Patent Publication Number: US-2005128559-A1

Title: Spatial light modulator and method for performing dynamic photolithography

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is related by subject matter to U.S. Utility applications for Patent Attorney Docket No. 10030518, entitled REAL TIME IMAGE RESIZING FOR DYNAMIC DIGITAL PHOTOLITHOGRAPHY; Ser. No. 10031375, entitled DEFECT MITIGATION IN SPATIAL LIGHT MODULATOR USED FOR DYNAMIC PHOTOLITHOGRAPHY; and Ser. No. 10040070, entitled LIQUID CRYSTAL CELL THAT RESISTS DEGRADATION FROM EXPOSURE TO RADIATION, each filed on an even date herewith. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Technical Field of the Invention  
      The present invention relates generally to photolithography, and more specifically, to dynamic photolithography systems.  
      2. Description of Related Art  
      Photolithography is a method of transferring a pattern or image onto a substrate. Some industrial uses of photolithography include the manufacture of products, such as flat panel displays, integrated circuits (ICs), IC packaging, planar lightwave circuits (photonics), printed circuit boards, flexible circuits/displays and wafer bumping. In its simplest form, a photolithography system operates by passing light through a mask or tool placed over a substrate having a photosensitive surface, such as a layer of photoresist. Typically, the mask is formed of a transparent material with a fixed opaque pattern inscribed on the surface. Due to the photosensitivity of the substrate surface, when placed in contact with the mask and exposed to light, the pattern inscribed on the mask is transferred onto the substrate surface.  
      Although the use of a mask provides for a high degree of precision and repeatability, traditional contact photolithography systems suffer from several limitations. One limitation is a manufacturing specification that restricts the size of the substrate to no greater than the size of the mask. For large substrates, it is difficult to produce and handle a mask of sufficient size to cover the entire substrate area. In addition, as technology has progressed, the size of features photolithographically transferred onto the substrate surface has decreased to 0.5 μm or smaller. To achieve such small feature sizes, more advanced systems use projection optics to separate the mask from the substrate, allowing for optical reduction in the transferred feature size. However, in order to transfer the pattern for the entire substrate using an optical reduction system, the size of the mask would necessarily be larger than the size of the substrate. Large masks are both unwieldy and expensive to produce. To overcome the problems associated with large masks, many photolithography systems use multiple masks that contain different portions of the total pattern. The pattern is stitched together on the substrate surface by altering the position of the substrate surface in relation to the mask.  
      However, the cost to design and embed a pattern on a mask is considerable, and therefore, creating a large number of masks may be cost-prohibitive. Likewise, in applications where frequent changes occur, creating a new mask each time a change occurs may not be cost effective. As a result, dynamic photolithography systems have developed that enable a manufacturer to dynamically change a mask pattern without requiring a new mask for each change. Dynamic photolithography systems commonly employ a spatial light modulator (SLM) to define a pattern that is imaged onto the substrate surface. SLMs are electrically controlled devices that include individually controllable light modulation elements that define pixels of an image in response to electrical signals.  
      Typically, at feature sizes of 0.5 μm or smaller, there are tens of millions of light modulation elements within an SLM that is not more than a few square centimeters in area. With the small SLM size, multiple exposures are generally required to image the entire area of the substrate. Since the image formed by the SLM is easily reconfigurable, it is a relatively simple process to divide the final image into sections, configure the SLM to transfer one of the image sections onto the appropriate area of the substrate surface, shift the relative position of the substrate and SLM and repeat the process for each image section until the entire image is transferred onto the substrate surface.  
      However, it is impracticable to assume that the SLM will be free from defects. Statistically, there will be at least a few of the tens of millions of light modulation elements of the SLM that are defective. As a result of the multiple imaging process, each defective light modulation element will produce numerous defects on the substrate surface. What is needed is a mechanism to mitigate the effect of defective light modulation elements.  
     SUMMARY OF THE INVENTION  
      Embodiments of the present invention provide a spatial light modulator for use in a photolithography system. The spatial light modulator includes memory elements configured to store data therein and move data therebetween. Light modulation elements are in communication with respective memory elements and are operable to be altered in response to the data stored in the respective memory elements. In one embodiment, to move the data between the memory elements, the memory elements can be configured as a shift register. In a further embodiment, the shift register configuration can be configured to shift the data bi-directionally. In another embodiment, each memory element can include a feedback element, where the feedback element is a “weak” feedback element that is utilized to contribute to maintaining a voltage to minimize photocurrent effects.  
      Other embodiments of the present invention provide a process for performing photolithography, in which data representing an image is loaded into memory elements in communication with respective light modulation elements. Certain ones of the light modulation elements are altered in response to the data loaded into the respective memory elements to transfer the image onto a substrate. The data is shifted between the memory elements, and additional ones of the light modulation elements are altered in response to the shifted data to transfer the image onto the substrate.  
      By shifting data through the memory elements of the spatial light modulator in the photolithography system, the amount of data loaded into the spatial light modulator for each image transfer is reduced, thereby increasing throughput rates. In addition, because the memory of the spatial light modulator can be configured to move the data bi-directionally, the substrate can be translated bi-directionally, which can further increase throughput rates.  
      Furthermore, the invention provides embodiments with other features and advantages in addition to or in lieu of those discussed above. Many of these features and advantages are apparent from the description below with reference to the following drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The disclosed invention will be described with reference to the accompanying drawings, which show sample embodiments of the invention and which are incorporated in the specification hereof by reference, wherein:  
       FIG. 1  illustrates a photolithography system utilizing a spatial light modulator to photolithographically transfer an image to a substrate in accordance with embodiments of the present invention;  
       FIG. 2A  is an exploded view of a spatial light modulator utilizing liquid crystal light modulation elements;  
       FIG. 2B  is a cross-sectional view of a liquid crystal light modulation element of  FIG. 2A ;  
       FIG. 3  is an illustration of a substrate that photolithographically receives a transferred image in image sections using the photolithography system of  FIG. 1 ;  
       FIG. 4  is an illustration of a mapping of image subsections to light modulation banks within the spatial light modulator;  
       FIGS. 5 and 6  are illustrations of a time-sequence for performing optical oversampling on the substrate by the spatial light modulator, in accordance with embodiments of the present invention;  
       FIG. 7A  is a flow chart illustrating an exemplary photolithography process for performing optical oversampling of the substrate, in accordance with embodiments of the present invention;  
       FIG. 7B  is a flow chart illustrating an exemplary photolithography process for performing multiple transfers of a portion of an image, in accordance with embodiments of the present invention;  
       FIG. 8  is a block diagram illustrating a computing system operable to control the photolithography system of  FIG. 1 ;  
       FIG. 9  is a schematic of exemplary spatial light modulator having memory elements in communication with light modulation elements for shifting data through the memory elements, in accordance with embodiments of the present invention;  
       FIG. 10  is a schematic of an alternative memory element for use in the spatial light modulator of  FIG. 9 ;  
       FIG. 11A  is a block diagram of an exemplary configuration of the spatial light modulator of  FIG. 9 ;  
       FIG. 11B  is a timing diagram for shifting data between the memory elements of  FIG. 11A ;  
       FIG. 12A  is a timing diagram that illustrates exemplary control signals for controlling liquid crystal light modulation elements and maintaining DC balance;  
       FIG. 12B  illustrates a data shifting technique to maintain DC balance in liquid crystal light modulation elements;  
       FIG. 13  illustrates an exemplary substrate exposure timing sequence;  
       FIG. 14  is a flow chart illustrating an exemplary method to dynamically photolithographically transfer an image onto a substrate by internally moving data; and  
       FIG. 15  is a flow chart illustrating an exemplary method for shifting data within a spatial light modulator to dynamically photolithographically transfer an image onto a substrate. 
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       FIG. 1  illustrates a dynamic photolithography system  100  for photolithographically transferring an image to a substrate  150  in accordance with embodiments of the present invention. The photolithography system  100  includes a light source  102  operable to output light  104 . The light source  102  can be a laser, such as an excimer laser, or other non-laser source, as understood in the art. The light source  102  is optically coupled to beam shaping optics  106 . The output of the beam shaping optics  106  is light  108  that is directed toward a spatial light modulator  110 . The spatial light modulator  110  includes light modulation elements (not shown) operable to selectively transfer the light  108 . The light modulation elements are described in more detail below in connection with  FIGS. 2A and 2B . In one embodiment, the light modulation elements are liquid crystal elements. However, it should be understood that in other embodiments, the light modulation elements are micromirrors or another type of optical device that can selectively transfer light by reflection, transmission or otherwise.  
      The output of the spatial light modulator  110  includes dark areas with no light and light areas made up of multiple light beams  112   a - 112   n  (collectively  112 ) that are transferred by selected light modulation elements to form at least a portion of an image containing a pattern. The light beams  112  are directed to projection optics  114 , which is optically aligned to direct the light beams  112  onto the substrate  150 . A photosensitive layer (not shown), such as a layer of photoresist, is on the surface of the substrate  150 . The photosensitive layer reacts in response to the light beams  112  to produce the pattern on the surface of the substrate  150 . In one embodiment, the substrate  150  is mounted on a scanning stage  120  to move the substrate  150  in any direction relative to the spatial light modulator  110 . The scanning stage  120  can be, for example, a high precision scanning stage. In another embodiment, the substrate  150  remains stationary and the optics and/or light beams  112  move relative to the substrate  150 . In either configuration, one of the substrate  150  and the spatial light modulator  110  is moved relative to the other to transfer the image onto the substrate  150 .  
      The spatial light modulator  110  further includes pixel drive circuits (not shown) that are uniquely coupled to the light modulation elements. The pixel drive circuits are described in more detail below in connection with  FIGS. 2A, 2B  and  9 . The pixel drive circuits store data that define the state of the light modulation elements. For example, light modulation elements that are reflective can be selectively altered to be in a reflective or non-reflective state such that the received light  108  is either reflected or not reflected onto the substrate  150  by storing data (e.g., logical LOW and HIGH data values) in pixel drive circuits associated with the light modulation elements. In effect, the spatial light modulator  110  operates as a dynamic mask that forms a pattern that is imaged onto the photosensitive layer of the substrate  150 .  
       FIGS. 2A and 2B  illustrate an example of an SLM  110  with liquid crystal (LC) light modulation elements  210  that define pixels of an image. The SLM in  FIGS. 2A and 2B  is a liquid crystal on silicon (LCOS) SLM  100  including individual LC light modulation elements  210  that selectively reflect light of a particular polarization to transfer an image of a pattern including one or more features onto a substrate.  FIG. 2A  is an exploded view of a portion of the LCOS SLM, and  FIG. 2B  is a cross-sectional view of an LC light modulation element  210  of the LCOS SLM  110 . As can be seen in  FIG. 2A , the LCOS SLM  110  includes a substrate  200  on which pixel electrodes  215  are located. The pixel electrodes  215  can be arranged in an array of rows and columns or in a nonorthogonal pattern. Within the substrate  200  below each pixel electrode  215  is located a pixel drive circuit  250  connected to drive the overlying pixel electrode  215 . Disposed above the substrate  200  is a transparent glass  230  coated with a layer  235  of transparent electrically conductive material, such as indium tin oxide (ITO). The ITO layer  235  is the common electrode of the LCOS SLM  110 . Encapsulated between the substrate  200  and the glass  230  is a layer  220  of liquid crystal material that reacts in response to electric fields established between the common electrode  235  and pixel electrodes  215 .  
      Thus, as shown in  FIG. 2B , the pixel electrodes  215  in combination with the liquid crystal material  220 , common electrode  235 , pixel drive circuits  250  and polarizer  260  form respective individual light modulation elements  210  that define pixels of an image. Depending on the voltages applied between the pixel electrodes  215  and common electrode  235 , the liquid crystal material  220  reacts at each light modulation element  210  to either change or not change the polarization state of incoming light. The light modulation elements  210  in combination with polarizer  260  of the SLM  110  allow light of a particular polarization to be reflected or not reflected onto the substrate  150  of  FIG. 1 . It should be understood that polarizer  260  includes one or more polarizers, as known in the art.  
      In another embodiment, the pixel electrodes  215  can be driven with voltages that create a partial reaction of the liquid crystal material  220  so that the light modulation element  210  is in a non-binary state (i.e., not fully ON or OFF) to produce a “gray scale” reflection. For example, the voltages that create a partial reaction of the liquid crystal material  220  are typically produced by applying signals on the pixel electrode  215  and common electrode  235  that not fully in or out of phase, thereby creating a duty cycle between zero and  100  percent, as understood in the art.  
      Although only a few light modulation elements  210  are illustrated in  FIGS. 2A and 2B , each LCOS SLM  110  typically include tens of millions of light modulation elements. For example, in one embodiment, the LCOS SLM  110  includes a matrix of 16,384 columns by 606 rows of light modulation elements. With such a large number of light modulation elements, it is difficult and expensive to produce a defect-free LCOS SLM  110 . In addition, the LCOS SLM  110  is typically not more than a few square centimeters in area.  
      Therefore, referring now to  FIG. 3 , multiple exposures are generally required to image the entire area of the substrate  150 . Each exposure transfers a different section  300   a - 300   g  . . .  300 N of the final image  300  onto a corresponding area  320  of the substrate  150 . For large substrates  150 , multiple passes over columns  320  of the substrate  150  may be required to image the entire substrate area. With a precision stage, the alignment of each exposure can be carefully controlled to seamlessly stitch the image sections  300   a - 300   g  . . .  300 N. However, for each defective light modulation element, a corresponding pixel defect  310  appears on the substrate surface. As a result of the multiple exposures, each defective light modulation element produces N pixel defects  310  on the substrate surface, where N is the number of sections  300   a - 300   g  . . .  300 N the final image  300  is divided into.  
      Therefore, in accordance with embodiments of the invention, as shown in  FIG. 4 , each image section (e.g., image section  300   a  from  FIG. 3 ) is divided into image subsections  400   a - 400   f , one or more of which correspond to a portion of the image, and the light modulation elements  210  of the spatial light modulator  110  are logically divided into light modulation banks  450   a - 450   f . In  FIG. 4 , the light modulation elements  210  of the SLM  110  are shown arranged in rows and columns. The number of rows and columns depends on the application. The light modulation banks  450   a - 450   f  can include one or more rows of light modulation elements  210 , one or more columns of light modulation elements  210  or any combination thereof. For example, in  FIG. 4 , the rows of light modulation elements  210  have been divided into six banks of rows  450   a - 450   f . Each bank  450   a - 450   f  transfers only one image subsection  400   a - 400   f . Thus, bank  450   a  transfers image subsection  400   a , bank  450   b  transfers image subsection  400   b , and so on. To minimize the effects of defective light modulation elements, each image subsection  400   a - 400   f  is transferred onto the substrate multiple times by two or more light modulation banks  450   a - 450   f  in the SLM  110 . This process is referred to herein as optical oversampling.  
      An example of optical oversampling is shown in  FIGS. 5 and 6 .  FIG. 5  illustrates an exemplary SLM  110  for photolithographically transferring image subsections of an image over a time sequence T 1 -T 3 , and  FIG. 6  illustrates a portion of an exemplary substrate for photolithographically receiving the transferred image subsections of the image over the same time sequence T 1 -T 3 . In  FIG. 5 , at time T 1 , all of the image subsections  400   a - 400   e  of image section  300   a  are shown loaded into respective banks  450   a - 450   f  of the SLM  110  for transfer to the substrate. At time T 2 , image subsection  400   a  has been moved out of the SLM  110 , while image subsections  400   b - 400   f  have been moved to banks  450   a - e , respectively, within the SLM  110 . In addition, an image subsection  500   a  of a new image section  300   b  has been loaded into bank  450   f  of the SLM  110 . At time T 3 , image subsection  400   b  has been moved out of the SLM  110 , while image subsections  400   c - 400   f  have been moved to banks  450   a - d , respectively, within the SLM  110 . In addition, image subsection  500   a  of image section  300   b  has been moved to bank  450   e  of the SLM  110  and a new image subsection  500   b  of image section  300   b  has been loaded into bank  450   f  of the SLM  110 .  
      Referring now to  FIG. 6 , a portion (e.g., column  320 ) of the substrate  150  is shown divided into multiple rows r 1 -r n . Each row r 1 -r n  defines an area of the substrate  150  that receives one of the image subsections of the image. Each row r 1 -r n  is exposed by no more than one of the banks  450   a - f  (shown in  FIG. 5 ) of the spatial light modulator at any time. In connection with the discussion of  FIG. 5 , at time T 1 , a footprint  600   a  of the SLM  110  is shown to cover six rows r 1 -r 6  of the substrate  150 , corresponding to the six banks  450   a - 450   f  of the SLM  110 . Each row of the substrate  150  within the footprint  600  is exposed by a flash or strobe of an illumination source (e.g., laser  102  of  FIG. 1 ) as a function of the state of the light modulation elements within the banks  450   a - f  of the SLM. The result is the transfer of image subsections  400   a - f  onto respective rows r 1 -r 6 . At time T 2 , the substrate  150  has moved relative to the spatial light modulator a distance equivalent to one row, and at the next strobe of the illumination source, the footprint  600   b  of the SLM  110  is shown to cover six rows r 2 -r 7  of the substrate  150 , corresponding to the six banks  450   a - 450   f  of the SLM  110 . The image subsections  400   b - f  and  500   a  stored in banks  450   a - 450   f  of the SLM are transferred onto respective rows r 2 -r 7  of the substrate  150 . At time T 3 , the substrate  150  has moved an additional row relative to the spatial light modulator, and at the next strobe of the illumination source, the footprint  600   c  of the SLM  110  is shown to cover six rows r 3 -r 8  of the substrate  150 , corresponding to the six banks  450   a - 450   f  of the SLM  110 . The image subsections  400   c - f  and  500   a - b  stored in banks  450   a - 450   f  of the SLM are transferred onto respective rows r 3 -r 8  of the substrate  150 . In general, as the rows of the substrate  150  shift upward due to the relative movement between the substrate  150  and the spatial light modulator, the image subsections stored in the light modulation banks of the spatial light modulator shift upward in the light modulation banks accordingly.  
      By moving the image subsections between banks  450   a - 450   f  a distance on the spatial light modulator  110  optically equivalent to the distance that the substrate  150  moves relative to the spatial light modulator  110 , each image subsection is transferred separately onto the substrate  150  by each bank, thereby imaging or transferring each image subsection multiple times. Over the course of six exposures (only three of which are shown in  FIG. 6 ), each row (e.g., rows r 1 -r n ) of the substrate  150  is exposed six different times by six different sets of light modulation elements (banks) of the spatial light modulator. This “oversampling” of the substrate of each image subsection minimizes defects in the resulting product as a result of defective “stuck OFF” light modulation elements. In further embodiments, to reduce the number of defects due to defective “stuck ON” light modulation elements, the photosensitive layer on the substrate  150  has a reaction threshold equivalent to two or more exposures.  
      Thus, even if light modulation bank  450   a  has a defective ON or OFF light modulation element, it is highly unlikely that the corresponding light modulation element by row and column in each of the remaining banks  450   b - f  is defective. Therefore, the probability of defects in the resulting transferred pattern on the substrate is low. It should be understood that the number of times that each area of the substrate  150  is exposed is dependent on the number of light modulation elements in the spatial light modulator and the manner in which the spatial light modulator  110  is divided into light modulation banks  450   a - 450   f . Furthermore, depending on the application, the substrate and image subsections can be moved bi-directionally for additional optical oversampling.  
      In addition to reducing defects in the transferred pattern on the substrate, optical oversampling also provides several other benefits. As a result of oversampling, the total amount of light energy to which the substrate is exposed is integrated over the multiple exposures, thereby allowing more energy to be impinged on the substrate. Optical oversampling can also be used to achieve grayscale in images when using an SLM in which the light modulation elements have a binary characteristic, such that they are either “ON” or “OFF.” The image subsections can be modified between exposures to alter the state of the light modulation elements to produce the desired grayscale. Another benefit of optical oversampling is speckle reduction in laser-based photolithography systems. As is understood in the art, due to the coherent nature of the light produced by lasers, interference patterns cause speckles or spatial variations in the light intensity, which can degrade the quality of the lithographic process. Optical oversampling reduces the effect of the speckle pattern on the substrate in the same manner as it reduces the effect of defective light modulation elements.  
       FIG. 7A  is a flow chart illustrating an exemplary photolithography process  700  for performing optical oversampling of the substrate, in accordance with embodiments of the present invention. The photolithography process starts at block  702 . At block  704 , a substrate having a photoresist layer is positioned in relation to an SLM. At block  706 , an area of the photoresist layer is exposed with a portion of an image defined by the states of a first set of light modulation elements of the SLM. At block  708 , the relative position of the substrate and SLM is altered. At block  710 , the same area of the photoresist layer is exposed with the same portion of the image defined by the states of a second set of light modulation elements of the SLM. In one embodiment, the states of individual light modulation elements within the second set of light modulation elements are the same as the states of corresponding light modulation elements within the first set of light modulation elements. In another embodiment, the states of individual light modulation elements within the second set of light modulation elements are modified relative to the states of corresponding light modulation elements within the first set of light modulation elements. The photolithography process ends at block  712 .  
       FIG. 7B  is a flow chart illustrating an exemplary photolithography process  750  for performing multiple transfers of a portion of an image, in accordance with embodiments of the present invention. The photolithography process starts at block  752 . At block  754 , an SLM is provided with a portion of an image to be photolithographically transferred onto an area of a substrate. At block  756 , the SLM transfers the portion of the image onto the area of the substrate using a first set of light modulation elements within the SLM. At block  758 , the SLM transfers the portion of the image onto the same area of the substrate using a second set of light modulation elements within the SLM. The photolithography process ends at block  760 .  
       FIG. 8  is a block diagram illustrating the configuration  800  of a computing system  802  operable to control the photolithography system  100  of  FIG. 1 . The computing system  802  includes a processing unit  804  operable to execute software  806 . The processing unit  804  can be any type of microprocessor, microcontroller, programmable logic device, digital signal processor or other processing device. The processing unit  804  is coupled to a memory unit  808  and input/output (I/O) unit  810 . The I/O unit  810  can be wired or wireless. The processing unit  804  is further coupled to a storage unit  812  and timing circuit  814  that generates timing signals  816  for the photolithography system  100 . An electronic display  820  is optionally coupled to the computing system  802  and operable to display an image (or portion of an image)  300  that is to be communicated to the spatial light modulator  110  for imaging onto the substrate  150  of  FIG. 1 .  
      In one embodiment, the timing signals  816  control the operation of the stage  120 , spatial light modulator  110  and laser  102  during exposure cycles. Examples of timing signals  816  include access control signals to sequentially clock data  822  representing a portion of an image  300  into the spatial light modulator  110 , strobe or exposure signals to initiate a flash of the laser  102 , and other clock signals to drive the spatial light modulator  110 , laser  102  and stage  120 . The processor  804  communicates with the timing circuit  814  and I/O unit  810  to communicate the data  822  and timing signals  816  to the spatial light modulator  110  and other components of the photolithography system  100 , such as the laser  102  and stage  120 . For example, during an exposure cycle, data  822  is transmitted from the computing system  802  to the spatial light modulator  110  with an access control signal, and the clock signals drive the SLM  110 , stage  120  and laser  102  to alter the state of light modulation elements within the SLM  110  as a function of the data  822 , to align the stage  120  with the SLM  110  for image transfer and to control the timing of the strobe or exposure signal to initiate the laser  102  flash.  
      To implement optical oversampling, the data  822  communicated to the SLM  110  during each exposure cycle includes at least one new image subsection of the image (as shown in  FIG. 4 ). In one embodiment, the data  822  includes both the new image subsection(s) and one or more image subsections transferred to the substrate during the previous exposure cycle. For example, if each image section is divided into six image subsections, the data  822  includes five image subsections previously transferred to the substrate and one new image subsection. However, with potentially tens of millions of light modulation elements, writing the data  822  required to represent all of the image subsections to the SLM  110  each time requires a large amount of data  822  to be communicated between the I/O unit  810  and the SLM  110 . As a result of such a large I/O bandwidth, the photolithography system  100  power consumption is high and the throughput speed is limited.  
      Therefore, in other embodiments, the data  822  communicated to the SLM  110  during each exposure cycle includes only the new image subsection(s) of the image and not any of the previously transferred image subsections in order to reduce bandwidth, thereby reducing power consumption and increasing throughput speed. The image subsections previously transferred to the substrate are stored within the SLM  110  and moved internally within the SLM  110 .  
       FIG. 9  is a schematic of a portion of an exemplary spatial light modulator  110  capable of moving data internally during a lithographic process. The SLM includes an array  900  of light modulation elements  210 , each including a memory element  902  corresponding to at least a portion of the pixel drive circuit  250  of  FIGS. 2A and 2B  in communication with an associated pixel controller  904  that is at least partially responsible for controlling the state of a pixel defined by the light modulation element  210 . In  FIG. 9 , each memory element  902  is a static memory element that includes an input line  906  and a forward access control element  908 . In the example shown, the forward access control element  908  is a transistor having a forward access control line  910  that is operable to control the state of the forward access control element  908  during a shift forward operation. Each memory element  902  further includes a reverse access control element  912  having a reverse access control line  914  operable to control the state of the reverse access control element  912  during a shift reverse operation. Thus, the memory elements  902  are configured to shift data bi-directionally between adjacent columns of the array  900 . In addition, although only a single row of light modulation elements  210  within the array  900  is shown, it should be understood that the memory elements  902  can be further configured to shift data between rows and to shift data bi-directionally between adjacent or non-adjacent rows and/or columns of the array  900 .  
      A common node  916  of the forward and reverse access control elements  908  and  912 , respectively, is coupled to a memory cell  917 . In one embodiment, the memory cell  917  is a bi-stable circuit or static latch utilized to store data representing one pixel of the image. The memory cell  917  is shown implemented as a latch (i.e., a switch and back-to-back inverters) that uses a ripple clock to propagate data between memory cells  917 . The ripple clock is described in more detail below with reference to  FIGS. 11A and 11B . However, in other embodiments, the memory cell  917  can be implemented as a master-slave flip-flop that does not require a ripple clock to propagate data between the memory cells  917 .  
      Each memory cell  917  includes a forward inverter  918  and a feedback inverter  920 . The feedback inverter  920  is a “weak” feedback element that is utilized to reinforce the current state (i.e., LOW or HIGH state) to a stable position. Thus, if the common node  916  is in a low voltage level (i.e., a LOW state), the forward inverter  918  inverts the LOW state to a HIGH state on the output coupled to output node  922 . The HIGH state on output node  922  is an input to the feedback inverter  920 , which outputs a low voltage level onto node  916 . The low voltage level output from the weak feedback inverter  920  reinforces, but does not control, the LOW state on node  916 . Similarly, a high voltage level output from the weak feedback inverter  920  reinforces, but does not control, the HIGH state on node  916 .  
      The output node  922  is coupled to the pixel controller  904  and is also the output node of the light modulation element  210 . In one embodiment, the pixel controller  904  is a pixel electrode of a LC light modulation element ( 215 , shown in  FIGS. 2A and 2B ). The voltage level on output node  922  is applied to the pixel electrode of the LC light modulation element to alter the state of the LC light modulation element when the voltage level applied to the pixel electrode differs from a voltage applied to the common electrode  235  of the LC light modulation element. In other embodiments, the pixel controller  904  is an electromechanical device controlling the state or position of a micromirror.  
      Multiple light modulation elements  210  are electrically interconnected. In one embodiment, the light modulation elements  210  are connected in a shift register configuration, as shown in  FIG. 9 . In the shift register configuration, the output node  922  of a first light modulation element (e.g., light modulation element  210   a ) is connected to the input line  906  of a second light modulation element (e.g., light modulation element  210   b ). The output node  922  of the second light modulation element  210   b  is connected to the input line of a third light modulation element (not shown), and so on until the output node of the (N-1)th pixel (not shown) is connected to the input line  906  of the Nth pixel (not shown), thereby forming a forward connection network. To load input data into the forward connection network, the input data is provided at the input line  906  of the first light modulation element  210   a , and data is shifted from the first light modulation element  210   a  to the second light modulation element  210   b , and so on. It should be understood that a parallel data loading and shifting configuration can be implemented for a reverse connection network, where data is input to the last light modulation element  210  in the array  900 .  
      In another embodiment, as shown in  FIG. 10 , a conventional dynamic memory cell  800  is used in the spatial light modulator  110  in place of the static memory cell  917  shown in  FIG. 9 . As shown, the memory cell  800  includes a charge capacitor  802  for storing the data. The charge capacitor  802  is coupled to an inverter  804 . However, the memory cell  800  suffers from photoinduced carriers generated by the illumination incident on the silicon of the memory cell  800 . The photoinduced carriers tend to increase the charge or voltage value of the charge capacitor  802 . The increased charge can cause a low state to unwantingly switch to a high state of the charge capacitor  802 . However, care can be taken to minimize the impact of the photogenerated carriers. Physical techniques, such as light shielding, can be used to reduce the magnitude of the problem. Other techniques that rely on gathering the unwanted carriers are described in U.S. Pat. No. 6,586,283, which is herein incorporated by reference. However, to substantially avoid problems associated with photoinduced carriers, the SLM  110  can be designed with the static memory cells  917  shown in  FIG. 9 .  
       FIG. 11A  is a block diagram of an exemplary configuration  1100  of the light modulation elements  210 . The light modulation elements  210  have forward access control lines  910  coupled thereto for causing data on the input lines  906  to propagate through the memory elements  902  (shown in  FIG. 9 ). The light modulation elements  210  can be viewed as elements N, N-1, N-2, N-3, and so forth, where the Nth light modulation element  210  is the last light modulation element and the (N-3)rd light modulation element  210  is the first light modulation element.  
       FIG. 11B  is a timing diagram  1105  for shifting data between the light modulation elements  210  of  FIG. 11A . As shown in  FIG. 11B , a sequence of non-overlapping pulses, produced by a ripple clock or otherwise, is utilized to shift the data through the light modulation elements. As shown, an access pulse  1102  is applied to the forward access control element  908  of the Nth light modulation element via forward access control  910  line between times t 1  and t 2  to move data out of the Nth light modulation element. Each of the other access pulses  1102  for the memory elements of the (N-1l)th, (N-2)th and (N-3)th light modulation elements are pulsed sequentially such that the data is moved serially from the (N-1)th light modulation element to the Nth light modulation element between times t 3  and t 4 , from the (N-2)th light modulation element to the (N-1)th light modulation element between times t 5  and t 6  and from the (N-3)the light modulation element to the (N-2)th light modulation element between times t 7  and t 8  so as to ensure the data is preserved as it is shifted through the light modulation elements. It should be understood that a similar shifting mechanism can be used to shift data in a reverse sequence to enable bi-directional data movement.  
      When the light modulation elements are liquid crystal elements, as shown in  FIGS. 2A and 2B , the net DC value across the liquid crystal elements should be zero to avoid damaging the liquid crystal elements. In embodiments of the present invention, DC balance can be achieved by alternating the voltage on the common electrode and inverting the data between exposure intervals.  
      For example,  FIG. 12A  is a timing diagram that illustrates an alternating common electrode voltage  1202 . As understood in the art, the state of a liquid crystal element is determined by the potential difference between the common electrode and the pixel electrode. In the example shown, an OFF state  1210  is one where no potential difference exists between a common electrode signal  1202  and a pixel electrode signal  1204 , and therefore no electric field is created, allowing the light to be reflected onto the substrate  150 . In an ON state  1212  (i.e., when there is a potential difference between the common electrode signal  1202  and the pixel electrode signal  1204 ), an electric field is created, and the light is not reflected onto the substrate  150 . In other configurations, the ON and OFF states can be reversed.  
      The sign of the electric field depends on the values of the common electrode signal  1202  and the pixel electrode signal  1204 . For example, the electric field can be obtained by placing either the pixel electrode signal  1204  at zero potential and the common electrode signal  1202  at unit potential (corresponding to logical one) or by placing the common electrode signal  1202  at zero potential and the pixel electrode signal  1204  at unit potential. In either case, a potential difference exists between the common electrode and the pixel electrode, and hence in the example shown in  FIG. 12A , the electric field is non-zero and the liquid crystal element is in an ON state. Although the sign of the electric field is unimportant in determining the state of the liquid crystal element, the net value of the electric field should average to zero to avoid ionization of the liquid crystal elements.  
      As shown in  FIG. 12A , during time interval t 1 , the common electrode signal  1202  has a voltage level of zero volts and the pixel electrode signal  1204  has a voltage level of zero volts. Because the voltage difference between the common and pixel electrode signals  1202  and  1204  is zero, the pixel state  1206  of the liquid crystal element is OFF  1210 . During time intervals t 2  and t 3 , the voltage differential between the common electrode signal  1202  and pixel electrode signal  1204  is also zero volts, thereby maintaining the pixel state  1206  at the OFF state  1210 . During time intervals t 4  and t 5 , the voltage differential between the common electrode signal  1202  and pixel electrode signal  1204  causes the pixel state  1206  to be ON  1212 . However, the ON state  1212  is achieved by alternating the common electrode signal  1202  with respect to the pixel electrode signal  1204 , and therefore, the sign of the electric field is opposite at times t 4  and t 5 . Therefore, DC balance is maintained. During time intervals t 6  and t 7 , the pixel state  1206  is again at OFF  1210 .  
      Since the common electrode is alternating with each data shift to maintain DC balance, a data inversion technique is needed to shift the data through the liquid crystal elements to preserve the correct pixel state for optical oversampling of the image.  FIG. 12B  illustrates an exemplary data inversion technique.  FIG. 12B  uses the exemplary pixel configuration shown in  FIG. 11A  to illustrate the shifting of data through the liquid crystal elements. As can be seen in  FIG. 12B , the voltage level (logical state) of the common electrode signal  1102  alternates over times t 1 -t 4 . As the data corresponding to the pixel electrode signal is propagated through the liquid crystal elements from the (N-3)th liquid crystal element to the (N-2)th liquid crystal element to the (N-1)th liquid crystal element to the Nth liquid crystal element, the logical state of the pixel electrode signal inverts with each shift to maintain the same pixel state. For example, at time t 1 , the common electrode signal is in a logical one state, while the pixel electrode signal at the (N-3)th liquid crystal element is also in a logical one state. Therefore, no electric field is created at the (N-3)th liquid crystal element at time t 1 , and the (N-3)th liquid crystal element is in an OFF state. At time t 2 , the data has shifted from the (N-3)th liquid crystal element to the (N-2)th liquid crystal element to cause the (N-2)th liquid crystal element to be in the same state at time t 2  (OFF state) as the (N-3)th liquid crystal element was at time t 1 . However, to maintain DC balance, the common electrode signal has inverted at time t 2  to be in a zero logical state. Therefore, when the data is shifted from the (N-3)th liquid crystal element to the (N-2)th liquid crystal element, the data is inverted, such that the (N-2)th liquid crystal element at time t 2  is also in a zero logical state, thereby allowing the (N-2)th liquid crystal element to be in an OFF state at time t 2 . Referring again to  FIG. 9 , such a data inversion technique is performed by the inverter  918  within the memory cells  917 .  
       FIG. 13  illustrates an exemplary substrate exposure timing sequence using optical oversampling and data shifting.  FIG. 13  shows a series of LC settling intervals  1302   a - 1302   e  (collectively  1302 ) during which the LC material settles after exposure. At the end of each LC settling interval  1302 , the laser is flashed (represented by  1310 ). Between each LC settling interval  1302 , there are transition time intervals tt 1 -tt 5 . During each of the transition time intervals tt 1 -tt 5 , data is moved between the memory elements within the SLM in preparation for the next exposure. The timing circuit  814  (shown in  FIG. 8 ) can be utilized to generate the timing signals to drive the data propagation via the access control signals on the access control lines  910  (shown in  FIG. 9 ), common electrode signal  1202  (shown in  FIGS. 12A and 12B ), and clock signal (not shown) to control the SLM, stage and laser.  
      The common electrode signal  1302  alternates between each time interval tt 1 -tt 5 . The transition intervals  1308   a - 1308   e  of the common electrode signal  1302  occur during the time intervals tt 1 -tt 5  after the laser flashes  1310 . In  FIG. 13 , two exemplary pixel electrode signals  1304  and  1306  are shown, where pixel electrode signal  1304  is illustrative of an ON liquid crystal element and pixel electrode signal  1306  is illustrative of an OFF liquid crystal element. The pixel electrode signal  1304  at each laser flash  1310  has the same potential on the pixel electrode as the common electrode and the pixel electrode signal  1306  has the opposite potential on the pixel electrode as the common electrode at the laser flashes  1310 . During the transition time intervals tt 1 -tt 5 , data inversions are performed as data is shifting through the memory array to maintain DC balance of the liquid crystal elements. In one embodiment, the data is shifted between the memory elements of the liquid crystal elements during the transition time intervals tt 1 -tt 5  in about 60 microseconds, which allows 940 microseconds of a one millisecond duty cycle for the liquid crystal material to respond to the electric field applied between the pixel electrode and the common electrode. A twenty-nanosecond (20 ns) flash of the laser  1310  occurs at the end of the LC settling intervals  1302  after the liquid crystal material has transitioned. It should be understood that other timings can be established to increase or decrease the LC settling intervals  1302  and data shifting rates based on the transition rate of the liquid crystal material and speed of the substrate moving with respect to the spatial light modulator.  
       FIG. 14  is a flow chart illustrating an exemplary process  1400  to dynamically photolithographically transfer an image onto a substrate by internally moving data. The photolithography process starts at block  1402 . At block  1404 , data representing an image is loaded into memory elements in communication with respective light modulation elements within a spatial light modulator. At block  1406 , the light modulation elements are altered in response to the data loaded in the memory. The altered light modulation elements are illuminated to direct an illumination pattern onto the substrate at block  1408 . At block  1410 , a determination is made whether the image has been transferred a requisite number of times. If not, the data is moved between the memory elements at block  1412 , and the relative positioning of the substrate and light modulation elements is altered at block  1414 . The process repeats to block  1406  to alter the light modulation elements again in response to the data as moved in the memory elements. If the image has been transferred the requisite number of times at block  1410 , the photolithography process ends at block  1416 .  
       FIG. 15  is a flow chart illustrating an exemplary process  1500  for shifting data within a spatial light modulator to dynamically photolithographically transfer an image onto a substrate. The photolithography process starts at block  1502 . At block  1504 , first data representing a first section of an image is loaded into a spatial light modulator. At block  1506 , the spatial light modulator is illuminated to direct the first section of the image onto the substrate. A portion of the first data is moved out of the spatial light modulator at block  1508 , the remaining data is moved within the SLM at block  1510  and a portion of second data representing a second section of the image is loaded into the spatial light modulator at block  1512 . At block  1514 , the spatial light modulator is illuminated to direct portions of the first and second image sections onto the substrate. The photolithography process ends at block  1516 .  
      The innovative concepts described in the present application can be modified and varied over a wide rage of applications. Accordingly, the scope of patented subject matter should not be limited to any of the specific exemplary teachings discussed, but is instead defined by the following claims.