Abstract:
Methods for a die to die and a die to database inspection of phase shifting masks is described. A layer of partially transmitting material, such as an anti-reflection coating, is formed on the mask covering the phase shifting mask elements. The mask is then illuminated by a light source and the light transmitted through the mask is detected. Defects in the pattern of phase shifting mask elements will cause a difference in the amount of light transmitted through the defect when compared to a defect free phase shifting mask element. This difference can be used to perform a die to die inspection or a die to database inspection of the pattern of phase shifting mask elements.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     This invention relates to a method of die to die or die to database inspection of phase shifting masks for defects in the transparent phase shifting mask elements. 
     (2) Description of the Related Art 
     In order to inspect masks having phase shifting mask elements, some means must be employed to make defects in the phase shifting mask elements detectable. The detectability of these defects is made difficult because the phase shifting mask elements are transparent and difficult to distinguish from the transparent mask substrate. The phase shifting mask elements are often the same material as the transparent mask substrate. 
     U.S. Pat. No. 5,446,540 to Lin describes the use of a phase-contrast microscope for the inspection of a phase shifting mask. 
     U.S. Pat. No. 5,786,112 to Okamoto et al. describes a method of inspecting photomasks having phase shifting mask elements using a first anomaly discrimination step, a second anomaly discrimination step, and an anomaly extraction step in inspecting the mask. 
     U.S. Pat. No. 5,439,767 to Yamashita et al. describes a method of inspection a phase shifter for transmittance error and phase shift angle error by exposing a layer of photoresist several times at varying focus, developing the photoresist, and measuring the width of the lines in the resulting pattern. 
     SUMMARY OF THE INVENTION 
     Levenson type phase shifting masks use quartz or other transparent phase shifting mask elements. The quality of these masks is very important so that inspection of the masks for mask defects is very important. Inspection of the masks is quite difficult, however, since the light transmission through the phase shifting mask elements is the same as through the transparent mask substrate. The phase shifting mask elements and the transparent mask substrate are often formed of the same material. 
     It is a principle objective of this invention to provide a method of inspecting for defects in transparent phase shifting mask elements using die to die inspection. 
     It is another principle objective of this invention to provide a method of inspecting for defects in transparent phase shifting mask elements using die to data base inspection. 
     These objectives are achieved by forming a layer of partially transmitting material, such as a layer of anti-reflective material, on the mask covering the phase shifting mask elements. The mask is then illuminated by a light source and the light transmitted through the mask is detected. Defects in the pattern of phase shifting mask elements will cause a difference in the amount of light transmitted through the defect when compared to a defect free phase shifting mask element. This difference can be used to perform a die to die inspection or a die to database inspection of the pattern of phase shifting mask elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a cross section view of a phase shifting mask having phase shifting mask elements formed of the same material as the transparent mask substrate and defects in the mask. 
     FIG. 2 shows a cross section view of a phase shifting mask having phase shifting mask elements formed of the same material as the transparent mask substrate, opaque mask elements, and defects in the mask. 
     FIG. 3 shows a cross section view of a phase shifting mask having phase shifting mask elements formed on a transparent mask substrate and defects in the mask. 
     FIG. 4 shows a cross section view of a phase shifting mask having phase shifting mask elements formed on a transparent mask substrate, opaque mask elements, and defects in the mask. 
     FIG. 5 shows a cross section view of a phase shifting mask having phase shifting mask elements formed of the same material as the transparent mask substrate, defects in the mask, and a layer of partially transmitting material formed on the mask. 
     FIG. 6 shows a cross section view of a phase shifting mask having phase shifting mask elements formed of the same material as the transparent mask substrate, opaque mask elements, defects in the mask, and a layer of partially transmitting material formed on the mask. 
     FIG. 7 shows a top view of a mask having four die positions. 
     FIG. 8 shows a schematic diagram of an inspection set up to perform a die to die mask inspection. 
     FIG. 9 shows a schematic diagram of an inspection set up to perform a die to data base mask inspection. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Refer now to FIGS. 1-8 for a description of the embodiment of the die to die method of this invention for inspecting phase shifting masks for defects. FIG. 1 shows a cross section view of a Levenson type phase shifting mask without opaque mask elements. The mask comprises a transparent mask substrate  10 , formed of a material such as quartz, and phase shifting mask elements  12  which are formed of the same material as the transparent mask substrate. The mask shown in FIG. 1 has a defect  14  which consists of phase shifting material which should have been removed but is present in a region of the mask which should have no phase shifting material present. The defect in this invention is extraneous phase shifting material and not contamination of other materials. It is difficult to detect the defect because the defect  14 , the transparent mask substrate  10 , and the phase shifting material  12  are transparent material. A light beam  18  passing through the transparent mask substrate  10  and the defect  14  will not be attenuated any more than a light beam  19  passing through the transparent mask substrate only. 
     FIG. 2 shows a cross section view of a Levenson type phase shifting mask with opaque mask elements. The mask comprises a transparent mask substrate  10 , opaque mask elements  16 , and phase shifting mask elements  12  which are formed of the same material as the transparent mask substrate. The transparent mask substrate is formed of a material such as quartz and the opaque mask elements are formed of a material such as chrome. The mask shown in FIG. 2 has a defect  14  which consists of phase shifting material which should have been removed but is present in a region of the mask which should have no phase shifting material present. The defect in this invention is extraneous phase shifting material and not contamination of other materials. It is difficult to detect the defect because the defect  14 , the transparent mask substrate  10 , and the phase shifting material  12  are transparent material. A light beam  18  passing through the transparent mask substrate  10  and the defect  14  will not be attenuated any more than a light beam  19  passing through the transparent mask substrate only. 
     FIG. 3 shows a cross section view of another Levenson type phase shifting mask without opaque mask elements. This mask comprises a transparent mask substrate  11 , formed of a material such as quartz, and phase shifting mask elements  22  which are formed of different transparent material from the transparent mask substrate. The mask shown in FIG. 3 has a defect  24  which consists of phase shifting material which should have been removed but is present in a region of the mask which should have no phase shifting material present. A light beam  18  passing through the transparent mask substrate  11  and the defect  24  will not be attenuated any more than a light beam  19  passing through the transparent mask substrate  11  only. Those skilled in the art will readily recognize that the method of this invention for inspecting the mask of FIG. 3 is the same as the method for inspecting the mask of FIG.  1  and the invention will be described using the mask of FIG.  1 . 
     FIG. 4 shows a cross section view of a Levenson type phase shifting mask with opaque mask elements. The mask comprises a transparent mask substrate  11 , opaque mask elements  26 , and phase shifting mask elements  22  which are formed of different transparent material as the transparent mask substrate. The mask shown in FIG. 4 has a defect  24  which consists of phase shifting material which should have been removed but is present in a region of the mask which should have no phase shifting material present. The defect in this invention is extraneous phase shifting material and not contamination of other materials. A light beam  18  passing through the transparent mask substrate  11  and the defect  24  will not be attenuated any more than a light beam  19  passing through the transparent mask substrate only. Those skilled in the art will readily recognize that the method of this invention for inspecting the mask of FIG. 4 is the same as the method for inspecting the mask of FIG.  2  and the invention will be described using the mask of FIG.  2 . 
     The key step of the invention is shown in FIGS. 5 and 6. FIG. 5 shows a cross section view of the phase shifting mask without opaque mask elements with a mask defect  14 . A layer of partially transmitting material  20 , such as an anti-reflection coating, is formed on the mask covering the phase shifting mask elements  12  and the defect  14 . When the mask is illuminated, the light  18  transmitted through the defect  14  has a shorter path  15  in the partially transmitting material than the path  17  of the light  19  passing through a corresponding part of the mask without a defect. This shorter path in the partially transmitting material will cause the light passing through the defect  14  to have less attenuation than light  19  not passing through the defect  14 . This difference in attenuation makes the defect detectable. 
     FIG. 6 shows a cross section view of the phase shifting mask with opaque mask elements  16  with a mask defect  14 . A layer of partially transmitting material  20 , such as an anti-reflection coating, is formed on the mask covering the phase shifting mask elements  12  and the defect  14 . When the mask is illuminated, the light  18  transmitted through the defect  14  has a shorter path  15  in the partially transmitting material than the path  17  of the light  19  passing through a corresponding part of the mask without a defect. This shorter path in the partially transmitting material will cause the light passing through the defect  14  to have less attenuation than light  19  not passing through the defect  14 . This difference in attenuation makes the defect detectable. 
     FIG. 7 shows a mask  30  having a number of identical dies  32 , in this example four. The die contains an image for an integrated circuit or die. The image in each die  32  is identical to the image in the other dies  32 . As shown in FIG. 8, the mask  30  is placed in a mask holder  34  and illuminated with a light source  36 . A first detector  40 , such as a first objective lens, detects the light transmitted through a segment of one of the dies. A second detector  41 , such as a second objective lens, detects the light transmitted through the corresponding segment of another of the dies. The first detector  41  and second detector  43  can be connected to a visual display unit  42  or other image processing unit where the image of the first detector  40  can be compared with the image of the second detector  41 . The first detector  40 , the second detector  41 , and the mask  30  can be moved so that the entire mask can be scanned and a die to die inspection of the mask can be carried out. 
     Refer now to FIGS. 1-7 and  9  for a description of the embodiment of the die to database method of this invention for inspecting phase shifting masks for defects. FIGS. 1-4 show the phase shifting masks with defects and are the same as described in the preceding embodiment. As in the preceding embodiment, the key step of the invention is shown in FIGS. 5 and 6. FIG. 5 shows a cross section view of the phase shifting mask without opaque mask elements with a mask defect  14 . A layer of partially transmitting material  20 , such as an anti-reflection coating, is formed on the mask covering the phase shifting mask elements  12  and the defect  14 . When the mask is illuminated, the light  18  transmitted through the defect  14  has a shorter path  15  in the partially transmitting material than the path  17  of the light  19  passing through a corresponding part of the mask without a defect. This shorter path in the partially transmitting material will cause the light passing through the defect  14  to have less attenuation than light  19  not passing through the defect  14 . This difference in attenuation makes the defect detectable. 
     FIG. 6 shows a cross section view of the phase shifting mask with opaque mask elements  16  with a mask defect  14 . A layer of partially transmitting material  20 , such as an anti-reflection coating, is formed on the mask covering the phase shifting mask elements  12  and the defect  14 . When the mask is illuminated, the light  18  transmitted through the defect  14  has a shorter path  15  in the partially transmitting material than the path  17  of the light  19  passing through a corresponding part of the mask without a defect. This shorter path in the partially transmitting material will cause the light passing through the defect  14  to have less attenuation than light  19  not passing through the defect  14 . This difference in attenuation makes the defect detectable. 
     FIG. 7 shows a mask  30  having a number of identical dies  32 , in this example four. The die contains an image for an integrated circuit or die. The image in each die  32  is identical to the image in the other dies  32 . As shown in FIG. 9, the mask  30  is placed in a mask holder  34  and illuminated with a light source  36 . A detector  40 , such as an objective lens, detects the light transmitted through a segment of one of the dies. The detector  40  is connected to an image processing unit  44 . The mask database  46  is also fed to the image processing unit  44  where a die to database comparison of the mask image can be made. The detector  40  and the mask  30  can be moved so that the entire mask can be scanned and a die to database inspection of the mask can be carried out. 
     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.