Patent Document

CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation of application Ser. No. 09/651,392, filed Aug. 29, 2000, pending, which is a continuation of application Ser. No. 09/310,521, filed May 12, 1999, now U.S. Pat. No. 6,165,650, issued Dec. 26, 2000, which is a divisional of application Ser. No. 08/921,656, filed Aug. 28, 1997, now U.S. Pat. No. 5,938,860, issued Aug. 17, 1999. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    This invention relates to integrated circuit fabrication tools and processes and, more particularly, to a method and apparatus for cleaning a pellicled reticle.  
           [0003]    Integrated circuits (IC) commonly are fabricated on a semiconductor wafer. The semiconductor wafer typically is subjected to doping, deposition, etching, planarizing and lithographic processes to form semiconductor devices in the wafer. The wafer typically is cut to form multiple semiconductor “IC chips”. Each chip includes many semiconductor devices. Although the label semiconductor is used, the devices are fabricated from various materials, including electrical conductors (e.g., aluminum, tungsten), electrical semiconductors (e.g., silicon) and electrical non-conductors (e.g., silicon dioxide).  
           [0004]    A reticle is used in a lithographic process to define a photomask. A lithographic process refers to a process in which a pattern is delineated in a layer of material (e.g., photoresist) sensitive to photons, electrons or ions. The principle is similar to that of a photocamera in which an object is imaged on a photo-sensitive emulsion film. While with a photo-camera the “final product” is the printed image, the image in the semiconductor process context typically is an intermediate pattern, which defines regions where material is deposited or removed. The lithographic process typically involves multiple exposing and developing steps, wherein at a given step the photoresist is exposed to photons, electrons or ions, then developed to remove one of either the exposed or unexposed portions of photoresist. Complex patterns typically require multiple exposure and development steps.  
           [0005]    A typical lithographic system includes a light source, optical system and transparent photomask. The light source emits light through the optical system and photomask onto a photoresist layer of a semiconductor wafer. The photomask defines the “intermediate pattern” used for determining where photoresist is to be removed or left in place. Conventional photomasks are transparent masks. A photomask typically is formed on a glass blank. The mask and blank together are referred to as a reticle. Conventional materials for the blank include soda lime, borosilicate glass or fused silica. The photomask is formed by a film of opaque material. Typically, the film is formed with chrome less than 100 nm thick and covered with an anti-reflective coating, such as chrome oxide. The purpose of the anti-reflective coating is to suppress ghost images from the light reflected by the opaque material.  
           [0006]    The photomask serves to define geometries for materials deposited or etched on the wafer or materials applied to the wafer. The patterned film on the reticle blank includes mask lines and line spacings of less than 10 microns. Depending on the reduction factor x, line width and line space geometries for a resulting semiconductor device are from less than 10 microns to less than 2 microns. Other mask line spacings and semiconductor line spacings also can be achieved. When working with such small geometries, it is important that the reticle and other components in the fabrication processes be free of foreign particles. A tiny speck of dust alters the desired pattern to be imaged onto the wafer. Conventionally, a thin transparent membrane, referred to as a pellicle membrane, is applied over the photomask portion of the reticle to keep the photomask portion free of foreign particles. The pellicle membrane typically is positioned at a height above the photomask. Such height is greater than the focal length of the light imaged onto the photomask. Thus, small particles on the pellicle membrane will not block light from reaching the photomask.  
           [0007]    Another problem caused by foreign particles is bad registration of the reticle. During a lithographic process, the reticle rests on a reticle table. The reticle table typically is part of a stepper device, which also includes a light source and a stepper control. The stepper control manages the relative position of the light source and the reticle table. Even the smallest of particles on the edge of the reticle can lift a portion of the reticle off the reticle table. Such offset of the reticle can result in bad registration of the light onto the wafer, which, in turn, can result in bad overlay from one pattern to another. Because the pellicle membrane typically is very fragile, the pellicle membrane is destroyed during the course of cleaning the reticle. Conventionally, the pellicle membrane is removed, the entire surface of the reticle is cleaned, and then the reticle undergoes requalification. Such a process is very time consuming and costly. Accordingly, there is a need for an alternative method and apparatus for cleaning a reticle.  
         BRIEF SUMMARY OF THE INVENTION  
         [0008]    According to the invention, a reticle having a pellicle is cleaned without removing or damaging the pellicle. The pellicle includes a pellicle membrane and a pellicle frame. A cover encases the pellicle, sealing it from the external environment during the cleaning process.  
           [0009]    According to one aspect of the invention, the cover fits around the periphery of the pellicle frame and covers the pellicle membrane. An edge of the cover in contact with the reticle (adjacent the pellicle frame) forms a seal. In a preferred embodiment, the edge includes a groove within which is an O-ring seal.  
           [0010]    According to another aspect of the invention, the reticle is fastened to reticle supports on a spin chuck during the cleaning process. An anchor plate presses the cover to the reticle, maintaining the pellicle sealed from the external environment. The anchor plate fastens to the spin chuck. Thus, the cover and reticle are sandwiched together between the anchor plate and spin chuck.  
           [0011]    According to another aspect of the invention, a system for cleaning a reticle having a pellicle frame and pellicle membrane is provided. The reticle has a pattern formed on a first surface, the pattern occurring within a first area of the first surface. The pellicle frame is attached to the first surface, defining a border of the pattern. The pellicle membrane is attached to the pellicle frame and elevated above the pattern, the pellicle membrane sealing the first area. The reticle is secured to a support. A lid encases the pellicle frame and pellicle membrane. The lid has a first surface in contact with the first surface of the reticle. A clamp pushes the lid to the reticle.  
           [0012]    According to another aspect of the invention, the lid has a first surface in contact with the first surface of the reticle. The lid has a groove formed within the lid&#39;s first surface. An O-ring seal is within the groove. The O-ring seal is pressed into contact with the reticle by the lid under a force of the clamp. The clamp is secured to the support.  
           [0013]    According to another aspect of the invention, a fluid under pressure is ejected onto the reticle, wherein the pellicle membrane is shielded from the fluid by the lid. A drive mechanism rotates the support, altering a portion of the reticle receiving the fluid under pressure.  
           [0014]    According to another aspect of the invention, the lid and reticle serve as an apparatus for encasing the pellicle. The lid has a recessed area, which is bordered peripherally by a first wall. The first wall is adjacent to a first edge. The first edge has a seal extending around a peripheral border of the recessed area. The first wall has a height greater than a height of the pellicle frame. The lid encases the pellicle membrane and pellicle frame within the recessed area with the seal making contact with the reticle on the first surface.  
           [0015]    According to another aspect of the invention, the seal is an O-ring seal within a groove along the first edge for sealing the recessed area of the lid and the enclosed pellicle frame and pellicle membrane from an environment of the reticle.  
           [0016]    According to another aspect of the invention, a method for cleaning a reticle without damaging or removing a pellicle membrane is performed. At one step, the pellicle is covered with a lid to separate the pellicle from an external environment of an uncovered portion of the reticle. At another step, a force is applied to the lid to seal the pellicle from the external environment. At another step, the reticle is secured to a base. At another step, fluid under pressure is ejected onto the uncovered portion of the reticle to clean the uncovered portion of foreign particles.  
           [0017]    An advantage of the invention is that a reticle, which does not accurately rest on a stepper table due to foreign particles, is cleaned without removing or damaging the pellicle. An effect of this advantage is that the reticle does not need to go through an extensive process of reapplying a pellicle frame and pellicle membrane and requalifying the reticle for use in a lithographic process. These and other aspects and advantages of the invention will be better understood by reference to the following detailed description taken in conjunction with the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is a block diagram of a conventional lithographic system with photomask and wafer;  
         [0019]    [0019]FIG. 2 is a perspective view of a conventional reticle with pellicle frame and pellicle membrane;  
         [0020]    [0020]FIG. 3 is a side view of the reticle, pellicle frame and pellicle membrane of FIG. 2;  
         [0021]    [0021]FIG. 4 is a diagram of a reticle cleaning system according to an embodiment of this invention;  
         [0022]    [0022]FIG. 5 is a plan view of a spin chuck of FIG. 4 according to an embodiment of this invention;  
         [0023]    [0023]FIG. 6 is a plan view of a pellicle cover of FIG. 4 according to an embodiment of this invention;  
         [0024]    [0024]FIG. 7 is a side view of the pellicle cover of FIG. 6, along with a side view of an O-ring; and  
         [0025]    [0025]FIG. 8 is a perspective view of an anchor cap of FIG. 4 according to an embodiment of this invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]    Overview  
         [0027]    [0027]FIG. 1 shows a block diagram of a conventional lithographic system  10 . The lithographic system  10  includes a light source  12  which emits light  13  (e.g., ultraviolet light, visible light, infrared light). The light passes through a mask formed on a reticle  14 , then through an opening in a reticle table  16 , and onto a semiconductor wafer  18 . A stepper controller  20  (also known as an aligner) controls the relative positioning of the light source  12  and reticle table  16 . Typically, the light  13  serves to develop portions of photoresist applied to the semiconductor wafer  18 . The mask defines a pattern distinguishing which portions of the photoresist are developed and which are not developed.  
         [0028]    [0028]FIGS. 2 and 3 show a reticle  14 . The reticle includes a transparent plate  22  or “blank” covered with a patterned film  24  of opaque material (i.e., the photomask). Although the size may vary, an exemplary reticle  14  is 6 inches by 6 inches and 0.25 inches thick. Conventional materials for the blank include soda lime, borosilicate glass or fused silica. The film of opaque material typically is a film of chrome less than 100 nm thick and covered with an anti-reflective coating such as chrome oxide. Within an area  26 , the film  24  defines masks  28  for respective portions of the semiconductor wafer  18 . For example, in one embodiment illustrated, fifteen masks are shown. Each mask  28  within the area  26  may be the same or different, so as to make the same or different integrated circuits.  
         [0029]    Attached to the reticle is a pellicle frame  30 . In an exemplary embodiment, the pellicle frame  30  is adhered to the reticle  14  by double back tape. Other adhesives structures may be used, however. The pellicle frame  30  encloses the area  26  of the reticle having the masks  28 . Adhered to the pellicle frame  30  is a thin membrane, referred to as a pellicle membrane  32  (FIG. 3). The pellicle membrane seals the area  26  from the external environment. As described in the background section, it is desirable to avoid foreign particles on a photomask. When a reticle with masks  28  is formed, the surface is cleaned and qualified to assure that the mask is accurate and that no foreign particles are present. As part of such a qualification process, the pellicle membrane  32  is adhered to the pellicle frame  30 . The pellicle membrane  32  protects the masks  28  from foreign particles. The pellicle membrane is formed of a conventional material, such as cellulose acetate or nitrous cellulose.  
         [0030]    As shown in FIG. 1, the reticle  14  rests on a reticle table  16  during the lithographic process. The lithographic processes often require that a given reticle  14  be replaced from the reticle table  16  with another reticle having a different mask pattern. This movement of reticles on and off the reticle table  16  can cause microscopic particles to adhere to the reticle  14 . Further, reticles typically are stored in a carrying case. Microscopic particles also may adhere to the reticle from rubbing along rails of the reticle carrying case. If there are any foreign particles on the reticle in the regions  33  (see FIG. 2) where the reticle  14  is supposed to contact the reticle table  16 , then the reticle may not be seated exactly. A portion of the reticle may be higher than another portion. This can result in bad registration of the light passing through a mask  28  onto a wafer, or in bad overlay from one mask to another mask. If such a problem is detected, the reticle is removed and cleaned. Because the pellicle membrane  32  typically is very fragile, the pellicle membrane is destroyed during the course of cleaning the reticle. Conventionally, the pellicle is removed and the entire surface of the reticle is cleaned. The pellicle frame and a pellicle membrane then are reapplied, and the structures  14 ,  30 ,  32  requalified for the desired lithographic operations.  
         [0031]    [0031]FIG. 4 shows a cleaning system  40 , according to an embodiment of this invention. During cleaning, the pellicle membrane  32  and pellicle frame  30  are covered to avoid damage. A lid  42  encases the pellicle membrane  32  and pellicle frame  30 , sealing the pellicle from the external environment of the cleaning system. In one embodiment, an O-ring  44  defines the seal between the lid  42  and the reticle  14  adjacent to the pellicle frame  30 . The O-ring  44  or seal is formed from silicon or another material.  
         [0032]    During a cleaning operation, the reticle  14  is secured to a spin chuck  46 . The spin chuck  46  includes reticle supports  48 . The reticle  14  rests on the reticle supports  48 . An anchor plate  50  resides on top of the lid  42 . The anchor plate  50  is bolted to the spin chuck  46 , pressing the lid  42  to the reticle  14  to maintain the seal, and pressing the reticle to the reticle supports  48 . Thus, the reticle  14  and lid  42  are sandwiched between the anchor plate  50  and spin chuck  46 .  
         [0033]    With the lid  42  and reticle  14  secure, a rotary drive  52  rotates the spin chuck  46 . In addition, a spray source  54  ejects a fluid to clean and rinse the reticle  14 . In one embodiment, de-ionized water or another fluid is ejected as a fan spray  57  to the upper surface of the anchor plate  50 , reticle  14  and spin chuck  46  assembly and as a rinse spray  59  to a lower surface of such assembly. Then a fluid under pressure (e.g., 500 psi) is ejected as a high pressure spray  56  onto at least the exposed portions of the reticle  14  to clean away any foreign particles on the reticle  14 . The fluid ejected from the fan spray  57  and rinse spray  59  is de-ionized water in one embodiment, although other liquid or gas fluids may be used. The fluid ejected from the high pressure spray  56  is ammonium hydroxide, de-ionized water and/or another liquid or gas fluid. In one method for cleaning the reticle, the spin chuck  46  rotates at 1500 revolutions per minute during the ejection of the fluids. The high pressure spray  56  then ceases, followed by cessation of the fan spray  57  and rinse spray  59 . The spin chuck  46  then increases the rotational rate (e.g., to 2000 rpm) during a drying time period. The speeds of revolution, the pressure of the fluids emitted from sprays  56 ,  57  and  59  and the time for spraying and drying the assembly may vary.  
         [0034]    The reticle  14 , being secured to the spin chuck  46 , rotates with the spin chuck  46 . Rotation of the reticle  14  places different exposed portions of the reticle  14  in the path of the high pressure fluid spray  56 . In a preferred embodiment, the portion of the reticle  14  which is in contact with the reticle table  16  during a lithographic process is exposed during the cleaning process. Specifically, such portion is not covered by the lid  42 .  
         [0035]    [0035]FIG. 5 shows a spin chuck  46  according to an embodiment of this invention. The spin chuck  46  serves as a base to which the other components are secured. The base  46 , either with or without the reticle supports  48 , serves as a support for the reticle  14  (e.g., in one embodiment supports are integral to the base). In one embodiment, the spin chuck  46  is of sufficient area that a portion of the spin chuck  46  is exposed when the reticle  14  is secured to the spin chuck. Openings  58  occur in the exposed areas along opposite edges  60 ,  62  of the reticle  14 . Such openings receive pins  64  (see FIG. 8), which secure the anchor plate  50  to the spin chuck  46 .  
         [0036]    In various embodiments, the spin chuck  46  has different shapes (e.g., circular, square, rectangular, or other shape). In the embodiment illustrated, the spin chuck is a ring  66  with spokes  68  extending from a central portion  70 . Multiple reticle supports  48  are attached to the spin chuck  46 . In one embodiment, the reticle supports  48  are bolted to the spin chuck  46 . In another embodiment, the reticle supports  48  are integral to the rest of the spin chuck  46 . Each reticle support has a distal surface or pin  72  upon which the reticle  14  rests during cleaning. The spin chuck  46  is rotated by the rotary drive  52 .  
         [0037]    [0037]FIGS. 6 and 7 show the lid  42  for covering the pellicle frame  30  and pellicle membrane  32 . The lid is generally planar, defining two faces  73 ,  76 . One face  76  defines a generally planar exterior surface. The contour of the exterior surface  76 , however, need not be planar and may vary. The other face  73  defines a distal surface  80  and a recessed area  74 . The recessed area is delimited by an interior surface  82  and a wall  84  and a distal surface  80 . The wall  84  extends from the interior surface  82  to the distal surface  80 . When the lid  42  is applied over the pellicle onto the reticle  14 , the pellicle frame  30  and pellicle membrane  32  are enclosed within the recessed area  74 . Accordingly, the height of the wall  84  relative to the interior surface  82  is greater than a height of the pellicle frame  30 . The lid  42  includes a seal along the distal surface  80 . In one embodiment, the seal is formed by a groove  86  and an O-ring  44 . In an exemplary embodiment, the groove is 0.07 inches wide with a depth of 0.04 inches. The distal surface  80  spans a width of 0.2 inches. Such dimensions, however, vary for differing embodiments. The O-ring  44  seats within the groove  86  and extends along the entire circumference of the distal wall  80  so as to form a seal all the way around the pellicle frame  30 . In other embodiments, an alternative sealing device is used, such as a gasket. Preferably, the seal and lid  42  are formed of material which does not readily chip. The advantage of such material is the avoidance of leaving foreign particles on the reticle  14  when the lid  42  is removed from the reticle  14 . When the lid  42  is applied to the reticle  14 , the pellicle frame  30  and pellicle membrane  32  are completely encased between the lid  42  and reticle  14 . When the lid  42  is pressed to the reticle  14 , the seal isolates the pellicle membrane  32  from the environment of the cleaning system  40 , and, in particular, from the ejected fluid. As the ejected fluid would break the pellicle membrane  32 , the lid  42  prevents the pellicle membrane  32  from being damaged during the cleaning process.  
         [0038]    [0038]FIG. 8 shows the anchor plate  50 , which clamps the lid  42  to the reticle  14  and holds the reticle  14  to the spin chuck  46 . The anchor plate  50  includes a recessed area  90  bordered by two opposing walls  92 ,  94 . In the embodiment illustrated, the recessed area  90  is not enclosed. The anchor plate  50  fits over the lid  42  with the lid  42  fitting between the walls  92 ,  94  of the recessed area  90 . In one embodiment, the lid  42  is mounted to the anchor plate  50  with screws. The walls  92 ,  94  fix the orientation of the lid  42  relative to the reticle  14 , so as to prevent movement, displacement or offset of the lid  42  by the ejected fluid during cleaning. The anchor plate  50  defines openings  96  which receive the pins  64 . The pins  64  pass through the openings  58  in the spin chuck  46  (see FIG. 5). In one embodiment, a respective screw  65  extends into a threaded opening of each pin  64 . The screw  65  pushes the anchor plate  50  toward the spin chuck  46 . In alternative embodiments, the pins are integral to either the anchor plate  50  or spin chuck  46 . In another embodiment, an alternative clamp (e.g., C-clamp; nut and bolt) is used to secure the anchor plate  50  to the spin chuck  46 .  
         [0039]    Meritorious and Advantageous Effects  
         [0040]    An advantage of the invention is that a reticle which does not accurately rest on a stepper table due to foreign particles is cleaned without removing or damaging the pellicle membrane. An effect of this advantage is that the reticle does not need to go through an extensive process of re-applying a pellicle frame and pellicle membrane and requalifying the reticle for use in a lithographic process. Although a preferred embodiment of the invention has been illustrated and described, various alternatives, modifications and equivalents may be used. Therefore, the foregoing description should not be taken as limiting the scope of the invention which is defined by the appended claims.

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