Abstract:
Exposure systems may use pellicles made of perfluoropoly-ether. These materials may exhibit reduced darkening upon repeated exposures compared to other materials.

Description:
BACKGROUND 
     This invention relates generally to fabrication of integrated circuits. 
     In the fabrication of integrated circuits, a mask is used to repeatedly illuminate a large number of wafers. An exposure system may be used to transfer a pattern on the mask repeatedly to one wafer after another. 
     To protect the mask, a frame may be positioned around the mask. Radiation that illuminates the mask may pass through a transparent pellicle attached to the frame. The frame and pellicle define a chamber that maintains a clean environment around the mask. This reduces the need to clean the mask, reducing damage to the mask from cleaning steps. 
     Existing 157 nanometer pellicles suffer from short lifetimes due to photochemical darkening induced by exposure to photons. Particularly, 157 nanometer pellicles use fluoropolymer materials which have relatively short lifetimes. One micron thick film existing materials typically transmit about 95 percent of the incident 157 nanometer radiation. This means there is an absorbence of about 0.02 per micron. The relatively high energy of the 157 nanometer photon is sufficient to break most chemical bonds. Thus, this absorbence leads to pellicle degradation, over a relatively short lifetime. 
     Thus, there is a need for better ways to form pellicles for advanced exposure systems. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of one embodiment of the present invention; and 
         FIG. 2  is a depiction of a perfluoropoly-ether that is suitable in one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , an exposure system  10  includes illuminator optics  12  that generate a radiation beam L 1  to expose a mask  14  having features thereon. The illuminator optics  12  may be any system including a system that uses wavelengths of 157 nanometers. After being exposed, the radiation beam L 2  extends through a chamber  18  defined by a frame  26  and a pellicle  20 . The radiation L 2  then passes through projection optics  22  to form the radiation L 3  that exposes a wafer  24 . 
     While an embodiment is depicted in  FIG. 1  in which the entire mask  14  is exposed simultaneously, the present invention is also applicable to systems which scan the beam L across the mask. The system  10  also includes a nitrogen purge system with a nitrogen inlet into the chamber  18  and an outlet  16 . Nitrogen purges may be used where high absorbence of oxygen and water found in the atmosphere occurs. 
     The pellicle  20  is formed using polymers based on perfluoropoly-ethers that, in liquid form, exhibit low absorbence to 157 nanometer radiation. Perfluoropoly-ethers have been shown to have high transparency and chemical inertness for use in immersion lithography (see M. Switkes and M. Rothschild, J. Vac. Sci. Technol. B 19(6), November/December 2001). 
     Typically, perfluoropoly-ethers are formulated as oils or lubricants, not as solids. At lower molecular weights, perfluoropoly-ethers are liquids, while at higher molecular weights they can be used as lubricating waxes. 
     Commercially available perfluoropoly-ethers may use straight chain polymers. Existing perfluoropoly-ethers include CF 3 —[(O—CF 2 —CF 2 ) a —(O—CF 2 ) b ]—O—CF 3 , which is commercially available as Fomblin® Z, distributed by Ausimont USA, Inc., Thorofare, N.J. Another commercially available perfluoropoly-ether, CF 3 —[(O—CF(CF 3 )—CF 2 ) a —(O—CF 2 ) b ]—O—CF 3  is sold as Fomblin® Y, also available from Ausimont. Other forms of perfluoropoly-ethers may also be applied, including CF 3 —[(O—CF 2 —CF 2 —CF 2 ) a —(O—CF 2 ) b ]—O—CF 3 , commercially available as Demnum® from Daikin America Orangeburg, N.Y. 
     The Fomblin® Z shows better stability under 157 nm. illumination, most probably due to the lack of branching CF 3  groups found in the Fomblin® Y formulation. It is postulated that the presence of the CF 3  groups in the Fomblin® Y allows for the formation of mobile CF 3  radicals that can easily move within the polymer, leaving a chemically active radical site on the main chain of the polymer. This radical site can react with other polymer sites and give unwanted chemical reaction. Thus, in general, it is believed to be desirable to provide sufficient cross-linking of the perfluoropoly-ether chains without creating sites for unwanted photochemistry. 
     Cross-linking of perfluoropoly-ether materials using functionalized Fomblin® may improve the physical properties of the fabricated membranes due to an increase in the molecular weight and more constrained molecular motion. An example of one cross-linking scheme, shown in  FIG. 2 , uses cross-linked Fomblin® Z. 
     In order to link perfluoropoly-ether chains, as shown in  FIG. 2 , functionalized Fomblin® derivatives may be used. For example, the backbone may be derived from Rf-[CF 2 —(O—C 2 F 4 )—(OCF 2 )—OCF 2 ]—Rf, where Rf are functional groups. Examples of suitable Fomblin® Z formulations include Fomblin® Z-DOL, where Rf is —CH 2 OH, Fomblin® Z DIAC, where Rf is —COOH, and Fomblin® Z DEAL, where Rf is —COOR H . Additional functional groups, Rf, that may be used include olefin, epoxide, and oxetane. 
     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.