Patent Publication Number: US-2003228529-A1

Title: Photomask and method for repairing defects

Description:
RELATED APPLICATIONS  
     [0001] This application claims the benefit of U.S. provisional patent application Serial No. 60/387,375 entitled “Photomask and Method For Repairing Defects On The Same” filed on Jun. 10, 2002. 
    
    
     
       TECHNICAL FIELD OF THE INVENTION  
       [0002] This invention relates in general to the field of lithography and, more particularly, to a photomask and method for repairing defects on the same.  
       BACKGROUND OF THE INVENTION  
       [0003] Today, photolithography generally requires short exposure wavelengths for successful imaging of very small semiconductor device dimensions on a wafer. At wavelengths in or below the deep ultraviolet (DUV) range, e.g., below two hundred nanometers, materials and techniques typically used to produce a photomask assembly have become increasingly important.  
       [0004] A typical fabrication process for a photomask may include imaging a circuit pattern into a resist layer, developing the resist layer, etching the resist layer and any uncovered regions of an opaque or semitransmissive layer and removing unetched portions of the resist layer. During the process, defects may be created if portions of the opaque or semitransmissive layer remain on the substrate in areas that should be free of such material. These defects may be repaired by removing the excess material but the substrate may be damaged during the repair process.  
       [0005] At least two techniques have previously been used to repair a photomask. Focused gallium ion beam photomask repair technology typically relies on ion detection to determine an endpoint for the defect repair process. In order to determine the endpoint, a substrate below the defect is sampled to detect the presence of gallium ions. Since sampling of the substrate does not indicate an endpoint until gallium ions are present in the substrate, undesired gallium contamination and/or pitting of the substrate may occur before the endpoint. Gallium contamination increasingly absorbs energy at wavelengths below 400 nm. Any associated damage to the substrate may be mitigated by reducing the dose of the ion beam and/or post processing after the ion beam repair process is complete. However, dose reduction may hinder overall quality of an image projected by the photomask and may reduce endpoint precision. Furthermore, post processing may result in localized phase errors. Repair techniques using focused Ga ion beam (such as Seiko—SIR3000x) may also have the drawback of possibly straining a transparent substrate.  
       [0006] Other repair techniques may use laser evaporation or ablation to remove defects. Laser repair techniques may cause divots in a substrate that can alter optical characteristics of the substrate and associated photomask. The endpoint for laser repair processes is often determined by the presence or absence of ions associated with removal of a defect. The endpoint for repair of nontransmissive material disposed on a substrate is often more difficult to determine if the nontransmissive material and the substrate have common ions. The presence of common ions in materials used to form an nontransmissive layer and an associated substrate often results in substrate damage in the form of quartz pits.  
       [0007] Substrate damage was often not a concern in lithography systems using exposure wavelengths above approximately four hundred nanometers. However, in lithography systems using exposure wavelengths below approximately four hundred nanometers, substrate damage may cause absorption of such exposure wavelengths and thus, decrease transmissive properties of an associated photomask.  
       [0008] TiSi-nitride based materials have previously been used to form embedded, attenuated phase shift photomask blanks and associated photomasks. Such materials are sometimes referred to as silicon nitride titanium nitride (SiNTiN). Silicon nitride (Si 3 N 4 )is a dielectric material frequently used in the semiconductor industry.  
       SUMMARY OF THE INVENTION  
       [0009] In accordance with teachings of the present invention, disadvantages and problems associated with repairing defects on a photomask have been substantially reduced or eliminated. For one embodiment, a photomask may be formed with a buffer layer that prevents an associated substrate from being damaged during a repair process. The buffer layer may also prevent electrostatic discharge (ESD) damage to the associated substrate. Another embodiment of the present invention may include a method for repairing defects on a photomask having a buffer layer and a nontransmissive layer formed on a substrate with the buffer layer disposed between the nontransmissive layer and the substrate. A pattern may be formed in the nontransmissive layer. If one or more defects are identified in the patterned nontransmissive layer, the defects may be repaired in the patterned nontransmissive layer while the buffer layer protects the substrate from damage during the repair process.  
       [0010] A further embodiment of the present invention may include a photomask assembly having a photomask pellicle assembly defined in part by a pellicle frame and a pellicle film attached to the pellicle frame. The photomask assembly may also include a photomask coupled to the pellicle assembly opposite from the pellicle film. The buffer layer may be used to protect the substrate from damage during repair of any defects in the nontransmissive layer. After repair of the nontransmissive layer, portions of the buffer layer corresponding with a pattern formed in the nontransmissive layer may be etched to expose adjacent portions of the substrate. The resulting patterned layer may be defined in part by etched portions of the nontransmissive layer and corresponding etched portion of the buffer layer.  
       [0011] In accordance with teachings of the present invention, a photomask may be formed with a buffer layer disposed on at least a portion of an associated substrate. The buffer layer may be formed from various materials which transmit, partially transmit, absorb and/or reflect electromagnetic energy. The photomask may further include a nontransmissive layer formed on the buffer layer. The nontransmissive layer may be formed from various materials which absorb, partially transmit, and/or reflect electromagnetic energy. A pattern may be formed in the nontransmissive layer using various lithography techniques. The buffer layer is preferably operable to prevent the substrate from being damaged during a repair process associated with the patterned nontransmissive layer.  
       [0012] Technical advantages of certain embodiments of the present invention may include a buffer layer that prevents a substrate of a photomask from being damaged during a repair process. During a photomask manufacturing process, defects may be formed in a nontransmissive layer and must be repaired. During the repair process, a repair beam may be used to remove such defects. Since the buffer layer is preferably located between the nontransmissive layer and the substrate, any damage from the repair process will generally effect only the buffer layer rather than the substrate.  
       [0013] Another technical advantage of certain embodiments of the present invention may include a buffer layer that reduces electrostatic discharge (ESD) damage during a manufacturing process. Traditionally, oxide materials have been used as a buffer material since oxide materials will typically remain intact during an etch of an associated nontransmissive layer. However, many oxide materials may also function as an insulator, which increases the risk of ESD damage by providing a dielectric material between a charged nontransmissive layer and an associated substrate. Accordingly, the present invention reduces the risk of ESD damage by using electrically conductive materials to form the buffer layer.  
       [0014] A further technical advantage of certain embodiments of the present invention may include a buffer layer that enables precise endpoint detection of a repair process using a focused ion beam (FIB) system to repair any damage to a nontransmissive layer. The buffer layer may be formed from material that is different from material used to form the nontransmissive layer. During an ion beam repair process, an associated repair tool may monitor the concentration of ions associated with the ion beam in the nontransmissive layer. The endpoint of the repair process may be determined when no ions associated with the ion beam are detected in the nontransmissive layer since the ion concentration will change when the defect has been removed and the ion beam reaches the surface of the buffer layer.  
       [0015] Other aspects of the present invention include using single ion beam deposition or dual ion beam deposition techniques to fabricate at least portions of an attenuating, embedded phase shift photomask blank capable of producing approximately one hundred eighty degree (180°) phase shifts at selected lithographic wavelengths less than four hundred (400) nanometers (nm). For some applications the phase shifts may vary plus or minus five degrees (±5°).  
       [0016] All, some, or none of these technical advantages may be present in various embodiments of the present invention. Other technical advantages will be readily apparent to one skilled in the art from the following figures, descriptions, and claims.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0017] A more complete and thorough understanding of the present invention and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings, in which like reference numbers indicate like features, and wherein:  
     [0018]FIG. 1 is a schematic drawing in section showing one example of a photomask assembly formed according to teachings of the present invention;  
     [0019]FIG. 2A is a schematic drawing in section with portions broken away showing one example of a photomask blank which may be used to form a photomask and/or photomask assembly in accordance with teachings of the present invention;  
     [0020]FIGS. 2B, 2C and  2 D are schematic drawings in section with portions broken away showing various views of a photomask formed from the photomask blank of FIG. 2A before and after a repair process has removed defects from a patterned layer according to teachings of the present invention; and  
     [0021]FIGS. 3A and 3B are schematic drawings in section showing one example of a photomask repaired by an ion beam repair process according to teachings of the present invention.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0022] Preferred embodiments of the present invention and its advantages may be understood by reference to FIGS. 1 through 3B, where like numbers are used to indicate like and corresponding parts.  
     [0023]FIG. 1 illustrates a cross-sectional view of photomask assembly  10  with photomask  12  coupled to pellicle assembly  14 . Substrate  16  and patterned layer  18  cooperate with each other to form photomask  12 , otherwise known as a mask or reticle. Photomask  12  may have a variety of sizes and shapes, including, but not limited to, round, rectangular or square. Photomask  12  may also be any variety of photomask types, including, but not limited to, a one-time master, a five-inch reticle, a six-inch reticle, a nine-inch reticle or any other appropriately sized reticle that may be used to project an image of a circuit pattern onto a semiconductor wafer (not expressly shown). Photomask  12  may further be a binary mask, a phase shift mask, an optical proximity correction (OPC) mask, or any other type of mask suitable for use in a lithography system. When photomask assembly  10  is placed in a lithography system, a circuit image defined in part by patterned layer  18  may be projected through substrate  16  and on to the surface of a semiconductor wafer.  
     [0024] For some applications, substrate  16  may be a transparent material such as quartz, synthetic quartz, fused silica, magnesium fluoride (MgF 2 ), calcium fluoride (CaF 2 ), or any other suitable material that transmits at least approximately seventy-five percent (75%) of incident light having a wavelength between approximately ten (10) nanometers (nm) and approximately 450 nm. In an alternative embodiment, substrate  16  may be a reflective material such as silicon or any other suitable materials that reflect greater than approximately fifty percent of incident light having a wavelength between approximately 10 nm and 450 nm.  
     [0025] In some embodiments, patterned layer  18  may be a metal material such as chrome, chromium nitride, a metallic oxy-carbo-nitride (M-O—C—N), where the metal is selected from the group consisting of chromium, cobalt, iron, zinc, molybdenum, niobium, tantalum, titanium, tungsten, aluminum, magnesium and silicon, and any other suitable material that absorbs and/or reflects electromagnetic energy with wavelengths in the ultraviolet (UV) range, deep ultraviolet (DUV) range, vacuum ultraviolet (VUV) range and extreme ultraviolet range (EUV). In an alternative embodiment, patterned layer  18  may be a partially transmissive material, such as molybdenum silicide (MoSi), which has a transmissivity of approximately one percent to approximately thirty percent in the UV, DUV, VUV and EUV ranges.  
     [0026] In other embodiments, patterned layer  18  may include at least one nontransmissive layer formed on at least one buffer layer. The nontransmissive layer and the buffer layer may have any of the transmissive characteristics described above in reference to patterned layer  18 . The buffer layer preferably prevents damage from occurring to substrate  16  during a defect repair process associated with the nontransmissive layer.  
     [0027] Frame  20  and pellicle film  22  cooperate with each other to form pellicle assembly  14 . Pellicle film  22  may be a thin film membrane formed of a material such as nitrocellulose, cellulose acetate, an amorphous fluoropolymer, such as Teflon® AF manufactured by E. I. du Pont de Nemours and Company or Cytop manufactured by Asahi Glass, or another suitable UV, DUV, VUV or EUV film. Pellicle film  22  may be prepared by conventional techniques such as spin casting. Frame  20  is typically formed of anodized aluminum. Alternatively, frame  20  be formed of stainless steel, plastic or other suitable materials.  
     [0028] Pellicle film  22  protects photomask  12  from dust particles by ensuring that the dust particles remain a defined distance away from photomask  12 . This may be especially important in a lithography system. During a lithography process, photomask assembly  10  is exposed to electromagnetic energy produced by a radiant energy source within the lithography system. The electromagnetic energy may include light of various wavelengths, such as wavelengths approximately between the I-line and G-line of a Mercury arc lamp, or DUV, VUV or EUV light. In operation, pellicle film  22  is preferably designed to allow a large percentage of incident electromagnetic energy to pass therethrough. Dust particles collected on pellicle film  22  will likely be out of focus on the surface of a wafer being processed using photomask assembly  10  and, therefore, the exposed image on the wafer (not expressly shown) should be clear.  
     [0029] Photomask  12  may be fabricated from a photomask blank that includes a layer of buffer material, a layer of nontransmissive material and a layer of resist material disposed on one surface of substrate  16 . One example is shown in FIG. 2A. For some applications, the buffer layer and the nontransmissive layer may be formed from multiple layers of material. Respective layers of buffer material, nontransmissive material, and resist material may be deposited on one surface of substrate  16  using physical vapor deposition (PVD), chemical vapor deposition (CVD), ion beam deposition (IBD), dual ion beam deposition (DIBD) or any other suitable deposition technique.  
     [0030] Photomask  12  may be formed from a photomask blank using various lithography processes. In a typical lithography process, a mask pattern file (not expressly shown) that includes data for patterned layer  18  may be generated from a circuit design pattern (not expressly shown). The desired pattern for patterned layer  18  may be imaged into a resist layer of the photomask blank using a laser, electron beam, X-ray lithography tool or other suitable source of electromagnetic energy. For example, a laser lithography tool may use an Argon-Ion laser that emits light having a wavelength of approximately 364 nanometers (nm). In alternative embodiments, a laser lithography tool may use laser emitting light at wavelengths from approximately 150 nm to approximately 300 nm.  
     [0031] As discussed later in more detail with respect to FIGS.  2 A- 2 D, an imaged pattern (not expressly shown) may be imaged on a resist layer. The resist layer and an associated nontransmissive layer may be etched to create at least a portion of corresponding etched pattern  18 . One or more defects (not expressly shown) which may occur in patterned layer  18  may be repaired in accordance of teachings of the present invention without damaging substrate  16 .  
     [0032]FIGS. 2A, 2B,  2 C and  2 D illustrate cross-sectional views of photomask blank  12   a  and associated photomask  12 . FIG. 2A shows photomask blank  12   a  prior to forming patterned layer  18  associated with photomask  12 . FIGS. 2B, 2C and  2 D show examples of some steps associated with repairing a defect in patterned nontransmissive layer  32  in accordance with teachings of the present invention.  
     [0033] Photomask  12  may be a phase shift mask (PSM), including, but not limited to, an alternating PSM, an attenuated PSM, and a multitone PSM. For some applications, photomask  12  may be formed from an embedded, attenuated phase shift photomask blank  12   a.  For some applications photomask blank  12   a  may be generally described as an embedded, attenuated phase shift photomask blank with repair buffer, etch control and electrostatic discharge (ESD) reducing layer  30 . However, the present invention is not limited to phase shift photomasks.  
     [0034] In FIG. 2A, photomask blank  12   a  is shown after buffer layer  30 , nontransmissive layer  32  and resist layer  60  have been formed on one surface of substrate  16 . Substrate  16  may be formed from transparent material, such as quartz, synthetic quartz or fused silica, or reflective material, such as silicon. Various lithography fabrication techniques may be used to form photomask blank  12 a with layers  30 ,  32  and  60 .  
     [0035] Materials used to fabricate layers  30 ,  32  and/or  60  on photomask blank  12   a  may be homogeneous, graded or multilayered as long as photomask blank  12   a  satisfies optical properties of a semitransparent medium providing desired transmission and phase shift characteristics. The structure of photomask blank  12   a  will generally have application for lithographic processes using wavelengths below 400 nm. For example some lithography processes use electromagnetic energy with wavelengths of 248 nm, 193 nm, 157 nm, 100 nm, and 50 nm.  
     [0036] Buffer layer  30  may act as a protective layer so that substrate  16  is not damaged during a repair process associated with nontransmissive layers  32 . Buffer layer  30  may also serve as an etch stop during etching processes associated with patterning of nontransmissive layer  32 . Materials used to form buffer layer  30  may be selected to enhance phase and transmission percentage uniformity of nontransmissive layer  32 . Furthermore, buffer layer  30  may be formed at least in part from conductive materials to reduce electrostatic discharge (ESD) effects on substrate  16  during fabrication of photomask blank  12   a,  photomask  12  and/or photomask assembly  10 .  
     [0037] Buffer layer  30  may be formed from any material that offers dry etch selectivity relative to both substrate  16  and nontransmissive layer  32 . Materials used to form buffer lay  30  preferably have optical properties that do not interfere with and preferably enhance overall optical characteristics of photomask  12 . The thickness of buffer lay  30  may vary in thickness between a few angstroms to a few nanometers depending on respective photomask repair technology used during manufacture of photomask  12 . The thickness of buffer layer  30  is preferably selected to minimize or prevent any straining of substrate  16 .  
     [0038] For some applications buffer layer  30  will preferably be formed from carbon type materials such as “Diamond Like Carbon” (DLC) because most commercially available dry etch processes associated with fabrication of embedded, attenuated phase shift photomasks will not etch DLC materials. Thus, DLC materials generally have good dry etch selectivity relative to materials used to form embedded, attenuated phase shift photomasks. In a second etch process, DLC materials preferably have good selectivity relative to substrate  16 . DLC materials often have very good electrical properties which prevent as critical dimensions become smaller (under 500 nm).  
     [0039] Hard carbon films or layers may sometimes be described as diamond like carbon (DLC) films or layers. DLC materials may be generally described as a mixture of diamond and graphite structures including, but not limited to, hard noncrystal carbon, hard amorphous carbon, amorphous carbon and i-carbon. A wide variety of DLC materials are commercially available for use in forming one or more layers  30  on photomask blank  12   a.  However, the present invention is not limited to buffer layers formed from DLC materials.  
     [0040] Layers  30  and  32  of photomask blank  12   a  may be formed from materials such as:  
     [0041] M a -Si x O y N z  disposed in either a generally homogeneous or graded structure, where M is a metal from Group IV, V or VI; or  
     [0042] M 1 O a N b /M 2 O c N d  multilayers having at least one layer of each. M 1  may be aluminum or silicon and “a” varies between (0 to 1) while “b” varies between (0 and 1-a). M 2  is a metal from Group IV, V or VI.  
     [0043] Layers  30  and  32  may be a combination of the above materials so that layer  32  functions as a nontransmissive layer and layer  30  functions as a buffer for substrate  16 .  
     [0044] Alternatively, layers  30  and  32  may be generally homogeneous or graded structures of MSi x O y N z  where M is a metal selected from Groups IV, V or VI or a multilayer structure of M 1 O a N b /M 2 O c N d  where M 1  is either aluminum (Al) or silicon (Si), M 2  is a metal from Group IV, V or IV, and a varies between 0 and 1 while b varies between 0 and 1-a. The multilayered structure may be a combination of the above materials such that at least one layer  32  is nontransmissive to the exposure wavelength and another layer  30  function as a buffer to protect an associated substrate. The resulting structure may be capable of producing a 180° phase shift at selected exposure wavelengths in a lithography system of less than 400 nanometers.  
     [0045] Buffer layer  30  may be formed from materials other than those used to form nontransmissive layer  32  to provide an etch stop for repair of defects in layer  32 . A repair tool may monitor a repair site for either the elimination of ions from nontransmissive material associated with the defect or for the presence of ions from the buffer layer material disposed below the defect. Once the stop condition is achieved the repair may be deemed complete. For example: An FIB repair tool may monitor for Si ions from a defect associated with a nontransmissive layer formed from SiNTiN material. Without buffer layer  30  the difference between ion yield of the SiNTiN defect and a substrate formed in part with silicon (Si) is typically too small to accurately define an end point. The Si yield from buffer layer  30  made of DLC materials would yield substantially zero Si ions. Thus, an end point for removing a defect associated with nontransmissive layer  32  may be defined more precisely by buffer layer  30  blocking or preventing production of secondary Si ions from substrate  16 .  
     [0046] Buffer layer  30  and nontransmissive layer  32  may be deposited using PVD, CVD, IBD or any other suitable deposition technique while simultaneously receiving a thermal treatment. In one embodiment, a single ion beam deposition (IBD) process may be used to deposit one or more buffer layers  30  and one or more nontransmissive layers  32 . The resulting photomask blank may be an attenuating embedded phase shift photomask blank capable of producing 180° phase shifts at selected lithographic wavelengths less than 400 nanometers. The process may include depositing at least one buffer layer  30  and at least one nontransmissive layer  32  or a combination thereof, on substrate  16  by ion beam sputtering of a target or targets by ions from a group of gases.  
     [0047] In a single IBD process, a plasma discharge may be contained in a separate chamber (ion “gun” or source) and ions extracted and accelerated by an electric potential impressed on a series of grids at the “exit port” of the gun (not expressly shown). The IBD process may also provide a cleaner process (fewer added particles) at the deposition surface on substrate  16  because the plasma that traps and transports charged particles to substrate  16  is generally not in proximity with either buffer layer  30  or nontransmissive layer  32 . Additionally, the IBD process generally operates at lower total gas pressure which results in reduced levels of chemical contamination. The IBD process also has the ability to independently control deposition flux and reactive gas ion flux (current) and energy.  
     [0048] During the single IBD process, an energized beam of ions (usually neutralized by an electron source) may be directed from a deposition gun (not expressly shown) to a target material located on a target holder. The target material is typically sputtered when bombarding ions have energy above a sputtering threshold energy for the specific material, which may be approximately fifty (50) eV. Ions from the deposition gun may be from an inert gas source such as He, Ne, Ar, Kr, Xe, although reactive gases such as O 2 , N 2 , CO 2 , F 2 , CH 3 , or combinations thereof, may also be used. When these ions are from an inert gas source, the target material may be sputtered and deposited as either nontransmissive layer  32  on buffer layer  30  or buffer layer  30  on substrate  16 . When these ions are produced by a reactive gas source, the ions may combine with the target material. Products of the chemical combination may be sputtered or deposited as either nontransmissive layer  32  on buffer layer  30  or buffer layer  30  on substrate  16 .  
     [0049] In a dual IBD process, ions from a second or “assist” gun (not expressly shown) are typically neutralized by an electron source directed at the surface of buffer layer  30  or substrate  16 . An ion beam from a first gun or deposition gun may also be directed at substrate  16  or buffer layer  30  similar to a single IBD process. The ions from the assist gun may originate from a reactive gas source such as O 2 , N 2 , CO 2 , F 2 , N 2 O, H 2 O, NH 3 , CF 4 , CHF 3 , CH 4 , C 2 H 2 , or any combination thereof. The energy of ions from the assist gun is usually lower than the energy of ions from the deposition gun. The assist gun provides an adjustable flux of low energy ions that react with sputtered atoms from the deposition gun at the surface of buffer layer  30  or substrate  16  to respectively form nontransmissive layer  32  or buffer layer  30 . In a dual ion beam deposition (DIBD) process the angles between a material target, substrate  16 , and associate deposition gun and assist gun (not expressly shown) may be adjusted to optimize film uniformity and film stress.  
     [0050] One example of a dual IBD process includes using a deposition gun to deposit at least one layer of optically transmitting material and at least one layer of optically absorbing material or a combination thereof, on substrate  16  by ion beam sputtering of a primary target by ions from a group of gases. An assist gun may also deposit portions of the at least one layer of optically transmitting material and the at least one layer of optically absorbing material, or a combination thereof, on substrate  16  by a secondary ion beam of a group of gases. The layers may be formed either directly, or by a combination of the gas ions from the assist gun and material deposited from the primary target on the substrate.  
     [0051] Another example of a dual IBD process for preparing an embedded, attenuated phase shift photomask blank capable of producing 180° phase shift at selected lithographic wavelengths less than 400 nanometers includes:  
     [0052] depositing at least one layer of optically transmitting material and at least one layer of optically absorbing material or a combination thereof, on substrate  16  by ion beam sputtering of a target or targets by ions from a group of gases; and  
     [0053] bombarding substrate  16  by a secondary ion beam from an assist source with ions from a reactive gas wherein the reactive gas is at least one gas selected from the group consisting of N 2 , O 2 , CO 2 , N 2 O, H 2 O, NH 3 , CF 4 , CHF 3 , F 2 , CH 4 , and C 2 H 2 .  
     [0054] After photomask blank  12 a has been formed as shown in FIG. 2A, a circuit design pattern may be imaged onto resist layer  60  using various lithographic techniques. Resist layer  60  may then be developed and exposed areas of resist layer  60  and adjacent portions of nontransmissive layer  32  etched to form a corresponding pattern in nontransmissive layer  32 . Any undeveloped portions of resist layer  60  may be removed as shown in FIG. 2B. Any defects which may be formed in nontransmissive layer  32  during the patterning process may be repaired in accordance with teachings of the present invention.  
     [0055] Nontransmissive layer  32  may include one or more defects  34  such as shown in FIG. 2B that were not removed during one or more etch processes associated with patterning nontransmissive layer  32 . Each defect  34  may be removed or repaired using repair beam  36 . Repair beam  36  may be a focused ion beam (FIB) that uses ion detection in buffer layer  30  to determine an endpoint for the repair process. A laser (not expressly shown) that evaporates or ablates material or any other suitable technique may also be used to repair nontransmissive layer  32  by removing defect  34 .  
     [0056] As shown in FIG. 2C, repair beam  36  may damage portions of buffer layer  30  directly below defect  34 . In the illustrated embodiment, repair beam  34  creates damaged portion  38  in buffer layer  30 . Damaged portion  38  may be gallium contamination and/or pitting created by FIB beam  36 . Damaged portions  38  may also be a divot created by a laser or any other type of damage created by an associated repair process.  
     [0057] As illustrated in FIG. 2D, once defect  34  has been removed from nontransmissive layer  32 , portions of buffer layer  30  may be removed from substrate  16  in uncovered areas or patterned areas of nontransmissive layer  32  to expose adjacent portions of substrate  16 . In one embodiment, portions of buffer layer  30  may be removed with a dry etch process that is different from the etch process or processes associated with patterning nontransmissive layer  32 . The resulting photomask  12  includes substrate  16  and patterned layer  18  defined in part by one or more buffer layers  30  and one or more nontransmissive layers  32  is shown in FIG. 20.  
     [0058]FIGS. 3A and 3B illustrate cross-sectional views of photomask  12  before and after repair by an FIB process. As illustrated in FIG. 3A, photomask  12  includes buffer layer  30  formed between nontransmissive layer  32  and substrate  16 . In one embodiment, nontransmissive layer  32  may be formed from SiNTiN based materials or any other suitable material that has appropriate transmissive characteristics or reflective characteristics when exposed to electromagnetic energy with a wavelength between approximately 10 nm and approximately 450 nm. Buffer layer  30  may be formed from diamond like carbon (DLC) materials or any other material that does not change optical characteristics of photomask  12  and has suitable dry etch selectivity relative to nontransmissive layer  32  and substrate  16 . For some applications, buffer layer  30  may have a thickness between approximately one hundred angstroms (100 Å) and three nanometers (3 nm) depending on respective repair processes used to remove any defects in nontransmissive layer  32 .  
     [0059] For one embodiment, buffer layer  30  may be made of DLC material with a thickness of approximately 150 angstroms. Nontransmissive layer  32  may be formed from SiNTiN based materials with a thickness of approximately six hundred thirty (630) angstroms. The combination of such materials may produce photomask  12  with approximately six percent transmission and a phase shift of approximately 180°±5° at an exposure wavelength of approximately 193 nanometers.  
     [0060] Focused ion beam (FIB)  40  may be used in a repair tool (not expressly shown) to remove defect  34 . Buffer layer  30  can be used to protect substrate  16  from gallium contamination. The required thickness of buffer layer  30  may be a function of the material used to form layer  30  and the associated FIB process. The thickness of layer  30  may be proportional to the acceleration voltage and the overall dose per pixel of FIB  40 .  
     [0061] During the repair process, silicon ions  42  may be produced as FIB  40  removes defect  34 . As illustrated in FIG. 3B, silicon ions  42  may not be present when defect  34  has been completely removed. The repair tool (not expressly shown), may monitor the concentration of silicon ions  42  to determine an end point for the repair process. When no silicon ions  42  are present, the repair tool may determine that defect  34  has been completely removed. Buffer layer  30 , therefore, provides a technique for determining the endpoint of the repair process in order to minimize possible damage to substrate  16  caused by FIB  40 . Furthermore, buffer layer  30  protects substrate  16  during the repair process because FIB  40  damages buffer layer  30  instead of substrate  16 . See for example defect  48  in buffer layer  30 .  
     [0062] Optical Properties  
     [0063] DLC  
       n (193)=1.757  k (193)=0.318  
     [0064] SiNTiN  
       n (193)=2.356  k (193)=0.5  
     [0065] Using the following Equations:  
     Phase=(2  Pi /λ)×Thickness×( n   Material −1)  
       T   s ≈(1 −R ) 2 exp(−4 πk   s   d   s /λ)  
     [0066] Although the present invention has been described in detail, it should be understood that various changes, substitutions, and alterations can be made without departing from the spirit and scope of the invention.