Patent Publication Number: US-2003228047-A1

Title: Photomask transparent substrate protection during removal of opaque photomask defects

Description:
FIELD OF THE INVENTION  
       [0001] This invention relates generally to the fabrication of photomasks for use in semiconductor fabrication photolithography, and more specifically to the repair of such photomasks.  
       BACKGROUND OF THE INVENTION  
       [0002] The accuracy of a photomask, also commonly referred to as a mask, is crucial for ensuring that the semiconductor devices formed are also accurate, and perform correctly. A photomask is generally a mask used in photolithography to block photoresist exposure in selected areas. A photomask generally includes chrome opaque areas that block light exposure, supported by a quartz plate that is transparent to light exposure. Opaque areas may also be a molybdenum silicide material, such as MoSiON, in lieu of chrome.  
       [0003] Defects in a photomask in particular can cause the semiconductor devices fabricated with the photomask to malfunction. Two common defects are shown in FIGS. 1A and 1B. In FIG. 1A, the mask  102  has a proper opaque region  104 , but an improper opaque spot  106 . Conversely, in FIG. 2B, the opaque region  110  of the mask  108  has an improper hole  112 . Other common mask defects include inclusions of opacity into a clear region, protrusions of clarity into an opaque region, clear breaks within opaque regions, and opaque bridges between one opaque region and another opaque region.  
       [0004] Clear or missing parts of a mask are typically repaired by “patching” them with a carbon deposit. Opaque or unwanted chrome regions are usually removed by sputtering from a focused ion beam (FIB). One type of focused ion beam is a gallium ion beam. A focused gallium ion beam is capable of milling away opaque defects and depositing carbon film for clear defects at desired locations. The gallium ion beam may be used to help form the opaque regions on a clear mask, as well as to repair opaque and clear defects on the formed mask. The gallium ion beam is a positive ion beam, since gallium ions are themselves positive ions. Thus, FIB systems use a focused beam of gallium ions that can be operated at high beam currents for site-specific sputtering or milling.  
       [0005] Furthermore, since the invention of the integrated circuit (IC), semiconductor chip features have become exponentially smaller and the number of transistors per device exponentially larger. Advanced IC&#39;s with hundreds of millions of transistors at feature sizes of 0.13 micron, 0.10 micron, and less are becoming routine. Improvement in overlay tolerances in optical photolithography, and the introduction of new light sources with progressively shorter wavelengths, have allowed optical steppers to significantly reduce the resolution limit for semiconductor fabrication far beyond one micron.  
       [0006] The reduction in feature size makes for fabrication of photomasks a more difficult and expensive process. Mask repair is especially difficult at small feature sizes. This is the case with repairing opaque defects on a photomask. In particular, it is difficult to focus the FIB only on the opaque defect to be removed, without also affecting some of the quartz substrate surrounding the defect. The result is that undesirable grooves in the quartz substrate surrounding the defects can occur, which is known as “riverbed” effects. Furthermore, extraneous gallium ions may stay on the quartz substrate, affecting the transparency of the substrate, in an effect known as gallium staining.  
       [0007]FIG. 3 illustratively shows an example of the riverbed effect on a photomask  202 . The photomask  202  has a transparent substrate  204 , which is typically quartz, and properly fabricated opaque regions  206  thereover, which may be chrome, MoSiON, and so on. However, an undesirable opaque defect  208  has been deposited between the opaque regions  206  on the substrate  204 . Therefore, an FIB  210  is directed towards the defect  208  to remove it, such as by milling. However, where the feature size of the photomask  202  is very fine, the resolution of the FIB  210  is not sufficiently fine to only remove the defect  208 . As a result, grooves  212  and  214  occur in the substrate  204 , which are undesirable. Gallium staining of the exposed substrate  204  may also result.  
       [0008] Therefore, there is a need for photomask repair that does not exhibit these problems. Specifically, there is a need for preventing riverbed and gallium staining effects when removing opaque mask defects. Such prevention should be achieved especially when employing an FIB to mill away or otherwise remove such opaque defects. For these and other reasons, there is a need for the present invention.  
       SUMMARY OF THE INVENTION  
       [0009] The invention relates to protecting the transparent substrate of a photomask when repairing opaque defects of the mask. The photomask includes an opaque defect on a transparent substrate. The photomask is coated with photoresist, such as negative photoresist. The mask is backside-exposed to a light source, to expose the photoresist where it is not blocked by the opaque defect and other opaque regions of the photomask. The photoresist is preferably developed to remove the photoresist where it was unexposed to the light source. Thus, the opaque defect and the other opaque regions of the mask are exposed through the photoresist, but the transparent regions of the mask —where the transparent substrate does not have the opaque defect or the other opaque regions thereon—remain protected by the photoresist.  
       [0010] The opaque defect is then removed, such as by using a focused ion beam (FIB). The photoresist over the exposed transparent substrate protects the substrate from riverbed effects and gallium staining, which is the advantage of the invention. The photoresist that remains, which is the photoresist that was exposed to the light source from the mask&#39;s backside, is then removed, such as by photoresist stripping. The transparent substrate of the photomask may be quartz, whereas the opaque defect and the other opaque regions of the mask may be chrome, a molybdenum silicide material such as MoSiON, or another material. Still other aspects, embodiments, and advantages of the invention will become apparent by reading the detailed description that follows, and by referencing the attached drawings. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0011]FIGS. 1A and 1B are diagrams showing an example opaque defect and an example clear defect, respectively, that can occur when fabricating photomasks.  
     [0012]FIG. 2 is a diagram showing how removing an opaque defect on a photomask by using a focused ion beam (FIB) can also cause riverbed effects, in addition to gallium staining effects.  
     [0013]FIG. 3 is a flowchart of a method for removing an opaque defect on a photomask by using a FIB, but avoiding riverbed and gallium staining effects, according to an embodiment of the invention.  
     [0014]FIGS. 4A, 4B,  4 C,  4 D, and  4 E are diagrams showing illustratively the performance of the parts of the method of FIG. 3, according to an embodiment of the invention.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0015] In the following detailed description of exemplary embodiments of the invention, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific exemplary embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized, and logical, mechanical, and other changes may be made without departing from the spirit or scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims.  
     [0016]FIG. 3 shows a method  300  for repairing an opaque defect from a photomask, according to an embodiment of the invention. The method  300  may be used in part to fabricate a photomask as well, such as to repair the photomask after it has been manufactured. A semiconductor device may also be at least in part fabricated by using a photomask that is fabricated and/or repaired according to the method  300 .  
     [0017] First, a preferably negative photoresist coating is applied  5  over a mask that has an opaque defect ( 302 ). This is illustratively shown in FIG. 4A. The photomask  400  includes a transparent substrate  402 , which may be quartz. The photomask  400  also includes desired opaque regions  404  and  406  over the transparent substrate  402 , and an undesired opaque defect  408 . The parts of the transparent substrate  402 , between either region  404  or  406  and the defect  408 , that do not have a corresponding opaque region or defect thereover are the transparent regions of the mask  400 .  
     [0018] Thus, the defect  408  is isolated, in that it is not immediately adjacent to, and does not touch, one of the desired opaque regions  404  and  406 . The opaque regions  404  and  406  and the opaque defect  408  may be chrome, a molybdenum silicide material such as MoSiON, or another material. They are opaque in that they substantially block light from passing through, whereas the transparent regions are transparent in that they substantially allow light to pass through.  
     [0019] Negative photoresist  410  preferably coats the entirety of the photomask  400 . The photoresist  410  is negative in that when it is partially exposed to a light source and subsequently developed, only the parts of the photoresist  410  that were not exposed to the light source are removed. This is in distinction with positive photoresist, which when developed only the exposed parts thereof are removed, as can be appreciated by those of ordinary skill within the art.  
     [0020] Referring back to FIG. 3, the photomask is backside exposed to a light source, preferably an ultraviolet (UV) light source ( 304 ). This is illustratively shown in FIG. 4B. From the backside, or bottom, of the photomask  400 , a light source shines, as indicated by the arrows  412 . All the light passes through the transparent substrate  402 , since this region is transparent and lets light pass. However, the opaque regions  404  and  406  and the opaque defect  408  do not let light pass. Therefore, only the parts  414  of the photoresist  410  are exposed to the light source, since these parts do not have an opaque region or an opaque defect underneath them. By comparison, the parts  416  of the photoresist  410  are not exposed to the light source.  
     [0021] Referring back to FIG. 3, the photoresist as has been partially exposed to a light source is developed, which removes the unexposed photoresist ( 306 ). As has been indicated, only the unexposed photoresist is removed, because the photoresist is negative photoresist preferably. This is illustratively shown in FIG. 4C. Only the parts  414  of the photoresist  410  that are not over the opaque regions  404  and  406  and the opaque defect  408  remain. That is, the parts  414  of the photoresist  410  were exposed to the backside-emitting light source, since they were not blocked by the opaque regions  404  and  406  and the opaque defect  408 , and lie directly over the transparent substrate  402  of the mask  400 . Thus, the parts of the transparent substrate  402  that are not covered with the opaque regions  404  and  406  and the opaque defect  408  are now protected/covered by the parts  414  of the photoresist  410 . The opaque regions  404  and  406  and the opaque defect  408  are exposed, resulting from the removal of the parts  416  of the photoresist  410 .  
     [0022] Referring back to FIG. 3, the opaque defect is removed, preferably by performing focused ion beam (FIB) mask repair ( 308 ). Such mask repair does not result in gallium staining or riverbed effects, because the photoresist remaining over the transparent substrate thereof protects the substrate from the FIB. This is illustratively shown in FIG. 4D. In the mask  400 , the opaque defect  408  has been removed, resulting from milling away by the FIB, as represented by the arrows  418 . Riverbedding and gallium staining that result in the prior art from such FIB application are prevented by the presence of the parts  414  of the photoresist  410  over the transparent substrate  402  that would otherwise be exposed to the FIB. The mask  400  still includes the desired opaque regions  404  and  406 , since these are sufficiently far away from the FIB that they are preferably not affected by the FIB.  
     [0023] Referring finally back to FIG. 3, the remaining photoresist is removed, such as by stripping ( 310 ). This is the photoresist that was exposed by the backside light, and that was thus not blocked by the opaque regions or the opaque defect during this backside light exposure. Photoresist removal is illustratively shown in FIG. 4D. The mask  400  now includes the desired opaque regions  404  and  406  on top of the transparent substrate  402 , without the presence of the opaque defect  408 . The parts  414  of the photoresist  410  have also been removed. Thus, the mask  400  as shown in FIG. 4D is the end product after photomask repair. The mask  400  may now be used for semiconductor device fabrication, or may itself be further manufactured.  
     [0024] It is noted that, although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement is calculated to achieve the same purpose may be substituted for the specific embodiments shown. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is manifestly intended that this invention be limited only by the claims and equivalents thereof.