy*X/L+Y 0.
 
     where L is the distance between two encoders. 
     The computer generates the data according to Y for the synchronization of each maskless composite engine. The maskless composite engines  507   514  each scan four times  518 , 506  for entire substrate exposure. 
       FIG. 6  illustrates a stitch area between two scans, in the case of  FIG. 5 , the maskless exposure system will expose multiple regions through the respective maskless optical engines  514 , 507  formed on the ultra-large substrate  517 . Multiple exposure regions  601 , 606  are arranged so that the sun of pixel lengths along the scanning direction  603 , 605  is constant over the direction perpendicular to the scanning direction  603 , 605 , so that the amount of exposure light is constant over the entire surface of the substrate  517 . Since the exposure regions  601 , 606  are tilted and the exposure regions  601 , 606  are arranged so that the sum of widths of exposure regions along the direction perpendicular to the scanning direction  603 , 605  is constant over the scantling direction  603 , 605 . There is a stitch area  602 , 607  between scans which is overlaid, by two scans  603 , 605 . Due to the tilt of rectangles  601 , 606 , the stitch area between lines  602 , 607  is smooth transition from one scan to next scan, so that multi-scan exposure can achieve a large exposure area with accurate and smooth exposure on the whole substrate. Also, since each maskless composite engine is compact, scanning exposure can be made while minimizing occurrence of aberrations and keeping excellent imaging performance. To increase the exposure speed, several rows of the maskless engines also can be added in a staggered arrangement. 
     In  FIG. 7  illustrates stitch areas from two staggered rows of maskless composite engines, in this case, one scan can be done for a whole substrate exposure. The exposure regions  701 , 721 , 720 , 719  are on the first row and the exposure regions  704 , 712 , 711  are on the second row. The first row will scan along the paths  703 , 705 , 708 , 710  and the second row will scan along the paths  705 , 707 , 709 . There are stitch areas as  702 , 714 , 715 , 716 , 717 , 718 . Since the pitch of the maskless engines is same as the effective scan width of each maskless optical engine, this staggered engine setup does not need an X stage. 
       FIG. 8 , is an alternate embodiment of the example shown in  FIG. 5 . The substrate  517  is translated at in the Y direction and maskless composite engines  507 , 514  are translated in the X direction. The maskless exposure system in  FIG. 8  utilizes same maskless composite engines as  FIG. 5  but the substrate  808  is moved, in both XY directions. Y direction in this instance is the scanning direction. There are four maskless composite engines  813 , 815 , 819 , 820  formed a matrix above the substrate  808 . The XY stage is also set on a granite base  801  which is supported by vibration isolators (not show here). There are two pairs of encoders  806 , 802 , 812 , 809  for X and Y direction positions. Since there is a 2×2 matrix of maskless optical engines, the XY stage travel length just need about ½×½ of the substrate size  808 . The maskless composite engines  813 , 815 , 819 , 820  are mounted above the substrate  808  with a Z axis or individual Z axis for focusing. The engine  813  scans along line  814  and the engine  819  scans along line  817 . There are two lines  818 , 816  which indicate an overlay area from two rows of engines  813 , 820  and  815 , 819 . Since this overlay area is in a direction perpendicular to the scanning direction, the tilt of maskless optical engines do not assist in smoothing the transition from one maskless engine exposure area to next maskless engine exposure area. One way of smoothing the transition is to use a grey scale exposure between lines  818 , 816  but in binary exposure pattern case, a grey scale exposure may not be applied. The solution is to decrease the light source output of the first maskless engines  813 , 820  and increase the light source output of the second maskless engines  815 , 819  between the lines  818 , 816 . The scan of engines  813 , 820  starts or stops at Y 3  line and the engines  818 , 816  are turned off when the scan speed is decreased to zero front Y 2  to Y 3 . The scan of engines  815 , 819  starts or stops at Y 0  line and the engines  815 , 819  are turned off when the scan speed is decreased to zero from Y 1  to Y 0 . 
       FIG. 9  illustrates the light intensity and scan speed change between Y 0  and Y 3 . The lines  902 , 903  indicate the light output of engines  813 , 820  versus positions and the lines  904 , 906  show the light output of the engines  815 , 819 . The curve  905  shows the speed of the engines  813 , 820  and the curve  901  shows the speed of the engines  815 , 819 . 
       FIG. 10  illustrates a reference mark plate for the system in  FIG. 8 . Due to the fact that the stage  807  moves in both the X and Y directions, the reference mark can&#39;t be same as the system in  FIG. 5 . To correct the position of stage  807  a plate is required that shows whole tracks of the maskless composite engines. In  FIG. 10 , the rows of cross marks  1001 , 1004  are the start points or stop points of the engines  813 , 820  and the rows of cross marks  1003 , 1006  are the start points or stop points of the engines  815 , 819 . The line  1002  is the scanning track of the engine  813 . The line  1005  is the scan track of the engine  815 , and line  1002  and  1005  are identical. The line  1010  and  1008  are the scanning tracks of the engines  820  and  819 . The rectangular area  1007  is the effective exposure area. 
       FIG. 11  illustrates the reference mark plate position in the maskless system  FIG. 8 . The reference mark plate  1106  is a transparent glass plate for the exposure light  101  and it is fixed on the XY stage  1102  by connectors  1101 . The maskless composite engine generates and projects an image on the surface  1103  of the substrate  808 . The reference marks can be on the surface  1104  or  1105 . The marks are transparent for the exposure light source  101  and reflect the vision system light  201 . The vision system focuses on the mark surface rather than the substrate surface  1103 . In other embodiments, the maskless composite engine can be replaced by the engines in  FIG. 3  and  FIG. 4 . There several methods to make the surface  1103  the focus plane of the vision system. A first method is to design the reference mark as hologram on the surface  1104  or  1105  and design a virtual reference mark are on the surface  1103 . A second method is to design the reference marks on the surface  1104  and coat the surface  1105  with a film that reflects light  201  but is transparent for the exposure light source  101 . Therefore, the surface  1105  acts as a mirror for the reference mark on the surface  1104 . If the distance between the surface  1104  and  1105  is equal to the distance between surface  1103  and  1105  then the virtual image of the reference marks on the surface  1104  appears to be located on the surface  1103 . Due to the fact that the marks on the reference plate are typically very fine lines or dots, the vast majority of vision system light  201  passes thru the surface  1104  without image quality degradation, so the light can reflect back from the surface  1105  to the detector  203 . 
       FIG. 12  illustrates an embodiment of the reference mark plate location in  FIG. 11 . The reference mark plate  1204  is placed on the top of the maskless composite engines and the vision system focuses on the top rather than the substrate surface  1202 . The reference mark plate  1204  is connected with the stage  1201  by the parts  1203 . The marks may not be transparent for the light source  101  and  201 . 
       FIG. 13  shows an embodiment of the reference mark plate location in  FIG. 11 . The reference mark plate  1303  is placed under the substrate and the vision system focuses on the reference marks thru the substrate surface  1302 . The reference mark plate  1204  is directly connected with the stage  1301 . In this case, the substrate must be transparent to the light source  201 . 
       FIG. 14  shows a system block diagram which includes basic parts for a maskless exposure system in the present system and method. 
       FIG. 15  illustrates the flow chart of a maskless exposure system. This is just for one scan. 
       FIG. 16  illustrates a perspective view of a system design of the present system and method. The system includes four maskless composite engines  1614 ,  1613 ,  1612 ,  1611  on the XYZ stage  1616 . The each engine can have an individual Z stage for auto-focus function. The substrate  1605  does not move on the base plate  1604 . The engines  1614 ,  1613 ,  1612 ,  1611  scan relative to the substrate  1605 . The bridge  1606  is driven by two linear motors  1603 ,  1607  with linear encoders  1601 , 1609  in Y direction and set on the linear bearings  1602 , 1608 . There are two reference position sensors  1615 , 1610  on the bridge  1606  to detect position errors relative to reference lines which are indicated in  FIG. 5 . The reference marks are fixed on the base plate  1604 . 
       FIG. 17  illustrates a top view of  FIG. 16 . The substrate  1605  does not move in this system. The track  1701  shows the scan path of the engine  1614  and other engines. 
       FIG. 18  illustrates a perspective view of a system design of the present system and method. There is 4×4 matrix of maskless composite engines  1805  which are mounted on the granite plate  1804 . The reference mark plate is put on the XY stage  1802  as in  FIG. 11  with connection  1801 . 
       FIG. 19  illustrates a front view of  FIG. 18 . As in  FIG. 11 , the reference mark plate  1803  is located between the matrix engines and substrate so the vision system in the engine  1805  focuses on the reference mark plate which is above the XY stage  1802 . 
       FIG. 20  illustrates a top view of  FIG. 18 . The rectangles  2002 , 2005 , 2007  show the scan areas for engines  2003 , 1805 , 2006  respectively. The track  2004  shows the scan path of the engine  1805 . Since there is a 4×4 matrix of the maskless engines, each engine exposes about 1/(4×4) area on the substrate. The XY stage just needs ¼ X and ¼ Y travel length of the substrate size. 
       FIG. 21  illustrates a perspective view of an ultra-large system design of the present system and method. The system includes 10×10 maskless engines  1805  in a matrix. The stage  2105  is set on the granite base  2104  and moves in both the X and Y directions. The XV position of the stage  2015  is measured by laser interferometers  2102 ,  2101 . The size of the substrate  2103  may be up to 3 meters×3 meters. The XY stage  2105  just needs to move 1/10 of the substrate size which means about 300 mm in this example. The reference mark plate can be located on the top of the matrix engine or above the substrate  2103 . 
     A calibration system may also be part of the system in which a first calibration light source emits a first calibration light whose wavelength spectra does not photo-react the photo sensitive material on the substrate and communicably coupled to the control system. The system may include a first beamsplitter, where the first calibration light is sent through the first beamsplitter and reflected from the second stage system or the substrate and a first camera system, where the first calibration light is sent back to the camera system through the first beamsplitter. The calibration system may also include a second calibration light source that emits a second calibration light whose wavelength spectra does not photo-react the photo sensitive, material on the substrate, the second calibration light source is communicably coupled to the control system, a second beamsplitter that splits an input light into the maskless optical engines, where the second calibration light is sent through the second beamsplitter and reflected from the second stage system or the substrate. The system may also include a third beamsplitter, and a second camera system, where the second calibration light is sent hack to the second camera system through the second and third beamsplitters. 
     While the system and method has been particularly shown and described with reference to the preferred embodiment thereof, it will be understood by those skilled in the art that various modifications, additions and substitutions in form and detail may be made therein without departing form the spirit and scope of the system and method, as set forth in the following claims.