Abstract:
A double-sided maskless exposure system and method consists of light sources which includes two light wavelength segments, maskless optical engines in which a 2D spatial light modulation (spatial light modulator) device, such as DMD, is generating a plurality of pixel array of the pattern, vision system, moving substrate and computer control system. The double-sided maskless exposure system at least includes two maskless optical engines with auto-calibration function which can correct any alignment error in-line. Each optical engine is for each side of the substrate. The optical engines are aligned each other in pairs and are simultaneously patterning on each side of the moving substrate. The system also includes a manipulator for moving, stepping or scanning the optical engines, relative to the substrate so that it can create a contiguous whole image on the both sides of the subject.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present patent application is related to and claims the benefit of provisional patent application No. 61/523,742, filed on Aug. 15, 2011, entitled D OUBLE  S IDE  M ASKLESS  E XPOSURE  S YSTEM , the entire contents of which is enclosed by reference herein. 
    
    
     BACKGROUND 
     The method and system are generally related to maskless lithography or direct-write digital image technologies. More specifically, it relates to a double-sided maskless exposure system capable of simultaneously exposing both surfaces of a subject plate, such as a substrate for a printed circuit board (PCB) or a sheet for lead frames. The system disclosed herein could be used to create double side exposures for PCB, IC packaging and LCD manufacturing. It could also include applications such as document printing and photographic reproduction etc. The following description focuses on PCB exposure equipment, although the specification can be applied by obvious extension to other fields of use as well. 
     The current PCB exposure industry mainly uses film masks. The technology suffers many disadvantages, e.g., film deformation, low alignment accuracy, limited line width around 4 millimeter, and difficulties in film storage and management etc. With the PCB industry moving towards high-density interconnection (HDI) board, multi-layered board and other trends, along with rising demands for high precision alignment, the traditional film mask (Mask) exposure lithography process is limited by the technical production bottleneck. To solve the yield and productivity problems, the PCB industry has paid more attention to the emerging maskless lithography technology or the direct imaging equipment (Direct-write Digital Imaging System). The maskless lithography technology is expected to grow into a mainstream technology in lithography. Conventional film mask lithography for PCB is relatively cheap but cannot overcome many problems such as the distortion between PCB layers and the scaling issue. The development of HDI multilayer and high-density boards highlights the many advantages of using the maskless technology. The technology enjoys at a high exposure speed on the traditional dry film photo-resist with distortion correction. A direct-write digital imager can achieve high output (high throughput), high yield, yet with the lowest overall cost. This new kind of PCB maskless exposure equipment is gaining popularity in the industry. 
     The method and system also relates to double-sided exposure systems examples of which include U.S. Pat. No. 5,337,151, U.S. Pat. No. 5,627,378, U.S. Pat. No. 5,923,403, U.S. Pat. No. 5,929,973, U.S. Pat. No. 5,933,216 and U.S. Pat. No. 6,211,942. The technology requires masks for imaging patterns onto photo resist coated sides of a subject. The subject plate may include, for example, a semiconductor substrate for manufacture of integrated circuits, metal substrate for etched lead frame manufacture, conductive plate for printed circuit board manufacture, or the like. A patterned mask or photo mask may include, for example, a plurality of lines, structures, or images. With conventional photolithography, the patterned masks or films for high resolution applications are typically very expensive and have a short lifetime. In addition, photomasks often require a long mask purchase lead time. The long mask purchase lead time creates a roadblock when a short product development cycle is desired. Further, when a particular mask design requires a design change in the pattern, regardless the size of the change, it requires a long lead time and associated mask modification cost. Frequent mask modifications can cause serious problems in PCB manufacturing. 
     A double-sided maskless exposure system is also advantageous over single side maskless exposure systems. Most PCBs need exposure on both sides. A single side maskless exposure system doubles the exposure for exposure on both sides of the board and requires additional alignment process to align the patterns on both sides. Poor alignment often reduces system productivity and yield. A double-sided maskless exposure system, however, does not require pattern alignment and is compatible with the conventional double-sided exposure equipment and other processes. It works especially well for flexible exposure subjects such as a lead frame. A lead frame is fed in a roll. The continuous exposure required for a lead frame make it difficult to use a single side maskless exposure system due to required pattern alignment process for both sides. It would require multiple pieces of equipment, which comes with higher cost. 
     A double side maskless system (U.S. Pat. No. 6,396,561, US2009/0279057) is apparently a better choice than a single side maskless exposure system. The key is how to achieve desired system stability and reliability after adding another side maskless exposure mechanism. There are at least two maskless optical engines in a double-sided exposure system, the distance between them is often too far to get accurate alignment of the two optical engines. Even if the maskless optical systems were aligned in fabrication, it could easily change due to vibration, temperature change, and other environmental condition. An auto-calibration system or self-check function is necessary to ensure the alignment of both sides and exposure quality. 
     There are currently a few types of double-sided maskless exposure systems in the market. One uses a laser beam scanning on a substrate surface; another is called direct imaging, which uses a 2D Spatial Light Modulation (SLM), such as Digital Mirror Device (DMD) to project a pixel array on a substrate. The system disclosed herein relates to the direct imaging method. When using the direct imaging method, each of the plurality of pixel elements of SLM is simultaneously focused on portions of the subject plate. The subject and pixel elements are then moved (e.g., by vibrating one or both the subject and pixel elements), and the sub-pattern is changed in response to the movement and the SLM pixel pattern. As a result, UV light can be projected into the sub-pattern to create the plurality of pixel elements on the subject, and the pixel elements can be moved and altered, according to the pixel-mask pattern, to create a contiguous image on the subject. In a 2D direct imaging method, there are three generations of maskless optical engines; the first generation is to directly image a 2D SLM on a substrate with enlarging or shrinking pixel size without any transformation. The second generation is called the point array method, which uses micro-lens to focus the light from each pixel and get the focus points on a substrate surface. The third generation is called sub-image array method, which uses a special optical system to divide the image of the whole 2D SLM into a sub-images array with shrinking pixel size on the substrate. The difference between the three generations is the imaging optical system. 
     SUMMARY 
     The method and system disclosed herein aim to solve the problems associated with the current designs as described above, and an objective of the present apparatus and method is to provide an optical and mechanical system that can expose the both sides of a subject plate without masks. 
     The present system is a double-sided maskless exposure system, which includes a UV light source, a spatial light modulator (SLM), an optical systems, a motion system, a vision system, control circuits, data conversion and data processing software and computer system. Using the present system, double-sided pattern data generated by a computer are imaged by at least one maskless optical system (maskless optical engine) for each side of the moving substrate, through a spatial light modulator (SLM), such as DMD, LCOS, LCD and other 2D display panels. It can also be arranged to use more optical engines to expose a large area within the same exposure time period. 
     The present system overcomes the many disadvantages of a conventional double exposure systems and improve productivity and quality. A technical advance is achieved by a novel optical system and method for photolithography. It produces a digital image from a pixel panel onto specific sites on a subject plate using a vision system to detect alignment errors between a pair of the optical engines on both sides of the plate. In one embodiment, the system includes a panel for generating the pattern and for creating a plurality of pixel elements on the substrate. A vision system is provided and positioned co-axial with the optical engine and monitors the position of the panel position in the optical engines. The function of the vision system is to align the optical systems on the both sides of the substrate and also align exposure pattern with the substrate position. 
     In order to achieve the objectives described above, a first aspect of the present system includes: a substrate with photo sensitive material on both sides for exposure; two maskless optical engines that are separately set in each side of the substrate; one exposure light source for each optical engine, or one light source shared by the two optical engines through a beam splitter; a vision system with a calibration light source and the light wavelength thereof is different from exposure light source and not sensitive to the photo sensitive material on the substrate; the vision system is mounted with one of the maskless optical engines, and its optical axis is aligned with the maskless optical system axis thru a beam splitter; the beam splitter is mostly transparent for the exposure light and about 50% reflective for the vision system light; the focus plane position of the vision system is same as the maskless optical engine; a XY motion stage controlled by a control system and synchronized with the maskless optical engines, and the stage can make relative movement between maskless optical engines and the substrate; the stage is transparent on the exposure area which allows exposure light to reach the substrate or plate; each maskless optical engine has an individually controlled Z stage to change the distance between the maskless optical engine and substrate. In the auto-calibration process, the substrate is not put on the stage, and the two maskless optical engines are already pre-aligned at the time of design and fabrication, or by the control system. First, the system sends the alignment pattern to each engine and turn on the exposure light source with proper light intensity. Second, it adjusts each optical engine&#39;s Z position to focus each alignment pattern on the stage surface, which is transparent and also reflect a little bit light from the surface. Therefore one of pattern is reflected back to the vision system, and the light from another maskless optical engine passes through the stage surface into the vision system. Then the vision system can have both of alignment patterns in the camera and read each alignment position to get the distance between the two optical engines. This position data will be saved in the computer system for data correction during the exposure process. In the exposure process, the exposure light sources are initially off, and the vision system calibration light source is on. The second step is to put the substrate on the stage. If the substrate has no pattern on the surfaces, the system can directly do exposure with the corrected data saved in the computer. If there is an existing pattern on the substrate surface, the vision system reads the marked positions, save the position data in the computer, correct the exposure data, then do the exposure. In this embodiment, because the light from the stage surface reflection is not very strong, the vision system is hard to read it in some cases. If so, a thin sheet can be put on the stage surface to assist the reading of the reflected pattern positions. 
     Another aspect of the present system includes: a substrate with photo sensitive material on both sides for exposing; two maskless optical engines separately set at each side of the substrate; one light source for each maskless optical engine, or two maskless optical engines share one light source by a beam splitter; the light source has two wavelength segments, one of which is for exposing, e.g., UV light, and the other is for calibration and not sensitive to the photo sensitive material on the substrate, e.g., yellow or red light; a vision system without a separate light source is mounted with one of the maskless optical engines, and its optical axis is aligned with the maskless optical system axis through a beam splitter; the beam splitter is mostly transparent for the exposure light and about 50% reflective for the vision system light; the focus plane of the vision system is the same as the maskless optical engine; a XY motion stage that is controlled by a control system and synchronized with the maskless optical engines; the stage can make relative moving between maskless optical engines and the substrate; the stage uses transparent material on the exposure area which allows exposure light to reach the substrate; one of the stage surfaces is coated for partial reflection of the calibration light; each maskless optical engine has an individually controlled Z stage to change the distance between the maskless optical engine and substrate. In the auto-calibration process, the substrate is not put on the stage, and the two maskless optical engines are already pre-aligned at the time of design and fabrication, or by the control system. First, the system sends the alignment pattern to each engine and turn on the calibration light source with proper light intensity. Second, it adjusts each optical engine&#39;s Z position to focus each alignment pattern on the stage surface, which is transparent and also reflect part of the calibration light, usually 50%, depending on camera sensitivity and the calibration light source. One of pattern is reflected back to the vision system, and the light from another maskless optical engine passes through the stage surface into the vision system as well because the light is just reflected partially; then vision system can have both alignment patterns in the camera and read each alignment position to get the distance between the two optical engines. This position data will be saved in the computer system for data correction in exposure process. In the exposure process, the exposure light sources are initially off, and the vision system send a total white pattern (all pixel-on) in maskless optical engines; the second step is to put the substrate on the stage. If the substrate has no pattern on the surfaces, the system can directly do exposure with the corrected data saved in the computer. If there is an existing pattern on the substrate surface, the vision system can read the marked positions, save the position data in the computer, correct exposure data, then do the exposure. In this embodiment, because the light from the stage surface reflection is much stronger, it is easier for the vision system to read. If the stage surface has no coating, a thin sheet can be put on the stage surface for reading the reflected pattern position. 
     Another aspect of the present system includes: a substrate with photo sensitive material on both sides for exposure; two DMD maskless optical engines separately set at each side of the substrate; one light source for each maskless optical engine, or two maskless optical engines share one light source by a beam splitter; the light source has two wavelength segments, one is for exposure, such as UV light, and the other is for calibration and not sensitive for the photo sensitive material on the substrate, such as yellow or red light; a vision system without a dedicated light source is mounted with one of the DMD maskless optical engines and aligned its optical axis with the DMD maskless optical system axis through a beam splitter; the beam splitter is transparent for exposure light and about half reflective for calibration light; the focus plane of the vision system is the same as the DMD maskless optical engine; a XY motion stage controlled by control system and synchronized with the maskless optical engines, the stage can make relative movement between DMD maskless optical engines and the substrate, and the stage uses transparent material on the exposure area which allows exposure light to reach the substrate; each DMD maskless optical engine has an individually controlled Z stage to change the distance between the DMD maskless optical engine and substrate. In the auto-calibration process, the substrate is not put on the stage, and the two DMD maskless optical engines are already pre-aligned at the time of design and fabrication or by the control system. First, the system sends the alignment pattern to each engine and turn on the calibration light source with proper light intensity. Second, one adjusts the Z position of the optical engine located on the other side of the vision system, so the light passes through the stage surface into the vision system. The vision system will then has the DMD position in the camera. It reads the alignment position, then power off the DMD maskless optical engine (pixel panel power-off). The light from another DMD optical engine will go through stage surface and the first optical engine lenses to reach the DMD panel surface that is power-off. The light is reflected back by the panel because the mirrors in this DMD are in a flat status. The reflected light will go through stage surface again, and part of the light will be reflected to the vision system by the beam splitter so that the vision system can read the power-on DMD position. The computer system will correct the data according to the two DMD positions. 
     In the exposure process, the exposure light sources are initially off, and the calibration light sources are on. For a substrate with no existing pattern, the system can directly do exposure with the correct data saved in the computer. If there is a pattern on the substrate surface, the vision system can read the mark positions, save the position data in the computer, correct exposure data, then do the exposure. In this embodiment, because the light from the DMD surface reflection is much stronger, it is much easier for the vision system read. The calibration process can be done any time when there is no substrate. 
     In some embodiments, the system can includes many maskless optical engines for bigger exposure area and higher productivity. 
     In some embodiments, the light source can use beam splitter to combine two different wavelength segments of light. 
     In some embodiments, the light source can use optical fiber to combine two different wavelength segments of light. 
     In some embodiments, the system also use one vision systems for each side of the stage. 
     In some embodiments, the system can include one vision system for each side, and each vision system has a dedicated light source. 
     In some embodiments, the stage may only allows movement in the X direction, but separate movements in the Y and Z directions for the maskless optical engines. 
     In some embodiments, the stage also allows X-Y stepping for exposing. 
     In some embodiments, the system also allows separate movements in the Y and Z directions for each maskless optical engine. 
     Therefore, one advantage of the present system is that it provides a double-sided maskless exposure system and method with increased productivity. 
     Another advantage of the present system is that it eliminates or reduces the double side alignment problems in the art associated with conventional masks. 
     Still another advantage of the present system is that it provides an improved double-sided exposure system, such improvements being the simple process, high yield, scaling compensation, distortion compensation requirement. 
     A further understanding of the nature and advantages of the present system may be assisted by the remaining portions of the specification and the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objectives, features and other advantages of the present system will be more clearly understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a basic diagram illustrating a double-sided maskless exposure system; 
         FIG. 2  illustrates a double-sided maskless exposure system with a vision system; 
         FIG. 3  illustrates a double-sided maskless exposure system with a co-axial vision system; 
         FIG. 4  illustrate another embodiment of  FIG. 3 ; 
         FIG. 5  illustrate a double-sided maskless exposure system with two co-axial vision system; 
         FIG. 6  illustrates DMD mirror status and light ray directions; 
         FIG. 7  illustrates a double-sided maskless exposure system with a co-axial vision system and two calibration light sources; 
         FIG. 8  illustrate a continuous double-sided maskless exposure system of the present system; 
         FIG. 9  illustrates another embodiment of the present system with a vacuum chuck to hold a substrate; 
         FIG. 10  illustrates another vertical and tilt embodiments of  FIG. 9 ; 
         FIG. 11  illustrates two types of two segment wavelength light source in the present system; 
         FIG. 12  illustrates a perspective view of a system design of the present system; 
         FIG. 13  illustrates a front view of  FIG. 12 ; 
         FIG. 14  illustrates a exploded view of  FIG. 13 ; 
         FIG. 15  illustrates a maskless optical engine with a co-axial vision system; 
         FIG. 16  illustrates a exploded view of  FIG. 15 ; 
         FIG. 17  illustrates a perspective view of a multi-maskless optical engines of the present system; 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure relates to double-sided maskless exposure systems, which can be used in PCB, LCD, lead frame, and semiconductor photolithographic processing. It includes a number of embodiments for implementing various features of the system and method. Specific examples of components and arrangements are described below to demonstrate the various configurations. It is understood that none of these examples is intended to limit the scope of the claims. 
     Reference will now be made in greater detail to an exemplary embodiment of the present system, examples of which are illustrated in the accompanying drawings. Wherever possible, similar reference numerals are used throughout the drawings and the description to refer to the similar parts to ease understanding. 
     With reference now to  FIG. 1 , a double-sided maskless exposure system includes two sets of maskless optical engines  100   a  and  100   b . Each optical engine includes a light source ( 101 ,  114 ) having an optical fiber ( 116 ,  113 ) and light collimator and homogenizer ( 102 ,  112 ). The light outputted from the light collimator and homogenizer ( 102 ,  112 ) is reflected by mirror ( 104 ,  110 ) to a spatial light modulator ( 103 ,  111 ), and an optical lens ( 105 ,  109 ). Images the spatial light modulator ( 103 ,  111 ) are directed to a subject  107 . A photoresist layer may be disposed on the both sides of the subject  107 . 
     The light collimator and homogenizer ( 102 ,  112 ) provides a uniform light beam onto the spatial light modulator ( 103 ,  111 ). A stage surface plate  108  holds the substrate  107  and is transparent directly under the substrate area. The stage surface plate  108  can be translated in an XY direction by a control system. The spatial light modulator ( 103 ,  111 ) creates a desired pixel pattern referred to as the pixel-mask pattern. The pixel-mask pattern may be resident at the spatial light modulator ( 103 ,  111 ) in synchronization with the stage surface plate movement. 
     Light emanating from the pixel-mask pattern of the spatial light modulator ( 103 ,  111 ) then passes through the optical system ( 105 ,  109 ). The light from the optical system  105  focuses onto the one side of the substrate  107  and the light from optical system  109  passes through the stage surface  108  and focuses on the other surface of the substrate. In this manner, the pixel-mask pattern is projected onto both sides of the substrate  107 . 
     The spatial light modulator ( 103 ,  111 ) and optical system ( 105 ,  109 ) may be aligned with each optical axis ( 106 ,  115 ). The substrate  107  may be a PCB board such as is used in conventional fabrication of printing circuit board, a glass substrate, plastic substrate or a semiconductor wafer. It is understood, however, that many different substrates can benefit from this apparatus and method, including for example, a non-flat substrate. 
     Z axis translators for focus adjustment of maskless optical engines  100   a ,  100   b  may be utilized. One potential issue is the optical axis ( 106 ,  115 ) alignment due to environment conditions such as vibration or temperature variations. Another potential issue is vision system alignment of PCB patterns if a pattern exists on the substrate  107 . 
       FIG. 2  shows a double side maskless exposure system with a traditional vision system. The vision system consists of a camera  203 , an image lens  204  and a light source  201 . A collimation lens  202  is used to collect the light from the light source  201  and illuminate the substrate  107  thru a beam splitter  206 . The beam splitter generally reflects approximately half of the light and transmits the other half. The light reflected back from the substrate  107  is directed into the camera lens  204  and camera  203 . The camera can then read the pattern on the substrate. In this example, the vision system is used for alignment between patterns as the vision system optical axis is not aligned with maskless optical system axis ( 106 ,  115 ). If the maskless optical system axis ( 106 ,  115 ) is misaligned, the vision system cannot perform an inline check. 
       FIG. 3  shows an approach to ascertain the maskless optical system alignment information from the vision system. In this example there is an additional beam splitter  301  added to the system  200 . The purpose of this additional beam splitter is to combine the vision system axis and maskless optical system axis  106 . The beam splitter  301  allows light to be directed from the light source  101  and receive reflected light from light source  201 . 
     The optical system  302  requires calibration for high image quality. In the auto-calibration process the two maskless optical axis ( 106 ,  115 ) are pre-aligned. The alignment pattern is sent to each spatial light modulator ( 103 ,  111 ) and the exposure light source  101 ( 114 ) is gated on. Each optical engine Z position is adjusted to focus an alignment pattern on the stage surface  303 . The light from the optical system  302  is directed through the beam splitter  301  and is reflected from the surface  303 . This reflected light is directed to the camera  203  by beam splitter  301 . The camera reads the pattern to ascertain the optical axis position  106 . The light from the other maskless optical engine passes through the stage surface plate  108  to the vision system. 
     The vision system can view the alignment patterns in the camera  203  and read each axis position. This position data can be saved for data correction in the exposure process. During exposure, the exposure light sources ( 101 , 114 ) are gated off and the vision system light source  201  are gated on. The substrate  107  is placed on the stage and if the substrate does not have a pattern, the system can directly expose utilizing the corrected data. If a pattern exists on the substrate surface  107 , the vision system can read the marked positions for future exposure correction. 
       FIG. 4  shows another approach which is related to  FIG. 3 . In this example the vision system and beam splitter  301  are placed proximate to the spatial light modulator. The camera lens  204  may be removed if the focal length of the optical system  401  is large enough. 
       FIG. 5  shows a double vision system for checking alignment. The beam splitters ( 301 , 501 ) receive light from the light sources ( 101 , 114 ) and from the vision system light sources ( 201 , 503 ). In the auto-calibration process, the substrate  107  is a highly transparent glass with indicia in the vision system view area onto the stage surface plate  108 . The two maskless optical systems ( 302 , 502 ) are pre-aligned. 
     The alignment pattern is sent to each spatial light modulator ( 103 ,  111 ) and the exposure light source ( 101 ,  114 ) is set to the proper light intensity. Each optical engine Z position is adjusted to focus the alignment pattern on the substrate mark. The light from the optical system  302  is directed through the beam splitter  301  and is reflected by beam splitter  501  to camera  507 . The camera  501  reads the pattern, and the substrate mark is read by camera  507 . Camera  507  ascertains the optical axis  106  position relative to the mark. The light from the other maskless optical system  502  passes through beam splitter  501  and the stage surface plate  108 . The camera  203  can read the substrate mark. The vision system ascertains the optical system axis  115  position relative to the same mark. Therefore the system ascertains both of the optical system axis positions ( 106 ,  115 ). 
       FIGS. 6   a  and  6   b  show a Digital Mirror Device (DMD) and light ray direction. In  FIG. 6   a , paths  601   a ,  601   b  and  601   c  are parallel to the incident light rays. Mirror  603   a  and  603   c  indicate on/off status of the DMD when powered-on. The mirror  603   b  shows the status of the DMD when powered off. The mirror  603   a  indicates an off status because the light ray  601   a  is reflected off of the mirror. The mirror  603   c  reflects a light ray  601   a  to the DMD normal direction  602   c . In  FIG. 6   b , a set of parallel incident light rays  604   a ,  604   b ,  604   c  are directed in a normal direction. The mirrors  607   a ,  607   c  reflect light rays from the normal direction as the light rays  605   a ,  605   b . The mirror  607   b  reflects the light ray  604   b . A normal incident light will be reflected back from DMD mirror if the DMD is powered-off. This normal reflection property is also very useful for maskless optical engine alignment in  FIGS. 3-5 . 
     In  FIG. 3 , if the spatial light modulators  102  and  111  are DMDs. In the system the auto-calibration process begins with the two maskless optical axis  106  and  115  are pre-aligned. An alignment pattern is resident on the spatial light modulator  111  and the exposure light source  114  is set to the proper light intensity. The optical engine Z position is adjusted to focus the alignment pattern from spatial light modulator  111  to the vision system camera  203 , so the camera can read the spatial light modulator  111  position. The spatial light modulator  111  and the light source  114  are gated off and the light source  101  and spatial light modulator  103  with the alignment pattern are gated on. The light from the optical system  302  is directed through the beam splitter  301 , optical system  109  to image the alignment pattern on the spatial light modulator  111 . The light is reflected back from the spatial light modulator  111  and is directed through optical system  109  to be reflected by beam splitter  301  to the camera  203 . At this point the camera can read the pattern and the optical axis  106  position. 
     The vision system view both alignment patterns in the camera  203  and read each axis position. A similar process can be also applied to  FIGS. 4 and 5 . In the case of Liquid Crystal On Silicon (LCOS), if the pixels are gated off and the spatial light modulator is gated-on, the light from the optical system  109  will be reflected back to the optical system  109 . In this instance this maskless optical engine is equivalent to a reflecting mirror on the stage surface plate  108 . 
     With reference to  FIG. 7 , the double side maskless exposure system includes two sets of maskless optical engines  700   a  and  700   b . Each optical engine includes light sources  101  and  114  with optical fibers  116  and  113 , and a light collimator and homogenizer  102  and  112  which provides a uniform light beam onto the spatial light modulator ( 103 ,  111 ). The system includes another light source ( 701 ,  704 ) which has a different light wavelength which does not affect the photo sensitive material on the substrate. 
     The beam combiner ( 703 ,  706 ) has an optical coating to combine two light beams together. The output light from ( 102 ,  112 ) is reflected by a mirror ( 104 ,  110 ) to a spatial light modulator ( 103 ,  111 ), such as DMD. An optical lens ( 105 ,  109 ) images the spatial light modulator ( 103 ,  111 ) to the subject  107 . A photo resist layer or coating may be disposed on the both sides of the subject  107 . 
     A stage surface plate  108  holds the substrate  107  and is transparent under the substrate area. The stage surface plate  108  can be translated in the XY direction by a control system. The spatial light modulator ( 103 ,  111 ) creates a desired pixel pattern (the pixel-mask pattern). The pixel-mask pattern may be resident at the spatial light modulator ( 103 ,  111 ) and synchronized with the stage surface plate movement. Light emanating from (or through) the pixel-mask pattern of the spatial light modulator ( 103 ,  111 ) then passes through the optical system ( 105 ,  109 ), which is optimized for both of the light sources ( 101 , 114 ) and ( 701 ,  704 ). 
     Light from optical system  105  focuses the image of the spatial light modulator  103  onto one side of the substrate  107  and the light from optical system  109  passes through the stage surface  108  and focuses the image of spatial light modulator  704  on the other surface of the substrate. In this manner, the pixel-mask pattern is projected onto the both sides of the substrate  107 . 
     The spatial light modulator ( 103 ,  111 ) and the optical system ( 105 ,  109 ), can be aligned for each optical axis ( 106 ,  115 ). In the engine  700   a , there is a beam splitter  301  and a vision system ( 204 ,  203 ). The beam splitter passes light ( 101 ,  114 ) and partially reflects light ( 701 ,  704 ). The substrate  107  may be a PCB board such as is used in conventional fabrication of printing circuit board or a wafer. 
     It may is desired to project a plurality of patterns on the substrate  107  using the maskless exposure system. In this example, there may be Z axis translators for focus adjustment of maskless optical engines  700   a ,  700   b . During the auto-calibration process the two maskless optical axis  106 , 115  are pre-aligned. An alignment pattern is placed on the spatial light modulator  111  and the light source  704  is illuminated. The optical engine  700   b  adjusts the Z position to focus the image of the alignment pattern from spatial light modulator  111  onto the vision system camera  203  through beam splitter  301  and camera lens  204 . In this way the camera  203  can read the spatial light modulator  111  position. The spatial light modulator  111  and the light source  704  are gated off and the light source  701  and spatial light modulator  103  with the alignment pattern are gated on. 
     The light from the spatial light modulator  103  passes through the optical system  301 , the beam splitter  301 , the stage surface plate  108 , the optical system  109  and images the alignment pattern onto the spatial light modulator  111 . The light will be reflected from the spatial light modulator  111  and passes through the optical system  109 , the stage surface plate  108  and be reflected by beam splitter  301  to the camera  203 . Since spatial light modulator  103 , spatial light modulator  111  and camera  203  are conjugated by optical systems  302 ,  109  and camera lens  204 , the image position of spatial light modulator  103  is insensitive to the optical system axis  115 ,  106  mis-alignment of the camera  203 . In this example, the maskless optical engine  109  is equivalent to a reflecting mirror on the stage surface plate  108  for the light from the optical system  302 . 
     The vision system can view the alignment patterns in the camera  203  and read each axis position. In the exposure process, the exposure light sources  101 , 114  are gated off and the light source  701  is gated on. The system can be used to expose the substrate with the corrected data if the substrate is un-patterned. If the substrate is patterned and has alignment marks on the substrate surface  107 , the vision system reads the mark positions then saves the position and exposure data. If there is an pattern on the spatial light modulator  103 , the camera can read this pattern and alignment mark at same time to get more accurate distance between spatial light modulator  103  and substrate marks. If the maskless optical engines do not mount on separated y moving stages, the calibration process can be done for each scanning start point or stepping position to get optimized correct exposure and alignment data. 
     Referring now to  FIG. 8 , is a modification of the embodiment of the  FIG. 7 . The double side maskless exposure system  800  in  FIG. 8  utilizes the same maskless optical engines as  700   a  and  700   b  but the substrate in this example is a flexible roll. A pair of rollers  803  and  805  controls the substrate  804  speed. Rollers  802  and  806  are for continuously receiving and supplying substrate  804 . The substrate can continuously move in the x direction  807 . The maskless optical engines may have Z stage for adjusting focus and Y stage for handling wide substrates. 
       FIG. 9  illustrates a double side maskless exposure system  900  using a vacuum chuck for holding the substrate  107 . This system may be used with substrates that are not flat enough for the depth of focus (DOF) of the optical system ( 302 ,  109 ). The vacuum chuck consists of top plate  901 , a base plate  903 , a vacuum hose  904  and a soft seal material  902 . The top plate and base plate are transparent in the substrate area. The surfaces may be coated with anti-reflective material. 
       FIGS. 10   a  and  10   b  illustrate two embodiments of  FIG. 9 . The double side maskless exposure system may be set at vertical or in a tilted direction. 
       FIGS. 11   a  and  11   b  illustrate two types of light source combiners. One is a dielectric coating and another is an optical coupling. In  FIG. 11   a , the light source  701  is collimated by lens  702  and then reflected by beam combiner  701 . In  FIG. 11   b , the light source  1103  and the exposure light source are separately coupling into optical fibers  1102  and  1101 . The optical fibers are then bundled together as one optical fiber beam in the front of the collimator and homogenizer  102 . The system shown by  FIG. 11   b  may be used for laser light sources. The system of  FIG. 11   a  may be suitable for LED light sources. 
       FIG. 12  illustrates a perspective view of a system design of the apparatus. The system consists of two sets of maskless optical engines  1207 ,  1205 , an optical engine moving stage  1206 ,  1202  and  1204 , a scanning stage  1203  and a base frame  1201 . 
       FIG. 13  is a front view of  FIG. 12 . The maskless optical engines  1205 ,  1207  have individual XZ stages  1204 ,  1206 ,  1302  and  1202 . The Y stage  1304  supports the substrate and synchronizes scanning with the maskless optical engines  1206  and  1202 . The Y stage table  1203  is mounted on a very heavy stone base  1304 . The stone base  1304  has a hole  1301  and an empty area under the stage surface plate  1203 . The stage surface plate  1203  is transparent for the exposure light. 
       FIG. 14  is an exploded view of  FIG. 13 . The maskless optical engine  1205  is mounted on the X1 stage  1204  and Z1 stage  1206  above the Y stage surface plate  1203 . The maskless optical engine  1207  is mounted on the X2  1302  and Z2 stage  1202 . 
       FIGS. 15 and 16  illustrate the maskless optical engine  1205 . The maskless optical engine  1205  includes an optical fiber input port  1603  and a spatial light modulator (in this example a DMD)  1601 . The mirror  1503  reflects the light from the optical fiber input port  1603  to the spatial light modulator  1601 . The DMD  1601  is mounted to the base of housing  1505  as are the mirror  1503 , the optical fiber input and lens system  1506 , a heat sink  1602 , an optical system  1506 , an adjustment mechanism  1604 , a beam splitter  1501 , a vision system  1502  and a base plate  1507 . The optical system  1506  can be a general imaging lens for scaling the spatial light modulator image on a substrate, additionally it can also can be a point array optical system or sub-image array optical system. 
       FIG. 17  shows a multi-optical engine double side maskless exposure system. There are two optical engines  1701 , 1703 , 1704 , 1705  on the each side of the substrate  1706 . 
     While the apparatus and method 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, as set forth in the following claims.