Abstract:
A mask set of two masks and a method of using these masks in a double exposure to avoid line shortening due to optical proximity effects is described. A pattern having pattern elements comprising a number of line segments, wherein each of the line segments has one or two free ends which are not connected to other mask pattern elements is to be transferred to a layer of resist. A first mask is formed by adding line extensions to each of the free ends of the line segments. A cutting mask is formed comprising rectangles enclosing each of the line extensions wherein one of the sides of said rectangles is coincident with the corresponding free end of said line segment. The first mask has opaque regions corresponding to the extended line segments. The cutting mask has transparent regions corresponding to the cutting pattern. In another embodiment a pattern having pattern openings comprising a number of line segments. In this embodiment the cutting pattern comprises rectangles having the same width as said line segments and add length to the line segments.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     This invention relates to masks and methods for optical proximity correction of pattern images, and more particularly to the correction of line end shortening caused by optical proximity effects. 
     (2) Description of the Related Art 
     An important concern in the manufacture of integrated circuit devices is pattern distortion caused by optical proximity effects, particularly as critical dimensions in these patterns decreases. Much work has been done in developing methods to compensate for this pattern distortion. 
     U.S. Pat. No. 5,807,649 to Liebmann et al. describes a lithographic patterning method and mask set using a phase shift trim mask having mask dimensions increased in block size so as to remove previous exposure defects. 
     U.S. Pat. No. 5,308,741 to Kemp describes a lithographic method using double exposures, physical mask shifting, and phase shifting masks. 
     U.S. Pat. No. 5,686,223 to Cleeves describes a lithographic patterning method using multiple exposures to provide for relatively reduced pitch for features of a single patterned layer. 
     SUMMARY OF THE INVENTION 
     As optical lithography is used to delineate 0.1 micron and smaller features, it inevitably works in a low k 1  region. For example, for a 193 nanometer wavelength and a numerical aperture of 0.63 the value of k 1 , which is equal to the feature size multiplied by the numerical aperture and divided by the wavelength is equal to 0.33. At this low value of k 1  the length of lines shortens when the width of the lines is kept at the desired dimension. Conventional solutions to this line shortening have included widening of the line at the line ends. This widening of the line ends takes additional space on the integrated circuit element however, which also limits the density of wiring in the circuit element. 
     It is a principle objective of this invention to provide a method of forming pattern line elements without line shortening without widening line ends, using double exposure and two masks. 
     It is another principle objective of this invention to provide a method of forming pattern line openings without line shortening without widening line ends, using double exposure and two masks. 
     It is another principle objective of this invention to provide a mask set of two masks for forming pattern line elements without line shortening without widening line ends, using double exposure and the two mask set. 
     It is another principle objective of this invention to provide a mask set of two masks for forming pattern line openings without line shortening without widening line ends, using double exposure and the two mask set. 
     These objectives are accomplished by forming a modified pattern mask and a cutting mask. A pattern having pattern elements comprising a number of line segments, wherein each of the line segments has one or two free ends which are not connected to other mask pattern elements is to be transferred to a layer of resist. Extended line segments are formed by adding line extensions to each of the free ends of the line segments, thereby forming a modified pattern. A cutting pattern is formed comprising rectangles enclosing each of the line extensions wherein one of the sides of said rectangles is coincident with the corresponding free end of said line segment. 
     A first mask having opaque regions corresponding to said modified pattern is then formed. A second mask having transparent regions corresponding to the cutting pattern is also formed. The layer of resist is then formed using a first exposure and the first mask. Next the layer of resist is exposed using a second exposure and the second mask for removing any excess line length. When the layer of resist is developed lines having the proper length result. 
     In another embodiment a pattern having pattern openings comprising a number of line segments, wherein each of the line segments has one or two free ends which are not connected to other mask pattern elements. A cutting pattern comprising rectangles having the same width as said line segments, add length to each of the free ends of the line segments, and overlap the free ends of said line segments is then formed. 
     A first mask having transparent regions corresponding to the pattern is formed. A second mask having transparent regions corresponding to the cutting pattern is also formed. The layer of resist is then exposed using a first exposure and the first mask. The layer of resist is then exposed using a second exposure and the second mask. When the layer of resist is developed lines openings having the proper length result. 
     The masks and method of this invention will work for forming images in either positive or negative resist. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a pattern having a number of collinear line segments. 
     FIG. 2 shows the modified pattern and cutting pattern for the pattern of FIG.  1 . 
     FIG. 3A shows the mask for the modified pattern of FIG.  2 . 
     FIG. 3B shows the cutting mask for the modified pattern of FIG.  2 . 
     FIG. 4 shows a pattern having a number of off set line segments. 
     FIG. 5 shows the modified pattern and cutting pattern for the pattern of FIG.  4 . 
     FIG. 6A shows the mask for the modified pattern of FIG.  5 . 
     FIG. 6B shows the cutting mask for the modified pattern of FIG.  5 . 
     FIG. 7 shows a pattern having a line segments in perpendicular directions. 
     FIG. 8 shows the modified pattern and cutting pattern for the pattern of FIG.  7 . 
     FIG. 9A shows the mask for the modified pattern of FIG.  8 . 
     FIG. 9B shows the cutting mask for the modified pattern of FIG.  8 . 
     FIG. 10 shows a pattern having a number of line segments in additional to an L pattern element. 
     FIG. 11 shows the modified pattern and cutting pattern for the pattern of FIG.  10 . 
     FIG. 12A shows the mask for the modified pattern of FIG.  11 . 
     FIG. 12B shows the cutting mask for the modified pattern of FIG.  11 . 
     FIG. 13 shows a pattern having a number of line segments arranged in an H pattern. 
     FIG. 14 shows the modified pattern and cutting pattern for the pattern of FIG.  13 . 
     FIG. 15A shows the mask for the modified pattern of FIG.  14 . 
     FIG. 15B shows the cutting mask for the modified pattern of FIG.  14 . 
     FIG. 16 shows a pattern having pattern openings to form line segments and an L segment. 
     FIG. 17 shows the modified pattern and cutting pattern for the pattern of FIG.  16 . 
     FIG. 18A shows the mask for the modified pattern of FIG.  17 . 
     FIG. 18B shows the cutting mask for the modified pattern of FIG.  17 . 
     FIG. 19 shows a schematic diagram of a mask alignment and projection system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Refer now to FIGS. 1-19 for a detailed description of the preferred embodiments of this invention. The key to this invention is to form a set of two masks to transfer a pattern comprising lines which will become shortened by the photolithographic process to a layer of resist. This is accomplished by extending the length of each of the free ends of the pattern elements, thereby forming a first mask. A second mask is then formed to cut the ends of the lines to the proper length. A layer of resist is first exposed using the first mask and a first exposure. The layer of resist is then exposed using the second mask and a second exposure. FIG. 19 shows a schematic diagram of a photolithographic mask alignment and projection system. A substrate  708 , usually an integrated circuit wafer, having a layer of resist  706  formed thereon is placed in a substrate holder  710 . The masks  702  are placed in a mask holder  704 . The mask holder  704  and substrate holder  710  are positioned to achieve the proper alignment between the masks  702  and layer of resist  706 . A radiation source  700 , usually a light source, supplies radiation to pass through the masks  702  and the lens  705 . The radiation passing through the masks  702  is focussed by the lens  705  to expose the layer of resist  706  so that the image of the masks  702  is transferred to the layer of resist  706 . After exposure using the first and second masks the substrate  708  is removed and the layer of resist  706  is developed. 
     FIG. 1 shows a top view of a pattern to be transferred to a layer of resist. In this embodiment the pattern elements,  100  and  102 , are to be transferred as remaining resist in a layer of developed positive resist or openings in the resist in a layer of developed negative resist. FIG. 1 shows the example of a pattern wherein some of the pattern elements  100  at the top of the pattern are collinear with one of the pattern elements  102  at the bottom of the pattern. 
     FIG. 2 shows a top view of both the first and second mask showing the rules used to form the masks. The first mask pattern is shown using solid lines in FIG.  2 . The bottom ends of the line segments at the top of the pattern, reference number  100  in FIG. 1, and the top ends of the line segments at the top of the pattern, reference number  102  in FIG. 1, are extended until they meet forming extended line segments, reference number  104  in FIG.  2 . The top ends of the line segments at the top of the pattern, reference number  100  in FIG. 1, and the bottom ends of the line segments at the top of the pattern, reference number  102  in FIG. 1, are extended a first distance  116 . The extended line segments,  104  in FIG. 2, will be opaque regions of the first mask. The first distance is greater than the expected reduction of length at one end of the line segments. In this example the line segments at the top of the pattern and the line segments at the bottom of the pattern are extended until they meet. This is done unless the distance between the bottom of the line segments at the top of the pattern and the top of the line segments at the bottom of the pattern is greater than the largest expected reduction of length at one end of the line segments multiplied by ten. 
     As shown by the dashed lines in FIG. 2 a cutting pattern is located at the positions where the length of the lines is to be cut. FIG. 2 shows a number of cutting pattern elements  106  at the top of the extended line segments  104 , cutting pattern elements  110  at the bottom of the extended line segments  104 , and cutting elements  108  at the center of the extended line segments  104 . The cutting elements,  106 ,  108 , and  110  are a number of rectangles. The cutting elements,  106  and  110 , at the top and bottom of the extended line segments  104  are positioned to cut the first distance  116  from the line segment ends. These cutting elements have a height  114  of at least two times the first distance. The cutting elements  108  at the center of the extended line segments  104  are located to cut the extended line segments to duplicate the line segments of the original pattern. The width of the cutting elements,  106 ,  108 , and  110 , is large enough to extend a distance  120  beyond the edge of the extended line segments  104  which is greater than the largest expected reduction of length at one end of the line segments. If the separation  120  between the extended line segments  104  is less than the largest expected reduction of length at one end of the line segments multiplied by ten the width of the cutting pattern elements,  106 ,  108 , and  110 , is extended until the cutting pattern elements meet. 
     FIG. 3A shows a top view of the first mask showing the extended line segments  104 . The extended line segments  104  will be opaque regions on the first mask. FIG. 3B shows a top view of the second mask or cutting mask showing the cutting elements,  106 ,  108 , and  110 . The cutting elements,  106 ,  108 , and  110  are transparent regions of the cutting mask. 
     FIG. 19 shows a schematic diagram of a photolithographic mask alignment and projection system. A substrate  708 , usually and integrated circuit wafer, having a layer of resist  706  formed thereon is placed in a substrate holder  710 . The masks  702  are placed in a mask holder  704 . The mask holder  704  and substrate holder  710  are positioned to achieve the proper alignment between the masks  702  and layer of resist  706 . A radiation source  700 , usually a light source, supplies radiation to pass through the masks  702  and expose the layer of resist  706 . The first mask, shown in FIG. 3A, is first placed in the mask holder  704  and the layer of resist is exposed. This leaves the extended line segments  104  unexposed. The second mask, shown in FIG. 3B, is then placed in the mask holder and the layer of resist is exposed a second time. This exposes the extended regions of the extended line segments  104  so that they will be removed during development of the resist. Since the line shortening has taken place before the exposure using the cutting pattern, this method avoids line shortening in the final resist pattern. 
     The method just described with reference to FIGS. 1-3B, and  19 , is the method used to avoid line shortening while forming the image of line segments in a layer of resist. Refer now to FIGS. 4-15B for a description of additional mask embodiments having opaque pattern elements in the first mask and transparent cutting elements in the cutting mask, and layout ground rules for forming these masks. These masks and this method will work for the case of either positive or negative resist. 
     FIG. 4 shows a pattern having line segments,  200  and  202 , which are not collinear but which would contact each other if the line segments were extended toward one another. FIG. 5 shows the diagram of the extended line segments  204  for the first mask and the cutting elements,  206 ,  208 , and  210 , for the second mask or cutting mask. In this embodiment the bottom ends of the lines  200  at the top of the pattern and the top ends of the lines  202  at the bottom of the pattern, see FIG. 4, are extended until the lines meet as shown in FIG.  5 . The meeting point  212  of the lines is located midway between the bottom ends of the lines  200  at the top of the pattern and the top ends of the lines  202  at the bottom of the pattern. The rules for extending the lines at the top ends of the lines  200  at the top of the pattern and the bottom ends of the lines  202  at the bottom of the pattern are the same as described in the previous example. The rules for forming the cutting elements  206 ,  208 , and  210  are also the same as described in the previous example. 
     FIG. 6A shows a top view of the first mask showing the extended line segments  204 . The extended line segments  204  will be opaque regions on the first mask. FIG. 6B shows a top view of the second mask or cutting mask showing the cutting elements  206 ,  208 , and  210 . The cutting elements  206 ,  208 , and  210  are transparent regions of the cutting mask. 
     FIG. 7 shows a top view of a mask having line segments  300  and  302  in a T pattern. FIG. 8 shows the layout pattern for the extended line segments  304  and  306  of the first mask and the cutting elements  308  and  310  for the cutting mask. The rules for forming the vertical extended line segments  304 , the horizontal extended line segment  306 , and the cutting element  310  used to trim the bottom end of the vertical extended line segments  304  are the same as described in previous examples. A new ground rule introduced in this example requires a gap  314  between the edge of the cutting element  308  used to cut the top ends of the vertical extended line segments  304  and the ends of the horizontal extended line segment  306 . This gap must be greater than twice the root mean square sum of the stage accuracy and mask overlay errors for the mask alignment and projection system shown in FIG.  19 . 
     FIG. 9A shows a top view of the first mask showing the extended line segments  304  and  306 . The extended line segments  304  and  306  will be opaque regions on the first mask. 
     FIG. 9B shows a top view of the second mask or cutting mask showing the cutting elements  308  and  310 . The cutting elements  308  and  310  are transparent regions of the cutting mask. 
     FIG. 10 shows a top view of a mask having straight line segments  400  and an L shaped line segment  402 . FIG. 11 shows the layout pattern for the extended line segments  404  and  408  of the first mask and the cutting elements  410  and  412  for the cutting mask. The rules for forming the extended line segments  404  and  408  and the cutting elements  410  and  412  are the same as described in previous examples. A new ground rule introduced in this example requires gaps  414  and  416  between the edge of the cutting element  410  and adjacent line segments  408 . These gaps must each be greater than twice the root mean square sum of the stage accuracy and mask overlay errors for the mask alignment and projection system shown in FIG.  19 . 
     FIG. 12A shows a top view of the first mask showing the extended line segments  404  and  408 . The extended line segments  404  and  408  will be opaque regions on the first mask. FIG. 12B shows a top view of the second mask or cutting mask showing the cutting elements  410  and  412 . The cutting elements  410  and  412  are transparent regions of the cutting mask. 
     FIG. 13 shows a top view of a mask having straight line segments  500 ,  502 , and  504  forming an H shaped pattern. FIG. 14 shows the layout pattern for the extended line segments  506  of the first mask and the cutting elements  508 ,  510 , and  512  for the cutting mask. The rules for forming the extended line segments  506  and the cutting elements  508 ,  510 , and  512  are the same as described in previous examples. In this pattern the interior cutting elements  510  must extend beyond the edge of the interior extended line segment  506  a distance  514  least 20% larger than the expected largest line shortening of the pattern line segments,  500 ,  502 , and  504  in FIG.  13 . There must also be a gap  516  between the edge of the interior cutting elements  510  and adjacent line segments  510 . This gap must each be greater than twice the root mean square sum of the stage accuracy and mask overlay errors for the mask alignment and projection system shown in FIG.  19 . 
     FIG. 15A shows a top view of the first mask showing the extended line segments  506 . The extended line segments  506  will be opaque regions on the first mask. FIG. 15B shows a top view of the second mask or cutting mask showing the cutting elements  508  and  510 . The cutting elements  508  and  510  are transparent regions of the cutting mask. 
     Refer now to FIGS. 16-18B for a description of an embodiment of the masks of this invention having pattern elements corresponding to transparent regions of the mask. FIG. 16 shows a top view of a mask having straight line segments  600  and an L shaped line segment  602 . FIG. 17 shows the layout pattern for the first mask having line segments  600  and  602  mask and the cutting elements  610  and  612  for the cutting mask. In this example the line segments  600  and  602  in. the first mask will be transparent regions in an otherwise opaque mask and will be the same size and shape as the line segments and L shaped segment in the original pattern mask. Since exposure of a layer of resist using this mask will shorten these pattern elements the cutting mask also has transparent cutting elements  610  and  610  which will lengthen these pattern elements in the second exposure. The cutting elements  610  and  612  have the same width as the corresponding line elements  600  and  602  but serve the purpose of extending these line segments in a second exposure. 
     FIG. 18A shows a top view of the first mask showing the line segments  600  and L shaped segment  602 . The line segments  600  and L shaped segment  602  are transparent regions of the first mask. FIG. 18B shows a top view of the second mask or cutting mask showing the cutting elements  610  and  612 . The cutting elements  610  and  612  are transparent regions of the cutting mask. 
     These embodiments have described first exposing a layer of resist using a first mask followed by exposure using a cutting mask. The method of this invention will work equally well by first exposing the layer of resist using the cutting mask followed by a second exposure using the first mask. The masks and method of all the embodiments described herein will work equally well for forming images in either positive or negative resist. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.