Abstract:
A photolithography mask is disclosed. The mask comprises a pattern layer that is selectively formed on a substrate in a photomask pattern. Next, a multilayer stack is formed on the pattern layer and the substrate. The multilayer stack is comprised of a plurality of pairs of thin films. Finally, an absorptive layer is disposed in trenches formed within the multilayer stack. The absorptive layer is absorptive of an EUV illuminating radiation. Further, the trenches are located substantially over the borders between the pattern layer and the substrate.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    The present invention relates to photomasks, and more particularly, to an alternative phase shift mask (APSM) having a damascene structure formed for use with extreme ultraviolet lithography (EUVL).  
         BACKGROUND OF THE INVENTION  
         [0002]    Photolithography is a common step used in the manufacture of integrated circuits. In photolithography, a photomask is placed above the wafer. The photomask (also known as a reticle) contains the pattern that is to be replicated onto the wafer. Illuminating radiation is then projected onto the photomask.  
           [0003]    In the case of a transmissive photomask, the mask pattern is created by transmissive portions and absorbing portions arranged in the pattern on the mask. A selected wavelength, for example, 248 nanometers (nm), of irradiating radiation is shined through the mask. The transmissive portions of the mask, which are transparent to the selective wavelength, allow the light to pass through the mask. The absorbing portions, which are opaque to and absorb the selected wavelength, block the transmission. The pattern on the mask is thereby replicated onto the photoresist on the device wafer.  
           [0004]    In another type of photomask, known as a reflective mask, the photomask surface contains reflective portions and absorbing portions. When light of a selected wavelength is applied to the photomask, the light is reflected off the reflecting portions. The reflected image from the mask usually is further reflected off of a mirror or lens system, then onto the wafer.  
           [0005]    Reflective photomasks are used when the illuminating radiation is in the EUV range. Patterning of the transmission mask using deep UV radiation, such as 193 nm wavelength, and vacuum UV radiation, such as 157 nm wavelength, are all currently being developed. Because EUV radiation is strongly absorbed by condensed matter, such as quartz, a reflective photomask is commonly used for EUVL.  
           [0006]    Another method of increasing the resolution of a photolithography system is to combine alternative phase shift mask (APSM) technology with a EUVL reflective photomask. In this method, selected portions of a photomask are manufactured to introduce a 180 degree phase shift in the reflected light. Thus, the reflected light from the phase shifted portions of the photomask will destructively interfere with the reflected light from the non-phase shifted portions. This destructive interference intensity pattern can be used to pattern the photoresist on a wafer. This technology is described in U.S. Pat. No. 5,328,784 to Fukuda and in “Optical Technology for EUV Lithography” by Ito et al., Optical Society of America, TOPS on Extreme Ultraviolet Lithography, Vol. 4 (1996).  
           [0007]    In the prior art reflective APSM, referring to FIG. 1, the APSM  101  includes a substrate  103  that has various layers formed thereon. First, a phase shifting pattern  105  is deposited onto the substrate  103 . The phase shifting pattern has a thickness of approximately ¼ of the illuminating radiation wavelength, i.e., λ/4. Next, a multilayer stack  107  comprising alternating thin film layers of molybdenum (Mo) and silicon (Si) is deposited. Typically, the multilayer stack  103  consists of 40 pairs of Mo/Si thin films, each pair of thin films approximately 7 nm in thickness. The multilayer stack  103  will reflect EUV radiation. Formed atop of the multilayer stack  103  is a patterned absorptive metal layer  109 . The patterned absorptive metal layer  109  covers the transitions between areas of the substrate  103  that have the phase shift patterns  105  and those that do not. By varying the widths of the absorptive metal layer  109 , features having different sizes can be patterned.  
           [0008]    This prior art photomask has some disadvantages. First, the photomask  101  of FIG. 1 introduces a shadowing effect. In EUVL, the incident radiation comes at an angle from normal due to the nature of a reflective mask. The combination of oblique illumination with a non-zero height of the absorptive metal layer  109  causes a shadowing effect, which needs to be corrected by adjusting the size of the photomask features. Typically, the photomask is biased toward a smaller dimension in order to compensate for the shadowing effect. As EUVL technology extends to smaller design rules, the biasing requirement may place a limitation on EUVL mask fabrication. Further, the prior art photomask  101  is not planar, leading to possible damage during cleaning of the surface of the photomask. Other disadvantages of the prior art photomask  101  will become apparent as the detailed description of the present invention is reviewed.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The invention is best understood by reference to the figures wherein references with like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements.  
         [0010]    [0010]FIG. 1 is a prior art reflective EUVL alternative phase shift mask (APSM).  
         [0011]    FIGS.  2 - 7  are cross sectional views illustrating a method for forming an APSM reflective EUVL photomask in accordance with the present invention.  
         [0012]    FIGS.  8 - 9  are cross sectional views of an alternative embodiment of the present invention using a contrast layer.  
         [0013]    FIGS.  10 - 13  are cross sectional views illustrating a method for forming an EUVL reflective APSM in accordance with an alternate embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]    In accordance with the present invention, a method for forming an extreme ultraviolet lithography (EUVL) reflective alternative phase shift mask (APSM) is disclosed. In the following description of the preferred embodiments, numerous specific details are provided to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, operations are not shown or described in detail to avoid obscuring aspects of the invention.  
         [0015]    Reference throughout this specification to “one embodiment”, “an embodiment”, or “preferred embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrase “in one embodiment”, “in an embodiment”, or “in a preferred embodiment” in various places throughout the specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristic may be combined in any suitable manner in one or more embodiments.  
         [0016]    Referring to FIG. 2, a substrate  201  is provided upon which a pattern layer  203  is deposited. In one embodiment, the substrate  201  may be silicon and the pattern layer  203  may be silicon dioxide. However, a host of other materials may be used for both structures, insofar as the purpose of the substrate  201  is primarily to provide a base upon which the reflective and absorptive portions of the photomask may be formed. Therefore, the substrate  201  may be formed from materials, such as quartz, ultra-low expansion titanium silicate glass (ULE), and zerodur which exhibit good thermal stability.  
         [0017]    Similarly, as will be seen below, the primary purpose of the pattern layer  203  is to provide raised areas that will cause a 180 degree phase shift in reflected illuminating radiation. Thus, the pattern layer  203  may be formed from several types of material. Some considerations for selecting the material comprising the pattern layer  203  are that (1) the material should be capable of being easily and uniformly deposited over the substrate  201  with near zero defect and (2) the material should have good etching selectivity to the substrate  201 . If the substrate is silicon or silicon dioxide, the pattern layer  203  may be formed, for example, from carbon. The advantage of using carbon as the pattern layer  203  is that optical inspection may be used to determine if defects in etching of the pattern layer  203  are present.  
         [0018]    Generally, the pattern layer  203  should have a thickness that will cause about a 180 degree phase shift in illuminating radiation reflected from the raised areas relative to illuminating radiation reflected from unraised areas of the substrate (where pattern layer  203  is not present). It has been found that a pattern layer  203  having a thickness of λ/(4 cos θ) is appropriate for generating a 180 degree phase shift, where λ is the wavelength of the illuminating radiation and θ is the incident angle. For 13.4 nm EUV wavelength and an incident angle of 5 degrees, the pattern layer  203  should thus have a thickness of about 3.36 nm.  
         [0019]    Next, turning to FIG. 3, the pattern layer  203  is then patterned and etched to leave raised areas on the substrate  201 . It can be appreciated that the pattern layer  203  shown in FIG. 3 is merely illustrative and that in actual practice, pattern layer  203  is typically a complicated network that defines the pattern of the photomask. Thus, pattern layer  203  is formed throughout the photomask in the desired photomask pattern.  
         [0020]    Next, turning to FIG. 4, a multilayer stack  205  is deposited over the substrate  201  and the remaining portions of the pattern layer  203 . In one embodiment, 40 pairs of molybdenum/silicon thin films comprise multilayer stack  205 . However, other materials may be used to form the pairs of thin films and the present invention should not be limited to molybdenum/silicon thin films. For example, molybdenum/beryllium, niobium/beryllium, ruthenium/beryllium, rhodium/beryllium, or silicon/ruthenium thin film pairs may be used. Further, the pairs of thin films may include an interlayer between the first and second films to increase thermal stability and to prevent interdiffusion. The interlayer may be, for example, carbon.  
         [0021]    Typically, each pair of molybdenum/silicon thin films is approximately 7 nm (or 70 angstroms) thick. Using known physical relationships, it has been found theoretically that 40 pairs (or 280 nm thickness) of molybdenum/silicon thin films will provide nearly 75% reflectivity for wavelengths in the EUV band (e.g., 13.4 nm).  
         [0022]    Still referring to FIG. 4, a buffer layer  207  is then deposited over the multilayer stack  205 . As will be seen below, the buffer layer  207  will be used as an etch stop. The buffer layer  207  should be of a material that has good etching selectivity between the buffer material and molybdenum/silicon. For example, carbon may be used. Nevertheless, other materials, such as oxide, chromium, or ruthenium may be used as the buffer layer. In an alternative embodiment, the buffer layer  207  is omitted.  
         [0023]    Still referring to FIG. 4, in accordance with the present invention, an additional number of pairs of molybdenum/silicon thin films are deposited over the buffer layer  207 . In one embodiment, 10 pairs of thin films are deposited, resulting in an additional 70 nm in thickness. These 10 pairs of molybdenum/silicon thin films are referred to herein as supplemental multilayer stack  209 .  
         [0024]    Nevertheless, as noted above, other types of reflective thin film combinations may be used for the supplemental multilayer stack  209 . Indeed, the thin film combination used for the supplemental multilayer stack  209  may be different from the thin film combination used for the multilayer stack  205 . As will be seen below, trenches will be formed in the stack of thin films.  
         [0025]    Next, still referring to FIG. 4, a planarizing cap layer  211  is deposited over the supplemental multilayer stack  209 . The cap layer  211  is preferably on the order of 40 to 120 angstroms thick. In one embodiment, the cap layer  211  is formed from silicon. Alternatively, the cap layer  211  may be formed from other materials, such as ruthenium.  
         [0026]    Next, referring to FIG. 5, trenches  501  are formed in the cap silicon layer  211  and supplemental multilayer stack  209 . The trenches  501  may be formed using conventional patterning and etching processes. The buffer layer  207  is used as an etch stop layer, and thus, the trenches  501  extend down through the entire supplemental multilayer stack  209  to the buffer layer  207 . Further, generally, the trenches  501  are formed to be substantially over the borders between the pattern layer  203  and the substrate  201 .  
         [0027]    Although not required, the buffer layer  207  is preferably formed to a thickness dependent upon the wavelength of the illuminating radiation, the real portion of the index of refraction of the etching stop layer (n), and the angle of incidence of the illuminating radiation (θ) by the following relationship: 
         Thickness= mλ/ (2 n  cos θ) 
         [0028]    where m is an integer.  
         [0029]    Thus, for an angle of incidence of 5 degrees, an exposure wavelength of 134 angstroms, using oxide as the buffer layer having a real index of refraction of 0.9735, the optimal thickness is about 7 nm. For a ruthenium buffer layer, the optimal thickness is still approximately 7 nm. The buffer layer  207  can be formed from an oxide, carbon, chromium, ruthenium, or other materials.  
         [0030]    The formula given above is valid for a multilayer stacks  205  and  209  that has a substantially uniform periodicity. Periodicity refers to a consistent pattern of thin film thickness for the molybdenum/silicon thin film pairs. In one example, this results in pairs of 2.8 nm molybdenum thin film and 4.2 nm silicon thin film. For other types of materials forming the thin film pairs, it can be appreciated that other thicknesses are used. In any event, a uniform periodicity refers to having consistent thicknesses in the thin film pairs throughout the multilayer stacks  205  and  209 .  
         [0031]    In broader terms, the thickness of the buffer layer, including for any overages or underages in thickness relative to uniform periodicity in the thin film layers directly adjoining the buffer layer, should have an optical path that is a multiple of 2π. Thus, as an example, assume that the topmost thin film layer in the multilayer stack  205  is 5.2 nm, instead of the nominal 4.2 nm thickness. In such a situation, 1.0 nm of this thickness should be accounted for as part of the buffer layer in calculating its optical path.  
         [0032]    The buffer layer  207  serves at least two functions. First, when the trenches  501  are formed in the supplemental multilayer stack  209 , the precise depth of the trenches  501  can be uniformly controlled. Second, if an error is made in the etching process of the trenches  501  is discovered, the supplemental multilayer stack  209  (and cap layer  211 ) can be stripped away and a new supplemental multilayer stack  209  can be formed on the multilayer stack  205 . Thus, errors in patterning can be corrected without destroying an expensive mask blank. Conceivably, mask blanks may be even reused.  
         [0033]    It can be appreciated that the trenches  501  shown in FIG. 5 are merely illustrative and that in actual practice, trenches  501  are typically a complicated network that defines the pattern of the photomask. Thus, trenches  501  are formed throughout the photomask in the desired photomask pattern.  
         [0034]    Next, turning to FIG. 6, the trenches  501  are filled with a metal layer  601 , typically using a blanket sputter process. Alternatively, physical vapor or chemical vapor deposition may also be used. The metal layer  601  may be, for example, tantalum nitride, tungsten, copper, chrome, tantalum, tantalum nitride, aluminum, germanium, or silicon germanium. It has been found preliminarily that germanium or aluminum provide a preferred level of performance as the metal layer  601 .  
         [0035]    Indeed, any material that is generally absorptive of EUVL radiation may be used. However, it should be noted that the more absorptive the material used, the lower the thickness of the supplemental multilayer stack  209  need be.  
         [0036]    Still referring to FIG. 6, the portion of the metal layer  601  that is outside trenches  501  is removed. Typically, this is accomplished using a chemical mechanical polishing (CMP) process, using the cap layer  211  as a polish stop. Thus, it is preferred that the cap layer  211  be formed from a material that has good polishing selectivity to the metal layer  601 . Still alternatively, the portion of the metal layer  601  that is outside trenches  501  is removed using an etch back process.  
         [0037]    Finally, turning to FIG. 7, as an optional step, a thin amorphous silicon layer  701  is deposited over the cap layer  211  and the metal layer  601  within the trenches  501 . The thickness of the amorphous silicon layer  701  is preferably a few angstroms, in the range of 40 to 120 angstroms.  
         [0038]    Several advantages of the photomask of FIG. 7 can be seen. First, because the incident EUV radiation is reflected by the supplemental multilayer stack  209 , which is at substantially the same planar level as the metal layer  601 , the shadowing effect is nearly nonexistent.  
         [0039]    Second, because the multilayer stack  203  and the supplemental multilayer stack  301  are conductive, the overall conductivity of the photomask is increased, which facilitates protection of the photomask from particle contamination.  
         [0040]    Third, the photomask design of the present invention is amenable to optical inspection and focused ion beam (FIB) repair technology for opaque etched defects. Other known techniques for repairing clear defects may also be used. For example, one method is described in U.S. Pat. No. 5,935,737 to Yan and assigned to the same assignee as the present invention.  
         [0041]    Fourth, because the surface of the photomask is substantially flat, it can be easily cleaned, when compared to prior art EUVL reflective alternative phase shift photomasks. Moreover, because of the cap layer  211  and the optional amorphous silicon layer  701 , cleaning processes will not damage the underlying supplemental multilayer stack  209  or the metal layer  601 .  
         [0042]    The present invention can be modified to aid in the inspection and repair of the photomask during its manufacture. In particular, turning to FIG. 8, in addition to the multilayer stack  205 , the buffer layer  207 , the supplemental multilayer stack  209 , and the cap layer  211  being deposited, a contrasting layer  801  is also deposited. The contrasting layer  801  is a material that can provide good contrast between the etched and unetched regions (for forming trenches  501 ) using optical inspection techniques. In one embodiment, the contrast layer  801  can be formed from carbon, assuming the underlying cap layer is made from silicon dioxide, or other material that contrasts well with carbon. Titanium nitride, tantalum nitride, or chromium may also be used as a contrast layer  801 .  
         [0043]    After patterning and etching of the photomask, optical inspection can be conducted because a high optical contrast can be obtained between the etched region and the unetched region covered by the contrast layer  801 . In FIG. 9, two types of defects are shown: a protrusion defect  901  and an intrusion defect  903 . A protrusion defect is an area of the photomask that should be etched, but has not been etched. An intrusion defect is an area of the photomask that should not be etched, but has been etched. A protrusion defect can be repaired using conventional focused ion beam (FIB) technology to remove the protrusion defect  803 . An intrusion defect  903  should be avoided in the patterning process by using other known techniques. For example, one method is described in U.S. Pat. No. 5,935,737 to Yan and assigned to the same assignee as the present invention. However, the use of the contrast layer  801  is useful for identifying the defects using optical techniques.  
         [0044]    The remaining steps shown in FIGS.  6 - 7  may then be carried out after the defects are removed. Thus, the metal layer  601  is deposited over the photomask and a metal CMP process is performed. The contrasting layer  801 , in one embodiment carbon, may also serve as a CMP stop layer. After the CMP process is stopped on the contrast layer  801 , the surface of the metal layer  601  is significantly more planar. This in turn insures increased uniformity control in the next polishing step to remove the contrast layer  801 . In this example, the cap layer  211  is used as the stop layer. Any carbon residue can be removed via oxygen plasma etching.  
         [0045]    Alternatively, the contrast layer  801  may be removed by a conventional lift-off process. In this case, the thickness of the contrast layer is preferably less than 20 nanometers. Although this method results in a surface that is not as planar as obtained using a CMP process, this technique is still more planar than a photomask fabricated using a subtractive metal technique. The advantage of removing contrast layer  801  by a lift-off process is to retain a good film uniformity of the capping layer  211 . A wet or dry etch process usually can achieve higher selectivity to the silicon capping layer as compared to the CMP process.  
         [0046]    Still alternatively, before removing the contrast layer by lift-off process, a blanket etch of the metal layer  601  may be performed so as to recess the metal layer  601  under the capping layer  211  or the supplemental multilayer stack  209 . The recess depth can be in a range of 0-30 nm. The contrast layer  801  is then removed using a lift-off process by either a dry or wet etch. For certain metals forming the metal layer  601 , the recessed metal layer has performance advantages.  
         [0047]    Finally, like the embodiment described above, a thin amorphous silicon coating may be placed over the photomask as an optional step. Another advantage of using the contrast layer  801  is that the contract layer  801  can be used as an “etching test layer”. Thus, any errors in the etching pattern to be imprinted onto the photomask can be determined by first etching the contrast layer  801 . If errors are found, then the contrast layer  801  can be repaired. Then, the contrast layer  801  can be used as a hard mask to etch the underlying supplemental multilayer stack  209 .  
         [0048]    Turning next to FIGS.  10 - 13 , an alternative embodiment of the present invention is shown. In this embodiment, instead of using a pattern layer  203  to raise portions of the multilayer stack  205 , the underlying substate is etched so as to leave recessed regions that will lower portions of the multilayer stack  205 .  
         [0049]    Turning to FIG. 10, a substrate  1001  is provided. The substrate is patterned and etched to provide recesses  1003 . In one embodiment, the substrate  1001  may be silicon or an oxide. As noted above, a host of other materials may be used for both structures, insofar as the purpose of the substrate  1001  is primarily to provide a base upon which the reflective and absorptive portions of the photomask may be formed. Therefore, the substrate  1001  may be formed from materials, such as quartz, ultra-low expansion titanium silicate glass (ULE), and zerodur which exhibits—good thermal stability.  
         [0050]    As will be seen below, the primary purpose of the recesses  1003  is to provide lowered areas that will cause a phase shift in reflected illuminating radiation. It can be appreciated that the recess  1003  shown in FIG. 10 are merely illustrative and that in actual practice, recess  1003  is typically a complicated network that defines the pattern of the photomask. Thus, recesses  1003  are formed throughout the photomask in the desired photomask pattern. One disadvantage of the embodiment of FIGS.  10 - 13  compared to the embodiment of FIGS.  2 - 7  is that optical inspection may not be used to determine if defects in etching of the recesses  1003  are present. This is because there is little to no optical difference between the recesses  1003  and the substrate  1001 .  
         [0051]    Generally, recesses  1003  should have a depth that will cause about a 180 degree phase shift in illuminating radiation reflected from the lowered areas relative to illuminating radiation reflected from unlowered areas of the substrate (where recesses  1003  are not present). It has been found that recesses  1003  having a depth of λ/(4 cos θ) is appropriate for generating a 180 degree phase shift, where λ is the wavelength of the illuminating radiation and θ is the incident angle. For 13.4 nm EUV wavelength and 5 degrees incident angle, the recesses  1003  should thus have a depth of about 3.36 nm.  
         [0052]    Next, still referring to FIG. 10, a multilayer stack  1005  is deposited over the substrate  1001  and into recesses  1003 . In one embodiment, 40 pairs of molybdenum/silicon thin films comprise multilayer stack  1005 . However, other materials may be used to form the pairs of thin films and the present invention should not be limited to molybdenum/silicon thin films. For example, molybdenum/beryllium, niobium/beryllium, ruthenium/beryllium, rhodium/beryllium, or silicon/ruthenium thin film pairs may be used. Further, the pairs of thin films may include an interlayer between the first and second films to increase thermal stability and to prevent interdiffusion. The interlayer may be, for example, carbon.  
         [0053]    Typically, each pair of molybdenum/silicon thin films is approximately 7 nm (or 70 angstroms) thick. Using known physical relationships, it has been found theoretically that 40 pairs (or 280 nm thickness) of molybdenum/silicon thin films will provide nearly 75% reflectivity for wavelengths in the EUV band (e.g., 13.4 nm).  
         [0054]    Still referring to FIG. 10, a buffer layer  1007  is then deposited over the multilayer stack  1005 . As will be seen below, the buffer layer  1007  will be used as an etch stop. The buffer layer  1007  should be of a material that has good etching selectivity between the buffer material and molybdenum/silicon. For example, carbon may be used. Nevertheless, other materials, such as silicon oxide, chromium, or ruthenium may be used as the buffer layer. In an alternative embodiment, the buffer layer  207  is omitted.  
         [0055]    Still referring to FIG. 10, in accordance with the present invention, an additional number of pairs of molybdenum/silicon thin films are deposited over the buffer layer  1007 . In one embodiment, 10 pairs of thin films are deposited, resulting in an additional 70 nm in thickness. These 10 pairs of molybdenum/silicon thin films are referred to herein as supplemental multilayer stack  1009 .  
         [0056]    Nevertheless, as noted above, other types of reflective thin film combinations may be used for the supplemental multilayer stack  1009 . Indeed, the thin film combination used for the supplemental multilayer stack  1009  may be different from the thin film combination used for the multilayer stack  1005 . As will be seen below, trenches will be formed in the stack of thin films.  
         [0057]    Next, a planarizing cap layer  1011  is deposited over the supplemental multilayer stack  1009 . The cap layer  1011  is preferably on the order of 40 to 120 angstroms thick. In one embodiment, the cap layer  1011  is formed from silicon. Alternatively, the cap layer  1011  may be formed from other materials, such as ruthenium.  
         [0058]    Next, referring to FIG. 11, trenches  1101  are formed in the cap layer  1011  and supplemental multilayer stack  1009 . The trenches  1101  may be formed using conventional patterning and etching processes. The buffer layer  1007  is used as an etch stop layer, and thus, the trenches  1101  extend down through the entire supplemental multilayer stack  1009  to the buffer layer  1007 .  
         [0059]    Although not required, the buffer layer  1007  is preferably formed to a thickness dependent upon the wavelength of the illuminating radiation, the real portion of the index of refraction of the etching stop layer (n), and the angle of incidence of the illuminating radiation (θ) by the following relationship: 
         Thickness= mλ/ (2 n  cos θ) 
         [0060]    where m is an integer.  
         [0061]    Thus, for an angle of incidence of 5 degrees, an exposure wavelength of 134 angstroms, using oxide as the buffer layer having a real index of refraction of 0.9735, the optimal thickness is about 7 nm. For a ruthenium buffer layer, the optimal thickness is still approximately 7 nm. The buffer layer  1007  can be formed from an oxide, carbon, chromium, ruthenium, or other materials.  
         [0062]    The formula given above is valid for a multilayer stacks  1005  and  1009  that has a substantially uniform periodicity. Periodicity refers to a consistent pattern of thin film thickness for the molybdenum/silicon thin film pairs. In one example, this results in pairs of 2.8 nm molybdenum thin film and 4.2 nm silicon thin film. For other types of materials forming the thin film pairs, it can be appreciated that other thicknesses are used. In any event, a uniform periodicity refers to having consistent thicknesses in the thin film pairs throughout the multilayer stacks  1005  and  1009 .  
         [0063]    In broader terms, the thickness of the buffer layer, including for any overages or underages in thickness relative to uniform periodicity in the thin film layers directly adjoining the buffer layer, should have an optical path that is a multiple of 2π. Thus, as an example, assume that the topmost thin film layer in the multilayer stack  1005  is 5.2 nm, instead of the nominal 4.2 nm thickness. In such a situation, 1.0 nm of this thickness should be accounted for as part of the buffer layer in calculating its optical path.  
         [0064]    The buffer layer  1007  serves at least two functions. First, when the trenches  1101  are formed in the supplemental multilayer stack  1009 , the precise depth of the trenches  1101  can be uniformly controlled. Second, if an error is made in the etching process of the trenches  1101  is discovered, the supplemental multilayer stack  1009  (and cap layer  1011 ) can be stripped away and a new supplemental multilayer stack  1009  can be formed on the multilayer stack  1005 . Thus, errors in patterning can be corrected without destroying an expensive mask blank. Conceivably, mask blanks may be even reused.  
         [0065]    It can be appreciated that the trenches  1101  shown in FIG. 11 are merely illustrative and that in actual practice, trenches  1101  are typically a complicated network that defines the pattern of the photomask. Thus, trenches  1101  are formed throughout the photomask in the desired photomask pattern. Further, generally, the trenches  1101  are formed to be substantially over the borders between the recesses  1003  and the substrate  1001 .  
         [0066]    Next, turning to FIG. 12, the trenches  1101  are filled with a metal layer  1201 , typically using a blanket sputter process. Alternatively, physical vapor or chemical vapor deposition may also be used. The metal layer  1201  may be, for example, tantalum nitride, tungsten, copper, chrome, tantalum, tantalum nitride, aluminum, germanium, or silicon germanium. It has been found preliminarily that germanium or aluminum provide a preferred level of performance as the metal layer  1201 .  
         [0067]    Indeed, any material that is generally absorptive of EUVL radiation may be used. However, it should be noted that the more absorptive the material used, the lower the thickness of the supplemental multilayer stack  1009  need be.  
         [0068]    Still referring to FIG. 12, the portion of the metal layer  1201  that is outside trenches  1101  is removed. Typically, this is accomplished using a chemical mechanical polishing (CMP) process, using the cap layer  1011  as a polish stop. Thus, it is preferred that the cap layer  211  be formed from a material that has good polishing selectivity to the metal layer  1201 . Still alternatively, the portion of the metal layer  1201  that is outside trenches  1101  is removed using an etch back process.  
         [0069]    Finally, turning to FIG. 13, as an optional step, a thin amorphous silicon layer  1301  is deposited over the cap layer  1011  and the metal layer  1201  within the trenches  1101 . The thickness of the amorphous silicon layer  1301  is preferably a few angstroms, in the range of 40 to 120 angstroms.  
         [0070]    While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the art will recognize. These modifications can be made to the invention in radiation of the detailed description. The terms used in the following claims should not be construed to limit the invention to specific embodiments disclosed in the specification and the claims. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.