Abstract:
A photolithography mask for use with extreme ultraviolet lithography (EUVL) irradiation is disclosed. The mask comprises a multilayer stack that is substantially reflective of said EUV irradiation, a supplemental multilayer stack formed atop the multilayer stack, and an absorber material formed in trenches patterned into the supplemental multilayer stack. The absorber material being substantially absorptive of the EUV irradiation.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    The present invention relates to photomasks, and more particularly, to a damascene structured photomask formed for use with extreme ultraviolet (EUV) light illuminating radiation.  
         BACKGROUND OF THE INVENTION  
         [0002]    Photolithography is a common step used in the manufacture of integrated circuits and is typically carried out in a tool known as a “stepper”. In photolithography, a silicon wafer substrate having a layer of film to be patterned is covered with a layer of photoresist. The wafer is then placed within a stepper onto a stage. A photomask is placed above and over the wafer. The photomask (also known as a reticle) contains the pattern that is to be replicated onto the wafer.  
           [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. Once exposed, the photoresist on the wafer is developed by rinsing in a solution that dissolves either exposed or unexposed portions of the photoresist, depending upon positive or negative tone of the photoresist, to create a pattern in the photoresist that matches the pattern of the photomask.  
           [0005]    As device integration increases, the dimensions of features in the integrated circuit devices also must shrink. Therefore, the illuminating radiation used in photolithography must have shorter and shorter wavelengths to pattern successfully in shrinking dimensions. Patterning using 193 nm and 157 nm as wavelengths are all currently being developed. These wavelengths are generically known as the deep UV range (193 nm) and vacuum UV range (157 nm). However, EUV radiation is strongly absorbed generally by condensed matter, such as quartz. Thus, a reflective photomask is commonly used for EUVL.  
           [0006]    Generally, the reflective mask consists of a multilayer stack of pairs of molybdenum and silicon thin films. The multilayer stack will reflect EUV radiation. Formed atop of the multilayer stack is a patterned absorptive metal layer. The patterned absorptive metal layer is patterned and etched from a blanket metal layer that is deposited onto the multilayer stack. This type of reflective mask is known as a subtractive metal reflective mask.  
           [0007]    Another type of reflective mask is known as a damascene reflective mask. In this type of mask, trenches are formed in a silicon base layer that is deposited atop of the multilayer. The trenches are then filled with an absorptive metal layer. One such damascene reflective mask is described in detail in U.S. Pat. No. 5,935,733 to Scott et al. and assigned to the assignee of the present invention.  
           [0008]    Specifically, turning to FIG. 1, a prior art photomask  101  is shown that includes a multilayer stack  103 , a silicon base layer  105 , a metal absorber layer  107 , and a cap silicon layer  109 . The multilayer stack  103  comprises alternating thin film layers of molybdenum (Mo) and silicon (Si). Typically, the multilayer stack  103  consists of 40 pairs of Mo/Si thin films, each pair of thin films approximately 7 nm in thickness. Next, the amorphous silicon base layer  105  is deposited onto the multilayer stack  103 . Trenches are etched into the silicon base layer  105  and an absorbing metal  107  is deposited into the trenches. Finally, a silicon capping layer  109  is deposited to protect the photomask from damage.  
           [0009]    In operation, incident EUV light  111  is reflected by the multilayer stack  103 . Incident light on to the absorbing metal layer  107  is absorbed. This prior art photomask has several disadvantages. First, the silicon layer  105  tends to attenuate the amount of EUV light  111  that is reflected by the multilayer stack  103 . For a 70 nm thickness of silicon, the attenuation is on the order of 22%. This attenuation will eventually lower the throughput of the stepper machine. In other words, a photoresist layer that may ordinarily take five seconds to expose, because of the attenuation in the silicon layer  105 , may require six to seven seconds to expose.  
           [0010]    A second disadvantage can be seen in FIG. 1 and is referred to as the shadowing effect. Because 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 metal layer  107 , a shadowing effect exists 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.  
           [0011]    Finally, for a subtractive metal reflective mask, while the absorptive metal layer is a conductive layer, its passivating silicon oxide layer is non-conducting. The non-conductive photomask may cause pinhole defects during mask transfer or handling processes due to charging. Any charge build up during handling and exposure can attract charged particles that are very difficult to remove.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    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.  
         [0013]    [0013]FIG. 1 is a prior art damascene EUVL photomask.  
         [0014]    FIGS.  2 - 7  are cross sectional views illustrating a method for forming a EUVL photomask in accordance with the present invention.  
         [0015]    [0015]FIG. 8 is a cross sectional view of a photomask during manufacture showing a protrusion defect and an intrusion defect.  
         [0016]    [0016]FIG. 9 is a cross sectional view of a photomask formed in accordance with an alternative embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]    In accordance with the present invention, a method for forming a extreme ultraviolet lithography (EUVL) photomask 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.  
         [0018]    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.  
         [0019]    Turning to FIG. 2, a EUVL mask blank  201  is provided. The EUVL mask blank consists of a plurality of pairs of molybdenum/silicon thin films. In the preferred embodiment, 40 pairs of molybdenum/silicon thin films comprise a multilayer stack  203  that forms the EUVL mask blank. 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.  
         [0020]    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).  
         [0021]    Next, turning to FIG. 3, in accordance with the present invention, an additional number of pairs of molybdenum/silicon thin films are deposited. 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  301 . However, it should be stated that whatever the terminology used, the present invention discloses the use of a stack of molybdenum/silicon thin films.  
         [0022]    Nevertheless, as noted above, other types of reflective thin film combinations may be used for the supplemental multilayer stack  301 . Indeed, the thin film combination used for the supplemental multilayer stack  301  may be different from the thin film combination used for the multilayer stack  203 . As will be seen below, trenches will be formed in the stack of thin films. Thus, unlike the prior art, the trenches are formed within the stack of thin films and not within a silicon base layer.  
         [0023]    Additionally, a silicon capping layer  303  is deposited onto the supplemental multilayer stack  301 . Preferably, the silicon capping layer  303  is on the order of 40 to 120 angstroms thick. The silicon capping layer  303  is useful in protecting the surface of the supplemental multilayer stack  301  during cleaning and other handling of the photomask.  
         [0024]    Next, referring to FIG. 4, a trench  401  is formed in the silicon capping layer  303  and the supplemental multilayer stack  301 . While in this embodiment the trench  401  does not necessarily extend into the multilayer stack  203 , the trench  401  may be of variable depth. The trench  401  is formed using conventional photoresist patterning and etching. Though not required, in this embodiment, the trench  401  extends down into the supplemental multilayer stack  301 , but not into the multilayer stack  203 . It can be appreciated that the trench  401  shown in FIG. 4 is merely illustrative and that in actual practice, trench  401  is typically a complicated network that defines the pattern of the photomask. Thus, trench  401  is formed throughout the photomask in the desired photomask pattern.  
         [0025]    Next, turning to FIG. 5, the trench  401  is filled with a metal layer  501 , typically using a blanket sputtering process. Alternative, physical vapor deposition or chemical vapor deposition may also be used. The metal layer  501  may be, for example, tantalum, tantalum nitride, tungsten, copper, chrome, aluminum, germanium, or silicon germanium. It has been found preliminarily that germanium and aluminum provide a preferred level of performance as the metal layer  501 .  
         [0026]    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  301  need be.  
         [0027]    Next, turning to FIG. 6, the portion of the metal layer  501  that is outside trench  401  is removed. Typically, this is accomplished using a chemical mechanical polishing (CMP) process, using the cap silicon layer  303  as a polishing stop. The result is shown in FIG. 6.  
         [0028]    Finally, turning to FIG. 7, as an optional step, a thin amorphous silicon layer  701  is deposited over the cap silicon layer  303  and the metal layer  501  within the trenches. The thickness of the amorphous silicon layer  701  is preferably a few nanometers, in the range of 4 to 12 nanometers. The resulting photomask is shown in FIG. 7.  
         [0029]    Several advantages of the photomask of FIG. 7 can be seen. First, unlike the prior art, there is no bulk silicon layer to act as a radiation attenuator. Instead, incident EUV radiation travels through, at most, the thin amorphous silicon layer  701  and the thin cap silicon layer  303  prior to being reflected by the supplemental multilayer stack  301 . The thickness of the silicon capping layer  303  and the thin amorphous silicon layer  701  is on the order of 10-15 nanometers, which results in significantly less attenuation than that of the prior art bulk silicon layer, which was typically on the order of 70 to 100 nm.  
         [0030]    Second, because the incident EUV radiation is reflected by the supplemental multilayer stack  301 , which is at substantially the same planar level as the metal layer  501 , the shadowing effect is nearly nonexistent. Any shadowing effect caused by the cap silicon layer  303  is negligible due to the thinness of the cap silicon layer  303 .  
         [0031]    Third, 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.  
         [0032]    Fourth, 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.  
         [0033]    Fifth, because the surface of the photomask is substantially flat, it can be easily cleaned, when compared to conventional subtractive metal processes used for photomask fabrication. Moreover, because of the cap silicon layer  303  and the optional amorphous silicon layer  701 , cleaning processes will not damage the underlying supplemental multilayer stack  301  nor the metal absorber  501 .  
         [0034]    The present invention can be modified slightly 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  203 , the supplemental multilayer stack  301 , and the cap silicon layer  303 , 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 using optical inspection techniques. In one embodiment, the contrast layer  801  can be formed from carbon. Titanium nitride, tantalum nitride, or chromium may also be used as a contrast layer  801 .  
         [0035]    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. 8, two types of defects are shown: a protrusion defect  803  and an intrusion defect  805 . 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  805  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 5 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.  
         [0036]    The remaining steps shown in FIGS.  2 - 7  may then be carried out after the defects are removed. Thus, the metal layer  501  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  501  is significantly more planar. This in turn insures increased uniformity control in the next contrast layer  801  polishing step to remove the contrast layer  801 . In this example, the cap silicon layer  303  is used as the stop layer. Any carbon residue can be removed via oxygen plasma etching.  
         [0037]    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 silicon capping layer  303 . A wet or dry etch process usually can achieve higher selectivity to the silicon capping layer as compared to the CMP process.  
         [0038]    Still alternatively, before removing the contrast layer by lift-off process, a blanket etch of the metal layer  501  may be performed so as to recess the metal layer  501  under the silicon capping layer  303  or the supplemental multilayer stack  301 . The recess depth can be in a range of 0-30 nm. The contrast layer is then removed using a lift-off process by either a dry or wet etch. For certain metals forming the metal layer  501 , the recessed metal layer has performance advantages.  
         [0039]    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 a “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  301 .  
         [0040]    Turning to FIG. 9, an alternative embodiment of the present invention is shown. In this embodiment, an etching stop layer  901  is formed between the supplemental multiplayer stack  301  and the multiplayer stack  203 . This etching stop layer  901  (also referred to herein as a buffer layer) is 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= m λ/(2 n  cos θ)  
         [0041]    where m is an integer.  
         [0042]    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 etch stop layer, the optimal thickness is still approximately 7 nm. The etching stop layer  901  can be formed from an oxide, carbon, chromium, ruthenium, or other materials.  
         [0043]    The formula given above is valid for a multilayer stacks  301  and  203  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  301  and  203 .  
         [0044]    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  203  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.  
         [0045]    The etching stop layer  901  serves at least two functions. First, when the trenches  401  are formed in the supplemental multilayer stack  301 , the precise depth of the trenches  401  can be uniformly controlled. Second, if an error is made in the etching process of the trenches  401  is discovered, the supplemental multilayer stack  301  can be stripped away and a new supplemental multilayer stack  301  can be formed on the multilayer stack  203 . Thus, errors in patterning can be corrected without destroying an expensive mask blank. Conceivably, mask blanks may be even reused.  
         [0046]    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.