Abstract:
A simple −1X, catadioptric projection relay system (e.g., a modified Wynne-Dyson relay) is combined with a linear scanning and object and image indexing systems to provide good imagery over a useful field which is two or more times wider than the field size of the projection system and arbitrarily long. The projection system has opposed and parallel object and image planes and produces an image in which object and image vectors in one direction are parallel and in a normal direction are opposed. The reticle and substrate are clamped and scanned together in the parallel direction and are indexed in the normal direction by equal and opposite amounts between scans. An example shows how a 2.5 micron resolution, i-line projection system with a 300 mm wide field could be used to expose a 550 mm wide substrate In two scans to yield a very high throughput.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a scanning micro-lithographic apparatus and method for projecting a large field-of-view image on a substrate and, more particularly, to such apparatus that preferably employs a wide-field catadioptric projection system (e.g., a projection system incorporating a Wynne Dyson optical relay). 
     2. Description of the Prior Art 
     Some micro-lithographic applications, like flat display panel manufacturing, require a fast, seamless, high-resolution (e.g., 2-3 micrometers), large area exposure. This kind of exposure can be achieved effectively by scanning the substrate under a large exposure projection system with a field-of-view for exposure. 
     It is well known that micro-lithography can be done over a large area by combining two −1X projection systems having parallel object and image planes with a raster scanning system that moves the mask and substrate together so as to produce one strip of imagery at a time. The precision requirements on the scanning system are relaxed if adjacent strips of imagery are not butted together, but overlapped in a small area containing a linear variation in exposure intensity, such as can be obtained using a tapered illumination slit. A micro-lithography approach employing this principle is described in U.S. Pat. No. 5,298,939 and its division U.S. Pat. No. 5,835,195, both of which are entitled “Method and Apparatus for Transfer of a Reticle Pattern onto a Substrate by Scanning”. The drawback with this approach is that to obtain a +1X relay system it is either necessary to employ two −1X relays in series or to add an odd number of folds to the optical system which is often inconvenient or impossible. Also the requirements on the raster scanning system lead to substantial expense and complexity in the stage components. 
     An alternate approach, described in U.S. Pat. No. 5,585,972, entitled “Arbitrarily Wide Lens Array with an Image Field to Span the Width of a Substrate,” employs an array of +1X projection systems, which span the width of the substrate, to reduce the scanning requirement to a single linear scan. The drawback with this approach is that it requires a great many projection optical systems all of which have to be aligned very accurately and the illumination apertures are a staggered array of rectangles which is difficult to illuminate uniformly. 
     Therefore, there is a need for a scanning micro-lithographic apparatus and method that derives a high-resolution, large area exposure over a large area with a greatly simplified projection and illumination optical system, while not introducing too much complexity in the scanning system. In this regard, the teachings of U.S. Pat. No. 5,852,693, entitled “Low-loss Light Redirection Apparatus” which discloses a uniform illumination system employing light guiding technology using multiple light-pipes, is incorporated herein by reference. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a scanning micro-lithographic apparatus that employs a fixed −1X projection system (preferably, a wide-field catadioptric projection system) having a limited field of view in which image patterns located in its object plane can be imaged onto a substrate located in its image plane. These patterns, which are contained on a reticle or mask, are wider than the limited field size of the projection system. The mask is illuminated over the area corresponding to the field of the projection system and is moved in the object plane reticle in a direction parallel to that in which a vector in the object plane would be imaged parallel to its image in the object plane. Synchronous motion of the substrate in the same direction, scanning, copies the pattern from the reticle onto the substrate. In this fashion, a strip of imagery spanning the length of the pattern on the reticle may be copied onto the substrate. Other strips of imagery may be added in subsequent scans but, because the magnification is −1X, it is necessary to re-register the reticle and substrate by stepping them equally in opposite directions normal to the scanning direction. A second synchronous scan of the reticle and substrate then results in a pattern exposed on the substrate that is congruent with the reticle pattern. Subsequent stepping and scanning operations can be used to copy an arbitrarily large pattern on a reticle to a similarly sized substrate using a −1X relay having a smaller field size than the reticle pattern. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  is a diagrammatic showing in the vertical section through the scanning microlithographic apparatus of the present invention comprising an embodiment of an imaging means in which a movable mask structure and a movable substrate structure of the apparatus are each shown in a first of two relative positions thereof with respect to a fixed −1X projection system of the imaging means; 
     FIG. 1 b  is a diagrammatic showing in the vertical section through the scanning microlithographic apparatus of the present invention in which the movable mask structure and the movable substrate structure of this embodiment of the imaging means are each shown in a second of the two relative positions thereof with respect to the fixed −1X projection system of the imaging means; 
     FIG. 1 c  indicates the respective directions of the 2 horizontal spatial coordinates X and Y and the vertical spatial coordinate Z with respect to the plane of the paper in which each of FIGS. 1 a  and  1   b  is shown; 
     FIG. 2 a  is a diagrammatic showing in the horizontal plane of the movable mask structure shown in FIG. 1 a;    
     FIG. 2 b  is a diagrammatic showing in the horizontal plane of the movable mask structure shown in FIG. 1 b;    
     FIG. 3 diagrammatically illustrates the successive steps of one complete exposure scanning cycle of the movable mask structure and movable substrate structure of the imaging means of the scanning micro-lithographic apparatus incorporating present invention; 
     FIG. 3 a  illustrates the inverted image exposed on the substrate of spatial patterns disposed on the mask of the movable mask structure at the end of one complete exposure scanning cycle; and 
     FIGS. 4 a ,  4   b ,  4   c  and  4   d  relate to a preferred embodiment of the imaging means shown in FIGS. 1 a  and  1   b  that employs a fixed −1X projection system comprising a design of a catadioptric optical system design employing a plurality of lenses defining a modified Wynne-Dyson relay. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Scanning microlithographic apparatus  100  shown in each of FIGS. 1 a  and  1   b  comprises fixed base  102 , scanning carriage  104 , movable mask structure  106 , fixed −1X projection system  108  and movable substrate structure  110 . 
     In FIG. 1 a , movable mask structure  106 , supported by scanning carriage  104  toward the top thereof, is shown in its extreme left indexed position and movable substrate structure  110 , supported by scanning carriage  104  toward the bottom thereof, is shown in its extreme right indexed position. In FIG. 1 b , movable mask structure  106 , supported by scanning carriage  104  toward the top thereof, is shown in its extreme right indexed position and movable substrate structure  110 , supported by scanning carriage  104  toward the bottom thereof, is shown in its extreme left indexed position. Movable mask structure  106  and movable substrate structure  110  may be switched to either the indexed position thereof shown in FIG. 1 a  or the indexed position thereof shown in FIG. 1 b  by the operation of unshown indexing means (e.g., magnetic indexing means). When movable mask structure  106  and movable substrate structure  110  occupy their positions shown in FIG. 1 a , they comprise elements of imaging means that results in light (indicated by arrows) illuminating the patterns defined by the right half of mask structure  106  to be imaged by fixed −1X projection system  108  on the left half of movable substrate structure  110 . Similarly, when movable mask structure  106  and movable substrate structure  110  occupy their positions shown in FIG. 1 b , they comprise elements of imaging means that results in light illuminating the patterns defined by the left half of mask structure  106  is imaged by fixed −1X projection system  108  on the right half of movable substrate structure  110 . 
     In operation, both mask structure  106  and substrate structure  110  are carried on common scanning carriage  104  that moves on a fixed axis that is accurately aligned with the axis of projection system  108 . The axis of projection system  108  is defined as the direction in which a vector in the object plane and its image in the image plane are parallel. The carriage is shown shaded in the figure and is guided by air bearings that operate against a fixed base  102  so that it is only free to move in the scanning direction. 
     As indicated in FIG. 1 c , FIGS. 1 a  and  1   b  lie in an X, Z vertical plane that is parallel to the plane of the paper. Scanning carriage  104 , mounted in fixed base  102  (e.g., by air bearings) and operated by motor means (e.g., a linear motor) is moved either forward or backward in the Y direction normal to the plane of the paper to effect a scan. Since movable mask structure  106  is supported by scanning carriage  104 , such a scan results in the exposure of roughly half of the substrate by the half of mask structure  106  then being illuminated. 
     FIGS. 2 a  and  2   b , which, respectively, correspond in position to FIGS. 1 a  and  1   b , show mask structure  106  in its X, Y horizontal plane. As indicated in detail in FIGS. 2 a  and  2   b , mask structure  106  comprises mask holder  200  which holds mask  202 . Mask  202  constitutes a reticle that defines the patterns which are to be imaged on a substrate included in substrate structure  110 . As shown in the plane of the paper, projecting from the corners of mask holder  200  are the single upper-left prominence  204 L, the single upper-right prominence  204 R, the double lower-left prominences  206 L and  208 L and the double lower-right prominences  206 R and  208 R. When mask structure  106  occupies its first position shown in FIG. 2 a , the prominences  204 L,  206 L and  208 L are situated so that solely a first set of three points of mask holder  200  of mask structure  106  are in contact against edges of the left and lower interior edges of scanning carriage  104 , thereby precisely controlling the position and the angular orientation of reticle  202  in the horizontal plane defined by the contact positions of the three points of this first set. Similarly, when mask structure  106  occupies its second position shown in FIG. 2 b , the three prominences  204 R,  206 R and  208 R are situated so that solely a second set of three points of mask structure  106  are in contact against edges of the right and lower interior edges of scanning carriage  104 , thereby precisely controlling the position and the angular orientation of reticle  202  in the horizontal plane defined by the contact positions of the three points of this second set. The left and right interior edges of scanning carriage  104  are in precise parallel relationship with one another and unshown means comprising air bearings is employed in the support of mask structure  106  while (1) in its first position, (2) while in its second position and (3) while being moved between its first and second positions. 
     Although not shown in the drawing, the design of substrate structure  110  in FIGS. 1 a  and  1   b  is substantially similar to that of above-described mask structure  106  shown in FIGS. 2 a  and  2   b  to permit substrate structure  110 , in its X,Y horizontal plane, to be moved from its first position to its second position and to be precisely angularly oriented when in its first position and also when in its second position. It is important that neither mask structure  106  nor substrate structure  110  rotate between index positions and that the index distance be the same for both. 
     Referring now to FIG. 3, there is diagrammatically illustrated each of the successive  8  steps  300   a  to  300   h  of one complete exposure scanning cycle of reticle  302  of movable mask structure  106  and substrate  304  of movable substrate structure  110  with respect to the fixed position of illumination slit  306 . The width of illumination slit  306  is substantially one-half the width of each of reticle  302  and substrate  304 . The spatial patterns defined by reticle  302  are represented by the letter “F” situated in the left half of reticle  302  and the letter “R” situated in the right half of reticle  302 . 
     Step  300   a  shows that prior to the beginning of a scanning cycle, reticle  302  and substrate  304  are disposed in their first positions and illumination slit  306  is situated in the plane of the paper above reticle  302  and substrate  304 . Step  300   b  shows that during the first half of a scanning cycle, reticle  302  and substrate  304  are moved upward in the plane of the paper (as indicated by the direction of the arrow in  300   b ), thereby exposing the spatial patterns, such as the letter“R”, situated on the right half of reticle  302 . Step  300   c  shows that at the end of the first half of a scanning cycle, reticle  302  and substrate  304  are still disposed in their first positions and illumination slit  306  is situated in the plane of the paper below reticle  302  and substrate  304 . Step  300   d  shows that next indexing takes place, wherein reticle  302  is to be switched from its first (i.e., left) position to its second (i.e., right) position and substrate  304  is to be switched from its first (i.e., right) position to its second (i.e., left) position (as indicated by the direction of the arrows in  300   d ). Step  300   e  shows that subsequent to such indexing and prior to the beginning of the second half of a scanning cycle, reticle  302  and substrate  304  are disposed in their second positions and illumination slit  306  is situated in the plane of the paper below reticle  302  and substrate  304 . Step  300   f  shows that during the second half of a scanning cycle, reticle  302  and substrate  304  are moved downward in the plane of the paper (as indicated by the direction of the arrow in  300   f ), thereby exposing the spatial patterns, such as the letter “F”, situated on the left half of reticle  302 . Step  300   g  shows that at the end of the second half of a scanning cycle, reticle  302  and substrate  304  are still disposed in their second positions and illumination slit  306  is situated in the plane of the paper above reticle  302  and substrate  304 . Step  300   h  shows that finally indexing takes place, wherein reticle  302  is to be switched from its second (i.e., right) position to its first (i.e., left) position and substrate  304  is to be switched from its second (i.e., left) position to its first (i.e., right) position (as indicated by the direction of the arrows in  300   h ). The accomplishment of this final indexing completes the performance of an entire scanning cycle. 
     FIG. 3 a  illustrates the inverted image exposed on substrate  304  of spatial patterns, represented by the letters “F” and “R”, disposed on the reticle  302  at the end of one complete exposure scanning cycle. 
     Reference is now made to FIGS. 4 a ,  4   b ,  4   c  and  4   d , which show various indicated views of a preferred embodiment of the FIGS. 1 a  and  1   b  imaging means that includes a design for the fixed −1X projection system  108  which comprises a catadioptric optical system employing a plurality of lenses defining a modified Wynne-Dyson relay. More specifically, as indicated in FIGS. 4 a  and  4   b , in the plane of the paper, shows a Y, Z-plane view through the optical axis of triangular fold-mirror element  400 , reflector element  402  and the four lenses  404 - 1 ,  404 - 2 ,  404 - 3  and  404 - 4  (which constitute the structure of fixed −1X projection system  108 ) together with reticle  412 , its protective window  414 , substrate  416  and its protective window  418 . Thus, as further indicated in FIG. 4 b , the X direction in FIG. 4 a  is oriented normal to the plane of the paper. FIG. 4 c  shows the X, Z-plane view of the shape of the periphery of any one of lenses  404 - 1 ,  404 - 2 ,  404 - 3  and  404 - 4  (which is designated lens  404 -k in FIG. 4 c ). FIG. 4 d  shows (1) the respective sizes of the X and Y dimensions of the substantially rectangular illumination field incident on reticle  412  (as well as the corresponding field imaged on substrate  416 ) and (2) the relative offset between the rectangular illumination field and the center of the field on projection lens  422 . In the example shown in FIG. 4 d  the offset between the center of the field of the projection lens and the closest edge of the illuminated field is 15 mm, This offset avoids vignetting that would otherwise occur on the folding prism  400 . In FIG. 4 a  the optical axis through the center of the lens would be folded by folding prism along the flat edge of lens element  404 - 1 . The offset between the folded axis and the field is apparent in the Y separation distance between the piano surface of lens element  404 - 1  and the illuminated field shwon on the reticle plane  412 . 
     More particularly, in the rectangular object field  420  shown in FIG. 4 d , the size of the X dimension is 300 millimeters (mm) and the size of the Y dimension is 45 mm. Further, rectangular object field  420  is positioned so that its upper side would form a chord of a circle having a radius of 165 mm (i.e., 330 mm diameter) which is centered on optical axis position  422 . This results, in the FIG. 4 d  view, in the lower side of rectangular object field  420  being situated substantially 15 mm above optical axis  422 , which, in the FIG. 4 a  view, causes the left surface of lens  404 - 1  being situated 15 mm to the right of the right side of reticle  412 . 
     While originally each of lenses  404 - 1 ,  404 - 2 ,  404 - 3  and  404 - 4  had a circular periphery somewhat larger than 300 mm in diameter, in accordance with the principles of this invention, a portion of both the top and bottom of these lenses has been sawed away. This results in the sawed periphery of each of these lenses having the shape of lens  404 -k shown in FIG. 4 c , which has a width in its X dimension which remains somewhat larger than 300 mm, but has a thickness in its Y dimension which (as shown in FIG. 4) may have been reduced to substantially one-half that of its X dimension. Sawing away the top and bottom portions of these lenses results in the dual advantages of both facilitating scanning of the reticle  412  and substrate  416  by aligning them parallel to each other, and reducing lens weight. 
     Returning to FIG. 4 a , reticle  412 , which defines the patterns to be imaged, lies in the object plane of the projection system. While reticle  412  is being scanned in the horizontal Y direction, illumination from above, which is directed in the vertical Z direction (as indicated by arrow  424 ), is incident on the reticle  412 . The shape and relative position of this incident light with respect to the folded position of the projection system optical axis is that shown in FIG. 4 d . The result is that light (indicated by dashed lines in FIG. 4 a ) passes downward in the vertical Z direction through reticle  412  and its protective window  114  and is incident on the top mirror surface  400 T (indicated by a thick line) of triangular fold-mirror element  400 . Top mirror surface  400 T, which is oriented at a 45 degree angle with respect to each of the horizontal Y and vertical Z directions, reflects the vertical Z direction light incident thereon into the horizontal Y direction. This horizontal Y direction light, subsequent to being acted upon by each of lenses  404 - 1 ,  404 -Z,  404 - 3  and- 404 - 4  as it passes left-to right therethrough, is incident on concave-mirror surface  402 C (indicated by a thick line) of reflector element  402 . The horizontal Y direction light reflected from concave mirror surface  402 C, subsequent to passing right-to-left back through each of lenses  404 - 4 ,  404 - 3 ,  404 - 2  and  404 - 1 , is incident on the bottom mirror surface  400 B (indicated by a thick line) of triangular fold-mirror element  400 . Bottom mirror surface  400 B, which is oriented at a 45 degree angle with respect to each of the horizontal Y and vertical Z directions, reflects the horizontal Y direction light incident thereon into the downward vertical Z direction through substrate protective window  418  to a focus on substrate  416 , while substrate  416  is being scanned together with reticle  412  in the horizontal Y direction. Operating in this manner, the imaging means shown in FIG. 4 a , is effective in imaging a −1X (i.e., inverted, same size) image of the patterns defined by reticle  412  on substrate  416  (as represented in above discussed FIG. 3 a ). 
     The following Table 1 lists illustrative optical design criteria for the imaging means shown in FIG. 4 a  (wherein each of the respective optical surfaces in FIG. 4 a , except for the fold-mirror surfaces, is identified by a reference number within parentheses in FIG. 4 a ). This optical design criteria (which ignores the presence of the fold mirrors), operates with light centered at a wavelength of 365 nanometers (nm) to provide a 0.1 numerical aperture fixed −1X projection system: 
     In Table 1, the axial separation 113.685329 mm represents the sum of the vertical axial distance between the lower surface ( 3 ) of reticle protective window  414  and fold-mirror surface  400 T and the horizontal axial distance between fold-mirror surface  400 T and the left surface ( 4 ) of lens  404 - 1 , as well as the sum of the vertical axial distance between the upper surface ( 14 ) of substrate protective window  418  and fold-mirror surface  400 B and the horizontal axial distance between fold-mirror surface  400 B and the left-surface ( 15 ) of lens  404 - 1 . 
     The imaging means shown in FIG. 4 a , employing the illustrative optical design criteria listed in Table 1 is capable of providing a resolution of better than 3 micrometers (mm); and, therefore, is capable of imaging fine patterns (such as needed for the manufacture of the majority of high resolution flat display panels) in a single complete scanning cycle without requiring any field stitching. 
     Further, the design of the imaging means shown in FIG. 4 a  has the following desirable characteristics: 
     1. The use of single triangular fold-mirror element  400  to provide both top and bottom fold-mirror surfaces  400 T and  400 B, instead of employing two separate prisms for these purposes, makes not only the lens assembly but also the final imaging means adjustments much easier. In order to make the scanning mechanism simple, a mirror bar folds horizontal light paths to vertical directions. Further, the folding of the direction of the light from the reticle from vertical to horizontal and the folding of the light to the substrate from horizontal to vertical permits a simplified scanning mechanism to be employed; however this orientation makes the total footprint of the tool rather large and it necessitates that the substrate be handled in the horizontal position. In some situations it is likely that the preferred embodiment will rotate the reticle and wafer planes so that they are vertical while retaining a horizontal axis for the scanning stage. 
     2. The imaging means design has a large enough working distance to allow reticle and substrate protective windows  414  and  416  to be installed, which not only provide a means to seal and protect the imaging system but also can be used for magnification correction. Bending these windows about the Y-axis changes the magnification in the X-direction. The magnification can be independently changed in the Y-direction by differentially moving the reticle with respect to the substrate in the Y-direction as the reticle and substrate are being scanned. Some control-of the magnification (+/−30 ppm) is desirable to compensate for small changes in the scale of the substrate as compressive and tensile layers are added during successive process steps. 
     3. The design adds the two lenses  404 - 3  and  404 - 4  to the conventional Wynne-Dyson design to reduce aberrations and telecentricity errors across the large light field of 45×300 mm 2 . Lens  404 - 3 , closer to the doublet lenses  404 - 1  and  404 - 2 , has a positive power and lens  404 - 4 , further from the doublet lenses  404 - 1  and  404 - 2 , has a negative power. 
     It is important that the large object field of 45 mm×300 mm be provided with substantially uniform illumination. While providing uniform illumination over such a wide field is challenging, it can be achieved by employing the light guiding technology using multiple lightpipes that is taught in the aforementioned U.S. Pat. No. 5,852,693, which teaching has been incorporated herein by reference. The light pipes provide a means of obtaining uniform illumination over a field equivalent to 100 mm by 135 mm, which can then be divided into 6 fields, each 45 mm by 50 mm, that can be rearranged to provide a 45 mm by 300 mm field size. In order to conserve material this would likely be done at higher numerical aperture and at a smaller scale, possibly at ⅓ the required scale, and then magnified with a relay to obtain the correct scale at the reticle plane. 
     The 300 mm wide strip of the image pattern projected on substrate  418  by the imaging means shown in FIG. 4 a  permits a 550 mm wide substrate to be exposed with the two scans comprising a complete cycle shown in FIG. 3. A little extra field provided by the projection system permits the use of a slit with tapered ends and some overlap between the exposures of the two scans. A 550 mm wide substrate was chosen as an example because this size is believed to be representative of current state-of-the-art flat panel substrates. However the present invention could be scaled up to handle substantially larger substrates For example, a scaled-up projection system could be employed to achieve a span of 380 mm wide field for use in accommodating a 720 mm wide substrate, which is expected to be the next standard substrate size. 
     Further, it is not intended that the present invention be limited to the above described case in which the movable mask structure and movable substrate structure is situated in a selected one of a plurality of only two indexed, positions. Rather, the present invention contemplates a scanning microlithographic apparatus in which the plurality of selectable indexed positions may be two or greater. In such a case, step  300   h  in FIG. 3, in which the mask structure is moved right-to-left and the substrate structure is moved left-to-right, is repeated three and possibly more times. In each case the mask structure is moved left-to-right and the substrate structure is moved right-to-left by an identical amount, thereby permitting a new scan to continue until the entire reticle pattern has been copied onto the substrate and a new cycle with a new substrate can commence. In general if more than two scans are required, the re-registration of the reticle and substrate between scans becomes more complex, and the time required for more scans becomes longer. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 S n   
                   
                   
                   
               
               
                 (Surface 
                 Surface 
                 S n  − S n−1 -mm 
                 Lens 
               
               
                 No.) 
                 Radius-mm 
                 (Axial Separation) 
                 Glass Type 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 (1) &amp; (12) 
                 Infinity 
                   
                   
               
               
                 (2) &amp; (13) 
                 Infinity 
                 20.000000 
               
               
                 (3) &amp; (14) 
                 Infinity 
                 10.000000 
                 BK7_SCHOTT 
               
               
                 (4) &amp; (15) 
                 Infinity 
                 113.685329 
               
               
                 (5) 
                 −258.71918 
                 152.627197 
                 FSL5Y_O&#39;HARA 
               
               
                 (6) 
                 −665.58119 
                 132.232497 
                 PBL6Y_O&#39;HARA 
               
               
                 (7) 
                 1513.72957 
                 53.185137 
               
               
                 (8) 
                 −2294.93750 
                 219.718892 
                 PBL6Y_O&#39;HARA 
               
               
                 (9) 
                 1185.57361 
                 100.903748 
               
               
                 (10)  
                 833.06562 
                 73.410281 
                 PBL6Y_O&#39;HARA 
               
               
                 (11)  
                 −1400.00000 
                 569.305564