Abstract:
Spraying a surface of a reticle with carbon dioxide snow cleans the surface and removes particles. Further spraying the surface of the reticle with carbon dioxide snow at a temperature below a carbon dioxide sublimation temperature forms a solid carbon dioxide layer on the surface. The solid carbon dioxide layer prevents particles from contacting the surface of the reticle. The solid carbon dioxide layer may be removed, and the reticle may be used in a extreme ultraviolet lithography tool.

Description:
BACKGROUND 
     A microchip manufacturing process may deposit various material layers on a wafer and form a photosensitive film or photoresist on one or more deposited layers. A lithography tool may transmit light through transmissive optics or reflect light from reflective optics to a reticle or patterned mask. Light from the reticle transfers a patterned image onto the photoresist. Portions of the photoresist which are exposed to light may be removed. Portions of the wafer which are not protected by the remaining photoresist may be etched to form transistor features. 
     The semiconductor industry may continue to reduce the size of transistor features to increase transistor density and improve transistor performance. This reduction in transistor feature size has driven a reduction in the wavelength of light used in lithography tools to define smaller transistor features on a photoresist. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates an example of a lithography tool. 
         FIGS. 2A–2G  illustrate a technique for using carbon dioxide (CO 2 ) snow to clean and protect a reticle, which may be used with the lithography tool of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Extreme Ultraviolet lithography (EUVL) may use a radiation wavelength of approximately 11–15 nanometers (nm). An EUV lithography tool may print a pattern on a photoresist with dimensions which are smaller than dimensions achieved by other lithography tools. An EUV lithography tool may also be called a “lithographic exposure system,” an “EUV scanner” or an “EUV stepper.” 
       FIG. 1  illustrates an example of a lithography tool  100 , such as an Extreme Ultraviolet lithography (EUVL) tool. The lithography tool  100  may include a laser  102 , an electric discharge or laser produced plasma source  104 , condenser optics  106 , a reflective reticle  107  with a pattern, and reflective reduction optics  108 . The laser  102  may produce radiation which reflects off the reticle  107  (also called a mask) to form a patterned image on an object  110 . The object  110  may be a silicon wafer with a photoresist layer. 
     It may be difficult to keep a surface of the reticle  107  clean as the reticle  107  is made and transported to a site of the lithography tool  100 . Handling an unprotected reticle  107  may produce particle contamination and defects on the reticle surface. As the reticle  107  is installed in the lithography tool  100 , it may be difficult to protect the reticle  107  from particle contamination. A particle falling on the reticle  107  may reduce the yield for an object  110  processed by the lithography tool  100 . A reticle used in an extreme ultraviolet lithography (EUVL) tool may be especially sensitive to particle contamination because EUV lithography uses very small wavelengths (11–15 nanometers). Some reticle cleaning methods, such as a liquid cleaning process, may not be suitable for use inside the lithography tool  100 . 
     A pellicle may be designed to protect the reticle  107  from particles. But pellicle materials and adhesives may absorb extreme ultraviolet radiation and degrade after repeated exposure. The use of a removable pellicle or cover may add undesirable particle defects to the reticle  107 . 
     The present application relates to using carbon dioxide (CO 2 ) to clean and protect a reticle. A removable CO 2  protective layer may be created on the reticle. 
       FIGS. 2A–2G  illustrate a technique for using carbon dioxide (CO 2 ) snow (described herein) to clean and protect the reticle  107  of  FIG. 1 , such as an EUVL reticle.  FIG. 2A  illustrates particles  202  on a surface  200  of the reticle  107  before the reticle  107  is installed in the lithography tool  100  ( FIG. 1 ). Particles  202  may fall on the reticle  107  during fabrication (e.g., at a “mask shop”) and during handling of the reticle  107 . 
       FIG. 2B  illustrates a nozzle  204  spraying “CO 2  snow”  206  onto the surface  200  of the reticle  107  at a grazing angle (i.e., less than 90 degrees) to clean the reticle  107 . A CO 2  snow cleaning device may be obtained from Applied Surface Technologies of New Providence, N.J. 
     “CO 2  snow” may be used to clean micron and sub-micron particles from sensitive optics. CO 2  snow cleaning may remove carbon and hydrocarbon-based contamination. There may be more than one technique of CO 2  snow cleaning. CO 2  snow cleaning may include expansion of either liquid or gaseous CO 2  through an orifice inside a nozzle. This expansion may lead to nucleation of small dry ice particles and a high velocity gas carrier stream. Upon impact with a dirty surface, the dry ice media may remove micron and submicron particles and hydrocarbons by momentum transfer via a transient solvent or a freeze fracture mechanism. The CO 2  high-velocity gas may blow the contaminants away. 
     The CO 2  snow  206  in  FIG. 2B  may clean the reticle surface  200  (i.e., remove particles  202 ) before the reticle  107  is placed in the lithography tool  100 . CO 2  snow cleaning of the reticle  107  may be better than liquid cleaning because CO 2  snow cleaning may not leave a residue (e.g., chemicals) on the surface  200 . CO 2  snow cleaning of the reticle  107  may avoid scratching or eroding the surface  200 .  FIG. 2C  illustrates a cleaned reticle  107 . 
       FIG. 2D  illustrates a cleaned reticle  107  placed on a stage or mount  208  and subjected to a cooling cycle. The cooling cycle may use a temperature which is sufficiently low to prevent CO 2  in  FIG. 2E  from sublimating into a gas. 
     CO 2  may sublimate, i.e., change from a solid phase directly to a gas phase without a liquid phase, at a specific temperature. The sublimation temperature of CO 2  may be about −109.3 degrees Fahrenheit or −78.5 degrees Celsius. For example, a block of “dry ice” may sublimate from a solid to a gas. 
       FIG. 2E  illustrates a nozzle  210  spraying CO 2  snow  212  onto the surface  200  of the cooled reticle  107  at a substantially normal (90-degree) angle to form a solid CO 2  protective layer  214 . The temperature around the reticle  107  may be held sufficiently low (e.g., −109.3 degrees F. or −78.5 degrees Celsius) to maintain the solid CO 2  protective layer  214  and prevent CO 2  from sublimating into a gas. The CO 2  protective layer  214  may protect the reticle  107  from particle contamination as the reticle  107  is transported. 
       FIG. 2F  illustrates a thermoelectrically-cooled carrier  216  adapted to allow the reticle  107  and its CO 2  protective layer  214  to be transported to a site of the lithography tool  100 . The carrier  216  may maintain a temperature around the reticle  107  to prevent the CO 2  protective layer  214  from sublimating into a gas. After the carrier  216  and reticle  107  are transported to a site with the lithography tool  100 , the carrier  216  may be removed. 
       FIG. 2G  illustrates the reticle  107  with the CO 2  protective layer  214  on a stage  218  in the lithography tool  100 . Before being used by the lithography tool  100 , the reticle  107  may be warmed to a selected temperature (e.g., room temperature or a temperature above −109.3 degrees F. or −78.5 degrees Celsius) to sublimate the CO 2  protective layer  214 . CO 2  may not leave a residue on the reticle  107  and may not damage the reticle  107 . 
     A nozzle  220  may spray CO 2  or some other gas  222  at a grazing angle to clean the surface  200  of the reticle  107 . 
       FIG. 2G  illustrates the reticle  107  on the stage  218  in the lithography tool  100 . The reticle  107  is cleaned and ready to be used for lithography. 
     In an embodiment, the techniques described above may form a CO 2  protective layer on a reticle, and then a pellicle may be placed over the reticle. The pellicle may be removable. The CO 2  protective layer may be removed inside the lithography tool  100  with or without removing the pellicle. 
     A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the application. Accordingly, other embodiments are within the scope of the following claims.