Patent Publication Number: US-2012038894-A1

Title: Lens Cleaning Module

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation-in-part of application Ser. No. 10/910,480, filed Aug. 3, 2004. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to photolithography processes used in the formation of integrated circuit (IC) patterns on photoresist in the fabrication of semiconductor integrated circuits. More particularly, the present invention relates td a lens cleaning module which cleans an objective lens of a lithography system exposure apparatus to enhance the integrity of circuit pattern images transferred from a mask to a wafer. 
     BACKGROUND OF THE INVENTION 
     Various processing steps are used to fabricate integrated circuits on a semiconductor wafer. These steps include deposition of a conducting layer on the silicon wafer substrate; formation of a photoresist or other mask such as titanium oxide or silicon oxide, in the form of the desired metal interconnection pattern, using standard lithographic or photolithographic techniques; subjecting the wafer substrate to a dry etching process to remove the conducting layer from the areas not covered by the mask, thereby etching the conducting layer in the form of the masked pattern on the substrate; removing or stripping the mask layer from the substrate typically using reactive plasma and chlorine gas, thereby exposing the top surface of the conductive interconnect layer; and cooling and drying the wafer substrate by applying water and nitrogen gas to the wafer substrate. 
     In a common IC fabrication technique known as a dual damascene technique, lower and upper dielectric layers are sequentially deposited on a substrate. A via opening is patterned and etched in the lower dielectric layer, and a trench opening is patterned and etched in the upper dielectric layer. At each step, a patterned photoresist layer is used to etch the trench and via openings in the corresponding dielectric layer. A conductive copper line is then formed in the trench and via openings, typically using electrochemical plating (ECP) techniques, to form the horizontal and vertical IC circuit interconnects on the substrate. 
     Photoresist materials are coated onto the surface of a wafer, or onto a dielectric or conductive layer on a wafer, by dispensing a photoresist fluid typically on the center of the wafer as the wafer rotates at high speeds within a stationary bowl or coater cup. The coater cup catches excess fluids and particles ejected from the rotating wafer during application of the photoresist. The photoresist fluid dispensed onto the center of the wafer is spread outwardly toward the edges of the wafer by surface tension generated by the centrifugal force of the rotating wafer. This facilitates uniform application of the liquid photoresist on the entire surface of the wafer. 
     During the photolithography step of semiconductor production, light energy is applied through a reticle or mask onto the photoresist material previously deposited on the wafer to define circuit patterns which will be etched in a subsequent processing step to define the circuits on the wafer. A reticle is a transparent plate patterned with a circuit image to be formed in the photoresist coating on the wafer. A reticle contains the circuit pattern image for only a few of the die on a wafer, such as four die, for example, and thus, must be stepped and repeated across the entire surface of the wafer. In contrast, a photomask, or mask, includes the circuit pattern image for all of the die on a wafer and requires only one exposure to transfer the circuit pattern image for all of the dies to the wafer. 
     Spin coating of photoresist on wafers, as well as the other steps in the photolithography process, is carried out in an automated coater/developer track system using wafer handling equipment which transport the wafers between the various photolithography operation stations, such as vapor prime resist spin coat, develop, baking and chilling stations. Robotic handling of the wafers minimizes particle generation and wafer damage. Automated wafer tracks enable various processing operations to be carried out simultaneously. Two types of automated track systems widely used in the industry are the TEL (Tokyo Electron Limited) track and the SVG (Silicon Valley Group) track. 
     A typical method of forming a circuit pattern on a wafer includes introducing the wafer into the automated track system and then spin-coating a photoresist layer onto the wafer. The photoresist is next cured by conducting a soft bake process. After it is cooled, the wafer is placed in an exposure apparatus, such as a stepper, which aligns the wafer with an array of die patterns etched on the typically chrome-coated quartz reticle. When properly aligned and focused, the stepper exposes a small area of the wafer, then shifts or “steps” to the next field and repeats the process until the entire wafer surface has been exposed to the die patterns on the reticle. The photoresist is exposed to light through the reticle in the circuit image pattern. Exposure of the photoresist to this image pattern cross-links and hardens the resist in the circuit pattern. After the aligning and exposing step, the wafer is exposed to post-exposure baking and then is developed and hard-baked to develop the photoresist pattern. 
     The circuit pattern defined by the developed and hardened photoresist is next transferred to an underlying metal layer using an etching process, in which metal in the metal layer not covered by the cross-linked photoresist is etched away from the wafer with the metal under the cross-linked photoresist that defines the device feature protected from the etchant. Alternatively, the etched material may be a dielectric layer in which via openings and trench openings are etched according to the circuit pattern, such as in a dual damascene technique. The via and trench openings are then filled with a conductive metal such as copper to define the metal circuit lines. As a result, a well-defined pattern of metallic microelectronic circuits, which closely approximates the cross-linked photoresist circuit pattern, is formed on the wafer. 
     One type of lithography which is used in the semiconductor fabrication industry is immersion lithography, in which an exposure apparatus includes a mask and lens which are provided over an optical transfer chamber. A water-containing exposure liquid is distributed through the optical transfer chamber. In operation, the optical transfer chamber is placed over an exposure field on a photoresist-coated wafer. As the exposure liquid is distributed through the optical transfer chamber, light is transmitted through the mask, lens and exposure liquid in the optical transfer Chamber, respectively, and onto the photoresist of the exposure field. The circuit pattern image in the mask is therefore transferred by the light transmitted through the exposure liquid to the photoresist. The exposure liquid in the optical transfer chamber enhances the resolution of the transmitted circuit pattern image on the photoresist. 
     Prior to distribution of the exposure liquid through the optical transfer chamber, the aqueous liquid is typically de-gassed to remove most of the microbubbles from the liquid. However, some of the microbubbles remain in the liquid during its distribution through the optical transfer chamber. These remaining microbubbles have a tendency to adhere to the typically hydrophobic surface of the photoresist, thereby distorting the circuit pattern image projected onto the photoresist. Accordingly, an apparatus and method is needed to substantially obliterate microbubbles in an exposure liquid during immersion lithography in order to prevent distortion of the circuit pattern image projected onto the photoresist in an exposure field. 
     An object of the present invention is to provide a novel apparatus for substantially eliminating microbubbles in an exposure liquid before or during immersion lithography. 
     Another object of the present invention is to provide a novel megasonic exposure apparatus which is capable of substantially eliminating microbubbles in an exposure liquid before or during immersion lithography. 
     Still another object of the present invention is to provide a novel megasonic exposure apparatus which enhances the quality of a circuit pattern image projected onto a photoresist during immersion lithography. 
     Yet another object of the present invention is to provide a novel megasonic exposure apparatus in which sonic waves are used to substantially obliterate microbubbles in an exposure liquid before or during immersion lithography. 
     A still further object of the present invention is to provide a novel megasonic immersion lithography exposure method in which sonic waves are used to substantially obliterate microbubbles in an exposure liquid before or during immersion lithography. 
     A still further object of the present invention is to provide a novel megasonic immersion lithography exposure method in which sonic waves are used to substantially obliterate microbubbles and particles on exposure lens before or during immersion lithography. 
     SUMMARY OF THE INVENTION 
     In accordance with these and other objects and advantages, the present invention is generally directed to a novel megasonic immersion lithography exposure apparatus for substantially eliminating microbubbles from an exposure liquid before, during or both before and during immersion lithography. In one embodiment, the apparatus includes an optical transfer chamber which is positioned over a resist-covered wafer, an optical housing which is fitted with a photomask and lens provided over the optical transfer chamber, and an inlet conduit for distributing an immersion liquid into the optical transfer chamber. At least one megasonic plate operably engages the inlet conduit to perpetuate sonic waves through the immersion liquid as the liquid is distributed through the inlet conduit and into the optical transfer chamber. The sonic waves substantially obliterate microbubbles in the exposure liquid such that the liquid enters the optical transfer chamber in a substantially bubble-free state, for the exposure step. In another embodiment, the apparatus includes an annular megasonic plate, which encircles the optical transfer chamber. 
     The present invention is further directed to a method for substantially eliminating microbubbles in an exposure liquid used in an immersion lithography process for transferring a circuit pattern image from a mask or reticle to a resist-covered wafer. The method includes propagating sound waves through an exposure liquid before, during or both before and during distribution of the exposure liquid through an optical transfer chamber of an immersion lithography exposure apparatus. The sound waves substantially obliterate microbubbles in the exposure liquid and remove microbubbles from the resist surface, thereby preventing microbubbles from adhering to the resist on the surface of a wafer and distorting the circuit pattern image transferred from the apparatus, through the exposure liquid and onto the resist. 
     The present invention is further directed to a method for substantially eliminating microbubbles and particle from exposure lens used in an immersion lithography process for transferring a circuit pattern image from a mask or reticle to a resist-covered wafer. The method includes propagating sound waves through an exposure liquid before, during or both before and during distribution of the exposure liquid through an optical transfer chamber of an immersion lithography exposure apparatus. The method also includes changing the exposure liquid before, during or both before and during exposure process. The sound waves substantially obliterate microbubbles and particles on the lens surface, thereby preventing microbubbles and particle from adhering to the surface of a emersion lens and distorting the circuit pattern image transferred from the apparatus, through the exposure liquid and onto the resist. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic view of a megasonic immersion lithography apparatus according to a first embodiment of the present invention; 
         FIG. 2  is a schematic view of a megasonic immersion lithography apparatus according to a second embodiment of the present invention; 
         FIG. 3A  is a flow diagram which illustrates sequential process steps carried out according to a first embodiment of the method of the present invention; 
         FIG. 3B  is a flow diagram which illustrates sequential process steps carried out according to a second embodiment of the method of the present invention; 
         FIG. 3C  is a flow diagram which illustrates sequential process step carried out according to a third embodiment of the method of the present invention. 
         FIG. 3D  is a flow diagram which illustrates sequential process step carried out according to a fourth embodiment of the method of the present invention. 
         FIG. 3E  is a flow diagram which illustrates sequential process step carried out according to a fifth embodiment of the method of the present invention. 
         FIG. 4  is a schematic view of an illustrative embodiment of a lens cleaning module according to the present invention; 
         FIG. 5  is a schematic view of another illustrative embodiment of a lens cleaning module according to the present invention; 
         FIG. 6  is a schematic view of an exposure apparatus which is compatible with the lens cleaning modules of the present invention; 
         FIG. 7  is a schematic view of still another illustrative embodiment of the lens cleaning module according to the present invention; 
         FIG. 8A  is a schematic view, partially in section, of another embodiment of the lens cleaning module according to the present invention; and 
         FIG. 8B  is a schematic view, partially in section, of yet another embodiment of the lens cleaning module according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention contemplates a novel megasonic immersion lithography exposure apparatus for substantially eliminating microbubbles from an exposure liquid before, during, or both before and during immersion lithography. In one embodiment, the apparatus includes an optical housing which is fitted with a photomask and a lens. An optical transfer chamber is provided beneath the lens of the optical housing. An inlet conduit is provided in fluid communication with the optical transfer chamber to distribute an immersion liquid into the chamber. At least one megasonic plate operably engages the inlet conduit to perpetuate sonic waves through the immersion liquid as the liquid is distributed through the inlet conduit and into the optical transfer chamber. In another embodiment, an annular megasonic plate encircles the optical transfer chamber of the apparatus. 
     In operation of the apparatus, the optical transfer chamber is positioned over an exposure field on a photoresist-coated wafer. The sonic waves generated by the megasonic plate or plates substantially obliterate microbubbles in the exposure liquid, such that the liquid enters the optical transfer chamber in a substantially bubble-free state. During the exposure step, light is transmitted through the photomask and lens, respectively, of the optical housing; through the exposure liquid in the optical transfer chamber; and onto the photoresist coated onto the wafer. The exposure liquid, substantially devoid of microbubbles, transmits the substantially distortion-free circuit pattern image onto the photoresist with high resolution. 
     The present invention is further directed to a method for substantially eliminating microbubbles in an exposure liquid used in an immersion lithography process exposure step to transfer a circuit pattern image from a mask or reticle to an exposure field on a resist-covered wafer. In a first embodiment, the method includes propagating sound waves through an exposure liquid to obliterate microbubbles in the liquid before the exposure step. In a second embodiment, the method includes propagating sound waves through the exposure liquid both before and during the exposure step. In a third embodiment, the method includes intermittently propagating sound waves through the exposure liquid during the exposure step. The megasonic power applied by the megasonic plate or plates to the exposure liquid is preferably about 10-1,000 kHz. 
     Any of a variety of exposure liquids are suitable for the megasonic immersion lithography method of the present invention. In one embodiment, the exposure liquid includes NH 4 , H 2 O 2  and H 2 O in a concentration by volume ratio of typically about 1:1:10˜1:1:1000. In another embodiment, the exposure liquid includes NH 4  and H 2 O in a concentration by volume ratio of typically about 1:10˜1:1000. In still another embodiment, the exposure liquid is deionized (DI) water. In yet another embodiment, the exposure liquid is ozonated (O 3 ) water, having an ozone concentration of typically about 1˜1000 ppm. The exposure liquid may include a non-ionic surfactant, an anionic surfactant or a cationic surfactant having a concentration in the range of typically about 1˜1000 ppm. 
     Referring initially to  FIG. 1 , a megasonic immersion lithography exposure apparatus, hereinafter exposure apparatus, of the present invention is generally indicated by reference numeral  10 . The exposure apparatus  10  includes a wafer stage  28  for supporting a wafer  34  having a photoresist layer (not shown) deposited thereon. An optical housing  12  contains an optical system having a laser (not shown) and the last objective lens  16  which is positioned above the wafer stage  28 . A mask or reticle(not shown) is removably inserted in the optical housing  12 , above the lens  16 . The mask or reticle includes a circuit pattern (not shown) which is to be transmitted onto the photoresist layer on the wafer  34  during a lithography process, which will be hereinafter described. An optical transfer water immersion chamber  18  is provided beneath the last objective lens  16  and is disposed above the wafer stage  28 . During lithography, the laser beam through the mask or reticle, which produces a circuit pattern image that is transmitted through the last objective lens  16  and the optical transfer water immersion chamber  18 , respectively, and onto the wafer  34 . 
     An inlet liquid reservoir  20 , from which extends an inlet conduit  22 , contains a supply of exposure liquid  32 . A discharge conduit  22   a  extends from the inlet conduit  22  and is provided in fluid communication with the optical transfer chamber  18 . An outlet liquid reservoir  26  is provided in fluid communication with the optical transfer chamber  18  through a collecting conduit  24   a  and an outlet conduit  24 , respectively. According to the present invention, a megasonic plate  30  is provided on the inlet conduit  22 , according to the knowledge of those skilled in the art, to generate sonic waves (not shown) in the exposure liquid  32  as the liquid  32  is distributed through the inlet conduit  22 . 
     In operation of the exposure apparatus  10 , as hereinafter further described, the exposure liquid  32  is distributed from the inlet liquid reservoir  20 , through the inlet conduit  22  and discharge conduit  22   a,  respectively, and into the optical transfer water immersion chamber  18 . The megasonic plate  30  generates sonic waves (not shown) in the exposure liquid  32 , obliterating all or most of the microbubbles in the exposure liquid  32 . The laser beam from the optical housing  12  which produces a circuit pattern image is transmitted through the lens last objective  16  and exposure liquid  32  contained in the optical transfer water immersion chamber  18 , respectively, and is projected onto the photoresist coated on the wafer  34 . The exposure liquid  32  is continuous pumped from the optical transfer water immersion chamber  18 , through the collecting conduit  24   a  and outlet conduit  24 , respectively, and into the outlet liquid reservoir  26 . 
     Referring next to  FIGS. 3A-3C , in conjunction with  FIG. 1 , the exposure apparatus  10  can be operated according to one of three modes. According to the flow diagram of  FIG. 3A , the optical transfer water immersion chamber  18  is initially positioned over an exposure field on the wafer  34 , as indicated in step  1 . The megasonic plate  30  is then turned on (step  2 ), followed by distribution of the exposure liquid  32  from the inlet liquid reservoir  20 , through the inlet conduit  22  and into the optical transfer water immersion chamber  18 , respectively (step  3 ). As the exposure liquid  32  passes through the inlet conduit  22 , the megasonic plate  30  induces the formation of sonic waves in the exposure liquid  32 . The sonic waves obliterate microbubbles in the exposure liquid  32 , such that the exposure liquid  32  is substantially devoid of microbubbles upon entry into the optical transfer chamber  18 , Furthermore, the sonic waves also obliterate the microbubbles on the resist surface through the sonic wave transfer from discharge conduit  22   a  to optical transfer water immersion chamber  18 . 
     As indicated in step  4 , the megasonic plate  30  is turned off prior to exposing the exposure field on the wafer  34  to the circuit pattern image transmitted through the exposure liquid  32  (step  5 ), the exposure liquid  32  transmits a high-resolution circuit pattern image, which is undistorted by microbubbles onto the surface of the photoresist on the wafer  34 . After completion of the exposure step  5 , the optical transfer chamber  18  is moved to the next exposure field on the wafer  34  and steps  1 - 5  are repeated, as indicated in step  6 . 
     According to the flow diagram of  FIG. 3B , the optical transfer water immersion chamber  18  is initially positioned over an exposure field on the wafer  34 , as indicated in step  1   a.  The megasonic plate  30  is then turned on (step  2   a ), followed by distribution of the exposure liquid  32  from the inlet liquid reservoir  20 , through the inlet conduit  22  and into the optical transfer water immersion chamber  18 , respectively (step  3   a ). The sonic waves generated by the megasonic plate  30  obliterate microbubbles in the exposure liquid  32  passing through the inlet conduit  22 , such that the exposure liquid  32  is substantially devoid of microbubbles upon entry into the optical transfer chamber  18  and the microbubbles adhered on the wafer  34  is therefore obliterate. 
     As indicated in step  4   a,  while the megasonic plate  30  remains on, the photoresist on the wafer  34  is exposed. Accordingly, during the exposure step (step  4   a ), the megasonic plate  20  continues to obliterate microbubbles in the exposure liquid  32  and on the wafer resist surface  34 . The circuit pattern image transmitted from the optical housing  12  through the optical transfer chamber  18  is therefore undistorted by microbubbles and is projected onto the surface of the photoresist on the wafer  34  with a high resolution. After completion of the exposure step  4   a,  the megasonic plate  30  may be turned off ( FIG. 5   a ). The optical transfer chamber  18  is then moved to the next exposure field on the wafer  34  and steps  1 - 5  are repeated, as indicated in step  6   a.    
     According to the flow diagram of  FIG. 3C , the optical transfer chamber  18  is initially positioned over an exposure field on the wafer  34 , as indicated in step  1   b.  The megasonic plate  30  is then turned on (step  2   b ), and the exposure liquid  32  is distributed from the inlet liquid reservoir  20 , through the inlet conduit  22  and into the optical transfer chamber  18 , respectively (step  3   b ). The sonic waves generated by the megasonic plate  30  obliterate microbubbles in the exposure liquid  32  and on the wafer resist surface  34 , such that the exposure liquid  32  is substantially devoid of microbubbles upon entry into the optical transfer chamber  18  and adhesion on top of the resist surface  34 . 
     As indicated in step  4   b,  the exposure step is carried out while the megasonic plate  30  is intermittently turned on and off. Accordingly, during exposure of the wafer  34 , the megasonic plate  20  continues to obliterate microbubbles in the exposure liquid  32 . After completion of the exposure step  4   b,  the optical transfer chamber  18  is moved to the next exposure field on the wafer  34  and steps  1 - 5  are repeated, as indicated in step  5   b.    
     According to the flow diagram of  FIG. 3D , the optical transfer water immersion chamber  18  is initially positioned over an exposure field on the wafer  34 , as indicated in step  1   c.  The megasonic plate  30  is then turned on (step  2   c ), followed by distribution of the exposure liquid  32  from the inlet liquid reservoir  20 , through the inlet conduit  22  and into the optical transfer water immersion chamber  18 , respectively (step  3   c ). The sonic waves generated by the megasonic plate  30  obliterate microbubbles in the exposure liquid  32  passing through the inlet conduit  22 , such that the exposure liquid  32  is substantially devoid of microbubbles upon entry into the optical transfer chamber  18  and the microbubbles adhered on the wafer  34  is therefore obliterate. 
     As indicated in step  4   a,  while the megasonic plate  30  remains on, the photoresist on the wafer  34  is exposed. Accordingly, during the exposure step (step  4   c ), the megasonic plate  20  continues to obliterate microbubbles in the exposure liquid  32  and on the wafer resist surface  34 . The circuit pattern image transmitted from the optical housing  12  through the optical transfer chamber  18  is therefore undistorted by microbubbles and is projected onto the surface of the photoresist on the wafer  34  with a high resolution. After completion of the exposure step  4   a,  the megasonic plate  30  may be still turned on. The optical transfer chamber  18  is then moved to the next exposure field on the wafer  34  and steps  4   c - 5   c  are repeated, as indicated in step  6   c.    
     According to the flow diagram of  FIG. 3E , the optical transfer water immersion chamber  18  is initially positioned over an exposure field on the wafer  34 , as indicated in step  1   d.  The megasonic plate  30  is then turned on (step  2   d ), followed by distribution of the first liquid  32  from the inlet liquid reservoir  20 , through the inlet conduit  22  and into the optical transfer water immersion chamber  18 , respectively (step  3   d ). The sonic waves generated by the megasonic plate  30  obliterate microbubbles in the exposure liquid  32  passing through the inlet conduit  22  and removing particle on the low surface of the last objective lens  108 , such that the exposure liquid  32  is substantially devoid of microbubbles upon entry into the optical transfer chamber  18  and the particles adhered on the low surface of the last objective lens  108  is therefore obliterate. 
     As indicated in step  4 D, while the megasonic plate  30  remains on, followed by distribution of the second liquid from the inlet liquid reservoir  20 , through the inlet conduit  22  and into the optical transfer water immersion chamber  18  to replace the first liquid (step  4   d ), the photoresist on the wafer  34  is exposed. Accordingly, during the exposure step (step  6   d ), the megasonic plate does not turn on (step  5   d ). The circuit pattern image transmitted from the optical housing  12  through the optical transfer water immersion chamber  18  is therefore undistorted by particles and is projected onto the surface of the photoresist on the wafer  34  with a high resolution. After completion of the exposure step  6   d,  the optical transfer chamber  18  is then moved to the next exposure field on the wafer  34  and steps  6   d - 7   d  are repeated, as indicated in step  6   d.    
     According to the flow diagram of  FIG. 3F , the optical transfer water immersion chamber  18  is initially positioned over an exposure field on the wafer  34 , as indicated in step  1   e.  The megasonic plate  30  is then turned on (step  2   e ), followed by distribution of the first liquid  32  from the inlet liquid reservoir  20 , through the inlet conduit  22  and into the optical transfer water immersion chamber  18 , respectively (step  3   e ). The sonic waves generated by the megasonic plate  30  obliterate microbubbles in the exposure liquid  32  passing through the inlet conduit  22  and removing particle on the low surface of the last objective lens  108 , such that the exposure liquid  32  is substantially devoid of microbubbles upon entry into the optical transfer chamber  18  and the particles adhered on the low surface of the last objective lens  108  is therefore obliterate. 
     As indicated in step  4   e,  while the megasonic plate  30  remains on, followed by distribution of the second liquid from the inlet liquid reservoir  20 , through the inlet conduit  22  and into the optical transfer water immersion chamber  18  to replace the first liquid (step  4   e ), the photoresist on the wafer  34  is exposed. Accordingly, during the exposure step (step  5   e ), the megasonic plate still turn on (step  2   e ). The circuit pattern image transmitted from the optical housing  12  through the optical transfer water immersion chamber  18  is therefore undistorted by particles and is projected onto the surface of the photoresist on the wafer  34  with a high resolution. After completion of the exposure step  5   e,  the optical transfer chamber  18  is then moved to the next exposure field on the wafer  34  and steps  5   e - 6   e  are repeated, as indicated in step  5   e.    
     Referring next to  FIG. 2 , in an alternative embodiment of the exposure apparatus, generally indicated by reference numeral  10   a,  an annular megasonic plate  30   a  is provided around the optical transfer water immersion chamber  18 . The exposure apparatus  10   a  can be operated according to the flow diagram of  FIG. 3A , wherein the annular megasonic plate  30   a  is operated after the exposure liquid  32  is distributed into the optical transfer water immersion chamber  18  and then turned off prior to the exposure step; according to the flow diagram of  FIG. 3B , wherein the annular megasonic plate  30   a  remains on during distribution of the exposure liquid  32  into the optical transfer water immersion chamber  18  and throughout the exposure process; or according to the flow diagram of  FIG. 3C , wherein the annular megasonic plate  30   a  is turned on intermittently during the exposure step. In any case, the exposure liquid  32  contained in the optical transfer chamber  18  is substantially devoid of microbubbles which could otherwise distort the circuit pattern image transmitted to the wafer  34  during the exposure step. 
     Referring next to  FIGS. 4 and 6 , an illustrative embodiment of a non-manual lens cleaning module according to the present invention is generally indicated by reference numeral  101  in  FIG. 4 . As shown in  FIG. 6 , the lens cleaning module  101  is suitable for implementation in conjunction with an exposure apparatus,  130 , which may be conventional. A UV source  131  which emits ultraviolet light is provided at one end of the exposure apparatus  130 . Preferably, the UV source  131  emits UV light having less than 480 nm. An objective lens  133  is provided at the opposite end of the exposure apparatus  130 . Preferably, the objective lens  133  has an N.A. of larger than about 0.35. A condenser element  132  is provided between the UV source  131  and the objective lens  133  to condense the ultraviolet light before it passes through the objective lens  133 . A mask  134  is provided between the condenser element  132  and the objective lens  133 . A wafer  135  is supported on a wafer stage (not shown) beneath or adjacent to the objective lens  133 . The lens cleaning module  101  may include a heating/drying module  114  for drying the lens  110  after cleaning. 
     In operation of the exposure apparatus  130 , the UV source  131  emits a beam of ultraviolet light, which passes first through the condenser element  132 , then through the mask  134  and objective lens  133 , respectively. The mask  134  enables passage of light which corresponds to the circuit pattern to be transferred to the wafer  135 . The objective lens  133  focuses the light, in the circuit pattern image defined by the mask  134 , on the wafer  135 . The lens cleaning module  101  can be incorporated into the exposure apparatus  130  to remove particles, liquid marks and residues from the objective lens  133  in order to enhance the exposure quality of the exposure apparatus  130 . 
     As shown in  FIG. 4 , the lens cleaning module  101  typically includes a scanning stage  102  which has bi-directional movement capability and is adapted to support a wafer  112  beneath the exposure apparatus (not shown), such as the exposure apparatus  130  which was heretofore described with respect to  FIG. 6 , for example. A dish  103  is provided above the scanning stage  102 . The dish  103  includes a central dish opening  104  having a beveled dish surface  105 . A cleaning fluid  108  is contained in the dish opening  104  of the dish  103 . The objective lens  110  of the exposure apparatus is seated against the beveled dish surface  105  of the dish  103  and contacts the cleaning fluid  108 . The mask  111  of the exposure apparatus is provided above the lens  110 . The cleaning fluid  108  may be acetone, IPA (isopropyl alcohol) or other solvent which does not contain water or fluoride and is incapable of damaging, corroding or reacting with the surface coating of the objective lens  110 . Accordingly, before, during and after exposure of the wafer  112  through the mask  111  and lens  110 , the fluid  108  removes particles, liquid marks and residues from the lens  110 , thereby enhancing the exposure quality of the exposure apparatus and the precision of circuit pattern images transferred from the mask  111  to the wafer  112 . The heating/drying module  114  may utilize thermal, gas spray or other methods known by those skilled in the art to facilitate the evaporation of the cleaning fluid  108  from the objective lens  110 . 
     Referring next to  FIG. 5 , another illustrative embodiment of the lens cleaning module, of the present invention is generally indicated by reference numeral  116 . The lens cleaning module  116  typically includes a scanning stage  117 , which may have bi-directional movement capability, as shown by the arrow, and is adapted to support a wafer  124 . A fluid retaining wall  118  is provided on the scanning stage  117  and is adapted to contain a cleaning fluid  119  on the scanning stage  117 . The lens  122  of the exposure apparatus contacts the cleaning fluid  119 , and the mask  123  is provided above the lens  122 . Accordingly, during exposure of the wafer  124 , the cleaning fluid  119  removes particles, liquid marks and residues from the lens  122 , thereby enhancing the exposure quality of the exposure apparatus and the precision of circuit pattern images transferred from the mask  123  to the wafer  124 . The lens cleaning module  116  may include a heating/drying module  126  which may utilize thermal, gas spray or other methods known by those skilled in the art to facilitate the evaporation of the cleaning fluid  119  from the objective lens  122 . 
     Referring next to  FIG. 7 , still another illustrative embodiment of the lens cleaning module of the present invention is generally indicated by reference numeral  140 . The lens cleaning module  140  includes a wafer stage  141  which is adapted to support a wafer  156 . The optical housing  142  of the exposure apparatus is disposed above the wafer stage  141 , and the lens  143  is provided on the optical housing  142 . A liquid supply tank  146  is provided at one side of the optical housing  142  and contains a supply of cleaning liquid  144 . A liquid supply conduit  147  extends from the liquid supply tank  146  to a liquid collecting area  148  beneath the lens  143 . A liquid recovery tank  150  is provided at the opposite side of the optical housing  142 . A liquid recovery conduit  149  extends from the liquid recovery tank  150  to the liquid collecting area  148 , typically opposite the liquid supply conduit  147 . A liquid sealing member  152  may be supported by a support  153  and engage the upper edge of the wafer stage  141 , beneath the liquid recovery tank  150 , to prevent the inadvertent flow of cleaning liquid  144  from the wafer stage  141 . The lens cleaning module  140  may include a heating/drying module  158  which may utilize thermal, gas spray or other methods known by those skilled in the art to facilitate the evaporation of the cleaning liquid  144  from the objective lens  143 . 
     In use of the lens cleaning module  140 , cleaning liquid  144  is distributed from the liquid supply tank  146 , through the liquid supply conduit  147  to the liquid collecting area  148 , respectively. Simultaneously, the cleaning liquid  144  is pumped from the liquid collecting area  148 , through the liquid recovery area  149  and into the liquid recovery tank  150 , respectively. Accordingly, the lens  143  is continually exposed to the cleaning liquid  144  flowing through the liquid collecting area  148 , thus removing particles, liquid marks and residues from the lens  122  and enhancing the exposure quality of the exposure apparatus and the precision of circuit pattern images transferred from the mask  123  to the wafer  124 . 
     Referring next to  FIG. 8A , yet another illustrative embodiment of the lens cleaning module according to the present invention is generally indicated by reference numeral  160 . The lens cleaning module  160  includes a scanning stage  161  for supporting a wafer (not shown). A cleaning stage  162 , which may be removable, is positional above the scanning stage  161 , and at least one cleaning unit  163  is provided on the upper surface of the cleaning stage  162 , beneath the objective lens  168  of the exposure apparatus. Each cleaning unit  163  may be fixedly or pivotally mounted on the cleaning stage  162 . Each cleaning unit  163  typically includes a central dispensing nozzle  164 , and a collecting annulus  165 , which encircles the dispensing nozzle  164 . An inlet conduit  166  extends through the cleaning stage  162  and is provided in fluid communication with the dispensing nozzle  164 . A supply reservoir (not shown) which contains a supply of cleaning liquid  169  is provided in fluid communication with the inlet conduit  166 . An outlet conduit  167  extends from the collecting annulus  165 . A stand-by area (not shown) for the cleaning stage  162  may be provided next to the lens cleaning module  160 . The lens cleaning module  160  may include a heating/drying module  182  which may utilize thermal, gas spray or other methods known by those skilled in the art to facilitate the evaporation of the cleaning liquid  169  from the objective lens  168 . 
     In use of the lens cleaning module  160 , the cleaning liquid  169  is distributed through the inlet conduit  166  and ejected from the dispensing nozzle  164  and against the lens  168  to remove particles, liquid marks and residues from the lens  168 . The cleaning liquid  169  falls into the collecting annulus  165  and is distributed through the outlet conduit  167  to a suitable receptacle or outlet (not shown). 
     Referring next to  FIG. 8B , still another embodiment of the lens cleaning module according to the present invention is generally indicated by reference numeral  170 . The lens cleaning module  170  includes a scanning stage  171  for supporting a wafer (not shown). A cleaning stage  172  is provided above the scanning stage  171 . At least one cleaning unit  179  is provided on the cleaning stage  172 . Each cleaning unit  179  may be fixedly or pivotally mounted on the cleaning stage  172 . Each cleaning unit  179  includes a dispensing nozzle  173  which is directed toward the objective lens  177  of the exposure apparatus and a collector  174  which is adjacent to the dispensing nozzle  173 . An inlet conduit  175  is provided in fluid communication with the dispensing nozzle  173  and is connected to a supply (not shown) of cleaning liquid  178 . An outlet conduit  176  extends from the collector  174 . The lens cleaning module  170  may include a heating/drying module  184  which may utilize thermal, gas spray or other methods known by those skilled in the art to facilitate the evaporation of the cleaning liquid  178  from the objective lens  177 . 
     In use of the lens cleaning module  170 , the cleaning liquid  178  is distributed through the inlet conduit  175  and ejected from the dispensing nozzle  173 , against the lens  177  to remove particles, liquid marks and residues from the lens  177 . After striking the lens  177 , the cleaning liquid  178  falls into the collector  174  and is distributed through the outlet conduit  176  to a suitable receptacle or outlet (not shown). 
     In the various embodiments, the lens cleaning modules of the present invention can be integrated with the lithography system of which they are a part for automated cleaning of the objective lens in the exposure apparatus. Accordingly, pre-cleaning and post-cleaning of the objective lens before and after exposure, respectively, is possible. The cycle time of each cleaning cycle may be set by recipe for automatic implementation. The frequency of lens cleaning can be as high as once per exposed wafer, thus decreasing periodic maintenance (PM) manpower and cycle time to maintain consistent maintenance quality. Furthermore, the lens cleaning module can be movable with respect to the exposure apparatus to facilitate cleaning and maintenance of the lens cleaning module, for example. Moreover, each lens cleaning module may utilize contact with a physical object such as a sponge, for example, alone or in combination with a cleaning fluid or immersion liquid, as was heretofore described. In that case, the lens cleaning module typically includes a contacting material such as a sponge; a cleaning fluid or solvent which is contacted by the contacting material prior to contact of the material with the lens; and a collecting system for collecting the fluid or solvent. Referring again to  FIG. 6 , each lens cleaning module may be adapted to additionally or alternatively clean the condenser element  132 , windows (not shown) or other element or elements of the exposure apparatus  130  of which they are a part. 
     While the preferred embodiments of the invention have been described above, it will be recognized and understood that various modifications can be made in the invention and the appended claims are intended to cover all such modifications which may fall within the spirit and scope of the invention.