Abstract:
A method for manufacturing a photomask material includes delivering a powder containing silicon dioxide into a plasma to produce silica particles and depositing the silica particles on a deposition surface to form glass.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application relates to U.S. patent application Ser. No. ______, entitled “Method for Making Photomask Material by Plasma Induction,” filed ______,in the names of Laura Ball and Sylvia Rakotoarison. 
     
    
     
       BACKGROUND OF INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The invention relates generally to methods for making fused silica. More specifically, the invention relates to a method for making a pure and water-free fused silica and use of the fused silica as photomask material.  
           [0004]    2. Background Art  
           [0005]    Photomasks are patterned substrates used in optical lithography processes for selectively exposing specific regions of a material to be patterned to radiation. FIG. 1A shows a photomask blank  1  which includes a substrate  3  made of high-purity quartz or glass. The most common type of glass used is soda line. Quartz is more expensive than soda line and is typically reserved for critical photomask applications. The substrate  3  is usually coated with a thin uniform layer of chrome or iron oxide  5 . A chemical compound  7 , known as “photo-resist,” is placed over the chrome or iron oxide layer  5 . Although not shown an anti-reflective coating may also be applied over the chrome or iron oxide layer  5  before applying the photo-resist  7 . To form the photomask, a pattern is exposed onto the photo-resist  7  using techniques such as electron beam lithography. The pattern is then etched through the chrome or iron oxide layer  5 . FIG. 1B shows a pattern etched in the chrome or iron oxide layer  5 .  
           [0006]    For production of integrated circuits, the finished photomask contains high-precision images of integrated circuits. The integrated circuit images are optically transferred onto semiconductor wafers using suitable exposure beams. The resolution of the projected image is limited by the wavelength of the exposure beam. Currently, advanced microlithography tools use 248-nm radiation (KrF) laser or 193-nm radiation (ArF) laser to print patterns with line width as small as 0.25 μm. New microlithography tools using 157-nm (F 2 ) radiation are actively under development.  
           [0007]    One of the primary challenges of developing 157-nm microlithography tools is finding suitable photomask material. Calcium fluoride is the main candidate for lens material at 157-nm but cannot be used for photomask because it has a high coefficient of thermal expansion. Other fluoride crystal materials that have large band gaps and transmit at 157 nm are MgF 2  and LiF. However, MgF 2  has a high birefringence, and the manufacturing and polishing of LiF is unknown. Fused silica is used in 248-nm and 193-nm microlithography lenses. However, the fused silica produced by current processes is not adequate for use at 157-nm because its transmission drops substantially at wavelengths below 185 nm. The drop in transmission has been attributed to the presence of residual water, ie., OH, H 2 , and H 2 O, in the glass, where the residual water is due to the hydrogen-rich atmosphere in which the glass is produced.  
           [0008]    High-purity fused silica is commonly produced by the boule process. The boule process involves passing a silica precursor into a flame of a burner to produce silica soot. The soot is then directed downwardly into a refractory cup, where it is immediately consolidated into a dense, transparent, bulk glass, commonly called a boule. This boule can be used as lens and photomask material at appropriate wavelengths. Because of environmental concerns, the silica precursor is typically a hydrogen-containing organic compound, such as octamethyltetrasiloxane (OMCTS) or silane, and the conversion flame is typically produced by burning a hydrogen-containing fuel, such as CH 4 . Halogen-based silica precursors, particularly SiCl 4 , are other types of silica precursors that can be used in the process. Flame combustion of SiCl 4  using a hydrogen-containing fuel produces toxic and environmentally-unfriendly gases such as HCl.  
           [0009]    U.S. patent application Ser. No. ______ by Laura Ball and Sylvia Rakotoarison, supra, discloses a process for making a water-free fused silica by plasma induction. The process involves injecting a silica precursor and oxygen into a plasma. The silica precursor is oxidized in the plasma to form silica particles which are deposited on a deposition surface. The deposition surface is heated to consolidation temperatures so that the silica particles immediately consolidate into glass. To make a water-free silica glass, a hydrogen-free silica precursor is used, and the process takes place in a controlled atmosphere that is substantially free of water vapor. One suitable hydrogen-free silica precursor for the process is SiCl 4 . However, oxidation of SiCl 4  produces chlorine gas, as shown by equation (1) below: 
           SiCl 4 (g)+O 2 (g)→SiO 2 (s)+Cl 2 (g)  (1) 
           [0010]    If chlorine is captured in the silica glass, the transmission for the 157-nm wavelength is decreased. In order to increase transmission of the silica glass at 157 nm, a chlorine-free precursor is desired.  
         SUMMARY OF INVENTION  
         [0011]    In one embodiment, the invention relates to a method of making fused silica which comprises generating a plasma, delivering a powder containing silicon dioxide into the plasma to produce silica particles, and depositing the silica particles on a deposition surface to form glass.  
           [0012]    In another embodiment, the invention relates to a method for manufacturing a photomask material which comprises delivering a powder comprising silicon dioxide into a plasma to produce silica particles and depositing the silica particles on a deposition surface to form glass.  
           [0013]    In another embodiment, the invention relates to a feedstock for making fused silica by plasma induction which comprises silica powder.  
           [0014]    In another embodiment, the invention relates to a feedstock for making fused silica by plasma induction which comprises quartz.  
           [0015]    In another embodiment, the invention relates to a photomask for use at 157-nm including a silica glass made by a method comprising generating a plasma, delivering a powder containing silicon dioxide into the plasma to produce silica particles, and depositing the silica particles on a deposition surface to form glass.  
           [0016]    Other features and advantages of the invention will be apparent from the following description and the appended claims. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0017]    [0017]FIG. 1A is a cross-section of a photomask blank.  
         [0018]    [0018]FIG. 1B is a cross-section of a photomask.  
         [0019]    [0019]FIG. 2 illustrates a system for producing fused silica by plasma induction using a chlorine-free precursor. 
     
    
     DETAILED DESCRIPTION  
       [0020]    Embodiments of the invention provide a method for making silica glass by plasma induction using a chlorine-free precursor. In a preferred embodiment, the chlorine-free precursor is dry silica or quartz powder. There are several sources of silica powder that can be used. The silica powder may be obtained, for example, by sol-gel synthesis, such as disclosed in European Patent A-0271281. The nominal grain size of the powder can range from 0.1 to 300 μm. Natural or synthetic quartz can be used. Because the plasma induction process is itself a purifying process, the purity of the silica can be variable. The following is a description of specific embodiments of the invention.  
         [0021]    [0021]FIG. 2 illustrates a system, generally designated by numeral  2 , for producing a chlorine-free silica glass by plasma induction. The system  2  comprises an induction plasma torch  6  mounted on a reactor  10 , e.g., a water-cooled, stainless steel reactor, and an injection system  4  for injecting a silica precursor into the plasma torch  6 . The injection system  4  includes a distributor  12  and an injector  14 . The distributor  12  includes a container l 6  which holds a dry chlorine-free silica (or quartz) powder  20 . The container  16  is connected to the injector  14  via a feed line  22 . The container  16  is mounted on a vibrator  24 , which controls the rate at which the silica powder  20  is supplied to the injector  14 . Gas flow  26  creates pressure in the distributor  12  which assists in transporting the powder  20  to the injector  14 . A heating ring  28  is provided to heat the container  16  and maintain the powder  20  in a dry condition.  
         [0022]    The plasma torch  6  includes a reaction tube  30  inside which a plasma production zone  32  is located. The reaction tube  30  may be made of high-purity silica or quartz glass to avoid contaminating the silica particles being made with impurities. The plasma production zone  32  receives plasma-generating gases  33  from a plasma-generating gas feed duct  34 . Examples of plasma-generating gases  33  include argon, oxygen, air, and mixtures of these gases. The reaction tube  30  is surrounded by an induction coil  38 , which generates the induction current necessary to sustain plasma generation in the plasma production zone  32 . The induction coil  38  is connected to a high-frequency generator (not shown).  
         [0023]    In operation, the plasma-generating gases  33  are introduced into the plasma production zone  32  from the feed duct  34 . The induction coil  38  generates high-frequency alternating magnetic field within the plasma production zone  32  which ionizes the plasma-generating gases to produce a plasma  40 . Water coolers  44  are used to cool the plasma torch  6  during the plasma generation.  
         [0024]    The injector  14  projects the powder  20  into the plasma  40 . The powder  20  is converted to fine silica particles in the plasma  40 . The silica particles are directed downwardly and deposited on a substrate  36  on a rotating table  42 . The substrate  36  is typically made of fused silica. In one embodiment, the plasma torch  6  heats the substrate  36  to consolidation temperatures, typically 1500 to 1800° F., so that the silica particles immediately consolidate into glass  48 . In other embodiments, the silica particles deposited on the substrate  36  may be consolidated into glass in a separate step.  
         [0025]    The rotating table  42  is located within the reactor  10 , and the atmosphere in the reactor  10  is sealed from the surrounding atmosphere. The atmosphere in the reactor  10  is controlled such that it is substantially free of water, e.g., the water vapor content in the atmosphere is less than 1 ppm by volume. This can be achieved, for example, by purging the reactor  10  with a dry and inert gas and using a desiccant, such as zeolite, to absorb moisture.  
         [0026]    The glass  48  can be used as photomask material for microlithography applications or other applications requiring chlorine-free glass. In alternate embodiments, the silica glass may be doped with small amounts of other elements, such as F, B, Al, Ge, Sn, Ti, P, Se, Er, Na, K, Ca and S. In FIG. 1, a dopant feed  46  is inserted through the wall of the reactor  10 . The dopant feed  46  can be used to supply the dopant materials toward or through the center of the plasma  40  at the same time that the injector  14  projects the powder  20  into the plasma  40 . Examples of dopant materials include, but are not limited to, fluorinated gases and compounds capable of being converted to an oxide of B, Al, Ge, Sn, Ti, P, Se, Er, or S. Examples of fluorinated gases include, but are not limited to, CF 4 , CF 6 , chlorofluorocarbons, e.g., CF x Cl 4−x , where x ranges from 1 to 3, NF 3 , SF 6 , SiF 4 , C 2 F 6 , and F 2 . In an alternate embodiment, a fluorine-doped silica glass can be made by doping the powder  20  with fluorine prior to injecting the powder  20  into the plasma  40 . This eliminates the use of toxic fluorinated gases in the plasma  40 .  
         [0027]    The invention provides several advantages. The chlorine-free silica glass produced by the method of the invention can be used as a photomask material for microlithography applications, particularly 157-nm microlithography applications. The chlorine-free silica glass produced by the method of the invention can also be used in other applications that are sensitive to chlorine-levels in the glass. Other applications that are not sensitive to chlorine-levels in the glass can also benefit from the invention. Using a chlorine-free silica precursor eliminates production of chlorine gas. Further, the silica glass can be produced in one step, i.e., deposition and consolidation into glass are done at the same time. For fluorine-doped glass, use of toxic fluorine gases during deposition can be eliminated by using silica precursor that already contains fluorine. The plasma induction process itself is a purification process. Therefore, the purity of the silica powder used as the silica precursor can be variable. Alternatively, natural or synthetic quartz can be used as the silica precursor.  
         [0028]    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.