Patent Publication Number: US-7719753-B2

Title: Method of operation for SLM-based optical lithography tool

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. Non-Provisional application Ser. No. 10/941,969 filed Sep. 14, 2004 now U.S. Pat. No. 7,295,362, which in turn is a divisional of U.S. Non-Provisional application Ser. No. 10/646,525 filed Aug. 21, 2003 now U.S. Pat. No. 7,167,296, which claims the benefit of U.S. Provisional Application No. 60/406,030 filed Aug. 24, 2002, incorporated in its entirety by reference herein. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to the field of optical lithography, and in particular to printing patterns on the following substrates: wafers; printed circuit boards; flat panel displays; masks; reticles; and plates used for the reproduction of magazines, newspapers and books. 
   2. Description of the Related Art 
   The semiconductor industry uses very expensive stepper tools for lithographic processing. Furthermore, very expensive reticles are used in this processing—the cost of the reticles is sufficient to make low volume production of chips (such as custom ASICs) prohibitively expensive. The semiconductor industry needs a lower cost lithography process. Furthermore, every time the lithography pattern changes, several days or more are required to produce a new reticle. The semiconductor industry needs a lithography process which can quickly accommodate pattern changes. 
   The printed circuit board (PCB) industry has similar problems with its lithography processes. Furthermore, the substrates used in the PCB industry undergo distortion during fabrication which limits the use of high resolution lithography processing to small area substrates and the use of steppers. A high resolution lithographic process is required for large PCB substrates in which the pattern can be quickly and economically adjusted to accommodate the distortions, where the distortions vary from one substrate to the next. 
   U.S. Pat. Nos. 5,330,878 5,523,193 5,482,818 and 5,672,464 to Nelson describe a method and apparatus for patterning a substrate. The apparatus uses a spatial light modulator (SLM), specifically the Texas Instruments deformable mirror device (DMD), in place of a reticle. The DMD is an array of individually controllable reflective elements. An image of the DMD is projected on the substrate by an imaging lens. Whether or not an individual element of the DMD reflects light into the imaging lens, such that it is projected on the substrate, is determined by computer; thus the pattern projected on the substrate is computer controlled and readily changed. Improvements are required to this approach in order to meet the high resolution and throughput requirements of both the semiconductor and PCB industries. Furthermore, advancements are available to reduce the cost of the apparatus, while increasing the throughput and meeting the high resolution requirements. 
   SUMMARY OF THE INVENTION 
   The present invention provides an apparatus and method for patterning photosensitive substrates. The apparatus includes a spatial light modulator (SLM), a light source for illuminating the SLM, imaging optics for projecting an image of the SLM on the substrate, and means for moving the image across the surface of the substrate. The SLM controls the pattern of light which reaches the substrate. The SLM comprises at least one array of individually switchable elements-switchable between two or more states. The SLM can be either a diffractive or a transmissive device. The light source can be a continuous light source, such as an arc lamp, LED or continuous laser; quasi-continuous lasers can also be used when the laser pulsing frequency is much higher than the switching frequency of the elements of the SLM. The means for moving the image can be a stage on which either the SLM or the substrate is mounted. When the substrate is in the form of a flexible film or similar, it may be moved using a reel to reel mechanism. While the image is moving across the surface of the substrate, elements of the spatial light modulator are switched such that a pixel on the surface of the substrate receives, in serial, doses of energy from multiple elements of the spatial light modulator, thus forming a latent image on the substrate surface. The imaging optics can be telecentric. 
   In preferred embodiments the imaging optics is configured to project a blurred image of the spatial light modulator on the substrate, enabling sub-pixel resolution feature edge placement. The blurring can be implemented by: adjusting the focus of the imaging optics; adjusting the numerical aperture of the imaging optics; adding a diffuser between the SLM and the substrate; adding a microlens array between the SLM and the substrate; or a combination of the aforementioned. 
   In preferred embodiments the spatial light modulator is continuously illuminated, an image of the spatial light modulator is continuously projected on the substrate, and the image is continuously moved across the surface of the substrate. 
   In some embodiments the SLM comprises a multiplicity of area arrays. The corresponding imaging optics can be a single projection lens system, or a multiplicity of projection lens systems. In the case of the latter, the number of the area arrays is greater than the number of the projection lens systems, and the number of projection lens systems is preferably a submultiple of the number of area arrays. Furthermore, the multiplicity of area arrays can be arranged in a line, or they can be arranged in multiple lines where the placement of the arrays is staggered from one line to another. The latter may utilize more of the imaging field of the projection optics, and can also result in a more efficient exposure of the substrate—reducing the need for a serpentine motion of the projected image of the SLM across the substrate during exposure. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1  is a schematic representation of an optical lithography tool with a movable substrate, in accordance with the invention. 
       FIG. 2  is a schematic representation of an optical lithography tool with a movable spatial light modulator, in accordance with the invention. 
       FIG. 3  is a schematic representation of an optical lithography tool with a flexible film substrate, in accordance with the invention. 
       FIG. 4  is a detailed schematic representation of a first embodiment of the optical lithography tool of  FIG. 1 , showing telecentric projection optics. 
       FIG. 5  is a detailed schematic representation of a second embodiment of the optical lithography tool of  FIG. 1 , showing a spatial light modulator with multiple area arrays and corresponding multiple sets of projection optics. 
       FIG. 6  is a detailed schematic representation of a third embodiment of the optical lithography tool of  FIG. 1 , showing a spatial light modulator with multiple area arrays and a single set of telecentric projection optics. 
       FIG. 7  is a diagrammatic cross-sectional view through part of a micro-mirror array in accordance with the invention, showing array elements in ‘on’ and ‘off’ positions. 
       FIG. 8  is a plan view of a substrate showing a serpentine path that can be followed by the projected image of a spatial light modulator in order to expose the entire substrate surface, in accordance with the invention. 
       FIG. 9  is a plan view of a substrate showing serpentine paths that can be followed by projected images from each of a multiplicity of area arrays that are used together to expose the entire substrate surface, in accordance with the invention. 
       FIG. 10  is a diagrammatic representation of the process of forming a latent image, in accordance with the invention. 
       FIG. 11  is a diagrammatic representation of the substrate array of  FIG. 10 . 
       FIG. 12  is a graph showing instantaneous light intensity distributions along the line segment AB on the substrate of  FIG. 10 , at equally spaced time intervals T/10, starting at T 3 . 
       FIG. 13  is a graph showing instantaneous light intensity distributions along the line segment AB on the substrate of  FIG. 10 , at equally spaced time intervals T/10, ending at T 4 . 
       FIG. 14  is a graph showing the integrated dose distribution along the line segment AB on the substrate of  FIG. 10 , due to light exposure between times T 3  and T 4 . 
       FIG. 15  is a graph showing the total dose distribution along the line segment AB on the substrate of  FIG. 10 , due to light exposure between times T 1  and T 7 . 
       FIG. 16  is a diagrammatic representation of the process of forming a latent image including a first example of edge shifting by one half of the projected width of a mirror, in accordance with the invention. 
       FIG. 17  is a diagrammatic representation of the process of forming a latent image including a second example of edge shifting by one half of the projected width of a mirror, in accordance with the invention. 
       FIG. 18  is a diagrammatic representation of the process of forming a latent image including an example of edge shifting by one quarter of the projected width of a mirror, in accordance with the invention. 
       FIG. 19  is a diagrammatic representation of the process of forming a latent image including an example of edge shifting by three quarters of the projected width of a mirror, in accordance with the invention. 
       FIG. 20  is a diagrammatic representation of the process of forming a latent image including an example of edge shifting in another direction by one quarter of the projected width of a mirror, in accordance with the invention. 
       FIG. 21  is a graph showing the integrated dose distributions along the line segment AB on the substrates of  FIGS. 10 ,  16 ,  17 ,  18  and  19 . 
       FIG. 22  is a further diagrammatic representation of the process of forming a latent image, in accordance with the invention. 
       FIG. 23  is a diagrammatic representation of the substrate array of  FIG. 22 . 
       FIG. 24  is a graph showing the integrated dose distributions along the line segments CD, EF, GH and IJ on the substrate of  FIG. 22 . 
       FIG. 25  is a diagrammatic representation of the process of forming a latent image including a further example of edge shifting, in accordance with the invention. 
       FIG. 26  is a diagrammatic representation of the substrate array of  FIG. 25 . 
       FIG. 27  is a graph showing the integrated dose distributions along the line segments KL, MN, OP, QR and ST on the substrate of  FIG. 25 . 
       FIG. 28  is a block diagram of an optical lithography system in accordance with the invention. 
       FIG. 29  is a plan view of an arrangement of multiple area arrays in accordance with an embodiment of the invention. 
       FIG. 30  is a schematic representation of another embodiment of the optical lithography tool of  FIG. 4 , showing a light switching mechanism  121  on the light path between the light source and the substrate. 
       FIG. 31  is a timing diagram of an optical lithography system with two spatial light modulators configured in serial on the light path, in accordance with the invention. 
       FIG. 32  is a diagrammatic representation of the process of forming a latent image using an optical lithography system with two spatial light modulators configured in serial on the light path, in accordance with the invention. 
       FIG. 33  is a timing diagram of an optical lithography system with a spatial light modulator and a light switching mechanism configured in serial on the light path, in accordance with the invention. 
       FIG. 34  is a schematic representation of an optical lithography tool with light optics configured to overlap projected images of two area arrays on the substrate surface, in accordance with the invention. 
       FIG. 35  is a timing diagram of the optical lithography system of  FIG. 34 . 
   

   DETAILED DESCRIPTION 
   With reference to  FIG. 1 , optical lithography tool  100 , which is an embodiment of the invention suitable for patterning a substrate  140  mounted on a movable stage  150 , is shown with a light source  110 , a spatial light modulator (SLM)  120  and imaging optics  130 . Coordinate axes  160  are shown with the z and y axes in the plane of the figure and the x axis perpendicular to the plane of the figure. The light path through the optical lithography tool is represented by rays  170 . The light source  110  continuously illuminates the SLM  120 . The light source may comprise an arc lamp, continuous laser (solid state or gas), light emitting diode (LED) or other type of continuous light source that has suitable spectral properties for exposure of the substrate  140 . Furthermore, a light source such as a quasi-continuous laser (a laser which is pulsed at MHz frequencies), may be suitable as a light source for this invention—a critical criteria is that the pulsing frequency be much higher than the switching frequency for elements of the spatial light modulator (typically 10 4  Hz); in this case the illumination of the SLM by the light source is effectively continuous. The light source may also comprise optical components for increasing the intensity of illumination and to improve illumination uniformity. These may include an elliptical mirror, both round and cylindrical lenses, and light pipes or fly&#39;s eye lens arrays. SLM  120  is one or more area arrays (generally rectangular) of elements that act on the light beam from the light source. An image of the SLM is continuously projected onto the substrate by the imaging optics  130 , which is also referred to as the projection optics. The elements can be individually switched between two or more states, under computer control, so as to control the light amplitude in the image. One embodiment of the invention includes an SLM which is an array of mirrors or diffractive elements that can switch incoming light rays between two or more angular states. A digital micro-mirror device (DMD), currently available from Texas Instruments, is an example of a suitable mirror-array that can switch between two angular states. An example of a diffractive SLM is the Grating Light Valve (GLV) currently manufactured by Silicon Light Machines. Other embodiments of the invention include SLMs which are Liquid Crystal Display (LCD) devices. If the elements of the SLM are transmissive, rather than reflective, then the optics will need to be rearranged; such a rearrangement will be obvious to those skilled in the art. Imaging lens system  130  may contain both reflective and refractive elements, and is typically telecentric. Substrate  140  either includes a photosensitive layer, such as a photoresist coating, or is itself a photosensitive material, such as a sheet of photosensitive polyimide. The stage  150  may be of a roller-bearing or air-bearing design and may have height adjustment (in z-direction), tilt and rotation capabilities. These types of stages are well know and commonly used in lithography systems. For simplicity of illustration the substrate is assumed to be planar. However, the invention will also work with other substrate shapes, such as cylindrical or spherical, along with a rotary rather than a planar stage. 
   With reference to  FIG. 2 , optical lithography tool  200 , which is an embodiment of the invention suitable for patterning a static substrate  140 , is shown with a light source  110 , SLM  120 , a stage  250  on which the SLM is mounted, and projection optics  130 . The method of operation is the same as for optical lithography system  100 , described above, except that stage  250  moves SLM  120  during exposure while substrate  140  is stationary. Imaging lens system  130  and/or illumination source  110  can also be attached to the stage  250  and move with the SLM. 
   With reference to  FIG. 3 , optical lithography tool  300 , which is an embodiment of the invention suitable for patterning a flexible substrate  340 , is shown with a light source  110 , SLM  120 , a stage  250  on which the SLM is mounted, projection optics  130 , and rotatable, spaced apart, axially parallel film drums  342  and  344 . The photosensitive flexible film substrate  340  is wrapped around and tensioned between film drums  342  and  344  such that the film can be moved in the y direction (referenced to stationary coordinate system  160 ). Two modes of exposure are possible. In the first mode, stage  250  moves the SLM at constant speed in the x direction while the substrate  340  is stationary. When an exposure pass is complete (for example, the edge of the substrate is reached), the film drums index the substrate forward in the y direction and the stage reverses direction for the next exposure pass. The result is a serpentine exposure path similar to path  850  shown in  FIG. 8 , and discussed in more detail below. In the second mode the roles of the stage and film drums are reversed. While the stage is stationary, the film drums move the substrate at constant speed in the y direction until the edge of the exposure region is reached. The stage then indexes the substrate forward in the x direction and the film drums reverse direction for the next exposure pass. Again, this results in a serpentine exposure path. Furthermore, if the width of the area to be exposed on the substrate is less than or equal to the width of the projected image of the SLM, then the stage can remain stationary, or be eliminated, and the film drums move the substrate at a constant speed, without the need to reverse direction. As in other embodiments, the projection optics may be carried on the stage. 
   Referring to  FIGS. 4 ,  5  and  6 , different embodiments of optical lithography tool  100  (see  FIG. 1 ) are shown in detail. 
     FIG. 4  is a schematic of a continuous direct-write optical lithography system with an arc lamp and a telecentric projection lens system. Continuous illumination from mercury arc lamp  410  is reflected from elliptical reflector  411 . Reflected light, as represented by light rays  170 , travels to a dichroic mirror  412 , which reflects wavelengths useful for exposure of the substrate  140  (for example 350 nm-450 nm) and is transparent to other wavelengths. Light not reflected from the dichroic mirror is absorbed in illumination beam dump  413 . Other types of lamps can be used, such as a Xenon arc lamp, depending on the exposure wavelengths and source brightness needed. A light pipe  415  is used to improve the illumination uniformity, but could be replaced with a fly&#39;s eye lens array. A light pipe lens system  414 , positioned before the light pipe  415 , is used to adjust the numerical aperture of the illumination system and to adjust the diameter of the light beam prior to entering the light pipe. Condenser lens system  416  captures light exiting from the light pipe and modifies the beam shape and angle to match the requirements of the SLM  120 . The condenser lens system contains an illumination aperture  417 . The light pipe lens system and condenser lens system are usually anamorphic and contain cylindrical lens elements. The continuous illumination mercury arc lamp, elliptical reflector, dichroic mirror, illumination beam dump, light pipe lens system, light pipe, condenser lens system and illumination aperture comprise an embodiment of illumination source  110 , as shown in  FIG. 1 . The SLM is one or more area arrays (generally rectangular) of small mirrors that can switch between two or more angular states under computer control. At least one of the angular states reflects light rays from the illumination source into a telecentric projection lens system  430  and at least one other angular state reflects light rays into an SLM beam dump  480 . A digital micro-mirror device (DMD), currently available from Texas Instruments, is an example of a suitable mirror-array that can switch between two angular states. Mirrors in the “on” state in the SLM are imaged on the substrate by the telecentric projection lens system. Light reflected from mirrors in the SLM in the “off” state travels to the SLM beam dump where it is absorbed. Further details of the operation of a SLM are provided below and in  FIG. 7 . The substrate either contains a photosensitive layer, such as a photoresist coating, or is itself a photosensitive material, such as a sheet of photosensitive polyimide. The substrate is attached to stage  150 , which moves continuously during exposure in straight-line segments in the x-y plane of stationary coordinate system  160 . The numerical aperture of the telecentric projection lens system is determined by a projection lens aperture  432 , which is optically conjugate to illumination aperture  417 . A double telecentric projection lens system is shown. However, a single telecentric or non-telecentric projection system will also work. A telecentric design is preferred because the magnification does not change with substrate height, which simplifies calibration of the lithography tool for each substrate. The telecentric projection lens system is a type of projection lens system  130 , as shown in  FIG. 1 . The stage can move in a plane x-y and also in the z direction of the stationary coordinate system  160 . The stage  150  can also have rotation and tilt capability; this may be required for proper substrate alignment (for example, when substrate flatness is an issue). Movement in the z direction will either focus or defocus the projected image on the substrate. A substrate height measuring system  450 , utilizing height detection medium  490 , can be used to determine the z position of the surface of the substrate  140 . The height measuring system can be optical, capacitance or air based. The preferred type is air. Focusing can also be accomplished by moving either the SLM or projection lens system in the z direction. 
     FIG. 5  is a schematic of a continuous direct-write optical lithography system with an arc lamp, an SLM with multiple area arrays, and multiple projection lens systems. The light source is arranged as described above for  FIG. 4 , except that a condenser lens system  516  and lens array  518  capture light exiting from light pipe  415 , so as to modify the beam shape and angle to match the requirements of the individual SLM area arrays  520  through  524 . The lens array maximizes the light intensity on the individual SLM area arrays; the lens array is configured to match the arrangement of the SLM area arrays, which may be arranged in a line, multiple lines (see  FIG. 29 ), or some other two dimensional arrangement. While not an essential component, incorporation of the lens array is preferred. The lens array may comprise lenses arranged correspondingly with the SLM area arrays; alternatively, the lenses in the lens array may be replaced with one or more diffractive elements. Light pipe lens system  414  and condenser lens system  516  are usually anamorphic and contain cylindrical lens elements. The continuous illumination mercury arc lamp  410 , elliptical reflector  411 , dichroic mirror  412 , illumination beam dump  413 , light pipe lens system  414 , light pipe  415 , condenser lens system  516  and lens array  518  comprise a type of continuous illumination source  110  as in  FIG. 1 . Each individual SLM area array  520  through  524  is a rectangular array of small mirrors that can switch between two or more angular states under computer control. A digital micro-mirror device (DMD) currently available from Texas Instruments is an example of a suitable mirror-array that can switch between two angular states. Mirrors in the “on” state in SLM area array  520  are imaged on the substrate  140  by projection lens  530 ; likewise for SLM area arrays  521  through  524  and their corresponding projection lenses  531  through  534 . Light reflected from mirrors in SLM area array  520  in the “off” state travels to SLM beam dump  480  where it is absorbed; likewise for SLM area arrays  521  through  524 . Five each SLM area arrays ( 520  through  524 ), projection lenses ( 530  through  534 ) and substrate height measuring systems ( 550  through  554 ) are shown in this example, but any number may be used. The projection lenses may contain both reflective and refractive elements, and are typically telecentric. The projection lens systems (any one of  530  through  534 ) may be the same as the projection optics  130  in  FIG. 1 . Substrate  140  either contains a photosensitive layer, such as a photoresist coating, or is itself a photosensitive material, such as a sheet of photosensitive polyimide. The substrate is attached to stage  150 , which moves continuously during exposure in straight-line segments in the x-y plane of stationary coordinate system  160 . As in other embodiments, the imaging optics may be carried on the stage. 
     FIG. 6  is a schematic of a continuous direct-write optical lithography system with a single telecentric objective lens system and a SLM with multiple area arrays. The light source  610  is the same as the light source described in  FIG. 5 , and is configured so as to provide illumination to match the requirements of the individual SLM area arrays  520  through  524 . Each individual SLM area array is one or more rectangular arrays of small mirrors that can switch between two or more angular states under computer control. A digital micro-mirror device (DMD) currently available from Texas Instruments is an example of a suitable mirror-array that can switch between two angular states. Mirrors in the “on” state in the SLM area arrays are imaged on the substrate  140  by telecentric projection lens system  630 . Light reflected from mirrors in the SLM area arrays in the “off” state travels to SLM beam dump  480  where it is absorbed. Five SLM area arrays are shown in this example but any number may be used. A double telecentric projection lens system  630  is shown. However, a single telecentric or non-telecentric projection system can also be used. A telecentric design is preferred because the magnification does not change with substrate height, which simplifies calibration of the lithography tool for each substrate. The telecentric projection lens system is a type of projection lens system  130 , as in  FIG. 1 . The stage can move in a plane x-y and also in the z direction of the stationary coordinate system  160 . The stage  150  can also have rotation and tilt capability; this may be required for proper substrate alignment (for example, when substrate flatness is an issue). Movement in the z direction will either focus or defocus the projected image on the substrate. A substrate height measuring system  450  can be used to determine the z position of the surface of the substrate  140 . The height measuring system can be optical, capacitance or air based. The preferred type is air. Focusing can also be accomplished by moving either the SLM area arrays  520  through  524  or telecentric projection lens system  630  in the z direction. Substrate  140  either contains a photosensitive layer, such as a photoresist coating, or is itself a photosensitive material, such as a sheet of photosensitive polyimide. 
   Further to the lithography systems of  FIGS. 5 &amp; 6 , other embodiments of the invention are envisaged which combine a SLM having multiple area arrays with a submultiple number of projection lens systems. For example, a lithography system may have 6 SLM area arrays and 2 projection lens systems, such that each projection lens system images 3 different SLM area arrays at once. Furthermore, the number of projection lens systems need not be limited to a mathematical submultiple—for example, a lithography system may have 7 SLM area arrays and 2 projection lens systems, such that a first projection lens system images 3 SLM area arrays and a second projection lens system images the remaining 4 SLM area arrays. The configuration of these embodiments will be apparent to those skilled in the art. Clearly, there are very many further combinations of SLM area arrays and projection lens systems which follow this teaching and will be apparent to those skilled in the art. 
   With reference to  FIG. 7 , a partial cross-section of a SLM  720  is shown. Mirrors  721  are shown in the ‘on’ position and mirrors  722  are shown in the ‘off’ position. Light rays  770  are reflected off the surface of the mirrors  721 , which are in the ‘on’ position, toward a substrate (rays  771 ) and are reflected off the surface of mirrors  722 , which are in the ‘off’ position, toward a beam stop (rays  772 ). For example, referring to both  FIGS. 4 and 7 , rays  771  travel through projection lens system  430  and then to the substrate  140 , whereas the rays  772  fall outside the acceptance aperture of projection lens system  430  and are collected by beamstop  480 . This is the preferred mode of operation, although, other modes of operation may be considered. For example, the rays  772  could fall partly within the acceptance aperture of projection lens system  430 , consequently an attenuated signal from the “off” state mirrors would reach the substrate, which may be tolerable. 
   With reference to  FIG. 8 , an example is shown of a serpentine path  850  that can be followed by the projected image of a SLM in order to expose the entire surface of the substrate  140 . The motion of the image is due to an image movement mechanism. The substrate or the SLM can be mounted on the image movement mechanism. An example of a suitable mechanism is a stage, such as shown in  FIGS. 1 ,  2  and  3 . In the case of a flexible substrate a suitable mechanism is a pair of rotatable, spaced apart, axially parallel film drums, such as shown in  FIG. 3 . In the explanation that follows a configuration of the lithography system in which the substrate is mounted on a stage is assumed. The following are shown: substrate  140 , serpentine path  850 , distance between straight line segments on the path  851 , substrate coordinate system  853  and stationary coordinate system  860 . The SLM is oriented in such a way that the columns of pixels in the projected image on the substrate are parallel to the straight-line segment portions of the serpentine path, which, for ease of illustration, are parallel to the x-axis of stationary coordinate system  860 . A stage positions the substrate  140  such that the center of the projected image of the SLM is at the beginning of path  850 . In this example, at the beginning of path  850  none of the projected image of the SLM falls on the substrate  140 . As the stage moves in the +x direction, referenced to stationary coordinate system  860 , the center of the projected image of the SLM moves in the −x s  direction, referenced to substrate coordinate system  853 , and traces the first straight section of the serpentine path. The exposure starts when the projected image of the SLM falls on the substrate. The exposure stops after the projected image clears the edge of the substrate. The stage then repositions the substrate in readiness to scan in the −x direction along the second straight section of the path, which is separated from the first straight section by a distance  851  in the y direction, all referenced to stationary coordinate system  860 . This is repeated until the entire substrate is exposed. Clearly, the projected width of the SLM must be greater than or equal to the distance  851  in order to expose the complete substrate. If only certain regions of the substrate need to be exposed, then it may be more efficient to execute a serpentine pattern for each individual region. Although a serpentine path is preferred, other paths could be used as long as they contained straight-line segments for the exposure. It will be clear to those skilled in the art that a serpentine path can also be achieved with a lithography system configuration in which the SLM is mounted on a stage and the substrate is static. 
   With reference to  FIG. 9 , an example is shown of a set of serpentine paths  950  through  954  that can be followed by the projected images of a corresponding set of SLM area arrays in order to expose the entire surface of the substrate  140 . The motion of the image is due to an image movement mechanism. The substrate or the SLM can be mounted on the image movement mechanism. An example of a suitable mechanism is a stage, such as shown in  FIGS. 1 ,  2  and  3 . In the case of a flexible substrate a suitable mechanism is a pair of rotatable, spaced apart, axially parallel film drums, such as shown in  FIG. 3 . In the explanation that follows a configuration of the lithography system in which the substrate is mounted on a stage is assumed. Each SLM area array is oriented in such a way that the columns of pixels in the projected image on the substrate  140  are parallel to the straight-line segment portions of the serpentine path, which for ease of illustration, are parallel to the x-axis of stationary coordinate system  860 . A stage positions the substrate  140  such that the centers of the projected images of the SLM area arrays are at the beginning of paths  950  through  954 . In this example, at the beginning of paths  950  through  954  none of the projected images of the SLM arrays fall on the substrate  140 . As the stage moves in the +x direction, referenced to stationary coordinate system  860 , the center of the projected images of the SLM area arrays move in the −x s  direction, referenced to substrate coordinate system  853 , and trace the first straight sections of the serpentine paths. The exposure along any path starts when the projected image of the SLM area array falls on the substrate. The exposure stops along any path after the projected image clears the edge of the substrate. After all exposures have stopped, the stage then repositions the substrate in readiness to scan in the −x direction along the second straight section of the path, which is separated from the first straight section by a distance  851  in the y direction, all referenced to stationary coordinate system  860 . If this does not cover the entire substrate, then the stage moves in the y direction, referenced to stationary coordinate system  860 , by the distance between paths  950  and  954 , and the above procedure is repeated. Clearly, the projected width of the SLM arrays must be greater than or equal to the distance  851  in order to expose the complete substrate. Note that in this example the separation between consecutive paths  950 ,  951 , . . .  954  is twice the spacing  851 ; should the separation exceed twice the spacing  851 , then a serpentine motion with more straight sections can be employed. This explanation is relevant to the multiple SLM area array lithography systems of  FIGS. 5 &amp; 6 , for which paths  950  through  954  correspond to SLM area arrays  520  through  524 . 
   Referring to  FIGS. 1 and 8 , patterns of elements in the “on” state that correspond to features printed on substrate  140  must shift across the SLM  120  in such a way that they appear stationary, on average, to the constantly moving substrate. If the stage  150  is moving at constant speed v along one of the straight-line segments of serpentine path  850  (the stage is moving in a patterning direction), then this is accomplished by shifting the SLM pattern by one row at regular time intervals, where the time interval T is given by: 
   
     
       
         
           
             
               
                 T 
                 = 
                 
                   pM 
                   v 
                 
               
             
             
               
                 ( 
                 1 
                 ) 
               
             
           
         
       
     
   
   where p is the row pitch of the elements (the Texas Instruments DMD mirrors have the same pitch for rows and columns) and M is the magnification of the projection lens system  130 . As an example, the Texas Instruments DMD is available with a mirror pitch of 13.7 microns and the minimum mirror cycle time is 102 microseconds. If the projection lens system  130  has a magnification of 2.0, then the stage speed is approximately 269 mm/s. If the dose delivered is inadequate to expose the substrate or the stage speed required is beyond the capability of the stage system, then the actual mirror cycle time used may need to be longer. However, the mirror cycle time and stage speed must always satisfy equation (1). 
     FIG. 10  illustrates the shifting of patterns on the SLM and the corresponding image on the substrate. In this example, the substrate is on a stage and moves at constant speed in the x direction during exposures. The following are shown, with reference also to  FIG. 1 : part of SLM  120 , which is an array of elements  1000  with an area of 4 rows by 6 columns; a corresponding part of substrate  140 , which is an array of pixels  1002  with an area of 4 rows by 6 columns; resultant image  1007  with projected row pitch (width of a pixel)  1008 . The resultant image shows one possible latent image on the substrate due to completion of the entire series of exposures. The edge placement and corner rounding in a latent image will be discussed in detail below. “Snapshots” of the corresponding parts of the SLM and substrate are shown at equally spaced times T 1  through T 7 , where the time interval satisfies equation (1); the parts of the SLM and substrate are indicated in the figure by M and S, respectively. The SLM array  1000 , the substrate array  1002  and the resultant image  1007  are drawn as if viewed from a position directly above them and looking down in the −z direction of stationary coordinate system  160 . For ease of illustration, in each “snapshot” the SLM and substrate arrays are shown next to each other. The projected row pitch  1008  in the resultant image  1007  is the row pitch in the SLM array  1000  times the magnification of the projection lens system  130 . However, for ease of illustration, in each “snapshot” the SLM and substrate arrays are shown having the same size and orientation. The grid shown on the arrays  1000  and  1002 , and the image  1007  is for reference only. A light square in  1000  corresponds to an SLM element in the “on” state, while a dark square corresponds to one in the “off” state. The light and dark areas in  1002  correspond to the states of the SLM elements for that “snapshot”. For example, at time T 1  the substrate is receiving light at pixels located at R 1 C 4  and R 1 C 5  from mirrors in the SLM array at positions R 4 C 4  and R 4 C 5  (where the nomenclature R 1 C 4  represents the pixel/element at row R 1  and column C 4 ). At time T 1  the bottom edge of the substrate array  1002  is aligned with substrate position coordinate  1 . At time T 2  the substrate has moved by one row and the bottom edge of the substrate is now aligned with substrate position coordinate  2 . The time elapsed between T 2  and T 1  satisfies equation (1). The particular feature pattern used as an example in  FIG. 10  is shown in its entirety at time T 4  on both the SLM and the substrate arrays. It can be seen that the edge of this feature pattern first appears at T 1 , scrolls across the SLM array  1000  between times T 2  and T 6  and has moved off the SLM array  1000  by T 7 . On the substrate array  1002 , the feature pattern does not appear to move. This can be most clearly seen at times T 3  and T 4 . However, because the substrate is moving at constant speed while the SLM is stationary, the projected pattern does in fact move on the substrate by the projected row pitch  1008  between any two consecutive snapshot times. Note that for ease of illustration the patterns shown on the substrate arrays  1002  do not show any blurring or optical interference effects. 
     FIG. 11  shows the substrate array  1002  with a line segment AB positioned in the center of column C 4 . Light intensity and resultant dose profiles will be determined on the surface of the substrate array in the position indicated by line segment AB. Note that the position of AB is such that it crosses the “trailing edge” of the exposure pattern shown in  FIG. 10 . 
   The consequences of the movement of the projected pattern across the substrate surface during exposure will now be examined.  FIG. 12  shows instantaneous light intensity distributions on substrate array  1002  from  FIG. 10 ; the distributions are along the position of line segment AB as shown in  FIG. 11 . Note that in  FIG. 12  the line segment AB is shown to extend from −2 to 1.5 on the abscissa. In  FIG. 12 , six distributions are shown at intervals of T/10 starting at T 3  and then every T/10, where T is defined in equation (1) above. The substrate is moving with constant velocity. The abscissa represents substrate displacement x s  (as shown in  FIGS. 8 and 9 ) measured in units of projected row pitch (as defined above in reference to  FIG. 10 ). The following are shown in  FIG. 12 : light intensity profiles  1200 ,  1201 ,  1202 ,  1203 ,  1204  and  1205 ; 50% light intensity marker  1209 ; 50% position marker  1210 ; and projected row pitch  1215 . The shape of light intensity profiles  1200 ,  1201 ,  1202 ,  1203 ,  1204  and  1205  is shown as being Gaussian; however, the actual shape depends on details of the optics. The instantaneous light intensity as a function of position on the substrate array  1002  at time T 3  is represented by light intensity profile  1200 . Light intensity profile  1200  is positioned such that the intersection of the 50% marker  1209  on the abscissa corresponds to the boundary between rows R 3  and R 4  on the substrate array. The region between −1 and 0 on the abscissa corresponds to R 4  on the substrate array, the region between 0 and 1 corresponds to R 3  and the region between 1 and 2 corresponds to R 2 . As the stage moves substrate array  1002  in the +x direction, the instantaneous light intensity profile advances across the substrate array in the −x s  direction. The light intensity profiles  1201 ,  1202 ,  1203 ,  1204  and  1205  are for times T 3  plus T/10, 2T/10, 3T/10, 4T/10 and 5T/10, respectively. The light intensity profile advances across the substrate in the −x s  direction by one-half of the projected row pitch during T/2. In this example, at T 3  plus T/2 the elements in SLM array  1000  switch from the pattern shown at T 3  to the pattern shown at T 4 . Looking specifically at the array elements responsible for generating the light intensity profiles: the element at C 4 R 4  switches from “on” to “off”, the elements at C 4 R 3  and C 4 R 2  remain “on” and the element at C 4 R 1  switches from “off” to “on”. The effect is to shift the light intensity profile from the position of  1205  to a new position which is one times the projected row pitch in the +x s  direction. 
     FIG. 13  follows on from  FIG. 12  showing the light intensity profiles for the next period T/2. After the elements switch at time T 3 +T/2 the light intensity profile moves from the position of  1205  (see  FIG. 12 ) to that of  1300  (see  FIG. 13 ). As the stage continues to move the substrate array  1002  in the +x direction, the instantaneous light intensity profile advances across the substrate array in the −x s  direction. The light intensity profiles  1301 ,  1302 ,  1303 ,  1304  and  1305  are for times T 3  plus 6T/10, 7T/10, 8T/10, 9T/10 and 10T/10, respectively. Light intensity profile  1305  is at time T 3 +T, which is the same as time T 4 . The light intensity profile advances across the substrate in the −x s  direction by one-half of the projected row pitch during T/2. Consequently, the position of light intensity profile  1305  at T 4  is the same as for profile  1200  at T 3 . 
     FIGS. 12 and 13  have shown how the light intensity distribution varies over the time interval between T 3  and T 4 .  FIG. 14  shows the resultant dose distribution for the same position on substrate array  1002 —along line segment AB. The light intensity distributions  1200  and  1300  in  FIGS. 12 and 13  are Gaussian with σ=0.43. One can see from  FIG. 14  that the resultant dose profile  1401  has a similar form to the original Gaussian. 
   The following are shown in  FIG. 14 : resultant dose profile  1401 , 50% resultant dose marker  1404 , 50% position marker  1405  and projected row pitch  1215 . Because the elements in SLM array  1000  in  FIG. 10  are switched when the substrate array  1002  has moved by one-half of the projected row pitch  1008 , the 50% resultant dose marker  1404  intersects the resultant dose profile  1401  at position  1405 , which is the same as position  1210  in  FIGS. 12 and 13 . This is due to the symmetrical nature of the process shown in  FIGS. 12 and 13 . Other choices of element switching time, besides Tn+T/2 (where n=1, 2, 3 . . . ), could be used (such as Tn+T/5). The shape of the resultant dose profile would be the same as  1401 , but the 50% resultant dose location on the abscissa would be shifted from 0. Clearly, modulating the switching time can be used to control the position of printed pattern edges. However, it is preferred to keep the switching time constant. The shape of the dose distribution will not usually be the same as the instantaneous light intensity profiles. This means that the dose profile for edges parallel to the direction of stage motion will differ from those that are orthogonal. Edges parallel to the direction of stage motion are not constantly moving, consequently the dose profile on the substrate for such an edge will be identical to its instantaneous light intensity profile. 
   Referring back to  FIG. 10 , the process of switching the elements in SLM array  1000  at times Tn+T/2 (where n=1, 2, 3, . . . ) is repeated until the pattern has completely scrolled off the SLM array  1000 , which in this example is at time T 7 . Since the time interval between any two consecutive “snapshot” times is equal to T from equation (1), the switching times are equal to (T 1 +T 2 )/2, (T 2 +T 3 )/2, (T 3 +T 4 )/2, (T 4 +T 5 )/2, (T 5 +T 6 )/2 and (T 6 +T 7 )/2 respectively. Because the dose is additive, the final dose profile along line segment AB will have the same shape as resultant dose profile  1401  in  FIG. 14 .  FIG. 15  shows the total dose profile  1501 . The following are shown in  FIG. 15 : total dose profile  1501 , 50% total dose marker  1504 , 50% position marker  1505 , exposed region  1506 , unexposed region  1507  and projected row pitch  1215 . It is preferred to adjust the total dose so that, after development, the edge of the printed feature is at the 50% position marker  1505 . In which case, the region with more than 50% total dose is the exposed region  1506 , while the region with less than 50% total dose is the unexposed region  1507 . Under these conditions, the final developed pattern would be similar to the resultant image  1007  in FIG.  10 —light areas correspond to exposed regions  1506  and dark areas correspond to unexposed regions  1507 . Except for some corner rounding, all the pattern edges line up with the reference grid. Slight changes in exposure dose will affect vertical and horizontal dimensions differently. This is a practical problem only if the slope of the light intensity profile at and around 50% light intensity is not steep enough (a steep slope allows sufficient line width control, assuming reasonable exposure and processing variations). 
   Consider a lithography tool as in  FIGS. 1 and 10  described above. The dose distribution along the x direction on the surface of the substrate is given by the following equation: 
                   D   ⁡     (   x   )       =     N   ⁢       ∫   0   T     ⁢         I   w     ⁡     (     x   ,   t     )       ⁢           ⁢     ⅆ   t                   (   2   )               
where N is a constant, I w (x,t) is the time dependent light intensity distribution at the surface of the substrate, and time T satisfies equation (1). If the substrate is moving at constant speed v, then the light intensity for the moving substrate, I w , is related to the light intensity for the substrate at rest, I, by:
   I   w ( x,t )= I ( x+vt,t )  (3) 
Between t=0 and t=T/2 the elements in the SLM are in one state and shift at t=T/2 by one row, i.e.;
   I ( x,t )= I   0 ( x ) 0&lt; t&lt;T/ 2   I ( x,t )= I   0 ( x−pM )  T/ 2 &lt;t&lt;T   (4) 
   Where I 0 (x) is the intensity distribution for a single SLM element with the substrate at rest, p is the row pitch of the SLM array elements and M is the projection lens system magnification. Using equations (1), (3) and (4), equation (2) can be written as: 
   
     
       
         
           
             
               
                 
                   D 
                   ⁡ 
                   
                     ( 
                     x 
                     ) 
                   
                 
                 = 
                 
                   N 
                   ⁡ 
                   
                     [ 
                     
                       
                         
                           ∫ 
                           0 
                           
                             T 
                             / 
                             2 
                           
                         
                         ⁢ 
                         
                           
                             
                               I 
                               0 
                             
                             ⁡ 
                             
                               ( 
                               
                                 x 
                                 + 
                                 vt 
                               
                               ) 
                             
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             ⅆ 
                             t 
                           
                         
                       
                       + 
                       
                         
                           ∫ 
                           
                             T 
                             / 
                             2 
                           
                           T 
                         
                         ⁢ 
                         
                           
                             
                               I 
                               0 
                             
                             ⁡ 
                             
                               ( 
                               
                                 x 
                                 + 
                                 vt 
                                 - 
                                 vT 
                               
                               ) 
                             
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             ⅆ 
                             t 
                           
                         
                       
                     
                     ] 
                   
                 
               
             
             
               
                 ( 
                 5 
                 ) 
               
             
           
         
       
     
   
   As an example, assume the distribution I 0 (x) is Gaussian in form. Then for 10 rows of elements in the “on” state the intensity distribution at the substrate would be: 
                     I   0     ⁡     (   x   )       =       1     2.51   ⁢           ⁢   σ       ⁢       ∫   0   10     ⁢       ⅇ         -       (     x   -   y     )     2       /   2     ⁢           ⁢     σ   2         ⁢           ⁢     ⅆ   y                   (   6   )               
where σ 2  is the variance.
 
   Equations (5) and (6) are examples of the form of equations used to calculate the dose distributions and intensity distributions shown in the Figures. 
   In order to make fine adjustments to the location of feature edges, a “gray level” technique can be used. When such a technique is implemented on apparatus such as that shown in  FIGS. 1 through 6 , it is required that the image of an individual element of the SLM produced by the projection lens system must be “blurred” i.e. the element is not clearly resolved. This “blurring” can be accomplished in various ways including defocusing, using a microlens array or a diffuser or, more commonly, by adjusting the numerical aperture of one of the lenses in the projection lens system to decrease the resolution to the desired value. The preferred method is defocusing. The technique can be understood by referring to  FIG. 16 . 
     FIGS. 16 through 19  illustrate examples of “gray level” edge shifting on a pattern edge that is orthogonal to the direction of substrate motion during exposure; in these examples the substrate is assumed to be moving in the same direction at constant speed during exposure.  FIGS. 16 through 19  are very similar to  FIG. 10 . The significant difference is the displacement of the “trailing edge” of the resultant image by a fraction of a pixel; for example, examination of the “trailing edge” of the resultant image  1600  in  FIG. 16  shows a displacement  1601  which is 0.5 times the row pitch  1008 . Note that for ease of illustration the patterns shown on the substrate arrays  1002  do not show any blurring or optical interference effects. 
   In  FIG. 16  the sequence of patterns on SLM arrays  1000  are identical to the patterns shown in  FIG. 10  at times T 1 , T 2 , T 3 , T 4  and T 6 . However, at time T 5  the elements in SLM array  1000  at locations R 3 C 2 , R 3 C 3 , R 3 C 4  and R 3 C 5  are in the “on” state in  FIG. 16  and in the “off” state in  FIG. 10 . Also, at time T 7  elements in SLM array  1000  at locations R 1 C 2 , R 1 C 3 , R 1 C 4  and R 1 C 5  are in the “on” state in  FIG. 16  and in the “off” state in  FIG. 10 . With reference to substrate section  1002  in  FIG. 16 , pixels R 4 C 2 , R 4 C 3 , R 4 C 4  and R 4 C 5  are exposed at times T 5  and T 7 , but not at times T 1 , T 2 , T 3 , T 4  or T 6 . All other rows of the pattern are exposed for four time periods—for example, pixels R 1 C 4  and R 1 C 5  were exposed at times T 1 , T 2 , T 3  and T 4 , while pixels R 2 C 2 , R 2 C 3 , R 2 C 4  and R 2 C 5  were exposed at times T 2 , T 3 , T 4  and T 5 . The effect of the two time period only exposure in row R 4  is to produce an edge displacement  1601  of roughly 0.5 times the width of the projected row pitch  1008 , as can be seen in the resultant image  1600 . 
   The sequence of exposures in  FIG. 17  produces an edge displacement  1701  of roughly 0.5 times the width of the projected row pitch  1008 , as can be seen in the resultant image  1700 . This resultant image  1700  is identical to the resultant image  1600  in  FIG. 16 ; however, the two resultant images are produced with different sets of exposure patterns. The exposure patterns in the 2 figures are different at times T 4 , T 5 , T 6  and T 7 . These 2 examples are certainly not exhaustive. One can easily imagine other sequences of exposure patterns that give the same resultant image. 
     FIG. 18  illustrates a further example of “gray level” edge shifting, in this example the trailing edge displacement  1801  is 0.25 times the row pitch  1008 . The sequence of patterns on SLM array  1000  shown in  FIGS. 10 and 18  at times T 1 , T 2 , T 3 , T 4 , T 6  and T 7  are identical. However, at time T 5  elements in SLM array  1000  at locations R 3 C 2 , R 3 C 3 , R 3 C 4  and R 3 C 5  are in the “on” state in  FIG. 18  and in the “off” state in  FIG. 10 . With reference to substrate section  1002  in  FIG. 18 , pixels R 4 C 2 , R 4 C 3 , R 4 C 4  and R 4 C 5  are exposed at time T 5 , but not at times T 1 , T 2 , T 3 , T 4 , T 6  or T 7 . All other rows of the pattern are exposed for four time periods. The effect of the one time period exposure in row R 4  at time T 5  is to produce an edge displacement  1801  of roughly 0.25 times the width of the projected row pitch  1008 , as can be seen in resultant image  1800 . 
     FIG. 19  illustrates a further example of “gray level” edge shifting, in this example the trailing edge displacement  1901  is 0.75 times the row pitch  1008 . The sequence of patterns on SLM array  1000  shown in  FIGS. 19 and 10  at times T 1 , T 2 , T 3  and T 4  are identical. However, at time T 5  elements in SLM array  1000  at locations R 3 C 2 , R 3 C 3 , R 3 C 4  and R 3 C 5  are in the “on” state in  FIG. 19  and in the “off” state in  FIG. 10 . At time T 6  elements in SLM array  1000  at locations R 2 C 2 , R 2 C 3 , R 2 C 4  and R 2 C 5  are in the “on” state in  FIG. 19  and are in the “off” state in  FIG. 10 . Also, at time T 7  elements in SLM array  1000  at locations R 1 C 2 , R 1 C 3 , R 1 C 4  and R 1 C 5 , are in the “on” state in  FIG. 19  and are in the “off” state in  FIG. 10 . With reference to substrate section  1002  in  FIG. 19 , pixels R 4 C 2 , R 4 C 3 , R 4 C 4  and R 4 C 5  are exposed at times T 5 , T 6  and T 7 , but not at T 1 , T 2 , T 3  or T 4 . All other rows of the pattern are exposed for four time periods. The effect of the three time period exposure in row R 4  at times T 5 , T 6  and T 7  is to produce an edge displacement  1901  of roughly 0.75 times the width of the projected row pitch  1008 , as can be seen in resultant image  1900 . 
     FIG. 20  illustrates an example of “gray level” edge shifting on a pattern edge that is parallel to the direction of substrate motion during exposure; in this example the substrate is assumed to be moving in the same direction at constant speed during exposure.  FIG. 20  is very similar to  FIG. 10 . The significant difference is the displacement of an edge of the resultant image by a fraction of a pixel; for example, examination of the edge of the resultant image  2000  in  FIG. 20  shows a displacement  2001  which is 0.25 times the column pitch  2003 . 
   In  FIG. 20  the sequence of patterns on SLM array  1000  shown in  FIGS. 20 and 10  at times T 1 , T 2 , T 4 , T 5 , T 6  and T 7  are identical. However, at time T 3  elements in SLM array  1000  at locations R 2 C 6 , R 3 C 6  and R 4 C 6  are in the “on” state in  FIG. 20  and in the “off” state in  FIG. 10 . With reference to substrate section  1002  in  FIG. 20 , pixels R 1 C 6 , R 2 C 6  and R 3 C 6  are exposed at time T 3  but not at times T 1 , T 2 , T 4 , T 5 , T 6  or T 7 . All other pixels on the substrate array  1002  are exposed for four time periods. The effect of the one time period exposure in column C 6  at time T 3  is to produce an edge displacement  2001  of roughly 0.25 times the width of the projected column pitch  2003 , as can be seen in resultant image  2000 . 
   Further to edge displacements, using one or more pixel exposures near a corner will affect the degree of corner rounding. For example, with reference to resultant image  1007  in  FIG. 10 , exposures at R 1 C 1  or at both R 1 C 2  and R 2 C 1  will change the corner rounding at location R 2 C 2 . 
   The edge displacements shown in the resultant images of  FIGS. 16 through 20  are only approximate; the actual displacements will depend on the detailed shape of the instantaneous light intensity distribution at the edges of the exposure patterns. A more accurate determination can be made by using a slightly modified form of equation (5) for the dose distribution, including the light intensity distribution appropriate to the mirror section states for each half of the 7 time periods. This modified form of equation (5) was used to calculate resultant dose distributions along the position of line segment AB on substrate array  1002  (see  FIG. 11 ) for the exposure pattern examples given in  FIGS. 10 ,  16 ,  17 ,  18  and  19 . In these calculations it is assumed that the instantaneous light intensity distribution shape is Gaussian with a σ value of 0.43. These resultant dose distributions are shown in  FIG. 21 . 
   In  FIG. 21  resultant dose profiles  2101 ,  2102 ,  2103  and  2104  correspond to  FIGS. 10 ,  16 ,  18  and  19 , respectively; resultant dose profile  2102  also corresponds to  FIG. 17 . 50% position markers  2105 ,  2106 ,  2107 ,  2108  are for dose profiles  2101 ,  2102 ,  2103  and  2104 , respectively. With reference also to the resultant images in  FIGS. 10 ,  16 ,  17 ,  18  and  19 , the regions between −1 and 0 and 0 and 1 on the abscissa in  FIG. 21  correspond to R 4  and R 3 , respectively, in the resultant images. 50% position marker  2105  of resultant dose profile  2101  was calculated for the example given in  FIG. 10  and intersects the abscissa at 0. This result is consistent with the resultant image  1007  shown in  FIG. 10 . 50% position marker  2106  of resultant dose profile  2102  was calculated for the example given in  FIG. 16  and intersects the abscissa at −0.5. This result is in agreement with the value of edge displacement  1601 . 50% position marker  2107  of resultant dose profile  2103  was calculated for the example given in  FIG. 18  and intersects the abscissa at −0.20. This result is slightly different from the edge displacement  1801  value of 0.25. 50% position marker  2108  of resultant dose profile  2104  was calculated for the example given in  FIG. 19  and intersects the abscissa at −0.80. This result is slightly different from the edge displacement  1901  value of 0.75. 
   It should be noted that the examples given above are simplistic and ignore interference effects from adjacent elements of the SLM, the rigorously correct shape of the light intensity distribution, and the finite contrast of the photosensitive substrate. In general, the correct dose for a particular edge displacement will need to be determined experimentally. However, once the relationship between dose and edge displacement is determined, the technique can be used to compensate for misalignment and distortion of the substrate, distortion and aberrations in the projection lens system, and non-uniform illumination. This technique could be used to relax the specification of the optics, thus reducing the cost of the optics. 
   The preferred SLM device is the two-state DMD from Texas Instruments which has a rectangular array of mirrors—1024 mirrors wide by 768 mirrors deep. The scan direction during exposure of the substrate is preferably orthogonal to the 1024 width in order to minimize the number of times the stage must reverse direction along its serpentine path (see  FIG. 8 ). Since the array is 768 rows deep, the exposure patterns will scroll across the array in 768 discrete steps and there will be 768 opportunities to adjust edge locations using the “gray level” technique outlined above. This allows for an edge placement resolution of 1/768 th  the size of the projected row pitch of the DMD in the resultant image. In practice, one rarely needs more than 1/32 nd . Consequently, 32 equally spaced edge positions can be chosen and the extra resolution can be used to compensate for non-uniform illumination of the substrate. 
   The minimum feature size that can be printed on the substrate depends on the characteristics of the light intensity profile. This will be explained with reference to  FIGS. 22 through 27 . 
     FIG. 22  illustrates another example of the shifting of patterns on the SLM and the corresponding image on the substrate. As in previous examples, the substrate is on a stage and moves at constant speed in the x direction during exposures. The following are shown, with reference also to  FIG. 1 : part of SLM  120 , which is an array of elements  2200  with an area of 5 rows by 6 columns; a corresponding part of substrate  140 , which is an array of pixels  2202  with an area of 5 rows by 6 columns; resultant image  2207  with projected row pitch (width of a pixel)  1008 . “Snapshots” of the corresponding parts of the SLM and substrate are shown at equally spaced times T 1  through T 8 , where the time interval satisfies equation (1). This figure is similar to  FIG. 10 . 
     FIG. 23  shows the substrate array  2202  with line segments CD, EF, GH and IJ positioned in the center of columns C 2 , C 3 , C 4  and C 5 . Light intensity and resultant dose profiles will be determined on the surface of the substrate array in the positions indicated by the line segments. Note that the positions of the line segments are such that they cross both the “trailing edge” and “leading edge” of the exposure pattern shown in  FIG. 22 . 
     FIG. 24  shows resultant dose distributions for the exposed substrate  2202 , as detailed in  FIG. 22 . A Gaussian shape with a σ value of 0.43 is assumed for the instantaneous light intensity distributions used to derive the resultant dose distributions. The following are shown in  FIG. 24 : resultant dose profiles  2400 ,  2401 ,  2402  and  2403  along line segments CD, EF, GH and IJ, respectively; 50% position markers  2405 ,  2406  and  2407  corresponding to dose profiles  2401 ,  2402  and  2403 , respectively; 50% position markers  2404  and  2408 , both corresponding to dose profile  2400 ; and projected row pitch  1215 . Note that the line segment CD is shown to extend from −2 to 6 on the abscissa; line segments EF, GH and IJ extend over the same values on the abscissa, but are not shown so as to avoid cluttering the figure. The regions between −1 and 0, 0 and 1, 1 and 2, 2 and 3, and 3 and 4 on the abscissa in  FIG. 24  correspond to R 5 , R 4 , R 3 , R 2 , and R 1 , respectively, on the resultant image  2207  in  FIG. 22 . If the total dose is adjusted such that the edge of printed features is at the 50% position markers, which is preferred, than the final developed pattern would be similar to the resultant image  2207  in  FIG. 22 . It should be noted that resultant dose profile  2400  in  FIG. 24  never rises higher than about 70% of dose profiles  2402  and  2403 , and that the distance between the 50% position markers  2404  and  2408  is slightly less than the projected row pitch  1008 . Clearly, under these conditions the minimum feature size is roughly the same as the projected row pitch  1008 . The “gray level” technique described earlier can be used to adjust the width of such a feature—for example, decreasing the total dose for pixel R 4 C 2  in substrate array  2202  of  FIG. 22  will reduce the height of resultant dose profile  2400 , which decreases the size of the printed feature. However, the feature dimension changes rapidly with small changes in dose near the top of dose profile  2400 . Furthermore, there is always some noise and uncertainty in the total dose which places a practical limit on this approach. 
     FIG. 25  illustrates a further example of the shifting of patterns on the SLM and the corresponding image on the substrate. As in previous examples, the substrate is on a stage and moves at constant speed in the x direction during exposures. In  FIG. 25  examples of “gray level” edge shifting on various sizes of feature are shown, where the shifted edges are orthogonal to the direction of substrate motion during exposure. The following are shown, with reference also to  FIG. 1 : part of SLM  120 , which is an array of elements  1000  with an area of 4 rows by 6 columns; a corresponding part of substrate  140 , which is an array of pixels  1002  with an area of 4 rows by 6 columns; resultant image  2507  with projected row pitch (width of a pixel)  1008 . “Snapshots” of the corresponding parts of the SLM and substrate are shown at equally spaced times T 1  through T 7 , where the time interval satisfies equation (1). This figure is similar to  FIG. 10 . 
     FIG. 26  shows the substrate array  1002  with line segments KL, MN, OP, QR and ST positioned in the center of columns C 2 , C 3 , C 4 , C 5  and C 6 . Light intensity and resultant dose profiles will be determined on the surface of the substrate array in the positions indicated by the line segments. Note that the positions of the line segments are such that they cross the “trailing edge” and “leading edge” of the exposure pattern shown in  FIG. 25 . 
     FIG. 27  shows resultant dose distributions for the exposed substrate  1002 , as detailed in  FIG. 25 . A Gaussian shape with a σ value of 0.43 is assumed for the instantaneous light intensity distributions used to derive the resultant dose distributions. The following are shown in  FIG. 27 : resultant dose profiles  2700 ,  2701 ,  2702 ,  2703  and  2704  along line segments KL, MN, OP, QR and ST, respectively; 50% position markers  2710  and  2716  both corresponding to dose profile  2704 ; 50% position markers  2710  and  2713  both corresponding to dose profile  2703 ; 50% position markers  2711  and  2714  both corresponding to dose profile  2702 ; 50% position markers  2712  and  2715  both corresponding to dose profile  2701 ; and projected row pitch  1215 . Note that the line segment KL is shown to extend from −2 to 5 on the abscissa; line segments MN, OP, QR and ST extend over the same values on the abscissa, but are not shown so as to avoid cluttering the figure. The regions between −1 and 0, 0 and 1, 1 and 2, and 2 and 3 on the abscissa in  FIG. 27  correspond to R 4 , R 3 , R 2 , and R 1 , respectively, on the resultant image  2507  in  FIG. 25 . If the total dose is adjusted such that the edge of printed features is at the 50% position markers, which is preferred, than the final developed pattern would be similar to the resultant image  2507  in  FIG. 25 . It should be noted that resultant dose profile  2700  in  FIG. 27  never rises higher than about 45% of dose profile  2704 , and therefore does not print. Resultant dose profile  2700  is due to exposure by alternating single adjacent pixels, as can be seen by investigating column C 2  of substrate section  1002  at times T 2 , T 3 , T 4  and T 5  in  FIG. 25 . This is in contrast to the example of  FIG. 22  where a single pixel exposure created a dose profile that did print. With reference to  FIGS. 25 and 27 , the features printed in columns C 3 , C 4  and C 5  are all roughly 1.5 times the projected row pitch  1008  in width as can be seen, for example, by examining the distance between 50% position markers  2711  and  2714  of resultant dose profile  2702 . It appears that when a feature is at some arbitrary location with respect to the projected SLM element grid then the minimum (practical) feature size is roughly 1.5 times the projected row pitch; this is in contrast to the minimum feature size of roughly 1.0 times the projected row pitch seen for features located on the projected SLM element grid—see  FIGS. 22 and 24 . 
   With reference to  FIG. 28 , a block diagram for an optical lithography system of the invention is shown. Design data, which resides on the design data storage device  2804 , describes what the system should print and is input to the data preparation computer  2805  for translating into a form suitable for the decompression electronics  2807 . The data preparation computer  2805  can also modify the data to compensate for previously measured substrate distortion. Substrate alignment system  2803  can be used to measure the substrate distortion. The design data is typically in a CAD (Computer Aided Design) format or a mask standard format such as GDSII. The design data storage device may be one or more tapes or disk drives. The data preparation computer can be any general-purpose computer such as an IBM PC. After computation by the data preparation computer, the data is stored on one or more fast disk drives  2806 . The preferred form of this data can be understood by reference to the resultant image in  FIG. 19 . The entire area of the substrate  140  is divided into small squares with a pitch equal to the magnified pitch of the SLM array  120 , substrate array  1002  provides a small-scale example. Each pixel in the array covering the substrate is assigned a dose value that is based on the feature pattern and a look-up table value. The look-up table values are determined experimentally and take into account the distortions and aberrations of projection lens system  130  and the illumination non-uniformity from the illumination source  110 . As an example, dose values are derived based on the feature pattern of resultant image  1900 , assuming 32 gray levels, where  31  corresponds to 100% exposure.
     The following pixels will have a dose value of 31:   R 1 C 4 , R 1 C 5 , R 2 C 2 , R 2 C 3 , R 2 C 4 , R 2 C 5 , R 3 C 2 , R 3 C 3 , R 3 C 4 , R 3 C 5     The following will have a dose value of 0:   R 1 C 1 , R 1 C 2 , R 1 C 3 , R 1 C 6 , R 2 C 1 , R 2 C 6 , R 3 C 1 , R 3 C 6 , R 4 C 1 , R 4 C 6     The following pixels will have a dose value intermediate between 0 and 31, based on the intended edge location  1901 :   R 4 C 2 , R 4 C 3 , R 4 C 4 , R 4 C 5 
 
For convenience, we will assign a value of 24 to the above. Next, a look-up table is used to modify the dose values to account for distortions, aberrations and illumination nonuniformity of the system. Since the preferred SLM, the Texas Instruments DMD device, can switch mirror states every 102 microseconds and has 1024 rows and 768 columns, this means that the fast disk drives  2806  need to deliver 1 row of 1024 pixels every 102 microseconds. With 32 gray levels this is a data rate of roughly 6.3 megabytes/second. This data rate is easily within present day capabilities of disk drive arrays.
   

   Again referring to  FIG. 28 , alignment of the substrate  140  to the stage  150  and projection lens system  130  is determined by reflecting substrate alignment system light  2892  off features on the substrate  140  into substrate alignment system  2803 . The substrate alignment system is preferably a “machine vision” system that compares arbitrary features on the substrate to previously stored images or idealized images, such as a cross or a circle, in order to find a match. The substrate alignment system light could come from illumination source  110  by way of SLM  120  and projection lens system  130 , or from an external source. After reflecting off features on the substrate the light could travel directly to the substrate alignment system, as shown, or could first travel through the projection lens system (“through the lens” alignment). The light reflected off features on the substrate could also travel through the projection lens system, reflect off the SLM and then pass into the substrate alignment system. Stage metrology system  2802  receives stage position information from stage position optical sensor  2891 , which can be based on laser interferometers or linear scales, and sends information to control computer  2801 . In turn, the control computer sends signals to the stage x, y motors which then servo to the correct location. If edge blurring is accomplished by defocusing, which is the preferred technique, then the control computer commands the stage to servo in z until a suitable gap value is achieved. The gap value is measured by the substrate height detector  450  by way of substrate height detection medium  490 , which is preferably air. Other types of detection techniques, such as optical or capacitance, would also work. The gap value (defocus) is chosen to produce the desired amount of feature edge blurring in the image projected onto the substrate. Constant servoing to maintain this gap value is needed to compensate for local substrate height variations. Rather than move the stage in the z-direction, it would also be acceptable to move the projection lens system  130  or SLM  120  in the z-direction instead. Next, the control computer commands the fast disk drives  2806  to send the first row of data to the decompression electronics  2807 , which loads the first frame of mirror state data to the SLM memory  2808 . 
   To understand the function of the decompression electronics  2807  it is necessary to first understand the requirements of the SLM  120 . All of the mirrors in the SLM switch states at the same time. The states of all mirrors are individually determined by values stored in the SLM memory  2808 . Therefore, the requirement for the decompression electronics is that it must load the entire SLM memory with new mirror-state values every mirror clock cycle. For the Texas Instruments DMD device, this is every 102 microseconds. The decompression electronics must translate the dose values for each image pixel into a sequence of mirror states that shift with the moving substrate. A simplified example based on  FIG. 19  can illustrate how this could be accomplished. For any pixel in the resultant image  1900 , 5 dose levels are possible due to the four mirror clock cycles used to shift each row across the mirror section  1000 . For example, pixel R 4 C 2  in substrate section  1002  can be exposed at time T 4 , T 5 , T 6  and T 7 , as can be seen by inspecting  FIG. 19 . The actual exposure was only at times T 5 , T 6  and T 7  for this pixel. Any of the five possible exposure sequences can be represented by a string of 0&#39;s and 1&#39;s that correspond to the mirror state at the 4 exposure times. For example, for R 4 C 2  the string would be 0111. A suitable set of 5 exposure sequences would be:
     0000 0001 0011 0111 1111
 
There are other possible sequences that give the same dose, such as 1000 rather than 0001. This degree of freedom can be used to compensate for illumination non-uniformity from illumination source  110 . The dose levels that correspond to the exposure sequences are defined to be 0, 1, 2, 3, and 4. Prior to the mirrors switching at time (T 4 +T 3 )/2, dose levels for all of the pixels in row  4  (R 4 ) of SLM array  1000  are sent from the fast disk drives  2806  to the decompression electronics  2807 . The sequences that correspond to each possible dose level are stored in a look-up table in the decompression electronics. Again using pixel R 4 C 2  as an example, its dose level would be 3 which corresponds to the sequence 0111. Starting in the state shown at T 3 , the SLM memory  2808  would have a 0 state loaded for the mirror in the fourth row and second column, R 4 C 2 , of the substrate array  1000 . After the mirrors switch to the state shown at T 4 , the decompression electronics loads the SLM memory with the second digit in the exposure sequence (1) in the third row and second column, R 3 C 2 , of the substrate array  1000 . The mirrors switch state at (T 5 +T 4 )/2. After the mirrors switch to the state shown at T 5 , the decompression electronics loads the SLM memory with the third digit in the exposure sequence (1) in the second row and second column, R 2 C 2 , of the substrate array  1000 . The mirrors switch state at (T 6 +T 5 )/2. After the mirrors switch to the state shown at T 6 , the decompression electronics loads the SLM memory with the fourth digit in the exposure sequence (1) in the first row and second column, R 1 C 2 , of the substrate array  1000 . The mirrors switch state at (T 7 +T 6 )/2. The principal of operation is the same for the much larger Texas Instruments DMD array. The decompression electronics must contain a memory large enough to hold a dose level code for each of the mirrors in the SLM and a look-up table. The decompression electronics also contains logic components to handle the bookkeeping. Because all of the mirror values need to be determined and loaded into the SLM memory during the 102 microseconds mirror clock cycle, many mirror values need to be computed in parallel. For example, if it takes 100 nanoseconds to calculate the next state for a single mirror, then the computations for roughly 800 mirrors must clearly be done in parallel.
   

   The control computer  2801  commands the stage  150  to move to the start location and accelerate to the correct constant speed. Control computer  2801  also commands illumination source  110  to emit light of the correct intensity to match the requirements of photosensitive substrate  140 . This is usually done with a variable optical attenuator. Data from the stage metrology system  2802  tells the control computer when the substrate is in the correct position to begin exposure. Again referring to  FIG. 19 , at time T 1  minus T/2, where T satisfies equation (1), the bottom of substrate array  1002  would be at substrate position ½. At this time, the control computer commands the spatial light modulator to switch all of the mirrors to the states corresponding to the new values stored in the SLM memory  2808 . At the same time the control computer  2801  commands the fast disk drives  2806  to send the next row of data to the decompression electronics  2807 , which loads the second frame of mirror state data into the SLM memory. This process is repeated until the edge of the substrate is reached, at which time the control computer commands the stage to execute a turn-around; the system is then ready to start exposing the next segment of the serpentine path, as shown in  FIG. 8 . This is repeated until the entire patterned area of the substrate has been exposed. 
   The method of operation discussed above with reference to  FIG. 28  can readily be extended to operate an optical lithography system of the invention comprising multiple SLM area arrays. 
   Some embodiments of the optical lithography tool have an SLM with multiple area arrays which are arranged in multiple rows, where all of the following apply: (1) the rows of area arrays are perpendicular to the direction of movement of the projected image of the SLM arrays on the substrate; (2) the area arrays are individually aligned so that the rows of elements in the arrays are also perpendicular to the direction of movement of the projected image of the SLM arrays on the substrate; and (3) the positions of the area arrays are staggered from one row to the next. An example of such an arrangement is shown in  FIG. 29 . In  FIG. 29  the area arrays  2910  are arranged in three rows, where the rows are perpendicular to the direction of movement  2950  of the projected image of the SLM arrays on the substrate (direction  2950  is also the direction in which pattern data is scrolled across the elements of the area arrays). The arrangement of the SLM area arrays shown in  FIG. 29  allows a substrate to be exposed without having to follow a serpentine path as shown in  FIG. 9  (the path in  FIG. 9  is suitable for a single row of SLM area arrays in which there will be gaps between arrays). The staggered arrangement allows the gaps between the arrays in one row to be covered by arrays in other rows. The example shown in  FIG. 29  shows coverage without gaps where there is no overlap of coverage in the 3 rows; however, some embodiments may have overlap in coverage. Furthermore the arrangement of SLM area arrays within a roughly circular area (indicated by circle  2960  in  FIG. 29 ) makes efficient use of the imaging optics, which will typically consist of circular components. For example an image of the seven SLM arrays in  FIG. 29  can all be simultaneously projected onto a substrate by a projection lens system comprising a single set of circular lenses. 
     FIG. 30  shows the optical lithography tool of  FIG. 4  with the addition of a mirror  485 , a light switching mechanism  121  and a second SLM beam dump  481 . In this example the light switching mechanism  121  is a second SLM. A light path from a light source (comprising components  410  through  417 ) to a substrate  140 , via a SLM  120  is indicated by light rays  170 . The light switching mechanism  121  is positioned in serial with the SLM  120  on the light path. In this example, a mirror  485  has also been inserted on the light path to accommodate the SLM  121  in the position shown. Clearly, many other optical configurations are possible that will accommodate the light switching mechanism on the light path between the light source and the SLM  120 . The SLM  121  is a mirror array with mirrors that have two states—an “on” state in which the light is reflected toward the SLM  120 , and an “off” state in which the mirror reflects light toward second SLM beam dump  481 . In this example all of the mirrors are switched as one. A discussion of most of the components of the tool in  FIG. 30  can be found in the text relating to  FIG. 4 . Further explanation of the operation of the tool is given with reference to  FIG. 31 . 
   In  FIG. 31 , the timing of the switching of the SLMs  120  and  121  is shown by waveforms  3120  and  3121 , respectively. When SLM  120  is in the “on” state, all of the elements of the SLM may be individually “on” or “off”, in other words an exposure pattern may be loaded on the SLM. When the SLM  120  is in the “off” state, all of the elements of the SLM are “off”. The same is true for SLM  121 , except all of the elements of the SLM are “on” when SLM  121  is in the “on” state. SLMs  120  and  121  have the same time interval T between switching, in other words the same switching frequency; however, they are shifted out of phase by a time shift of T(1−1/n). All elements of both SLMs are switched “off” every other time interval. Only when both SLMs are in the “on” state can light reach the substrate, which is for a time span T/n every other time interval. During this time span the projected image must move across the surface of the substrate a distance of one projected mirror pitch pM (which is the same as one pixel&#39;s length on the substrate surface). This results in a stage speed v, given by the equation:
 
 v=npM/T   (7)
 
where n is a constant. The time between exposures of the substrate is 2T, during which time the pattern on the SLM  120  will have shifted by 2n rows. In principle n can have any value greater than 1; however, practical choices for n will typically be integers greater than one and less than 10.
 
     FIG. 32  illustrates the shifting of patterns on the SLM and the corresponding image on the substrate. In this example, the substrate is on a stage and moves at constant speed in the x direction during exposures. The following are shown, with reference also to  FIG. 30 : part of SLM  120 , which is an array of elements  3200  with an area of 12 rows by 6 columns; a corresponding part of substrate  140 , which is an array of pixels  3202  with an area of 4 rows by 6 columns; resultant image  3207  with projected row pitch (width of a pixel)  1008 . The resultant image shows one possible latent image on the substrate due to completion of the entire series of exposures. “Snapshots” of the corresponding parts of the SLM and substrate are shown at equally spaced times T 1  through T 7 , where the time interval is T (the times T 1  through T 5  are also labeled in the timing diagram,  FIG. 31 , for reference). The parts of the SLM and substrate are indicated in  FIG. 32  by M and S, respectively. The SLM array  3200 , the substrate array  3202  and the resultant image  3207  are drawn as if viewed from a position directly above them and looking down in the −z direction of stationary coordinate system  160 . For ease of illustration, in each “snapshot” the SLM and substrate arrays are shown next to each other. The projected row pitch  1008  in the resultant image is the row pitch in the SLM array  3200  times the magnification of the projection lens system  430 . However, for ease of illustration, in each “snapshot” the SLM and substrate arrays are shown having the same size and orientation. The grid shown on the arrays  3200  and  3202 , and the image  3207  is for reference only. A light square in  3200  corresponds to an SLM element in the “on” state, while a dark square corresponds to one in the “off” state. The light and dark areas in  3202  correspond to the states of the SLM elements for that “snapshot”. The example shown in  FIG. 32  is for n=2. An exposure is made every 2 T and the pattern on the SLM array is seen to have moved by 4 rows during this time period. The resultant image is the same as seen in  FIG. 10 , even though the substrate in  FIG. 32  was moving twice as fast during exposure. 
   The approach described above with reference to  FIGS. 30 through 32  is an example of how to increase the throughput of substrates, without having to reduce the switching time of the SLM. This is important when the minimum switching time for the SLM is already being used, since the throughput of substrates can still be increased. The cost of this increase in throughput is a more complex lithography tool, including a light switching mechanism and an SLM with a larger number of rows (to accommodate the movement of 2n rows between exposures). 
   Clearly, the tool of  FIG. 30  can be used to implement gray level techniques, as described previously. The tool of  FIG. 30  can be modified and operated in many ways, as described earlier for the tools of  FIGS. 1 through 6 . For example a variety of image movement mechanisms, as shown in  FIGS. 2 and 3 , can be integrated into the tool of  FIG. 30 . 
   The light switching mechanism  121  in  FIG. 30  can clearly be effective in different positions, both in front of and beyond the SLM  120  on the light path, providing appropriate optical adjustments are made. The light switching mechanism may be integrated into the light source, and may even be an intrinsic property of the light source (for example a pulsed laser). The light switching mechanism can be a SLM, a shutter, a rotating mirror, or any other optical component capable of controlling the passage of light along the light path. Those skilled in the art will be aware of the many ways in which these light switching mechanisms can be incorporated and used in the many embodiments of the optical lithography tool of the invention. For example, the addition of some lenses between the SLM  121  and SLM  120  shown in  FIG. 30  would allow an image of the pixels of SLM  121  to be focused, in one-to-one correspondence, onto the pixels of SLM  120 —this would allow the SLM  121  to be used to control the passage of light independently for different blocks of array elements or even to control the passage of light on an individual element basis. 
   Now to consider the case in which the light switching mechanism can be switched faster than the SLM  120 . In  FIG. 33 , the timing of the switching of the SLM  120  and the light switching mechanism is shown by waveforms  3320  and  3321 , respectively. When SLM  120  is in the “on” state, all of the elements of the SLM may be individually “on” or “off”, in other words an exposure pattern may be loaded on the SLM. When the SLM  120  is in the “off” state, all of the elements of the SLM are “off”. The pattern on the SLM can be switched every time interval T The light switching mechanism is configured as a simple two state “on”/“off” switch. Only when both the SLM and the light switching mechanism are in the “on” state can light reach the substrate. The light switching mechanism provides the limiting time span T/n during which light can reach the substrate. During this time span the projected image must move across the surface of the substrate a distance of one projected mirror pitch pM (which is the same as one pixel&#39;s length on the substrate surface). This results in a stage speed v, given by equation (7). The time between exposures of the substrate is T, during which time the pattern on the SLM  120  will have shifted by n rows. In principle n can have any value greater than 1; however, practical choices for n will typically be integers greater than one and less than 20, in which case the time span will be a submultiple of said switching time interval. 
     FIG. 34  shows an optical lithography tool with optics configured to allow the projected images from two SLM area arrays,  3420  and  3421 , to overlap on the surface of a substrate. If desired, the overlapping images may be brought into register—superimposed exactly, pixel for pixel. A light source  110  and prisms  3410  through  3413  provide illumination to two SLM area arrays  3420  and  3421 . The light reflected from the SLM area arrays is combined by prisms  3410  through  3413  and then projected by imaging optics  3430  onto the photosensitive surface of a substrate  140 . The substrate  140  is carried by a stage  150  which moves the substrate in the x-y plane of coordinate axes  160 . The optical configuration of  FIG. 34  may be modified to include more SLM area arrays. An example of an optical configuration allowing for the projected images of three SLM area arrays to overlap on the surface of a substrate is shown in U.S. Pat. No. 6,582,080 to Gibbon et al., incorporated by reference herein. Those skilled in the art will appreciate that the tool shown in  FIG. 34  may be modified along the lines of the apparatus shown in  FIGS. 1 through 6 , thus providing many further embodiments of the invention. The apparatus of  FIG. 34  can be operated in a similar manner to that of  FIG. 30 . Further explanation of the operation of the tool is given with reference to  FIG. 35 . 
   In  FIG. 35 , the timing of the switching of the area arrays  3420  and  3421  is shown by waveforms  3520  and  3521 , respectively. When array  3420  is in the “on” state, all of the elements of the array may be individually “on” or “off”, in other words an exposure pattern may be loaded on the array. When the array  3420  is in the “off” state, all of the elements of the array are “off”. The same is true for array  3421 . Arrays  3420  and  3421  have the same time interval T between switching, in other words the same switching frequency; however, they are shifted out of phase by a time shift of T(1−1/n). All elements of both arrays are switched “off” every other time interval. Both area arrays are in the “on” state and a double dose of light reaches the substrate for a time span T/n every other time interval. During this time span the projected image must move across the surface of the substrate a distance of one projected mirror pitch pM (which is the same as one pixel&#39;s length on the substrate surface). This results in a stage speed v, given by equation (7). The time between double dose exposures of the substrate is 2T, during which time the pattern on the SLM  120  will have shifted by 2n rows. Adjustment of the dose and development conditions of the photosensitive surface of the substrate are made to ensure that only pixels which have received sufficient double dose exposures will form the developed pattern. 
   Clearly, the tool of  FIG. 34  can be used to implement gray level techniques, as described previously. The tool of  FIG. 34  can be modified and operated in many ways, as described earlier for the tools of  FIGS. 1 through 6  and  30 . For example a variety of image movement mechanisms, as shown in  FIGS. 2 and 3 , can be integrated into the tool of  FIG. 34 . 
   An alternative mode of operation for the optical lithography tool of  FIG. 34  is to have the area arrays  3420  and  3421  operating in phase. In this case the speed of the substrate will be limited by equation (1). This mode of operation may be useful when a single area array is unable to deliver a large enough dose per unit time. 
   Referring to the description of  FIGS. 8 and 9 , an SLM area array is oriented in such a way that the columns of pixels in the projected image on the substrate are parallel to the direction of movement of the image itself. This results in blurring of the edges of the pixels which are orthogonal to the direction of movement; however, the edges parallel to the direction of movement are not blurred by the movement. In order to implement gray level techniques, the edges parallel to the direction of movement must also be blurred. Blurring of the parallel edges can be achieved in many ways as described earlier, all of which involve projecting a blurred image of the SLM onto the substrate surface. There is an alternative approach to achieving blurred edges which can be used with all of the embodiments of the optical lithography tool disclosed above—the SLM area array is oriented in such a way that the columns of pixels in the projected image on the substrate are not parallel to the direction of movement of the image itself. For example, the columns in the projected image may be at an angle of 45 degrees to the direction of movement, in which case all of the edges of the square pixels will be equally blurred due to the movement alone. 
   While the invention has been described with reference to particular embodiments, this description is solely for the purpose of illustration and is not to be construed as limiting the scope of the invention claimed below.