Abstract:
A lithographic projection apparatus includes a substrate table configured to hold a substrate, a projection system configured to project a patterned beam onto a target portion of the substrate, liquid being provided to a space between the projection system and the substrate, and a shutter configured to isolate the space from the substrate or a space to be occupied by a substrate. The shutter is separable from the remainder of the apparatus.

Description:
RELATED APPLICATIONS  
       [0001]     This is a Divisional of U.S. patent application Ser. No. 11/259,061 filed Oct. 27, 2005, which in turn is a Divisional of U.S. patent application Ser. No. 11/237,721 filed Sep. 29, 2005, which is a Continuation of International Application No. PCT/IB2004/001259 filed Mar. 17, 2004, which claims the benefit of U.S. Provisional Application No. 60/462,499 filed on Apr. 11, 2003. The entire disclosures of the prior applications are incorporated herein by reference in their entireties. 
     
    
     BACKGROUND  
       [0002]     Lithography systems are commonly used to transfer images from a reticle onto a semiconductor wafer during semiconductor processing. A typical lithography system includes an optical assembly, a reticle stage for holding a reticle defining a pattern, a wafer stage assembly that positions a semiconductor wafer, and a measurement system that precisely monitors the position of the reticle and the wafer. During operation, an image defined by the reticle is projected by the optical assembly onto the wafer. The projected image is typically the size of one or more die on the wafer. After an exposure, the wafer stage assembly moves the wafer and then another exposure takes place. This process is repeated until all the die on the wafer are exposed. The wafer is then removed and a new wafer is exchanged in its place.  
         [0003]     Immersion lithography systems utilize a layer of immersion fluid that completely fills a gap between the optical assembly and the wafer during the exposure of the wafer. The optic properties of the immersion fluid, along with the optical assembly, allow the projection of smaller feature sizes than is currently possible using standard optical lithography. For example, immersion lithography is currently being considered for next generation semiconductor technologies including 65 nanometers, 45 nanometers, and beyond. Immersion lithography therefore represents a significant technological breakthrough that will likely enable the continued use of optical lithography for the foreseeable future.  
         [0004]     After a wafer is exposed, it is removed and exchanged with a new wafer. As currently contemplated in immersion systems, the immersion fluid would be removed from the gap and then replenished after the wafer is exchanged. More specifically, when a wafer is to be exchanged, the fluid supply to the gap is turned off, the fluid is removed from the gap (i.e., by vacuum), the old wafer is removed, a new wafer is aligned and placed under the optical assembly, and then the gap is re-filled with fresh immersion fluid. Once all of the above steps are complete, exposure of the new wafer can begin.  
         [0005]     Wafer exchange with immersion lithography as described above is problematic for a number of reasons. The repeated filling and draining of the gap may cause variations in the immersion fluid and may cause bubbles to form within the immersion fluid. Bubbles and the unsteady fluid may interfere with the projection of the image on the reticle onto the wafer, thereby reducing yields. The overall process also involves many steps and is time consuming, which reduces the overall throughput of the machine.  
         [0006]     An apparatus and method for maintaining immersion fluid in the gap adjacent to the projection lens when the wafer stage moves away from the projection lens, for example during wafer exchange, is therefore needed.  
       SUMMARY  
       [0007]     An apparatus and method maintain immersion fluid in the gap adjacent to the projection lens in a lithography machine. The apparatus and method include an optical assembly that projects an image onto a work piece and a stage assembly including a work piece table that supports the work piece adjacent to the optical assembly. An environmental system is provided to supply and remove an immersion fluid from the gap. After exposure of the work piece is complete, an exchange system removes the work piece and replaces it with a second work piece. An immersion fluid containment system is provided to maintain the immersion fluid in the gap when the work piece table moves away from the projection lens. The gap therefore does not have to be refilled with immersion fluid when the first work piece is replaced with a second work piece. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The invention will be described in conjunction with the following drawings of exemplary embodiments in which like reference numerals designate like elements, and in which:  
         [0009]      FIG. 1  is an illustration of an immersion lithography machine having features of the invention;  
         [0010]      FIG. 2  is a cross section of an immersion lithography machine according to one embodiment of the invention;  
         [0011]      FIGS. 3A and 3B  are a cross section and a top down view of an immersion lithography machine according to another embodiment of the invention;  
         [0012]      FIGS. 4A and 4B  are cross section views of an immersion lithography machine according to another embodiment of the invention;  
         [0013]      FIGS. 5A and 5B  are top down views of two different twin wafer stages according to other embodiments of the invention;  
         [0014]      FIG. 6A  is a top down view of a twin stage lithography machine according to another embodiment of the invention;  
         [0015]      FIGS. 6B-6E  are a series of diagrams illustrating a wafer exchange according to the invention;  
         [0016]      FIG. 7A  is a flow chart that outlines a process for manufacturing a work piece in accordance with the invention; and  
         [0017]      FIG. 7B  is a flow chart that outlines work piece processing in more detail. 
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0018]      FIG. 1  is a schematic illustration of a lithography machine  10  having features of the invention. The lithography machine  10  includes a frame  12 , an illumination system  14  (irradiation apparatus), an optical assembly  16 , a reticle stage assembly  18 , a work piece stage assembly  20 , a measurement system  22 , a control system  24 , and a fluid environmental system  26 . The design of the components of the lithography machine  10  can be varied to suit the design requirements of the lithography machine  10 .  
         [0019]     In one embodiment, the lithography machine  10  is used to transfer a pattern (not shown) of an integrated circuit from a reticle  28  onto a semiconductor wafer  30  (illustrated in phantom). The lithography machine  10  mounts to a mounting base  32 , e.g., the ground, a base, or floor or some other supporting structure.  
         [0020]     In various embodiments of the invention, the lithography machine  10  can be used as a scanning type photolithography system that exposes the pattern from the reticle  28  onto the wafer  30  with the reticle  28  and the wafer  30  moving synchronously. In a scanning type lithographic machine, the reticle  28  is moved perpendicularly to an optical axis of the optical assembly  16  by the reticle stage assembly  18 , and the wafer  30  is moved perpendicularly to the optical axis of the optical assembly  16  by the wafer stage assembly  20 . Scanning of the reticle  28  and the wafer  30  occurs while the reticle  28  and the wafer  30  are moving synchronously.  
         [0021]     Alternatively, the lithography machine  10  can be a step-and-repeat type photolithography system that exposes the reticle  28  while the reticle  28  and the wafer  30  are stationary. In the step and repeat process, the wafer  30  is in a constant position relative to the reticle  28  and the optical assembly  16  during the exposure of an individual field. Subsequently, between consecutive exposure steps, the wafer  30  is consecutively moved with the wafer stage assembly  20  perpendicularly to the optical axis of the optical assembly  16  so that the next field of the wafer  30  is brought into position relative to the optical assembly  16  and the reticle  28  for exposure. Following this process, the images on the reticle  28  are sequentially exposed onto the fields of the wafer  30 , and then the next field of the wafer  30  is brought into position relative to the optical assembly  16  and the reticle  28 .  
         [0022]     However, the use of the lithography machine  10  provided herein is not necessarily limited to a photolithography for semiconductor manufacturing. The lithography machine  10 , for example, can be used as an LCD photolithography system that exposes a liquid crystal display work piece pattern onto a rectangular glass plate or a photolithography system for manufacturing a thin film magnetic head. Accordingly, the term “work piece” is generically used herein to refer to any device that may be patterned using lithography, such as but not limited to wafers or LCD substrates.  
         [0023]     The apparatus frame  12  supports the components of the lithography machine  10 . The apparatus frame  12  illustrated in  FIG. 1  supports the reticle stage assembly  18 , the wafer stage assembly  20 , the optical assembly  16  and the illumination system  14  above the mounting base  32 .  
         [0024]     The illumination system  14  includes an illumination source  34  and an illumination optical assembly  36 . The illumination source  34  emits a beam (irradiation) of light energy. The illumination optical assembly  36  guides the beam of light energy from the illumination source  34  to the optical assembly  16 . The beam illuminates selectively different portions of the reticle  28  and exposes the wafer  30 . In  FIG. 1 , the illumination source  34  is illustrated as being supported above the reticle stage assembly  18 . Typically, however, the illumination source  34  is secured to one of the sides of the apparatus frame  12  and the energy beam from the illumination source  34  is directed to above the reticle stage assembly  18  with the illumination optical assembly  36 .  
         [0025]     The illumination source  34  can be a g-line source (436 nm), an i-line source (365 nm), a KrF excimer laser (248 nm), an ArF excimer laser (193 nm) or a F 2  laser ( 157  nm). Alternatively, the illumination source  34  can generate an x-ray.  
         [0026]     The optical assembly  16  projects and/or focuses the light passing through the reticle  28  to the wafer  30 . Depending upon the design of the lithography machine  10 , the optical assembly  16  can magnify or reduce the image illuminated on the reticle  28 . The optical assembly  16  need not be limited to a reduction system. It could also be a 1× or greater magnification system.  
         [0027]     Also, with an exposure work piece that employs vacuum ultra-violet radiation (VUV) of wavelength 200 nm or lower, use of a catadioptric type optical system can be considered. Examples of a catadioptric type of optical system are disclosed in Japanese Laid-Open Patent Application Publication No. 8-171054 and its counterpart U.S. Pat. No. 5,668,672, as well as Japanese Laid-Open Patent Publication No. 10-20195 and its counterpart U.S. Pat. No. 5,835,275. In these cases, the reflecting optical system can be a catadioptric optical system incorporating a beam splitter and concave mirror. Japanese Laid-Open Patent Application Publication No. 8-334695 and its counterpart U.S. Pat. No. 5,689,377 as well as Japanese Laid-Open Patent Application Publication No. 10-3039 and its&#39; counterpart U.S. patent application Ser. No. 873,605 (Application Date: Jun. 12, 1997) also use a reflecting-refracting type of optical system incorporating a concave mirror, etc., but without a beam splitter, and also can be employed with this invention. The disclosures of the above-mentioned U.S. patents and applications, as well as the Japanese Laid-Open patent application publications are incorporated herein by reference in their entireties.  
         [0028]     The reticle stage assembly  18  holds and positions the reticle  28  relative to the optical assembly  16  and the wafer  30 . In one embodiment, the reticle stage assembly  18  includes a reticle stage  38  that retains the reticle  28  and a reticle stage mover assembly  40  that moves and positions the reticle stage  38  and reticle  28 .  
         [0029]     Each stage mover assembly  40 ,  44  can move the respective stage  38 ,  42  with three degrees of freedom, less than three degrees of freedom, or more than three degrees of freedom. For example, in alternative embodiments, each stage mover assembly  40 ,  44  can move the respective stage  38 ,  42  with one, two, three, four, five or six degrees of freedom. The reticle stage mover assembly  40  and the work piece stage mover assembly  44  can each include one or more movers, such as rotary motors, voice coil motors, linear motors utilizing a Lorentz force to generate drive force, electromagnetic movers, planar motors, or some other force movers.  
         [0030]     In photolithography systems, when linear motors (see U.S. Pat. Nos. 5,623,853 or 5,528,118 which are incorporated by reference herein in their entireties) are used in the wafer stage assembly or the reticle stage assembly, the linear motors can be either an air levitation type employing air bearings or a magnetic levitation type using Lorentz force or reactance force. Additionally, the stage could move along a guide, or it could be a guideless type stage that uses no guide.  
         [0031]     Alternatively, one of the stages could be driven by a planar motor, which drives the stage by an electromagnetic force generated by a magnet unit having two-dimensionally arranged magnets and an armature coil unit having two-dimensionally arranged coils in facing positions. With this type of driving system, either the magnet unit or the armature coil unit is connected to the stage base and the other unit is mounted on the moving plane side of the stage.  
         [0032]     Movement of the stages as described above generates reaction forces that can affect performance of the photolithography system. Reaction forces generated by the wafer (substrate) stage motion can be mechanically transferred to the floor (ground) by use of a frame member as described in U.S. Pat. No. 5,528,100 and Japanese Laid-Open Patent Application Publication No. 8-136475. Additionally, reaction forces generated by the reticle (mask) stage motion can be mechanically transferred to the floor (ground) by use of a frame member as described in U.S. Pat. No. 5,874,820 and Japanese Laid-Open Patent Application Publication No. 8-330224. The disclosures of U.S. Pat. Nos. 5,528,100 and 5,874,820 and Japanese Paid-Open Patent Application Publication Nos. 8-136475 and 8-330224 are incorporated herein by reference in their entireties.  
         [0033]     The measurement system  22  monitors movement of the reticle  28  and the wafer  30  relative to the optical assembly  16  or some other reference. With this information, the control system  24  can control the reticle stage assembly  18  to precisely position the reticle  28  and the work piece stage assembly  20  to precisely position the wafer  30 . The design of the measurement system  22  can vary. For example, the measurement system  22  can utilize multiple laser interferometers, encoders, mirrors, and/or other measuring devices.  
         [0034]     The control system  24  receives information from the measurement system  22  and controls the stage assemblies  18 ,  20  to precisely position the reticle  28  and the wafer  30 . Additionally, the control system  24  can control the operation of the components of the environmental system  26 . The control system  24  can include one or more processors and circuits.  
         [0035]     The environmental system  26  controls the environment in a gap (not shown) between the optical assembly  16  and the wafer  30 . The gap includes an imaging field. The imaging field includes the area adjacent to the region of the wafer  30  that is being exposed and the area in which the beam of light energy travels between the optical assembly  16  and the wafer  30 . With this design, the environmental system  26  can control the environment in the imaging field. The desired environment created and/or controlled in the gap by the environmental system  26  can vary accordingly to the wafer  30  and the design of the rest of the components of the lithography machine  10 , including the illumination system  14 . For example, the desired controlled environment can be a fluid such as water. Alternatively, the desired controlled environment can be another type of fluid such as a gas. In various embodiments, the gap may range from 0.1 mm to 10 mm in height between top surface of the wafer  30  and the last optical element of the optical assembly  16 .  
         [0036]     In one embodiment, the environmental system  26  fills the imaging field and the rest of the gap with an immersion fluid. The design of the environmental system  26  and the components of the environmental system  26  can be varied. In different embodiments, the environmental system  26  delivers and/or injects immersion fluid into the gap using spray nozzles, electro-kinetic sponges, porous materials, etc. and removes the fluid from the gap using vacuum pumps, sponges, and the like. The design of the environmental system  26  can vary. For example, it can inject the immersion fluid at one or more locations at or near the gap. Further, the immersion fluid system can assist in removing and/or scavenging the immersion fluid at one or more locations at or near the work piece  30 , the gap and/or the edge of the optical assembly  16 . For additional details on various environmental systems, see U.S. provisional patent application 60/462,142 entitled “Immersion Lithography Fluid Control System” filed on Apr. 9, 2003, 60/462,112 entitled “Vacuum Ring System and Wick Ring System for Immersion Lithography” filed on Apr. 10, 2003, 60/500,312 entitled “Noiseless Fluid Recovery With Porous Material” filed on Sep. 3, 2003, and 60/541,329 entitled “Nozzle Design for Immersion Lithography” filed on Feb. 2, 2004, all incorporated by reference herein in their entireties.  
         [0037]     Referring to  FIG. 2 , a cross section of a lithography machine illustrating one embodiment of the invention is shown. The lithography machine  200  includes a optical assembly  16  and a stage assembly  202  that includes a wafer table  204  and a wafer stage  206 . The wafer table  204  is configured to support a wafer  208  (or any other type of work piece) under the optical assembly  16 . An environmental system  26 , surrounding the optical assembly  16 , is used to supply and remove immersion fluid  212  from the gap between the wafer  208  and the last optical element of the optical assembly  16 . A work piece exchange system  216 , including a wafer loader  218  (i.e., a robot) and an alignment tool  220  (i.e., a microscope and CCD camera), is configured to remove the wafer  208  on the wafer table  204  and replace it with a second wafer. This is typically accomplished using the wafer loader  218  to lift and remove the wafer  208  from the wafer table  204 . Subsequently, the second wafer (not shown) is placed onto the wafer chuck  218 , aligned using the alignment tool  220 , and then positioned under the optical assembly  16  on the wafer table  204 .  
         [0038]     With this embodiment, the wafer stage  206  includes an immersion fluid containment system  214  that is configured to maintain the immersion fluid  212  in the gap adjacent to the last optical element of the optical assembly  16  during wafer exchange. The immersion fluid containment system  214  includes a pad  222  that is adjacent to the wafer table  204 . A support member  224 , provided between the pad  222  and the wafer stage  206 , is used to support the pad  222 . The wafer table  204  has a flat upper surface that is coplanar with a surface of the wafer  208 . The pad  222  also has a flat upper surface that is coplanar with the upper surface of the wafer table  204  and the wafer surface. The pad  222  is arranged adjacent to the wafer table  204  with a very small gap (e.g., 0.1-1.0 mm) so that the immersion fluid  212  is movable between the wafer table  204  and the pad  222  without leaking. During a wafer exchange, the wafer stage  206  is moved in the direction of arrow  226  so that the pad  222  is positioned under the optical assembly  16  in place of the wafer table  204 , maintaining the fluid in the gap or maintaining the size of the fluid gap. After the new wafer has been aligned, the wafer stage is moved back to its original position so that the pad  222  is removed from the gap as the second wafer is positioned under the optical assembly  16 . In various embodiments, the pad  222  is disposed continuously adjacent to the wafer table  204  with no gap. Vertical position and/or tilt of the wafer table  204  can be adjusted so that the wafer table surface is coplanar with the pad surface, before the wafer table  204  is moved out from under the optical assembly  16 . Maintaining the gap between the pad  222  and the optical assembly  16  is not limited to just a wafer exchange operation. The pad  222  can be large enough to maintain the immersion fluid  212  in the space between the pad  222  and the optical assembly  16  during an alignment operation or a measurement operation. In those operations, a part of the area occupied by the immersion fluid  212  may be on the upper surface of the wafer table  204 .  
         [0039]     Referring to  FIGS. 3A and 3B , a cross section and a top down view of another immersion lithography machine according to another embodiment of the present invention are shown. The lithography machine  300  includes an optical assembly  16  and a stage assembly  302  that includes a wafer table  304  and a wafer stage  306 . The wafer table  304  is configured to support a wafer  308  (or any other type of work piece) under the optical assembly  16 . An environmental system  26 , surrounding the optical assembly  16 , is used to supply and remove immersion fluid  312  from the gap between the wafer  308  and the lower most optical element of the optical assembly  16 . A work piece exchange system  316 , including a wafer loader  318  and an alignment tool  320 , is configured to remove the wafer  308  on the wafer table  304  and replace it with a second wafer. This is accomplished using the wafer loader  318  to remove the wafer  308  from the wafer table. Subsequently, the second wafer (not shown) is placed onto the wafer chuck  318 , aligned using the alignment tool  320 , and then positioned under the optical assembly  16 . As best illustrated in  FIG. 3B , a set of motors  322  are used to move the wafer assembly  302  including the wafer table  304  and wafer stage  306  in two degrees of freedom (X and Y) during operation. As noted above, the motors  322  can be any type of motors, such as linear motors, rotary motors, voice coil motors, etc.  
         [0040]     The immersion lithography machine  300  also includes an immersion fluid containment system  324  that is configured to maintain the immersion fluid  312  in the space below the optical assembly  16  while the wafer table  304  is away from under the optical assembly. The immersion fluid containment system  324  includes a pad  326 , a motor  328 , and a control system  330 . The pad  326  can be positioned adjacent to the optical assembly  16  and the wafer table  304 . The wafer table  304  has a flat upper surface that is coplanar with a surface of the wafer  308 . The pad  326  has a flat upper surface that is coplanar with the upper surface of the wafer table  304  and the wafer surface. The pad  326  is movable in the X and Y directions using the motor  328 , which is controlled by the control system  330 . The motor  328  can be any type of motor as well as the motors  322 . The pad  326  is positioned under the optical assembly  16  when the wafer table  304  (the wafer stage  306 ) is away from under the optical assembly  16 . During a wafer exchange, the wafer table  304  moves away from the optical assembly  16 . Simultaneously, the control system  330  directs the motor  328  to move pad  326  under the optical assembly  16 , replacing the wafer table  304 . The pad  326  thus retains the immersion fluid  312  within the gap under the optical assembly  16 . After the new wafer has been aligned using the alignment tool  320 , the wafer table  304  is repositioned under the optical assembly  16 . At the same time, the control system  330  directs the motor  328  to retract the pad  326  from the gap, preventing the escape of the immersion fluid  312 . In the wafer exchange operation, the control system  330  moves the wafer table  304  and the pad  326  with a small gap between the wafer table  304  and the pad  326 , while the immersion fluid  312  below the optical assembly  16  moves between the wafer table  304  and the pad  326 . The immersion fluid containment system  324  thus maintains the immersion fluid  312  from the gap during wafer exchange. In this embodiment, the wafer table  304  (the wafer stage  306 ) and the pad  326  are movable separately. Therefore, the wafer table  304  is movable freely while the immersion fluid  312  is maintained in the space between the pad  326  and the optical assembly  16 . In various embodiments of the invention, the control system  330  may be a separate control system or it can be integrated into the control system used to control the motors  322  for positioning the wafer stage  306  and wafer table  304 . Vertical position and/or tilt of at least one of the wafer table  304  and the pad  326  may be adjusted so that the wafer table surface is coplanar with the pad surface, before the wafer table is moved out from under the optical assembly  16 . The operation, in which the wafer table  304  is away from the optical assembly  16 , is not necessarily limited to a wafer exchange operation. For example, an alignment operation, a measurement operation or other operation may be executed while maintaining the immersion fluid  312  in the space between the pad  326  and the optical assembly  16 .  
         [0041]     Referring to  FIGS. 4A and 4B , two cross sections of an immersion lithography machine are shown. The lithography machine  400  includes an optical assembly  16  and a stage assembly  402  that includes a wafer table  404  and a wafer stage  406 . The wafer table  404  is configured to support a wafer  408  (or any other type of work piece) under the optical assembly  16 . An environmental system  26  ( 410 ), surrounding the optical assembly  16 , is used to supply and remove immersion fluid  412  from the gap between the wafer  408  and the lower most optical element of the optical assembly  16 . A work piece exchange system  416 , including a wafer loader  418  and an alignment tool  420 , is configured to remove the wafer  408  on the wafer table  404  and replace it with a second wafer. This is accomplished using the wafer loader  418  to remove the wafer  408  from the wafer table  404 . Subsequently, the second wafer (not shown) is placed onto the wafer chuck  418 , aligned using the alignment tool  420 , and then positioned under the optical assembly  16  as illustrated in the  FIG. 4A .  
         [0042]     The immersion lithography machine  400  also includes an immersion fluid containment system  424  that is configured to maintain the immersion fluid  412  in the space below the optical assembly  16  while the wafer table  404  is away from under the optical assembly  16 . The immersion fluid containment system  424  includes a pad  426 , a first clamp  428  provided on the optical assembly  16  and a second clamp  430  provided on the wafer table  404 . When the immersion fluid  412  is between the optical assembly  16  and the wafer table  404  (or the wafer  408 ), the pad  426  is held by the second clamp  430  in place on the wafer table  404 . When the wafer table  404  is away from the optical assembly  16 , for example during a wafer exchange operation, the pad  426  is detached from the wafer table  404  and held by the first clamp  428  to maintain the immersion fluid  412  between the optical assembly  16  and the pad  426 . The wafer table  404  has a flat upper surface that is coplanar with a surface of the wafer  408 . The pad  426  held on the wafer table  404  also has a flat upper surface that is coplanar with the upper surface of the wafer table  404  and the wafer surface. Therefore, the immersion pad  426  and wafer  408  can be moved under the optical assembly without the immersion fluid leaking. In various embodiments, the clamps  428  and  430  can be vacuum clamps, magnetic, electrostatic, or mechanical.  
         [0043]     As best illustrated in  FIG. 4A , the pad  426  is positioned on the wafer table  404  during exposure of the wafer  408 . The second clamp  430  is used to hold the pad  426  in place on the table  404  during the wafer exposure. During a wafer exchange as illustrated in  FIG. 4B , the wafer table  404  is moved in the direction of arrow  432  so that the pad  426  is positioned under the optical assembly  16  in place of the wafer  408 . When this occurs, the second clamp  430  holding the pad  426  to the wafer table  404  is released while first clamp  428  clamps the pad  426  to the optical assembly  16 . As a result, the immersion fluid  412  is maintained under the optical assembly while the wafer  408  is exchanged. After the new wafer has been aligned, the wafer table  404  is moved in the direction opposite arrow  432  so that the new wafer is positioned under the optical assembly. Prior to this motion, the first clamp  428  is released while the second clamp  430  again clamps the pad  426  to the wafer table  404 . In this embodiment, the wafer table  404  is freely movable while the pad  426  is clamped by the first clamp  428 .  
         [0044]     In various embodiments, the operation, in which the pad  426  is clamped by the first clamp  428 , is not limited to only a wafer exchange operation. An alignment operation, a measurement operation, or any other operation can be executed while the immersion fluid  412  is maintained in the space between the optical assembly  16  and the pad  426  clamped by the first clamp  428 . Also, the clamp  428  can be provided on the frame  12  or other support member, and the clamp  430  can be provided on the wafer stage  406 . The pad  426  can be held on a movable member other than the stage assembly  402 .  
         [0045]      FIGS. 5A and 5B  are top down views of two different twin stage immersion lithography systems according to other embodiments of the present invention. For the basic structure and operation of the twin stage lithography systems, see U.S. Pat. No. 6,262,796 and U.S. Pat. No. 6,341,007. The disclosures of U.S. Pat. No. 6,262,796 and U.S. Pat. No. 6,341,007 are incorporated herein by reference in their entireties. In both embodiments, a pair of wafer stages WS 1  and WS 2  are shown. Motors  502  are used to move or position the two stages WS 1  and WS 2  in the horizontal direction (in the drawings), whereas motors  504  are used to move or position the stages WS 1  and WS 2  in the vertical direction (in the drawings). The motors  502  and  504  are used to alternatively position one stage under the optical assembly  16  while a wafer exchange and alignment is performed on the other stage. When the exposure of the wafer under the optical assembly  16  is complete, then the two stages are swapped and the above process is repeated. With either configuration, the various embodiments of the invention for maintaining immersion fluid in the gap under the optical assembly  16  as described and illustrated above with regard to  FIGS. 2 through 4 , can be used with either twin stage arrangement. With regard the embodiment of  FIG. 2  for example, each wafer stage SW 1  and SW 2  of either  FIG. 5A  or  5 B can be modified to include a pad  222  and a support member  224 . With regard to the embodiment of  FIG. 3 , a single pad  326 , motor  328 , and control system  330  could be used adjacent to the optical assembly  16 . The pad  326  is movable separately from the stages SW 1  and SW 2 . During the time when stages SW 1  and SW 2  are to be swapped, the pad  326  is moved to under the optical assembly  16  to maintain the immersion fluid  312  below the optical assembly  16 . Finally with the embodiment of  FIG. 4 , a detachable single pad can be used. During the time when stages SW 1  and SW 2  are to be swapped, the pad  426  is used to maintain the immersion fluid in the gap as illustrated in  FIG. 4B . On the other hand during exposure, the pad is clamped onto the wafer table on the wafer stage that is being exposed. In this manner, only a single pad is needed for the two stages WS 1  and WS 2 . Alternatively, as described below, the second stage can also be used as the pad.  
         [0046]     Referring to  FIG. 6A , a top down view of a twin stage lithography machine illustrating one embodiment of practicing the invention is shown. In this embodiment, the immersion lithography system  600  includes first stage  604  and second stage  606 . The two stages are moved in the X and Y directions by motors  602 . In this embodiment, the stages  604  and  606  themselves are used to contain the immersion fluid in the gap. For example as shown in the Figure, the first stage  604  is positioned under the optical assembly  16 . When it is time for the work piece to be exchanged, the motors  602  are used to position the second stage  606  with a second work piece adjacent to the first stage  604 . With the two stages positioned side-by-side, they substantially form a continuous surface. The motors  602  are then used to move the two stages in unison so that the second stage  604  is position under the optical assembly  16  and the first stage is no longer under the optical assembly  16 . Thus when the first work piece is moved away from the optical assembly  16 , the immersion fluid in the gap is maintained by the second stage  606 , which forms the substantially continuous surface with the first stage. In various alternative embodiments, the second stage  606  could also be a “pad” stage that contains a pad that is used to maintain the immersion liquid in the gap while a second work piece is being placed onto the first stage  604 . Similarly, the motor arrangement shown in either  FIG. 5A  or  5 B could be used.  
         [0047]     Referring to  FIGS. 6B-6E , a series of diagrams illustrating a work piece exchange according to one embodiment of the invention is illustrated.  FIG. 6B  shows a wafer on stage  604  after exposure is completed.  FIG. 6C  shows the second stage  606  in contact (or immediately adjacent) with the first stage  604  under the optical assembly  16 .  FIG. 6C  shows a transfer taking place, i.e., the second stage  606  is positioned under the optical assembly  16 . Finally, in  FIG. 6E , the first stage  604  is moved away from the optical assembly  16 . As best illustrated in  FIGS. 6C and 6D , the two stages  604  and  606  provide a continuous surface under the optical assembly  16  during a transfer, thus maintaining the immersion fluid in the gap. In the embodiment shown, the second stage  606  is a pad stage. This stage, however, could also be a work piece stage as noted above.  
         [0048]     In the various embodiments described above, the pad can be made of a number of different materials, such as ceramic, metal, plastic. These materials may also be coated with Teflon according to other embodiments. The size of the pad also should be sufficient to cover the area occupied by the immersion fluid. In the various embodiments described above, the surface of the last optical element of the optical assembly  16  is constantly under immersion fluid environment, preventing the formation of a fluid mark (e.g. “a water mark”).  
         [0049]     Semiconductor wafers can be fabricated using the above described systems, by the process shown generally in  FIG. 7A . In step  701  the work piece&#39;s function and performance characteristics are designed. Next, in step  702 , a mask (reticle) having a pattern is designed according to the previous designing step, and in a parallel step  703  a wafer is made from a silicon material. The mask pattern designed in step  702  is exposed onto the wafer from step  703  in step  704  by a photolithography system described hereinabove in accordance with the invention. In step  705  the semiconductor work piece is assembled (including the dicing process, bonding process and packaging process); finally, the work piece is then inspected in step  706 .  
         [0050]      FIG. 7B  illustrates a detailed flowchart example of the above-mentioned step  704  in the case of fabricating semiconductor work pieces. In  FIG. 7B , in step  711  (oxidation step), the wafer surface is oxidized. In step  712  (CVD step), an insulation film is formed on the wafer surface. In step  713  (electrode formation step), electrodes are formed on the wafer by vapor deposition. In step  714  (ion implantation step), ions are implanted in the wafer. The above mentioned steps  711 - 714  form the preprocessing steps for wafers during wafer processing, and selection is made at each step according to processing requirements.  
         [0051]     At each stage of wafer processing, when the above-mentioned preprocessing steps have been completed, the following post-processing steps are implemented. During post-processing, first, in step  715  (photoresist formation step), photoresist is applied to a wafer. Next, in step  716  (exposure step), the above-mentioned exposure work piece is used to transfer the circuit pattern of a mask (reticle) to a wafer. Then in step  717  (developing step), the exposed wafer is developed, and in step  718  (etching step), parts other than residual photoresist (exposed material surface) are removed by etching. In step  719  (photoresist removal step), unnecessary photoresist remaining after etching is removed.  
         [0052]     Multiple circuit patterns are formed by repetition of these preprocessing and post-processing steps.  
         [0053]     While the particular lithography machines as shown and disclosed herein are fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that they are merely illustrative embodiments of the invention, and that the invention is not limited to these embodiments.