Abstract:
A method for making extreme ultraviolet lithography tool glass substrates includes generating a plasma, delivering reactants comprising a silica precursor and a titania precursor into the plasma to produce titania and silica particles, and depositing the titania and silica particles on a deposition surface to form a homogeneous titania-doped silica. The invention provides for homogeneous glass substrates that are free of striae variations and provides for beneficial extreme ultraviolet lithography reflective optics.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    The present application claims priority under 35 U.S.C. §119 to French Patent Application No. 02 07034, filed Jun. 7, 2002; and to U.S. Provisional Patent Application No. 60/392,486, filed Jun. 28, 2002, each of which is incorporated herein by reference in its entirety. 
     
    
     
       BACKGROUND OF INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The invention relates generally to projection lithographic methods and systems for producing integrated circuits and forming patterns with extremely small feature dimensions. The invention relates particularly to a method for making thermally-stable extreme ultraviolet lithography structure objects such as optical mirror element substrate structures and reflective substrate structures. The invention relates particularly to a method for producing titania-doped silica glass and use of the titania-doped glass in fabricating extreme ultraviolet lithography structure objects.  
           [0004]    2. Background Art  
           [0005]    Extreme ultraviolet lithography is emerging as one of the next-generation lithography techniques that will allow high-volume production of integrated circuits with sub-100-nm features. Extreme ultraviolet lithography as currently contemplated involves producing electromagnetic radiation at around 13 nm. The extreme ultraviolet radiation may be produced, for example, using a 1064-nm neodymium-YAG laser, which produces a xenon gas plasma, a synchrotron source, discharge pumped x-ray lasers, or an electron-beam driven radiation source. FIG. 1 shows a schematic of an extreme ultraviolet lithography imaging system  1 . As shown, a condenser system, including mirrors  2 , collects, shapes, and filters radiation from an extreme radiation source  3  to achieve a highly uniform intense beam. The beam is projected onto a mask  4  containing a pattern to be replicated on a silicon wafer  5 . The mask  4  reflects the extreme ultraviolet radiation into a reduction imaging system including an assembly of reflective mirrors  6 . The reflective mirrors  6  image the mask pattern and focus the mask pattern onto a resist-coated silicon wafer  5 . The pattern is later transferred to the silicon wafer by etching.  
           [0006]    [0006]FIG. 2 shows a cross-sectional view of a typical mask structure. As shown, the mask structure includes a substrate  8 , a reflective multilayer stack  9  formed on the substrate  8 , and an absorber  10  formed on the reflective multilayer stack  9 . Typically, the reflective multilayer stack  9  includes alternating layers of Mo and Si or Mo and Be. The absorber  10  defines the pattern to be replicated on a silicon wafer. The mask blank  8  may be made of silicon or glass or other suitable material. However, it is important that the material used for the mask blank  8  has a low coefficient of thermal expansion so that the mask blank  8  does not distort under exposure to the extreme ultraviolet radiation. It is also important that the material used for the mask blank  8  has low absorption at the exposure wavelength. Otherwise, the mask blank  8  could heat up and cause distortion and pattern placement errors at the wafer.  
           [0007]    Titania-doped silica glass can be made to have a very low coefficient of thermal expansion, i.e., lower than pure fused silica with the potential for a coefficient of thermal expansion that approximates zero. Titania-doped silica glass is commercially produced by the boule process. The boule process involves passing a mixture of a silica precursor and a titania precursor into a flame of a burner to produce titania-doped silica soot. The soot is then directed downwardly into a refractory cup at consolidation temperatures, typically 1200 to 1900° C., so as to allow the silica particles to immediately consolidate into a dense glass. The glass can be used as a mask blank at appropriate wavelengths. However, it is difficult to achieve a glass with a uniform composition using the boule process. Compositional variations in the glass would result in the glass having non-uniform thermal expansion properties. Extreme ultraviolet lithography requires very low variations in coefficient of thermal expansion within the glass, e.g., 0±5 ppb/° C. Therefore, a method for producing titania-doped silica glass that favors homogeneity in the glass is desirable.  
         SUMMARY OF INVENTION  
         [0008]    In one aspect, the invention relates to a method for making titania-doped silica which comprises generating a plasma, delivering reactants comprising a silica precursor and a titania precursor into the plasma to produce titania and silica particles, and depositing the particles on a deposition surface to form a homogeneous titania-doped silica.  
           [0009]    In another aspect, the invention relates to a method for making titania-doped silica which comprises generating a plasma, delivering reactants comprising a chlorine-free silica precursor and a chlorine-free titania precursor into the plasma to produce titania and silica particles, and depositing the particles on a deposition surface to form a homogeneous titania-doped silica.  
           [0010]    In another aspect, the invention relates to a method for forming an extreme ultraviolet lithography glass substrate which comprises generating a plasma, delivering reactants comprising a silica precursor and a titania precursor into the plasma to produce titania and silica particles, and consolidating the titania and silica particles into a homogeneous titania-doped silica glass having a dopant level in a range from 6 to 9% by weight and a homogeneous coefficient of thermal expansion in a range from +30 to −30 ppb/° C. at 20-25° C.  
           [0011]    In another aspect, the invention relates to a method for forming a lithography glass substrate which comprises generating a plasma, delivering reactants comprising a silica precursor and a titania precursor into the plasma to produce titania and silica particles, and consolidating the titania and silica particles into a homogeneous titania-doped silica glass having a dopant level in a range from 6 to 9% by weight and a homogeneous coefficient of thermal expansion in a range from +30 to −30 ppb/° C. at 20-25° C.  
           [0012]    In another aspect, the invention relates to a mask blank for extreme ultraviolet lithography produced by a method comprising generating a plasma, delivering reactants comprising a silica precursor and a titania precursor into the plasma to produce titania and silica particles, depositing the particles on a deposition surface, consolidating the particles into glass, and finishing the glass into a mask blank.  
           [0013]    Other features and advantages of the invention will be apparent from the following description and the appended claims. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0014]    [0014]FIG. 1 is a schematic of an extreme ultraviolet lithography system.  
         [0015]    [0015]FIG. 2 is a cross-sectional view of the mask shown in FIG. 1.  
         [0016]    [0016]FIG. 3 illustrates a system for producing titania-doped silica glass by plasma induction.  
         [0017]    [0017]FIG. 4 shows a distribution system for distributing a silica precursor to the injector shown in FIG. 3. 
     
    
     DETAILED DESCRIPTION  
       [0018]    Embodiments of the invention provide a method for producing a titania-doped silica glass with uniform composition using plasma induction. The glass produced by the method of the invention is striae-free and therefore avoids the striae problem seen when glass produced by the boule process is formed, i.e., ground and polished, into a curved mirror surface that cuts across the planar striae levels. The glass produced by the method of the invention has a low coefficient of thermal expansion and is therefore useful in making extreme ultraviolet lithography structure objects such as optical mirror element substrate structures and reflective mask element substrate structures. PCT patent publications WO0108163 A1 (“EUV Soft X-ray Projection Lithographic Method System and Lithography Elements” by Davis et al. of Corning Incorporated) and WO0107967 A1 (“EUV Soft X-ray Projection Lithographic Method and Mask Devices” by Davis et al. of Corning Incorporated), hereby incorporated by reference, show extreme ultraviolet lithography mirror element and mask structures. The method of the invention can also be used to produce a water-free, titania-doped silica glass, which can be used as lens material at vacuum ultraviolet wavelengths, i.e., below 157 nm.  
         [0019]    Specific embodiments of the invention will now be described with reference to the accompanying drawings. FIG. 3 illustrates a system  12  for making titania-doped silica glass by plasma induction according to one embodiment of the invention. The system  12  includes an induction plasma torch  14  mounted on a reactor  16 , e.g., a water-cooled, stainless reactor. The plasma torch  14  can be a cold cage torch made of copper or quartz. The system  12  also includes an injector  18  for injecting silica precursor  20  into a plasma flame  22 . In the illustrated embodiment, the injector  18  is inserted through a lateral side of the reactor  16  to project the silica precursor  20  into the plasma flame  22 . Alternatively, the injector  18  may be inserted through the plasma torch  14  to deposit the silica precursor  20  through the center of the plasma flame  22 . The system  12  also includes an injector  24  for injecting titania precursor  26  into the plasma flame  22 . In the illustrated embodiment, the injector  24  is inserted through a lateral side of the reactor  16  to project the titania precursor  26  into the plasma flame  22 . In alternate embodiments, the titania precursor  26  may be mixed with the silica precursor  20 , and the mixture may be delivered into the plasma flame  22  by the injector  18 . If necessary, an oxidant may also be delivered to the plasma flame  22  using any of the injectors  18  and  24 .  
         [0020]    The silica precursor  20  and titania precursor  26  may be deposited in the plasma flame  22  in vapor, liquid, or solid form. The silica precursor  20  can be any compound containing silicon, such as SiCl 4  or octamethylcyclotetrasiloxane (OMCTS). The titania precursor  26  can be any compound containing titanium, such as titanium isoproxide or TiCl 4 . However, it is usually desirable to use a silica precursor and a titania precursor that is free of chlorine because chlorine is harmful to the environment and causes absorption at low wavelengths. It is also desirable to use precursors that do not polymerize during the process. To produce a water-free silica glass, it is desirable to use a silica precursor and a titania precursor that is free of hydrogen. In a preferred embodiment, the silica precursor  20  is silica powder, and the titania precursor  26  is titania powder. The nominal grain size of the silica powder and titania powder can range from 0.1 to 300 μm. Natural or synthetic quartz may also be used as the silica precursor.  
         [0021]    [0021]FIG. 4 shows a distribution system  28  for distributing silica powder  20  to the injector  18 . The distributor system  28  includes a container  30  for holding the silica powder  20 . The container  30  is connected to the injector  18  via a feed line  31 . The container  30  is mounted on a vibrator  32  that controls the rate at which the powder  20  is supplied to the injector  18 . Gas flow  34  creates pressure in the distribution system  28 , which assists in transporting the silica powder  20  to the injector  18 . A heating ring  36  heats the container  30  so that the silica powder  20  is maintained in a dry condition. Although not shown, a similar distribution system is provided for distributing titania powder  26  to the injector  24 . The distribution system could also maintain the titania powder  26  in a dry condition. This allows an essentially water-free glass to be produced. It should be noted that different distribution systems are needed if the silica precursor and titania precursor are in liquid or vapor form.  
         [0022]    The plasma torch  14  includes a reaction tube  40  that defines a plasma production zone  42 . The reaction tube  40  may be made of high-purity silica or quartz glass to avoid contaminating the silica glass with impurities. In operation, plasma-generating gases  44  are introduced into the plasma production zone  42  through a feed duct  46 . Examples of plasma-generating gases  44  include argon, oxygen, air, and mixtures of these gases. An induction coil  48  surrounding the reaction tube  40  generates high-frequency alternating magnetic field within the plasma production zone  42  which ionizes the plasma-generating gases  44  to produce the plasma flame  22 . The induction coil  48  is connected to a high-frequency generator (not shown). Water coolers  50  are used to cool the plasma torch  14  during the plasma generation. The injectors  18  and  24  project the silica precursor  20  and titania precursor  26  into the plasma flame  22 . The silica precursor  20  and titania precursor  26  are converted to fine titania-doped silica particles in the plasma flame  22 .  
         [0023]    The titania and silica particles are deposited on a substrate  52 . Typically, the substrate  52  is made of high-purity silica. The substrate  52  is mounted on a rotating table  54 , which allows the silica particles to be deposited evenly on the substrate  52 . The rotating table  52  is located within the reactor  16 . The atmosphere in the reactor  16  is controlled and sealed from the surrounding atmosphere so that a glass that is substantially free of water can be produced. In one embodiment, the atmosphere in the reactor  16  is controlled so that a water vapor content in the reactor is less than 1 ppm by volume. This can be achieved by purging the reactor  16  with an inert gas or dry air and/or using a desiccant, such as zeolite, to absorb moisture. In one embodiment, the plasma flame  22  heats the substrate  52  to consolidation temperatures, typically 1500 to 1800° F. so that the titania-doped silica particles deposited on the substrate  52  immediately consolidate into glass  56 . In other embodiments, the titania-doped silica particles deposited on the substrate  52  may be consolidated into glass in a separate step.  
         [0024]    The plasma induction process allows uniform doping of the silica with titania prior to deposition on the substrate  52 . Preferably, the homogeneous titania-doped silica glass produced by the plasma induction process has a titania dopant level in the range from 6 to 9% by weight and a coefficient of thermal expansion (CTE) in the range from +30 ppb/° C. to −30 ppb/° C. at 20-25° C., more preferably +20 ppb/° C. to −20 ppb/° C. at 20-25° C., with a CTE variation preferably lower than 10 ppb/° C. Preferably, the titania dopant level in the titania-doped silica particles and the consolidated titania-doped silica glass is in the range from 6 to 8% by weight, more preferably 6.8 to 7.5% by weight, and the CTE is in the range from +10 ppb/° C. to −10 ppb/° C. at 20-25° C., with a CTE variation preferably lower than 5 ppb/° C. For extreme ultraviolet lithography substrates, the homogeneous titania-doped silica glass preferably has a titania dopant level in the range from 6% to 9% by weight and has a coefficient of thermal expansion in the range from +30 ppb/° C. to −30 ppb/° C. at 20-25° C., more preferably +20 ppb/° C. to −20 ppb/° C. at 20-25° C., more preferably +10 ppb/° C. to −10 ppb/° C. at 20-25° C., and more preferably +5 ppb/° C. to −5 ppb/° C. at 20-25° C., with a variation in CTE less than 10 ppb/° C.  
         [0025]    The titania dopant level in the glass or soot can be adjusted by changing the amount of the titania precursor  26  delivered to the plasma flame  22 . Other dopants, such as fluorinated gases and compounds capable of being converted to an oxide of B, F, Al, Ge, Sn, P, Se, Er, or S, may be delivered to the plasma flame  22  together with the silica precursor  20  and titania precursor  26 . These dopants can be deposited in the plasma flame  22  using either of the injectors  18  or  24  or a separate dopant feed. Examples of fluorinated gases include, but are not limited to, CF 4 , chlorofluorocarbons, e.g., CF x Cl 4−x , where x ranges from 1 to 3, NF 3 , SF 6 , and SiF 4 . The glass formed by the process above can be used as mask blank or lens material. Finishing of the glass may include cutting the glass into a desired shape, polishing the surfaces of the glass, and cleaning the glass.  
         [0026]    The invention provides one or more advantages. The titania-doped silica glass produced by the method of the invention has a uniform composition, a low variation in coefficient of thermal expansion, and a low CTE. Therefore, the titania-doped silica glass is suitable for use as mask blank for reflective masks used in extreme ultraviolet lithography tools. The titania-doped silica glass is also suitable as lens material for extreme ultraviolet lithography tools and for other applications operating at wavelengths of 157 mn and shorter. Water-free titania-doped silica glass can be made using the process described above. The titania-doped silica glass can be produced in one step, i.e., deposition and consolidation into glass can be achieved at the same time. Precursors can be used in liquid, gas, or solid form. There is less contamination if the plasma torch is made out of quartz.  
         [0027]    While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.