Abstract:
A fused silica pellicle for use on photomasks having increased durability and improved transmission uniformity and birefringence properties. The pellicle may be secured to the photomask using an adhesive or a slide rail system, or may be held in place using a static charge.

Description:
BACKGROUND OF INVENTION 
     Photomasks, also called masks, are used in the semiconductor industry to transfer micro-scale images defining a semiconductor circuit onto a silicon or gallium arsenide wafer. In general, a photomask is comprised of a transparent substrate to which a masking material layer is affixed and patterned. The pattern of the masking material is a scaled master of the image desired to be formed on the semiconductor wafer. 
     The transfer the photomask image to the semiconductor wafer occurs through a process commonly referred to as photolithography. More specifically, a wafer exposure system is used to interpose the photomask between a semiconductor wafer which is coated with a layer of photosensitive material and an optical energy source. Energy from the wafer exposure system is inhibited from passing through the areas of the photomask in which the masking material is present. However, energy generated by the water exposure system passes through the portions of the substrate of the photomask not covered by the masking material and causes a reaction in the photosensitive material on the semiconductor wafer. Through subsequent processing, the image created on the photosensitive material is transferred to the semiconductor wafer. 
     Since the masking image on the photomask directly correlates to the image created in the semiconductor wafer, any foreign substance or contamination on the surface of the mask during the photolithographic process will cause unwanted images of these artifacts to be printed on the semiconductor wafer. To reduce or eliminate photomask surface contamination, a thin, transparent membrane or film commonly referred to as a pellicle is stretched across an anodized aluminum frame mounted on the photomask before the photolithographic process is begun. 
     FIGS. 1A and 1B depict a top and side view of a typical photomask configured for use in the photolithographic process. As shown, photomask  2  (typically six inches by six inches in size and one-quarter inch thick) is comprised of transparent substrate  4  (e.g., fused silica) and the pattern layer of masking material  6  (e.g., chromium) defining the desired image to be created on the semiconductor wafer. Pellicle frame  8  extends around the perimeter of the patterned masking material  6  and is affixed to the substrate  4  via vapor deposition as well known in the art. Pellicle membrane  10  is stretched over and affixed to the upper surface of frame  8 . As shown, the surface of pellicle membrane  10  is generally parallel to the surface of the photomask and covers the entire patterned area of masking material  6 . Thus, any contamination which would otherwise land on the photomask instead falls on the pellicle membrane  10  staying out of the wafer exposure system focal plane. 
     Pellicle membranes known in the prior art are made of organic material such as nitocellulose or other fluorocarbon based polymers. Non-uniformities in transmission and birefringence caused by pellicle membranes result in pattern fidelity errors which become more prevalent when feature sizes patterned into the semiconductor wafer are in the sub-wavelength regime and may ultimately result in diminished device performance or failure. 
     The prior art pellicle membranes are susceptible to being scratched and torn, and any damage to the thin pellicle membrane requires the entire pellicle to be removed and replaced. Of course, during the time the pellicle membrane is being removed and replaced, the photomask cannot be used for semiconductor fabrication. Additionally, the extensive rework procedure required to remove and replace damaged pellicles sometimes results in the ultimate rejection of the entire photomask. Further, as discussed above, the pellicle membrane  10  prevents contaminants from reaching the photomask surface and therefore must be cleaned occasionally. Pellicles are typically cleaned using a nitrogen gun. However, due to their somewhat fragile nature, the prior art pellicle membranes have a propensity to break or otherwise become damaged during the cleaning process requiring their removal and replacement. Also, defects that cannot be removed with a nitrogen gun also cannot be removed mechanically for fear or scratching or tearing the membrane. Here again, during the pellicle replacement process, the photomasks cannot be used for semiconductor fabrication and there is a risk of rejection of the entire photomask 
     SUMMARY OF INVENTION 
     Accordingly, it is the object of the present invention to overcome the shortcoming of the prior art by providing a pellicle for use on a photomask having improved uniformity of transmission and birefringence thereby increasing pattern fidelity. 
     It is a further object of the present invention to provide a pellicle which is less susceptible to damage and therefore can be easily cleaned. 
     It is a further object of the present invention to provide a reusable pellicle which can be easily removed, cleaned, and re-installed on a photomask. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1A is a cross-sectional view of a prior art photomask configured for use in a photolithographic process. 
     FIG. 1B is a top-level view of a prior art photomask configured for use in a photolithographic process. 
     FIG. 2 is a cross-sectional view of a photomask configured in accordance with the present invention for use in a photolithographic process. 
     FIG. 3 is a cross-sectional view of a photomask configured in accordance with the present invention having a removable frame assembly. 
    
    
     It will be appreciated by those skilled in the art that FIGS. 1A through 3 are for illustrative purposes and therefore are not per scale. 
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2 depicts a photomask configured in accordance with the present invention. As shown, photomask  20  comprises a substantially transparent substrate  22  to which a patterned layer of masking material  24  is affixed. The pattern layer of masking material  24  represents a scaled image of the pattern desired to be created on the semiconductor wafer. As discussed above, the substrate may be comprised of fused silica and the masking material may be comprised of chromium. Those skilled in the art will understand that other materials are used to create photomasks, and that the present invention is not limited for use with photomasks having fused silica substrates and chromium masking material. Further, those skilled in the art will understand that the pellicle of the instant invention can be used in conjunction with all types of photomasks including, but not limited to, binary masks (discussed above) and phase shift masks (PSM). 
     Again with reference to FIG. 2, photomask  20  also includes a pellicle frame or ring  26  which extends around the perimeter of the patterned masking material  24 . In the preferred embodiment frame  26  is made from anodized aluminum, however, other materials may be used as well. Although shown as a continuous ring, such is not a requirement of the present invention, and frame  26  may include various gaps or vents to ensure that pressure comes to equilibrium at the end user site. Frame  26  is affixed to substrate  22  using adhesive  27 , types of which being well known in the art. 
     Pellicle  28  is comprised of a flat, polished, low birefringence slice of fused silica dimensioned to generally conform to the dimensions of frame  26 . One or more of the edges or corners  30  of the fused silica pellicle  28  may be beveled or rounded for safety reasons. The overall thickness of fused silica pellicle  28  may be varied, the only restriction being that the overall thickness of the photomask frame  26 , adhesive  27 , and pellicle  28  be such that the entire assembly fit in the wafer exposure system. Typically this would require the overall thickness of the assembly to be less than or equal to 7 mm. In general, the thicker the fused silica pellicle the more durable it will be. 
     The fused silica pellicle  28  may be affixed to the upper surface of frame  26  using adhesives which are well known in the art which may include, e.g., SAG, acrylics and SEBs. Alternatively to enhance removeability, the fused silica pellicle may to affixed to the upper surface of frame  26  using a reusable adhesive examples of which are known in the art. Additionally, pellicle  28  may be secured to the upper surface of frame  26  by means of a static charge. 
     In yet another embodiment, the pellicle may be secured to the frame using a removable frame assembly so that the pellicle can be easily removed and cleaned. For example, as shown in the cross-sectional view of FIG. 3, frame  42  made from anodized aluminum is affixed to substrate  22  by means of an adhesive, applicable types of which being well known in the art. Those skilled in the art will understand frame  42  can be made from materials other than anodized aluminum. In the preferred embodiment frame  42  extends around the entire perimeter of the patterned masking material, however, frame  42  need not be contiguous and may include one or more gaps. Frame  42  includes a first receptive area  44  which forms a shelf parallel to the surface of substrate  22  for receiving the lower surface of the outer edges of pellicle  28 . Frame  42  also includes a second receptive area or detent  46  which receives lower protrusion  52  of flexible retainer  50  which may be constructed from a variety of materials including plastics and teflon. An upper protrusion  54  of retainer  50  extends over the first receptive area  44  of frame  42  and over the upper surface of the outer edge of pellicle  28  thereby holding pellicle  28  securely in place. Accordingly, in this embodiment there is no need to for adhesive to affix the pellicle to the frame. For aid in the installation and removal of flexible retainer  50 , the corners of retainer  50  may include flexible tabs  56 . When an upward force is exerted on flexible tabs  56 , lower protrusion  52  is decoupled from second receptive area  46  of frame  42 . With lower protrusion  52  decoupled from frame  42 , retainer  50  can be removed thereby enabling pellicle  28  to be removed as well. 
     In this embodiment, no vent is necessary in frame  42  since pressure can be relieved through the gaps between frame  42 , pellicle  28 , and retainer  50 . Additionally, since no adhesive is used to secure the pellicle to the frame, the pellicle can be more readily removed, cleaned, and/or replaced. 
     Various additional modifications and improvements thereon will become readily apparent to those skilled in the art. For example, rather than be comprised of fused silica, the pellicle may be made from F-doped fused silica for 157 nm applications or Si 3 N 4  for EPL and NGL applications. Accordingly, the spirit and scope of the present invention is to be construed broadly and limited only by the appended claims, and not by the foregoing specification.